Kwon, Eun Young, John P Dunne, and Kitack Lee, March 2024: Biological export production controls upper ocean calcium carbonate dissolution and CO2 buffer capacity. Science Advances, 10(13), DOI:10.1126/sciadv.adl0779. Abstract
Marine biogenic calcium carbonate (CaCO3) cycles play a key role in ecosystems and in regulating the ocean’s ability to absorb atmospheric carbon dioxide (CO2). However, the drivers and magnitude of CaCO3 cycling are not well understood, especially for the upper ocean. Here, we provide global-scale evidence that heterotrophic respiration in settling marine aggregates may produce localized undersaturated microenvironments in which CaCO3 particles rapidly dissolve, producing excess alkalinity in the upper ocean. In the deep ocean, dissolution of CaCO3 is primarily driven by conventional thermodynamics of CaCO3 solubility with reduced fluxes of CaCO3 burial to marine sediments beneath more corrosive North Pacific deep waters. Upper ocean dissolution, shown to be sensitive to ocean export production, can increase the neutralizing capacity for respired CO2 by up to 6% in low-latitude thermocline waters. Without upper ocean dissolution, the ocean might lose 20% more CO2 to the atmosphere through the low-latitude upwelling regions.
Estimating global river solids, nitrogen (N), and phosphorus (P), in both quantity and composition, is necessary for understanding the development and persistence of many harmful algal blooms, hypoxic events, and other water quality issues in inland and coastal waters. This requires a comprehensive freshwater model that can resolve intertwined algae, solid, and nutrient dynamics, yet previous global watershed models have limited mechanistic resolution of instream biogeochemical processes. Here we develop the global, spatially explicit, and process-based Freshwater Algae, Nutrient, and Solid cycling and Yields (FANSY) model and incorporate it within the Land Model (LM3). The resulting model, LM3-FANSY v1.0, is intended as a baseline for eventual linking of global terrestrial and ocean biogeochemistry in next-generation Earth system models to project global changes that may challenge empirical approaches. LM3-FANSY explicitly resolves interactions between algae, N, P, and solid dynamics in rivers and lakes at 1° spatial and 30 min temporal resolution. Simulated suspended solids (SS), N, and P in multiple forms (particulate or dissolved, organic or inorganic) agree well with measurement-based yield (kg km−2 yr−1), load (kt yr−1), and concentration (mg L−1) estimates across a globally distributed set of large rivers, with an accuracy comparable to other global nutrient and SS models. Furthermore, simulated global river loads of SS, N, and P in different forms to the coastal ocean are consistent with published ranges, though regional biases are apparent. River N loads are estimated to contain approximately equal contributions by dissolved inorganic N (41 %) and dissolved organic N (39 %), with a lesser contribution by particulate organic N (20 %). For river P load estimates, particulate P, which includes both organic and sorbed inorganic forms, is the most abundant form (64 %), followed by dissolved inorganic and organic P (25 % and 11 %). Time series analysis of river solid and nutrient loads in large US rivers for the period ∼ 1963–2000 demonstrates that simulated SS and N loads in different N forms covary with variations of measurement-based loads. LM3-FANSY, however, has less capability to capture interannual variability of P loads, likely due to the lack of terrestrial P dynamics in LM3. Analyses of the model results and sensitivity to components, parameters, and inputs suggest that fluxes from terrestrial litter and soils, wastewater, and weathering are the most critical inputs to the fidelity of simulated river nutrient loads for observation-based estimates. Sensitivity analyses further demonstrate a critical role of algal dynamics in controlling the ratios of inorganic and organic nutrient forms in freshwaters. While the simulations are able to capture significant cross-watershed contrasts at a global scale, disagreement for individual rivers can be substantial. This limitation is shared by other global river models and could be ameliorated through further refinements in nutrient sources, freshwater model dynamics, and observations. Current targets for future LM3-FANSY development include the additions of terrestrial P dynamics, freshwater carbon, alkalinity, enhanced sediment dynamics, and anthropogenic hydraulic controls.
Lennon, Jay T., Rose Z Abramoff, Steven D Allison, Rachel M Burckhardt, Kristen M DeAngelis, and John P Dunne, et al., May 2024: Priorities, opportunities, and challenges for integrating microorganisms into Earth system models for climate change prediction. mBio, 15(5), DOI:10.1128/mbio.00455-24. Abstract
Climate change jeopardizes human health, global biodiversity, and sustainability of the biosphere. To make reliable predictions about climate change, scientists use Earth system models (ESMs) that integrate physical, chemical, and biological processes occurring on land, the oceans, and the atmosphere. Although critical for catalyzing coupled biogeochemical processes, microorganisms have traditionally been left out of ESMs. Here, we generate a “top 10” list of priorities, opportunities, and challenges for the explicit integration of microorganisms into ESMs. We discuss the need for coarse-graining microbial information into functionally relevant categories, as well as the capacity for microorganisms to rapidly evolve in response to climate-change drivers. Microbiologists are uniquely positioned to collect novel and valuable information necessary for next-generation ESMs, but this requires data harmonization and transdisciplinary collaboration to effectively guide adaptation strategies and mitigation policy.
We present a variable-resolution global chemistry-climate model (AM4VR) developed at NOAA's Geophysical Fluid Dynamics Laboratory (GFDL) for research at the nexus of US climate and air quality extremes. AM4VR has a horizontal resolution of 13 km over the US, allowing it to resolve urban-to-rural chemical regimes, mesoscale convective systems, and land-surface heterogeneity. With the resolution gradually reducing to 100 km over the Indian Ocean, we achieve multi-decadal simulations driven by observed sea surface temperatures at 50% of the computational cost for a 25-km uniform-resolution grid. In contrast with GFDL's AM4.1 contributing to the sixth Coupled Model Intercomparison Project at 100 km resolution, AM4VR features much improved US climate mean patterns and variability. In particular, AM4VR shows improved representation of: precipitation seasonal-to-diurnal cycles and extremes, notably reducing the central US dry-and-warm bias; western US snowpack and summer drought, with implications for wildfires; and the North American monsoon, affecting dust storms. AM4VR exhibits excellent representation of winter precipitation, summer drought, and air pollution meteorology in California with complex terrain, enabling skillful prediction of both extreme summer ozone pollution and winter haze events in the Central Valley. AM4VR also provides vast improvements in the process-level representations of biogenic volatile organic compound emissions, interactive dust emissions from land, and removal of air pollutants by terrestrial ecosystems. We highlight the value of increased model resolution in representing climate–air quality interactions through land-biosphere feedbacks. AM4VR offers a novel opportunity to study global dimensions to US air quality, especially the role of Earth system feedbacks in a changing climate.
Pelagic tunicates (salps, pyrosomes) and fishes generate jelly falls and/or fecal pellets that sink roughly 10 times faster than bulk oceanic detritus, but their impacts on biogeochemical cycles in the ocean interior are poorly understood. Using a coupled physical-biogeochemical model, we find that fast-sinking detritus decreased global net primary production and surface export, but increased deep sequestration and transfer efficiency in much of the extratropics and upwelling zones. Fast-sinking detritus generally decreased total suboxic and hypoxic volumes, reducing a “large oxygen minimum zone (OMZ)” bias common in global biogeochemical models. Newly aerobic regions at OMZ edges exhibited reduced transfer efficiencies in contrast with global tendencies. Reductions in water column denitrification resulting from improved OMZs improved simulated nitrate deficits relative to phosphate. The carbon flux to the benthos increased by 11% with fast-sinking detritus from fishes and pelagic tunicates, yet simulated benthic fluxes remained on the lower end of observation-based estimates.
Mariotti, Annarita, David Bader, Susanne E Bauer, Gokhan Danabasoglu, John P Dunne, Brian D Gross, L Ruby Leung, S Pawson, William M Putman, V Ramaswamy, Gavin A Schmidt, and Vijay Tallapragada, June 2024: Envisioning U.S. climate predictions and projections to meet new challenges. Earth's Future, 12(6), DOI:10.1029/2023EF004187. Abstract
In the face of a changing climate, the understanding, predictions, and projections of natural and human systems are increasingly crucial to prepare and cope with extremes and cascading hazards, determine unexpected feedbacks and potential tipping points, inform long-term adaptation strategies, and guide mitigation approaches. Increasingly complex socio-economic systems require enhanced predictive information to support advanced practices. Such new predictive challenges drive the need to fully capitalize on ambitious scientific and technological opportunities. These include the unrealized potential for very high-resolution modeling of global-to-local Earth system processes across timescales, reduction of model biases, enhanced integration of human systems and the Earth Systems, better quantification of predictability and uncertainties; expedited science-to-service pathways, and co-production of actionable information with stakeholders. Enabling technological opportunities include exascale computing, advanced data storage, novel observations and powerful data analytics, including artificial intelligence and machine learning. Looking to generate community discussions on how to accelerate progress on U.S. climate predictions and projections, representatives of Federally-funded U.S. modeling groups outline here perspectives on a six-pillar national approach grounded in climate science that builds on the strengths of the U.S. modeling community and agency goals. This calls for an unprecedented level of coordination to capitalize on transformative opportunities, augmenting and complementing current modeling center capabilities and plans to support agency missions. Tangible outcomes include projections with horizontal spatial resolutions finer than 10 km, representing extremes and associated risks in greater detail, reduced model errors, better predictability estimates, and more customized projections to support next generation climate services.
Global Earth system models are often enlisted to assess the impacts of climate variability and change on marine ecosystems. In this study, we compare high frequency (daily) outputs of potential ecosystem stressors, such as sea surface temperature and surface pH, and associated variables from an Earth system model (GFDL ESM4.1) with high frequency time series from a global network of moorings to directly assess the capacity of the model to resolve local biogeochemical variability on time scales from daily to interannual. Our analysis indicates variability in surface temperature is most consistent between ESM4.1 and observations, with a Pearson correlation coefficient of 0.93 and bias of 0.40°C, followed by variability in surface salinity. Physical variability is reproduced with greater accuracy than biogeochemical variability, and variability on seasonal and longer time scales is more consistent between the model and observations than higher frequency variability. At the same time, the well-resolved seasonal and longer timescale variability is a reasonably good predictor, in many cases, of the likelihood of extreme events. Despite limited model representation of high frequency variability, model and observation-based assessments of the fraction of days experiencing surface T-pH and T-Ωarag multistressor conditions show reasonable agreement, depending on the stressor combination and threshold definition. We also identify circumstances in which some errors could be reduced by accounting for model biases.
Schultz, Cristina, John P Dunne, Xiao Liu, Elizabeth J Drenkard, and Brendan R Carter, February 2024: Characterizing subsurface oxygen variability in the California Current System (CCS) and its links to water mass distribution. Journal of Geophysical Research: Oceans, 129(2), DOI:10.1029/2023JC020000. Abstract
The California current system (CCS) supports a wide array of ecosystem services with hypoxia historically occurring in near-bottom waters. Limited open ocean data coverage hinders the mechanistic understanding of CCS oxygen variability. By comparing three different models with varying horizontal resolutions, we found that dissolved oxygen (DO) anomalies in the CCS are propagated from shallower coastal areas to the deeper open ocean, where they are advected at a density and velocity consistent with basin-scale circulation. Since DO decreases have been linked to water mass redistribution in the CCS, we conduct a water mass analysis on two of the models and on biogeochemical Argo floats that sampled multiple seasonal cycles. We found that high variability in biogeochemical variables (DO and nutrients) seen in regions of low variability of temperature and salinity could be linked to water mass mixing, as some of the water masses considered had higher gradients in biogeochemical variables compared to physical variables. Additional DO observations are needed, therefore, to further understand circulation changes in the CCS. We suggest that increased DO sampling north of 35˚N and near the shelf break would benefit model initialization and skill assessment, as well as allow for better assessment of the role of equatorial waters in driving DO in the northern CCS.
We describe the baseline model configuration and simulation characteristics of the Geophysical Fluid Dynamics Laboratory (GFDL)'s Land Model version 4.1 (LM4.1), which builds on component and coupled model developments over 2013–2019 for the coupled carbon-chemistry-climate Earth System Model Version 4.1 (ESM4.1) simulation as part of the sixth phase of the Coupled Model Intercomparison Project. Analysis of ESM4.1/LM4.1 is focused on biophysical and biogeochemical processes and interactions with climate. Key features include advanced vegetation dynamics and multi-layer canopy energy and moisture exchanges, daily fire, land use representation, and dynamic atmospheric dust coupling. We compare LM4.1 performance in the GFDL Earth System Model (ESM) configuration ESM4.1 to the previous generation component LM3.0 in the ESM2G configuration. ESM4.1/LM4.1 provides significant improvement in the treatment of ecological processes from GFDL's previous generation models. However, ESM4.1/LM4.1 likely overestimates the influence of land use and land cover change on vegetation characteristics, particularly on pasturelands, as it overestimates the competitiveness of grasses versus trees in the tropics, and as a result, underestimates present-day biomass and carbon uptake in comparison to observations.
Buchovecky, Benjamin, Graeme A MacGilchrist, Mitchell Bushuk, F Alexander Haumann, Thomas L Frölicher, Natacha Le Grix, and John P Dunne, October 2023: Potential predictability of the spring bloom in the Southern Ocean sea ice zone. Geophysical Research Letters, 50(20), DOI:10.1029/2023GL105139. Abstract
Every austral spring when Antarctic sea ice melts, favorable growing conditions lead to an intense phytoplankton bloom, which supports much of the local marine ecosystem. Recent studies have found that Antarctic sea ice is predictable several years in advance, suggesting that the spring bloom might exhibit similar predictability. Using a suite of perfect model predictability experiments, we find that November net primary production (NPP) is potentially predictable 7 to 10 years in advance in many Southern Ocean regions. Sea ice extent predictability peaks in late winter, followed by absorbed shortwave radiation and NPP with a 2 to 3 months lag. This seasonal progression of predictability supports our hypothesis that sea ice and light limitation control the inherent predictability of the spring bloom. Our results suggest skillful interannual predictions of NPP may be achievable, with implications for managing fisheries and the marine ecosystem, and guiding conservation policy in the Southern Ocean.
Cornwall, Christopher E., Steeve Comeau, Simon D Donner, Chris Perry, John P Dunne, Ruben von Hooidonk, James S Ryan, and Cheryl A Logan, June 2023: Coral adaptive capacity insufficient to halt global transition of coral reefs into net erosion under climate change. Global Change Biology, 29(11), DOI:10.1111/gcb.166473010-3018. Abstract
Projecting the effects of climate change on net reef calcium carbonate production is critical to understanding the future impacts on ecosystem function, but prior estimates have not included corals' natural adaptive capacity to such change. Here we estimate how the ability of symbionts to evolve tolerance to heat stress, or for coral hosts to shuffle to favourable symbionts, and their combination, may influence responses to the combined impacts of ocean warming and acidification under three representative concentration pathway (RCP) emissions scenarios (RCP2.6, RCP4.5 and RCP8.5). We show that symbiont evolution and shuffling, both individually and when combined, favours persistent positive net reef calcium carbonate production. However, our projections of future net calcium carbonate production (NCCP) under climate change vary both spatially and by RCP. For example, 19%–35% of modelled coral reefs are still projected to have net positive NCCP by 2050 if symbionts can evolve increased thermal tolerance, depending on the RCP. Without symbiont adaptive capacity, the number of coral reefs with positive NCCP drops to 9%–13% by 2050. Accounting for both symbiont evolution and shuffling, we project median positive NCPP of coral reefs will still occur under low greenhouse emissions (RCP2.6) in the Indian Ocean, and even under moderate emissions (RCP4.5) in the Pacific Ocean. However, adaptive capacity will be insufficient to halt the transition of coral reefs globally into erosion by 2050 under severe emissions scenarios (RCP8.5).
Deposition of mineral dust plays an important role in upper-ocean biogeochemical processes, particularly by delivering iron to iron-limited regions. Here we examine the impact of dynamically changing iron deposition on tropical Pacific Ocean biogeochemistry in fully coupled earth system model projections under several emissions scenarios. Projected end-of-21st-century increases in central tropical Pacific dust and iron deposition strengthen with increasing emissions/radiative forcing, and are aligned with projected soil moisture decreases in adjacent land areas and precipitation increases over the equatorial Pacific. Increased delivery of soluble iron results in a reduction in, and eastward contraction of, equatorial Pacific phytoplankton iron limitation and shifts primary production and particulate organic carbon flux projections relative to a high emissions projection (SSP5-8.5) wherein soluble iron deposition is prescribed as a static climatology. These results highlight modeling advances in representing coupled land-air-sea interactions to project basin-scale patterns of ocean biogeochemical change.
Dunne, John P., November 2023: Physical mechanisms driving enhanced carbon sequestration by the biological pump under climate warming. Global Biogeochemical Cycles, 37(11), DOI:10.1029/2023GB007859. Abstract
As ocean Carbon Dioxide Removal techniques are being considered, it is critical that they be evaluated against our scientific understanding of the global biological carbon pump. In a recent paper Nowicki et al. (2022, https://doi.org/10.1029/2021GB007083) provide an innovative and comprehensive breakdown of the different mechanistic pathways of carbon sequestration through the present-day biological pump but then speculate that “These results suggest that ocean carbon storage will weaken as the oceans stratify and the subtropical gyres expand due to anthropogenic climate change.” Essentially, the authors combine their steady state result that oligotrophic subtropical gyres have lower residence times than other areas with the climate change result of these areas increasing under climate warming and extrapolate—assuming “all else is equal”—that the overall ocean will suffer a reduction in carbon sequestration efficiency. Expressing global changes in carbon sequestered by the ocean's biological pump as the summation of local changes in the sequestered carbon, timescale of return to the surface, and biogeographical area, I discuss how all three terms are tightly coupled, and summarize decades of climate change modeling consistently indicating that the global scale physical sequestration response is an increase - in opposition of what one would infer from changes in subtropical area alone.
Dunne, John P., Helene T Hewitt, Susann Tegtmeier, Catherine A Senior, Tatiana Ilyina, Baylor Fox-Kemper, and Eleanor O'Rourke, November 2023: Climate Projections in Next Phase of the Coupled Model Intercomparison Project. World Meteorological Organization Bulletin, 72(2), . Abstract
There is an urgent need for large ensemble climate models with high resolution and fidelity as nations consider local climate mitigation and adaptation efforts for energy efficiency and resource transitioning.
Stratospheric injections of sulfur dioxide from major volcanic eruptions perturb the Earth's global radiative balance and dominate variability in stratospheric sulfur loading. The atmospheric component of the GFDL Earth System Model (ESM4.1) uses a bulk aerosol scheme and previously prescribed the distribution of aerosol optical properties in the stratosphere. To quantify volcanic contributions to the stratospheric sulfur cycle and the resulting climate impact, we modified ESM4.1 to simulate stratospheric sulfate aerosols prognostically. Driven by explicit volcanic emissions of aerosol precursors and non-volcanic sources, we conduct ESM4.1 simulations from 1989 to 2014, with a focus on the Mt. Pinatubo eruption. We evaluate our interactive representation of the stratospheric sulfur cycle against data from Moderate Resolution Imaging Spectroradiometer, Multi-angle Imaging SpectroRadiometer, Advanced Very High Resolution Radiometer, High Resolution Infrared Radiation Sounder, and Stratospheric Aerosol and Gas Experiment II. To assess the key processes associated with volcanic aerosols, we performed a sensitivity analysis of sulfate burden from the Mt. Pinatubo eruption by varying injection heights, emission amount, and stratospheric sulfate's dry effective radius. We find that the simulated stratospheric sulfate mass burden and aerosol optical depth in the model are sensitive to these parameters, especially volcanic SO2 injection height, and the optimal combination of parameters depends on the metric we evaluate.
Jiang, Li-Qing, and John P Dunne, et al., March 2023: Global Surface Ocean Acidification Indicators From 1750 to 2100. Journal of Advances in Modeling Earth Systems, 15(3), DOI:10.1029/2022MS003563. Abstract
Accurately predicting future ocean acidification (OA) conditions is crucial for advancing OA research at regional and global scales, and guiding society's mitigation and adaptation efforts. This study presents a new model-data fusion product covering 10 global surface OA indicators based on 14 Earth System Models (ESMs) from the Coupled Model Intercomparison Project Phase 6 (CMIP6), along with three recent observational ocean carbon data products. The indicators include fugacity of carbon dioxide, pH on total scale, total hydrogen ion content, free hydrogen ion content, carbonate ion content, aragonite saturation state, calcite saturation state, Revelle Factor, total dissolved inorganic carbon content, and total alkalinity content. The evolution of these OA indicators is presented on a global surface ocean 1° × 1° grid as decadal averages every 10 years from preindustrial conditions (1750), through historical conditions (1850–2010), and to five future Shared Socioeconomic Pathways (2020–2100): SSP1-1.9, SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5. These OA trajectories represent an improvement over previous OA data products with respect to data quantity, spatial and temporal coverage, diversity of the underlying data and model simulations, and the provided SSPs. The generated data product offers a state-of-the-art research and management tool for the 21st century under the combined stressors of global climate change and ocean acidification.
Planchat, Alban, Lester Kwiatkowski, Laurent Bopp, Olivier Torres, James R Christian, Momme Butenschön, Tomas Lovato, Roland Séférian, Matthew A Chamberlain, Olivier Aumont, Michio Watanabe, Akitomo Yamamoto, Andrew Yool, Tatiana Ilyina, Hiroyuki Tsujino, Kristen M Krumhardt, Jörg Schwinger, Jerry Tjiputra, John P Dunne, and Charles A Stock, April 2023: The representation of alkalinity and the carbonate pump from CMIP5 to CMIP6 Earth system models and implications for the carbon cycle. Biogeosciences, 20(7), DOI:10.5194/bg-20-1195-2023. Abstract
Ocean alkalinity is critical to the uptake of atmospheric carbon in surface waters and provides buffering capacity towards the associated acidification. However, unlike dissolved inorganic carbon (DIC), alkalinity is not directly impacted by anthropogenic carbon emissions. Within the context of projections of future ocean carbon uptake and potential ecosystem impacts, especially through Coupled Model Intercomparison Projects (CMIPs), the representation of alkalinity and the main driver of its distribution in the ocean interior, the calcium carbonate cycle, have often been overlooked. Here we track the changes from CMIP5 to CMIP6 with respect to the Earth system model (ESM) representation of alkalinity and the carbonate pump which depletes the surface ocean in alkalinity through biological production of calcium carbonate and releases it at depth through export and dissolution. We report an improvement in the representation of alkalinity in CMIP6 ESMs relative to those in CMIP5, with CMIP6 ESMs simulating lower surface alkalinity concentrations, an increased meridional surface gradient and an enhanced global vertical gradient. This improvement can be explained in part by an increase in calcium carbonate (CaCO3) production for some ESMs, which redistributes alkalinity at the surface and strengthens its vertical gradient in the water column. We were able to constrain a particulate inorganic carbon (PIC) export estimate of 44–55 Tmol yr−1 at 100 m for the ESMs to match the observed vertical gradient of alkalinity. Reviewing the representation of the CaCO3 cycle across CMIP5/6, we find a substantial range of parameterizations. While all biogeochemical models currently represent pelagic calcification, they do so implicitly, and they do not represent benthic calcification. In addition, most models simulate marine calcite but not aragonite. In CMIP6, certain model groups have increased the complexity of simulated CaCO3 production, sinking, dissolution and sedimentation. However, this is insufficient to explain the overall improvement in the alkalinity representation, which is therefore likely a result of marine biogeochemistry model tuning or ad hoc parameterizations. Although modellers aim to balance the global alkalinity budget in ESMs in order to limit drift in ocean carbon uptake under pre-industrial conditions, varying assumptions related to the closure of the budget and/or the alkalinity initialization procedure have the potential to influence projections of future carbon uptake. For instance, in many models, carbonate production, dissolution and burial are independent of the seawater saturation state, and when considered, the range of sensitivities is substantial. As such, the future impact of ocean acidification on the carbonate pump, and in turn ocean carbon uptake, is potentially underestimated in current ESMs and is insufficiently constrained.
Sharp, Jonathan D., Andrea J Fassbender, Brendan R Carter, Gregory C Johnson, Cristina Schultz, and John P Dunne, October 2023: GOBAI-O2: Temporally and spatially resolved fields of ocean interior dissolved oxygen over nearly 2 decades. Earth System Science Data, 15(10), DOI:10.5194/essd-15-4481-20234481–4518. Abstract
For about 2 decades, oceanographers have been installing oxygen sensors on Argo profiling floats to be deployed throughout the world ocean, with the stated objective of better constraining trends and variability in the ocean's inventory of oxygen. Until now, measurements from these Argo-float-mounted oxygen sensors have been mainly used for localized process studies on air–sea oxygen exchange, upper-ocean primary production, biological pump efficiency, and oxygen minimum zone dynamics. Here, we present a new four-dimensional gridded product of ocean interior oxygen, derived via machine learning algorithms trained on dissolved oxygen observations from Argo-float-mounted sensors and discrete measurements from ship-based surveys and applied to temperature and salinity fields constructed from the global Argo array. The data product is called GOBAI-O2, which stands for Gridded Ocean Biogeochemistry from Artificial Intelligence – Oxygen (Sharp et al., 2022; https://doi.org/10.25921/z72m-yz67); it covers 86 % of the global ocean area on a 1∘ × 1∘ (latitude × longitude) grid, spans the years 2004–2022 with a monthly resolution, and extends from the ocean surface to a depth of 2 km on 58 levels. Two types of machine learning algorithms – random forest regressions and feed-forward neural networks – are used in the development of GOBAI-O2, and the performance of those algorithms is assessed using real observations and simulated observations from Earth system model output. Machine learning represents a relatively new method for gap filling ocean interior biogeochemical observations and should be explored along with statistical and interpolation-based techniques. GOBAI-O2 is evaluated through comparisons to the oxygen climatology from the World Ocean Atlas, the mapped oxygen product from the Global Ocean Data Analysis Project and to direct observations from large-scale hydrographic research cruises. Finally, potential uses for GOBAI-O2 are demonstrated by presenting average oxygen fields on isobaric and isopycnal surfaces, average oxygen fields across vertical–meridional sections, climatological seasonal cycles of oxygen averaged over different pressure layers, and globally integrated time series of oxygen. GOBAI-O2 indicates a declining trend in the oxygen inventory in the upper 2 km of the global ocean of 0.79 ± 0.04 % per decade between 2004 and 2022.
Takano, Yohei, Tatiana Ilyina, Jerry Tjiputra, Yassir A Eddebbar, Sarah Berthet, Laurent Bopp, Erik T Buitenhuis, Momme Butenschön, James R Christian, John P Dunne, Matthias Gröger, Hakase Hayashida, Jenny Hieronymus, Torben Koenigk, and John P Krasting, et al., November 2023: Simulations of ocean deoxygenation in the historical era: insights from forced and coupled models. Frontiers in Marine Science, 10, DOI:10.3389/fmars.2023.1139917. Abstract
Ocean deoxygenation due to anthropogenic warming represents a major threat to marine ecosystems and fisheries. Challenges remain in simulating the modern observed changes in the dissolved oxygen (O2). Here, we present an analysis of upper ocean (0-700m) deoxygenation in recent decades from a suite of the Coupled Model Intercomparison Project phase 6 (CMIP6) ocean biogeochemical simulations. The physics and biogeochemical simulations include both ocean-only (the Ocean Model Intercomparison Project Phase 1 and 2, OMIP1 and OMIP2) and coupled Earth system (CMIP6 Historical) configurations. We examine simulated changes in the O2 inventory and ocean heat content (OHC) over the past 5 decades across models. The models simulate spatially divergent evolution of O2 trends over the past 5 decades. The trend (multi-model mean and spread) for upper ocean global O2 inventory for each of the MIP simulations over the past 5 decades is 0.03 ± 0.39×1014 [mol/decade] for OMIP1, −0.37 ± 0.15×1014 [mol/decade] for OMIP2, and −1.06 ± 0.68×1014 [mol/decade] for CMIP6 Historical, respectively. The trend in the upper ocean global O2 inventory for the latest observations based on the World Ocean Database 2018 is −0.98×1014 [mol/decade], in line with the CMIP6 Historical multi-model mean, though this recent observations-based trend estimate is weaker than previously reported trends. A comparison across ocean-only simulations from OMIP1 and OMIP2 suggests that differences in atmospheric forcing such as surface wind explain the simulated divergence across configurations in O2 inventory changes. Additionally, a comparison of coupled model simulations from the CMIP6 Historical configuration indicates that differences in background mean states due to differences in spin-up duration and equilibrium states result in substantial differences in the climate change response of O2. Finally, we discuss gaps and uncertainties in both ocean biogeochemical simulations and observations and explore possible future coordinated ocean biogeochemistry simulations to fill in gaps and unravel the mechanisms controlling the O2 changes.
Tebaldi, Claudia, Guðfinna Aðalgeirsdóttir, Sybren Drijfhout, and John P Dunne, et al., May 2023: The hazard components of representative key risks. The physical climate perspective. Climate Risk Management, 40, DOI:10.1016/j.crm.2023.100516. Abstract
The framework of Representative Key Risks (RKRs) has been adopted by the Intergovernmental Panel on Climate Change Working Group II (WGII) to categorize, assess and communicate a wide range of regional and sectoral key risks from climate change. These are risks expected to become severe due to the potentially detrimental convergence of changing climate conditions with the exposure and vulnerability of human and natural systems. Other papers in this special issue treat each of eight RKRs holistically by assessing their current status and future evolution as a result of this convergence. However, in these papers, such assessment cannot always be organized according to a systematic gradation of climatic changes. Often the big-picture evolution of risk has to be extrapolated from either qualitative effects of “low”, “medium” and “high” warming, or limited/focused analysis of the consequences of particular mitigation choices (e.g., benefits of limiting warming to 1.5 or 2C), together with consideration of the socio-economic context and possible adaptation choices.
In this study we offer a representation – as systematic as possible given current literature and assessments – of the future evolution of the hazard components of RKRs.
We identify the relevant hazards for each RKR, based upon the WGII authors’ assessment, and we report on their current state and expected future changes in magnitude, intensity and/or frequency, linking these changes to Global Warming Levels (GWLs) to the extent possible. We draw on the assessment of changes in climatic impact-drivers relevant to RKRs described in the 6th Assessment Report by Working Group I supplemented when needed by more recent literature.
For some of these quantities - like regional trends in oceanic and atmospheric temperature and precipitation, some heat and precipitation extremes, permafrost thaw and Northern Hemisphere snow cover - a strong and quantitative relationship with increasing GWLs has been identified. For others - like frequency and intensity of tropical cyclones and extra-tropical storms, and fire weather - that link can only be described qualitatively. For some processes - like the behavior of ice sheets, or changes in circulation dynamics - large uncertainties about the effects of different GWLs remain, and for a few others - like ocean pH and air pollution - the composition of the scenario of anthropogenic emissions is most relevant, rather than the warming reached. In almost all cases, however, the basic message remains that every small increment in CO2 concentration in the atmosphere and associated warming will bring changes in climate phenomena that will contribute to increasing risk of impacts on human and natural systems, in the absence of compensating changes in these systems’ exposure and vulnerability, and in the absence of effective adaptation. Our picture of the evolution of RKR-relevant climatic impact-drivers complements and enriches the treatment of RKRs in the other papers in at least two ways: by filling in their often only cursory or limited representation of the physical climate aspects driving impacts, and by providing a fuller representation of their future potential evolution, an important component – if never the only one – of the future evolution of risk severity.
Tiny phytoplankton are the base of ocean production and thus critical to carbon storage, carbon fluxes and living marine resources. Now, research suggests that the vertical migration of these organisms provides a previously under-recognized resiliency to climate warming.
Fassbender, Andrea J., Sarah Schlunegger, Keith B Rodgers, and John P Dunne, June 2022: Quantifying the role of seasonality in the marine carbon cycle feedback: An ESM2M case study. Global Biogeochemical Cycles, 36(6), DOI:10.1029/2021GB007018. Abstract
Observations and climate models indicate that changes in the seasonal amplitude of sea surface carbon dioxide partial pressure (A-pCO2) are underway and driven primarily by anthropogenic carbon (Cant) accumulation in the ocean. This occurs because pCO2 is more responsive to seasonal changes in physics (including warming) and biology in an ocean that contains more Cant. A-pCO2 changes have the potential to alter annual ocean carbon uptake and contribute to the overall marine carbon cycle feedback. Using the GFDL ESM2M Large Ensemble and a novel analysis framework, we quantify the influence of Cant accumulation on pCO2 seasonal cycles and sea-air CO2 fluxes. Specifically, we reconstruct alternative evolutions of the contemporary ocean state in which the sensitivity of pCO2 to seasonal thermal and biophysical variation is fixed at preindustrial levels, however the background, mean-state pCO2 fully responds to anthropogenic forcing. We find near-global A-pCO2 increases of >100% by 2100, under RCP8.5 forcing, with rising Cant accounting for ∼100% of thermal and ∼50% of nonthermal pCO2 component amplitude changes. The other ∼50% of nonthermal pCO2 component changes are attributed to modeled changes in ocean physics and biology caused by climate change. Cant-induced A-pCO2 changes cause an 8.1 ± 0.4% (ensemble mean ± 1σ) increase in ocean carbon uptake by 2100. This is because greater wintertime wind speeds enhance the impact of wintertime pCO2 changes, which work to increase the ocean carbon sink. Thus, the seasonal ocean carbon cycle feedback works in opposition to the larger, mean-state feedback that reduces ocean carbon uptake by ∼60%.
The extension of seasonal to interannual prediction of the physical climate system to include the marine ecosystem has a great potential to inform marine resource management strategies. Along the east coast of Africa, recent findings suggest that skillful Earth system model (ESM)-based chlorophyll predictions may enable anticipation of fisheries fluctuations. The mechanisms underlying skillful chlorophyll predictions, however, were not identified, eroding confidence in potential adaptive management steps. This study demonstrates that skillful chlorophyll predictions up to two years in advance arise from the successful simulation of westward-propagating off-equatorial Rossby waves in the Indian ocean. Upwelling associated with these waves supplies nutrients to the surface layer for the large coastal areas by generating north- and southward propagating waves at the east African coast. Further analysis shows that the off-equatorial Rossby wave is initially excited by wind stress forcing caused by El Niño/Southern Oscillation-Indian Ocean teleconnections.
Ocean acidification is a consequence of the absorption of anthropogenic carbon emissions and it profoundly impacts marine life. Arctic regions are particularly vulnerable to rapid pH changes due to low ocean buffering capacities and high stratification. Here, an unsupervised machine learning methodology is applied to simulations of surface Arctic acidification from two state-of-the-art coupled climate models. We identify four sub-regions whose boundaries are influenced by present-day and projected sea ice patterns. The regional boundaries are consistent between the models and across lower (SSP2-4.5) and higher (SSP5-8.5) carbon emissions scenarios. Stronger trends toward corrosive surface waters in the central Arctic Ocean are driven by early summer warming in regions of annual ice cover and late summer freshening in regions of perennial ice cover. Sea surface salinity and total alkalinity reductions dominate the Arctic pH changes, highlighting the importance of objective sub-regional identification and subsequent analysis of surface water mass properties.
The El Niño-Southern Oscillation (ENSO) strongly influences phytoplankton in the tropical Pacific, with El Niño conditions suppressing productivity in the equatorial Pacific (EP) and placing nutritional stresses on marine ecosystems. The Geophysical Fluid Dynamics Laboratory's (GFDL) Earth System Model version 4.1 (ESM4.1) captures observed ENSO-chlorophyll patterns (r = 0.57) much better than GFDL's previous ESM2M (r = 0.23). Most notably, the observed post-El Niño “chlorophyll rebound” is substantially improved in ESM4.1 (r = 0.52). We find that an anomalous increase in iron propagation from western Pacific (WP) subsurface to the cold tongue via the equatorial undercurrent (EUC) and subsequent post-El Niño surfacing, unresolved in ESM2M, is the primary driver of chlorophyll rebound. We also find that this chlorophyll rebound is augmented by high post-El Niño dust-iron deposition anomalies in the eastern EP. This post-El Niño chlorophyll rebound provides a previously unrecognized source of marine ecosystem resilience independent from the La Niña that sometimes follows.
Lim, Hyung-Gyu, John P Dunne, Charles A Stock, and Minho Kwon, October 2022: Attribution and predictability of climate-driven variability in global ocean color. Journal of Geophysical Research: Oceans, 127(10), DOI:10.1029/2022JC019121. Abstract
For over two decades, satellite ocean color missions have revealed spatio-temporal variations in marine chlorophyll. Seasonal cycles and interannual changes of the physical environment drive the nutrient and chlorophyll variations. In order to identify contributions of seasonal and interannual components on chlorophyll, the present study investigates total chlorophyll variance (TCV) of a 24 year records (September 1997 to December 2021) across satellite generations. First-order contributions of the seasonal cycle in the mid-latitude (25°–35°) oceans in the Northern and Southern Hemispheres explain 59.5% and 69.9% of TCV, respectively. In contrast, the contribution of seasonal cycle only explain 30.9% in the tropical oceans (20°N–20°S). Both seasonal cycle- and climate-driven variability (26.3%) explain 57.2% on TCV in the tropical oceans. A multiple linear regression model was forced by instantaneous and delayed effects of oceanic memory of eight climate indices based on sea surface temperature anomalies to reconstruct chlorophyll anomalies. Delayed climate effects generally boost the anomaly correlation coefficients (ACC) between the observed and reconstructed chlorophyll timeseries (ACC skills: 0.64 to 0.72 in the Indian Ocean, 0.74 to 0.82 in off-equatorial Northern Pacific, and 0.58 to 0.71 in the off-equatorial Southern Pacific). Such delayed climate effects provide a source of predicted chlorophyll ACC (ACC_predic) skills one season ahead in some ocean regions (ACC_predic skill: 0.63 in the overall tropical ocean, 0.67 in the tropical Pacific, and 0.60 in the Indian Ocean). The attribution of chlorophyll variability indicates promising avenues for improving marine ecosystem predictions with Earth system models by incorporating delayed climate effects.
The pelagic tunicates, gelatinous zooplankton that include salps, doliolids, and appendicularians, are filter feeding grazers thought to produce a significant amount of particulate organic carbon (POC) detritus. However, traditional sampling methods (i.e., nets), have historically underestimated their abundance, yielding an overall underappreciation of their global biomass and contribution to ocean biogeochemical cycles relative to crustacean zooplankton. As climate change is projected to decrease the average plankton size and POC export from traditional plankton food webs, the ecological and biogeochemical role of pelagic tunicates may increase; yet, pelagic tunicates were not resolved in the previous generation of global earth system climate projections. Here we present a global ocean study using a coupled physical-biogeochemical model to assess the impact of pelagic tunicates in the pelagic food web and biogeochemical cycling. We added two tunicate groups, a large salp/doliolid and a small appendicularian to the NOAA-GFDL Carbon, Ocean Biogeochemistry, and Lower Trophics version 2 (COBALTv2) model, which was originally formulated to represent carbon flows to crustacean zooplankton. The new GZ-COBALT simulation was able to simultaneously satisfy new pelagic tunicate biomass constraints and existing ecosystem constraints, including crustacean zooplankton observations. The model simulated a global tunicate biomass of 0.10 Pg C, annual tunicate production of 0.49 Pg C y-1 in the top 100 m, and annual tunicate detritus production of 0.98 Pg C y-1 in the top 100 m. Tunicate-mediated export flux was 0.71 Pg C y-1, representing 11% of the total export flux past 100 m. Overall export from the euphotic zone remained largely constant, with the GZ-COBALT pe-ratio only increasing 5.3% (from 0.112 to 0.118) compared to the COBALTv2 control. While the bulk of the tunicate-mediated export production resulted from the rerouting of phytoplankton- and mesozooplankton-mediated export, tunicates also shifted the overall balance of the upper oceans away from recycling and towards export. Our results suggest that pelagic tunicates play important trophic roles in both directly competing with microzooplankton and indirectly shunting carbon export away from the microbial loop.
Climate change can drive shifts in the seasonality of marine productivity, with consequences for the marine food web. However, these alterations in phytoplankton bloom phenology (initiation and peak timing), and the underlying drivers, are not well understood. Here, using a 30-member Large Ensemble of climate change projections, we show earlier bloom initiation in most ocean regions, yet changes in bloom peak timing vary widely by region. Shifts in both initiation and peak timing are induced by a subtle decoupling between altered phytoplankton growth and zooplankton predation, with increased zooplankton predation (top-down control) playing an important role in altered bloom peak timing over much of the global ocean. Only in limited regions is light limitation a primary control for bloom initiation changes. In the extratropics, climate-change-induced phenological shifts will exceed background natural variability by the end of the twenty-first century, which may impact energy flow in the marine food webs.
Zhang, Wenxia, John P Dunne, Hui Wu, Feng Zhou, and Daji Huang, January 2022: Using timescales of deficit and residence to evaluate near-bottom dissolved oxygen variation in coastal seas. Journal of Geophysical Research: Biogeosciences, 127(1), DOI:10.1029/2021JG006408. Abstract
We identify the local, vertical and lateral processes that cause bottom oxygen variation, and characterize timescales for each process using oxygen budget analysis based on a coupled physical-biogeochemical model for the coastal seas in east China. Local oxygen deficit often occurs in the summer season due to the faster local consumption than the vertical replenishment in seasonal timescale. Lateral transport of oxygen-rich ambient water replenishes local deficit of bottom dissolved oxygen. Competition between local deficit and lateral exchange determines seasonal hypoxia formation and sustainment. Short local consumption timescale is favorable for hypoxia formation, and transient hypoxia often forms when the local deficit and lateral exchange processes act on comparable timescales, such as the East China Sea in which the bottom hypoxia usually lasts for days. Extremely long lateral exchange timescale suggests that dissolved oxygen variation is predominantly controlled by local processes, and short local consumption timescale often causes increasingly severe seasonal hypoxia until the onset of the relaxation of local deficit, such as the bottom hypoxia in the Bohai Sea. Using these timescales to evaluate local deficit relative to vertical and lateral residence time and their variability is a convenient and potentially powerful general mechanistic framework to evaluate strategies to mitigate coastal hypoxia worldwide.
Zhang, Wenxia, John P Dunne, Hui Wu, and Feng Zhou, June 2022: Regional projection of climate warming effects on coastal seas in east China. Environmental Research Letters, 17(7), DOI:10.1088/1748-9326/ac7344. Abstract
The coastal region in east China experiences massive anthropogenic eutrophication, and the bottom water off the Changjiang River Estuary in the East China Sea faces the threat of severe seasonal hypoxia. We find that projected future climate changes will work in parallel with anthropogenic eutrophication to exacerbate current hypoxia and increase shelf vulnerability to bottom hypoxia. We use a coupled physical-biogeochemical regional model to investigate the differences of shelf hydrography and oxygen dynamics between present and future projected states. Model results indicate that the Yellow Sea Cold Water Mass which plays essential roles in nekton migration and shellfish farming practically disappears by the end of the 21st century, and shelf vertical stratification strengthens by an average of 12.7%. Hypoxia off the Changjiang River Estuary is exacerbated with increased (by one month) hypoxia duration, lower dissolved oxygen minima, and significant lateral (202%) and vertical (60%) expansions of hypoxic water. Reduced oxygen solubility, and accelerated oxygen consumption under increased primary production and rising water temperature contribute 42% and 58%, respectively, of bottom dissolved oxygen decrease in the East China Sea. Model results also show increased vertical diffusion of oxygen, despite vertical stratification strengthening, due to increased surface-bottom oxygen concentration gradient associated with increased oxygen release in surface water and exacerbated oxygen consumption in subsurface water.
Eyring, Veronika, Nathan P Gillett, Krishna M Achuta Rao, Rondrotiana Barimalala, Marcelo Barreiro Parrillo, Nicolas Bellouin, Christophe Cassou, Paul J Durack, Yu Kosaka, Shayne McGregor, Seung-Ki Min, Olaf Morgenstern, Ying Sun, and John P Dunne, et al., August 2021: Human Influence on the Climate System In Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, , Cambridge, United Kingdom and New York, NY, USA, Cambridge University Press, DOI:10.1017/9781009157896.005423-552.
Ilyina, Tatiana, Hongmei Li, Aaron Spring, Wolfgang A Müller, Laurent Bopp, Megumi O Chikamoto, Gokhan Danabasoglu, Mikhail Dobrynin, and John P Dunne, et al., March 2021: Predictable variations of the carbon sinks and atmospheric CO2 growth in a multi‐model framework. Geophysical Research Letters, 48(6), DOI:10.1029/2020GL090695. Abstract
Inter‐annual to decadal variability in the strength of the land and ocean carbon sinks impede accurate predictions of year‐to‐year atmospheric carbon dioxide (CO2) growth rate. Such information is crucial to verify the effectiveness of fossil fuel emissions reduction measures. Using a multi‐model framework comprising prediction systems initialized by the observed state of the physical climate, we find a predictive skill for the global ocean carbon sink of up to 6 years for some models. Longer regional predictability horizons are found across single models. On land, a predictive skill of up to 2 years is primarily maintained in the tropics and extra‐tropics enabled by the initialization of the physical climate. We further show that anomalies of atmospheric CO2 growth rate inferred from natural variations of the land and ocean carbon sinks are predictable at lead time of 2 years and the skill is limited by the land carbon sink predictability horizon.
Lee, June-Yi, Jochem Marotzke, Govindasamy Bala, Long Cao, Susanna Corti, John P Dunne, Francois Engelbrecht, Erich Fischer, John C Fyfe, Christopher Jones, Amanda Maycock, Joseph Mutemi, Ousmane Ndiaye, Swapna Panickal, and Tianjun Zhou, et al., August 2021: Future Global Climate: Scenario-based Projections and Near-term Information In Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, Cambridge, United Kingdom and New York, NY, USA, Cambridge University Press, DOI:10.1017/9781009157896.006553-672.
Lim, Hyung-Gyu, Jong-Yeon Park, John P Dunne, and Charles A Stock, et al., May 2021: Importance of human-induced nitrogen flux increases for simulated Arctic warming. Journal of Climate, 34(10), DOI:10.1175/JCLI-D-20-0180.13799-3819. Abstract
Human activities such as fossil fuel combustion, land-use change, nitrogen (N) fertilizer use, emission of livestock, and waste excretion accelerate the transformation of reactive N and its impact on the marine environment. This study elucidates that anthropogenic N fluxes (ANFs) from atmospheric and river deposition exacerbate Arctic warming and sea ice loss via physical–biological feedback. The impact of physical–biological feedback is quantified through a suite of experiments using a coupled climate–ocean–biogeochemical model (GFDL-CM2.1-TOPAZ) by prescribing the preindustrial and contemporary amounts of riverine and atmospheric N fluxes into the Arctic Ocean. The experiment forced by ANFs represents the increase in ocean N inventory and chlorophyll concentrations in present and projected future Arctic Ocean relative to the experiment forced by preindustrial N flux inputs. The enhanced chlorophyll concentrations by ANFs reinforce shortwave attenuation in the upper ocean, generating additional warming in the Arctic Ocean. The strongest responses are simulated in the Eurasian shelf seas (Kara, Barents, and Laptev Seas; 65°–90°N, 20°–160°E) due to increased N fluxes, where the annual mean surface temperature increase by 12% and the annual mean sea ice concentration decrease by 17% relative to the future projection, forced by preindustrial N inputs.
Over the past century, human activities have resulted in substantial global changes that threaten the stability and functionality of coastal habitats. One of these impacts was through nutrient pollution of river runoffs, which have triggered harmful algal blooms and caused low-oxygen conditions in many coastal regions. However, it is challenging for models to simulate coastal impacts of increasing river nutrient loads, especially on a global scale and over a long period of time. Here we take advantage of some recent modeling advances to provide a global perspective on coastal ecosystem responses to increasing river nitrogen loads over the half-century between 1961 and 2010. Overall, we show that the global coastal ocean accumulated more nitrogen over time as river nitrogen loads increased. This caused the primary production of the global coastal system (i.e., the conversion of inorganic to organic materials through photosynthesis) to increase as well. However, we found that the sensitivity of each coastal ecosystem to comparable changes in nitrogen loads varied considerably. This variability was to a large extent related to two factors: the rate of exchange between coastal waters and the adjacent ocean waters, and whether nutrients are limited for phytoplankton to conduct photosynthesis in that system.
Logan, Cheryl A., John P Dunne, James S Ryan, Marissa L Baskett, and Simon D Donner, May 2021: Quantifying global potential for coral evolutionary response to climate change. Nature Climate Change, DOI:10.1038/s41558-021-01037-2. Abstract
Incorporating species’ ability to adaptively respond to climate change is critical for robustly predicting persistence. One such example could be the adaptive role of algal symbionts in setting coral thermal tolerance under global warming and ocean acidification. Using a global ecological and evolutionary model of competing branching and mounding coral morphotypes, we show symbiont shuffling (towards taxa with increased heat tolerance) was more effective than symbiont evolution in delaying coral-cover declines, but stronger warming rates (high emissions scenarios) outpace the ability of these adaptive processes and limit coral persistence. Acidification has a small impact on reef degradation rates relative to warming. Global patterns in coral reef vulnerability to climate are sensitive to the interaction of warming rate and adaptive capacity and cannot be predicted by either factor alone. Overall, our results show how models of spatially resolved adaptive mechanisms can inform conservation decisions.
Morgan, Eric J., Manfredi Manizza, Ralph F Keeling, Laure Resplandy, Sara E Mikaloff-Fletcher, Cynthia D Nevison, Yuming Jin, Jonathan D Bent, Olivier Aumont, Scott C Doney, John P Dunne, Jasmin G John, Ivan D Lima, Matthew C Long, and Keith B Rodgers, August 2021: An atmospheric constraint on the seasonal air–sea exchange of oxygen and heat in the extratropics. Journal of Geophysical Research: Oceans, 126(8), DOI:10.1029/2021JC017510. Abstract
Typically, the surface of the ocean releases oxygen to the atmosphere during summer and takes it up during winter. This cycle is driven by circulation, biology (photosynthesis and respiration), and the seasonal cycle in water temperature, which changes the solubility of oxygen in surface water. We have used measurements of two atmospheric tracers, one which tracks oxygen and one which tracks heat, to estimate the amount of oxygen taken up or released by a change in ocean heat content. By looking at ocean models and atmospheric observations of the two atmospheric tracers, we find that the oxygen exchange between the ocean and atmosphere in the Southern Hemisphere is more responsive to changes in heat content than in the Northern Hemisphere. These hemispheric metrics are useful tests of how ocean models simulate some biological and physical processes.
Ranasinghe, Roshanka, Alex C Ruane, Robert Vautard, Nigel Arnell, Erika Coppola, Faye Abigail Cruz, Suraje Dessai, Akm Saiful Islam, Mohammad Rahimi, Daniel Ruiz Carrascal, Jana Sillmann, Mouhamadou Bamba Sylla, Claudia Tebaldi, Wen Wang, Rashyd Zaaboul, and John P Dunne, et al., August 2021: Climate Change Information for Regional Impact and for Risk Assessment In Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Che, Cambridge, United Kingdom and New York, NY, USA, Cambridge University Press, DOI:10.1017/9781009157896.0141767-1926.
Saba, Grace K., Adrian B Burd, and John P Dunne, et al., May 2021: Toward a better understanding of fish-based contribution to ocean carbon flux. Limnology and Oceanography, 66(5), DOI:10.1002/lno.11709. Abstract
Fishes are the dominant vertebrates in the ocean, yet we know little of their contribution to carbon export flux at regional to global scales. We synthesize the existing information on fish-based carbon flux in coastal and pelagic waters, identify gaps and challenges in measuring this flux and approaches to address them, and recommend research priorities. Based on our synthesis of passive (fecal pellet sinking) and active (migratory) flux of fishes, we estimated that fishes contribute an average (± standard deviation) of about 16.1% (± 13%) to total carbon flux out of the euphotic zone. Using the mean value of model-generated global carbon flux estimates, this equates to an annual flux of 1.5 ± 1.2 Pg C yr−1. High variability in estimations of the fish-based contribution to total carbon flux among previous field studies and reported here highlight significant methodological variations and observational gaps in our present knowledge. Community-adopted methodological standards, improved and more frequent measurements of biomass and passive and active fluxes of fishes, and stronger linkages between observations and models will decrease uncertainty, increase our confidence in the estimation of fish-based carbon flux, and enable identification of controlling factors to account for spatial and temporal variability. Better constraints on this key component of the biological pump will provide a baseline for understanding how ongoing climate change and harvest will affect the role fishes play in carbon flux.
Tittensor, Derek P., Camilla Novaglio, Cheryl S Harrison, Ryan F Heneghan, Nicolas Barrier, Daniele Bianchi, Laurent Bopp, Andrea Bryndum-Buchholz, Gregory L Britten, Matthias Büchner, William W L Cheung, Villy Christensen, Marta Coll, John P Dunne, Tyler D Eddy, Jason D Everett, Jose A Fernandes-Salvador, Elizabeth A Fulton, Eric D Galbraith, Didier Gascuel, Jerome Guiet, Jasmin G John, Jason S Link, Heike K Lotze, Olivier Maury, Kelly Ortega-Cisneros, Juliano Palacios-Abrantes, Colleen M Petrik, Hubert du Pontavice, Jonathan Rault, Anthony J Richardson, Lynne Shannon, Yunne-Jai Shin, Jeroen Steenbeek, Charles A Stock, and Julia L Blanchard, October 2021: Next-generation ensemble projections reveal higher climate risks for marine ecosystems. Nature Climate Change, DOI:10.1038/s41558-021-01173-9. Abstract
Projections of climate change impacts on marine ecosystems have revealed long-term declines in global marine animal biomass and unevenly distributed impacts on fisheries. Here we apply an enhanced suite of global marine ecosystem models from the Fisheries and Marine Ecosystem Model Intercomparison Project (Fish-MIP), forced by new-generation Earth system model outputs from Phase 6 of the Coupled Model Intercomparison Project (CMIP6), to provide insights into how projected climate change will affect future ocean ecosystems. Compared with the previous generation CMIP5-forced Fish-MIP ensemble, the new ensemble ecosystem simulations show a greater decline in mean global ocean animal biomass under both strong-mitigation and high-emissions scenarios due to elevated warming, despite greater uncertainty in net primary production in the high-emissions scenario. Regional shifts in the direction of biomass changes highlight the continued and urgent need to reduce uncertainty in the projected responses of marine ecosystems to climate change to help support adaptation planning.
July 2019 saw record-breaking wildfires burning 3,600 km2 in Alaska. The GFDL Earth system model indicates a threefold increased risk of Alaska’s
extreme fires during recent decades due to primarily anthropogenic ignition and secondarily climate-induced biofuel abundance.
The Southern Ocean south of 30° S represents only one-third of the total ocean area, yet absorbs half of the total ocean anthropogenic carbon and over two-thirds of ocean anthropogenic heat. In the past, the Southern Ocean has also been one of the most sparsely measured regions of the global ocean. Here we use pre-2005 ocean shipboard measurements alongside novel observations from autonomous floats with biogeochemical sensors to calculate changes in Southern Ocean temperature, salinity, pH and concentrations of nitrate, dissolved inorganic carbon and oxygen over two decades. We find local warming of over 3 °C, salinification of over 0.2 psu near the Antarctic coast, and isopycnals are found to deepen between 65° and 40° S. We find deoxygenation along the Antarctic coast, but reduced deoxygenation and nitrate concentrations where isopycnals deepen farther north. The forced response of the Earth system model ESM2M does not reproduce the observed patterns. Accounting for meltwater and poleward-intensifying winds in ESM2M improves reproduction of the observed large-scale changes, demonstrating the importance of recent changes in wind and meltwater. Future Southern Ocean biogeochemical changes are likely to be influenced by the relative strength of meltwater input and poleward-intensifying winds. The combined effect could lead to increased Southern Ocean deoxygenation and nutrient accumulation, starving the global ocean of nutrients sooner than otherwise expected.
We compare equilibrium climate sensitivity (ECS) estimates from pairs of long (≥800‐year) control and abruptly quadrupled CO2 simulations with shorter (150‐ and 300‐year) coupled atmosphere‐ocean simulations and slab ocean models (SOMs). Consistent with previous work, ECS estimates from shorter coupled simulations based on annual averages for years 1–150 underestimate those from SOM (−8% ± 13%) and long (−14% ± 8%) simulations. Analysis of only years 21–150 improved agreement with SOM (−2% ± 14%) and long (−8% ± 10%) estimates. Use of pentadal averages for years 51–150 results in improved agreement with long simulations (−4% ± 11%). While ECS estimates from current generation U.S. models based on SOM and coupled annual averages of years 1–150 range from 2.6°C to 5.3°C, estimates based longer simulations of the same models range from 3.2°C to 7.0°C. Such variations between methods argues for caution in comparison and interpretation of ECS estimates across models.
We describe the baseline coupled model configuration and simulation characteristics of GFDL's Earth System Model Version 4.1 (ESM4.1), which builds on component and coupled model developments at GFDL over 2013–2018 for coupled carbon‐chemistry‐climate simulation contributing to the sixth phase of the Coupled Model Intercomparison Project. In contrast with GFDL's CM4.0 development effort that focuses on ocean resolution for physical climate, ESM4.1 focuses on comprehensiveness of Earth system interactions. ESM4.1 features doubled horizontal resolution of both atmosphere (2° to 1°) and ocean (1° to 0.5°) relative to GFDL's previous‐generation coupled ESM2‐carbon and CM3‐chemistry models. ESM4.1 brings together key representational advances in CM4.0 dynamics and physics along with those in aerosols and their precursor emissions, land ecosystem vegetation and canopy competition, and multiday fire; ocean ecological and biogeochemical interactions, comprehensive land‐atmosphere‐ocean cycling of CO2, dust and iron, and interactive ocean‐atmosphere nitrogen cycling are described in detail across this volume of JAMES and presented here in terms of the overall coupling and resulting fidelity. ESM4.1 provides much improved fidelity in CO2 and chemistry over ESM2 and CM3, captures most of CM4.0's baseline simulations characteristics, and notably improves on CM4.0 in (1) Southern Ocean mode and intermediate water ventilation, (2) Southern Ocean aerosols, and (3) reduced spurious ocean heat uptake. ESM4.1 has reduced transient and equilibrium climate sensitivity compared to CM4.0. Fidelity concerns include (1) moderate degradation in sea surface temperature biases, (2) degradation in aerosols in some regions, and (3) strong centennial scale climate modulation by Southern Ocean convection.
Simulation of coupled carbon‐climate requires representation of ocean carbon cycling, but the computational burden of simulating the dozens of prognostic tracers in state‐of‐the‐art biogeochemistry ecosystem models can be prohibitive. We describe a six‐tracer biogeochemistry module of steady‐state phytoplankton and zooplankton dynamics in Biogeochemistry with Light, Iron, Nutrients and Gas (BLING version 2) with particular emphasis on enhancements relative to the previous version and evaluate its implementation in Geophysical Fluid Dynamics Laboratory's (GFDL) fourth‐generation climate model (CM4.0) with ¼° ocean. Major geographical and vertical patterns in chlorophyll, phosphorus, alkalinity, inorganic and organic carbon, and oxygen are well represented. Major biases in BLINGv2 include overly intensified production in high‐productivity regions at the expense of productivity in the oligotrophic oceans, overly zonal structure in tropical phosphorus, and intensified hypoxia in the eastern ocean basins as is typical in climate models. Overall, while BLINGv2 structural limitations prevent sophisticated application to plankton physiology, ecology, or biodiversity, its ability to represent major organic, inorganic, and solubility pumps makes it suitable for many coupled carbon‐climate and biogeochemistry studies including eddy interactions in the ocean interior. We further overview the biogeochemistry and circulation mechanisms that shape carbon uptake over the historical period. As an initial analysis of model historical and idealized response, we show that CM4.0 takes up slightly more anthropogenic carbon than previous models in part due to enhanced ventilation in the absence of an eddy parameterization. The CM4.0 biogeochemistry response to CO2 doubling highlights a mix of large declines and moderate increases consistent with previous models.
Frölicher, Thomas L., L Ramseyer, C C Raible, Keith B Rodgers, and John P Dunne, April 2020: Potential predictability of marine ecosystem drivers. Biogeosciences, 17(7), DOI:10.5194/bg-17-2061-2020. Abstract
Climate variations can have profound impacts on marine ecosystems and the socio-economic systems that may depend upon them. Temperature, pH, oxygen (O2) and net primary production (NPP) are commonly considered to be important marine ecosystem drivers, but the potential predictability of these drivers is largely unknown. Here, we use a comprehensive Earth system model within a perfect modelling framework to show that all four ecosystem drivers are potentially predictable on global scales and at the surface up to 3 years in advance. However, there are distinct regional differences in the potential predictability of these drivers. Maximum potential predictability (> 10 years) is found at the surface for temperature and O2 in the Southern Ocean and for temperature, O2 and pH in the North Atlantic. This is tied to ocean overturning structures with memory or inertia with enhanced predictability in winter. Additionally, these four drivers are highly potentially predictable in the Arctic Ocean at surface. In contrast, minimum predictability is simulated for NPP (< 1 years) in the Southern Ocean. Potential predictability for temperature, O2 and pH increases with depth to more than 10 years below the thermocline, except in the tropical Pacific and Indian Ocean, where predictability is also three to five years in the thermocline. This study indicating multi-year (at surface) and decadal (subsurface) potential predictability for multiple ecosystem drivers is intended as a foundation to foster broader community efforts in developing new predictions of marine ecosystem drivers.
Frölicher, Thomas L., M T Aschwanden, Nicolas Gruber, Samuel Jaccard, John P Dunne, and David J Paynter, August 2020: Contrasting upper and deep ocean oxygen response to protracted global warming. Global Biogeochemical Cycles, 34(8), DOI:10.1029/2020GB006601. Abstract
It is well established that the ocean is currently losing dissolved oxygen (O2) in response to ocean warming, but the long‐term, equilibrium response of O2 to a warmer climate is neither well quantified nor understood. Here, we use idealized multi‐millennial global warming simulations with a comprehensive Earth system model to show that the equilibrium response in ocean O2 differs fundamentally from the ongoing transient response. After physical equilibration of the model (>4000 yr) under a two‐times preindustrial CO2 scenario, the deep ocean is better ventilated and oxygenated compared to preindustrial conditions, even though the deep ocean is substantially warmer. The recovery and overshoot of deep convection in the Weddell Sea and especially the Ross Sea after ~720 yr causes a strong increase in deep ocean O2 that overcompensates the solubility‐driven decrease in O2. In contrast, O2 in most of the upper tropical ocean is substantially depleted owing to the warming‐induced O2 decrease dominating over changes in ventilation and biology. Our results emphasize the millennial‐scale impact of global warming on marine life, with some impacts emerging many centuries or even millennia after atmospheric CO2 has stabilized.
We describe the baseline model configuration and simulation characteristics of the Geophysical Fluid Dynamics Laboratory (GFDL)'s Atmosphere Model version 4.1 (AM4.1), which builds on developments at GFDL over 2013–2018 for coupled carbon‐chemistry‐climate simulation as part of the sixth phase of the Coupled Model Intercomparison Project. In contrast with GFDL's AM4.0 development effort, which focused on physical and aerosol interactions and which is used as the atmospheric component of CM4.0, AM4.1 focuses on comprehensiveness of Earth system interactions. Key features of this model include doubled horizontal resolution of the atmosphere (~200 to ~100 km) with revised dynamics and physics from GFDL's previous‐generation AM3 atmospheric chemistry‐climate model. AM4.1 features improved representation of atmospheric chemical composition, including aerosol and aerosol precursor emissions, key land‐atmosphere interactions, comprehensive land‐atmosphere‐ocean cycling of dust and iron, and interactive ocean‐atmosphere cycling of reactive nitrogen. AM4.1 provides vast improvements in fidelity over AM3, captures most of AM4.0's baseline simulations characteristics, and notably improves on AM4.0 in the representation of aerosols over the Southern Ocean, India, and China—even with its interactive chemistry representation—and in its manifestation of sudden stratospheric warmings in the coldest months. Distributions of reactive nitrogen and sulfur species, carbon monoxide, and ozone are all substantially improved over AM3. Fidelity concerns include degradation of upper atmosphere equatorial winds and of aerosols in some regions.
Kwiatkowski, Lester, O Torres, Laurent Bopp, Olivier Aumont, Matthew A Chamberlain, James R Christian, John P Dunne, Marion Gehlen, Tatiana Ilyina, Jasmin G John, A Lenton, Hongmei Li, Nicole S Lovenduski, James C Orr, Julien Palmieri, Jörg Schwinger, Roland Séférian, and Charles A Stock, et al., July 2020: Twenty-first century ocean warming, acidification, deoxygenation, and upper ocean nutrient decline from CMIP6 model projections. Biogeosciences, 17(13), DOI:10.5194/bg-17-3439-2020. Abstract
Anthropogenic climate change leads to ocean warming, acidification, deoxygenation and reductions in near-surface nutrient concentrations, all of which are expected to affect marine ecosystems. Here we assess projections of these drivers of environmental change over the twenty-first century from Earth system models (ESMs) participating in the Coupled Model Intercomparison Project Phase 6 (CMIP6) that were forced under the CMIP6 Shared Socioeconomic Pathways (SSPs). Projections are compared to those from the previous generation (CMIP5) forced under the Representative Concentration Pathways (RCPs). 10 CMIP5 and 13 CMIP6 models are used in the two multi-model ensembles. Under the high-emission scenario SSP5–8.5, the model mean change (2080–2099 mean values relative to 1870–1899) in sea surface temperature, surface pH, subsurface (100–600 m) oxygen concentration and euphotic (0–100 m) nitrate concentration is +3.48 ± 0.78 °C, −0.44 ± 0.005, −13.27 ± 5.28 mmol m−3 and −1.07 ± 0.45 mmol m−3, respectively. Under the low-emission, high-mitigation scenario SSP1–2.6, the corresponding changes are +1.42 ± 0.32 °C, −0.16 ± 0.002, −6.36 ± 2.92 mmol m−3 and −0.53 ± 0.23 mmol m−3. Projected exposure of the marine ecosystem to these drivers of ocean change depends largely on the extent of future emissions, consistent with previous studies. The Earth system models in CMIP6 generally project greater surface warming, acidification, deoxygenation and euphotic nitrate reductions than those from CMIP5 under comparable radiative forcing, with no reduction in inter-model uncertainties. Under the high-emission CMIP5 scenario RCP8.5, the corresponding changes in sea surface temperature, surface pH, subsurface oxygen and euphotic nitrate concentration are +3.04 ± 0.62 °C, −0.38 ± 0.005, −9.51 ± 2.13 mmol m−3 and −0.66 ± 0.49 mmol m−3, respectively. The greater surface acidification in CMIP6 is primarily a consequence of the SSPs having higher associated atmospheric CO2 concentrations than their RCP analogues. The increased projected warming results from a general increase in the climate sensitivity of CMIP6 models relative to those of CMIP5. This enhanced warming results in greater increases in upper ocean stratification in CMIP6 projections, which contributes to greater reductions in euphotic nitrate and subsurface oxygen ventilation.
Anthropogenically forced changes in ocean biogeochemistry are underway and critical for the ocean carbon sink and marine habitat. Detecting such changes in ocean biogeochemistry will require quantification of the magnitude of the change (anthropogenic signal) and the natural variability inherent to the climate system (noise). Here we use Large Ensemble (LE) experiments from four Earth system models (ESMs) with multiple emissions scenarios to estimate Time of Emergence (ToE) and partition projection uncertainty for anthropogenic signals in five biogeochemically important upper-ocean variables. We find ToEs are robust across ESMs for sea surface temperature and the invasion of anthropogenic carbon; emergence time scales are 20–30 yr. For the biological carbon pump, and sea surface chlorophyll and salinity, emergence time scales are longer (50+ yr), less robust across the ESMs, and more sensitive to the forcing scenario considered. We find internal variability uncertainty, and model differences in the internal variability uncertainty, can be consequential sources of uncertainty for projecting regional changes in ocean biogeochemistry over the coming decades. In combining structural, scenario, and internal variability uncertainty, this study represents the most comprehensive characterization of biogeochemical emergence time scales and uncertainty to date. Our findings delineate critical spatial and duration requirements for marine observing systems to robustly detect anthropogenic change.
Séférian, Roland, Sarah Berthet, Andrew Yool, Julien Palmieri, Laurent Bopp, Alessandro Tagliabue, Lester Kwiatkowski, Olivier Aumont, James R Christian, John P Dunne, Marion Gehlen, Tatiana Ilyina, Jasmin G John, Hongmei Li, Matthew C Long, Jessica Y Luo, Hideyuki Nakano, Anastasia Romanou, Jörg Schwinger, and Charles A Stock, et al., August 2020: Tracking Improvement in Simulated Marine Biogeochemistry Between CMIP5 and CMIP6. Current Climate Change Reports, 6, DOI:10.1007/s40641-020-00160-095-119. Abstract
Purpose of Review:
The changes or updates in ocean biogeochemistry component have been mapped between CMIP5 and CMIP6 model versions, and an assessment made of how far these have led to improvements in the simulated mean state of marine biogeochemical models within the current generation of Earth system models (ESMs).
Recent Findings:
The representation of marine biogeochemistry has progressed within the current generation of Earth system models. However, it remains difficult to identify which model updates are responsible for a given improvement. In addition, the full potential of marine biogeochemistry in terms of Earth system interactions and climate feedback remains poorly examined in the current generation of Earth system models.
Summary:
Increasing availability of ocean biogeochemical data, as well as an improved understanding of the underlying processes, allows advances in the marine biogeochemical components of the current generation of ESMs. The present study scrutinizes the extent to which marine biogeochemistry components of ESMs have progressed between the 5th and the 6th phases of the Coupled Model Intercomparison Project (CMIP).
This contribution describes the ocean biogeochemical component of the Geophysical Fluid Dynamics Laboratory's Earth System Model 4.1 (GFDL‐ESM4.1), assesses GFDL‐ESM4.1's capacity to capture observed ocean biogeochemical patterns, and documents its response to increasing atmospheric CO2. Notable differences relative to the previous generation of GFDL ESM's include enhanced resolution of plankton food web dynamics, refined particle remineralization, and a larger number of exchanges of nutrients across Earth system components. During model spin‐up, the carbon drift rapidly fell below the 10 Pg C per century equilibration criterion established by the Coupled Climate‐Carbon Cycle Model Intercomparison Project (C4MIP). Simulations robustly captured large‐scale observed nutrient distributions, plankton dynamics, and characteristics of the biological pump. The model overexpressed phosphate limitation and open ocean hypoxia in some areas but still yielded realistic surface and deep carbon system properties, including cumulative carbon uptake since preindustrial times and over the last decades that is consistent with observation‐based estimates. The model's response to the direct and radiative effects of a 200% atmospheric CO2 increase from preindustrial conditions (i.e., years 101–120 of a 1% CO2 yr−1 simulation) included (a) a weakened, shoaling organic carbon pump leading to a 38% reduction in the sinking flux at 2,000 m; (b) a two‐thirds reduction in the calcium carbonate pump that nonetheless generated only weak calcite compensation on century time‐scales; and, in contrast to previous GFDL ESMs, (c) a moderate reduction in global net primary production that was amplified at higher trophic levels. We conclude with a discussion of model limitations and priority developments.
Walworth, N G., E J Zakem, and John P Dunne, et al., March 2020: Microbial evolutionary strategies in a dynamic ocean. Proceedings of the National Academy of Sciences, 117(11), DOI:10.1073/pnas.1919332117. Abstract
Marine microbes form the base of ocean food webs and drive ocean biogeochemical cycling. Yet little is known about the ability of microbial populations to adapt as they are advected through changing conditions. Here, we investigated the interplay between physical and biological timescales using a model of adaptation and an eddy-resolving ocean circulation climate model. Two criteria were identified that relate the timing and nature of adaptation to the ratio of physical to biological timescales. Genetic adaptation was impeded in highly variable regimes by nongenetic modifications but was promoted in more stable environments. An evolutionary trade-off emerged where greater short-term nongenetic transgenerational effects (low-γ strategy) enabled rapid responses to environmental fluctuations but delayed genetic adaptation, while fewer short-term transgenerational effects (high-γ strategy) allowed faster genetic adaptation but inhibited short-term responses. Our results demonstrate that the selective pressures for organisms within a single water mass vary based on differences in generation timescales resulting in different evolutionary strategies being favored. Organisms that experience more variable environments should favor a low-γ strategy. Furthermore, faster cell division rates should be a key factor in genetic adaptation in a changing ocean. Understanding and quantifying the relationship between evolutionary and physical timescales is critical for robust predictions of future microbial dynamics.
GFDL's new CM4.0 climate model has high transient and equilibrium climate sensitivities near the middle of the upper half of CMIP5 models. The CMIP5 models have been criticized for excessive sensitivity based on observations of present‐day warming and heat uptake and estimates of radiative forcing. An ensemble of historical simulations with CM4.0 produces warming and heat uptake that are consistent with these observations under forcing that is at the middle of the assessed distribution. Energy budget‐based methods for estimating sensitivities based on these quantities underestimate CM4.0's sensitivities when applied to its historical simulations. However, we argue using a simple attribution procedure that CM4.0's warming evolution indicates excessive transient sensitivity to greenhouse gases. This excessive sensitivity is offset prior to recent decades by excessive response to aerosol and land use changes.
We document the configuration and emergent simulation features from the Geophysical Fluid Dynamics Laboratory (GFDL) OM4.0 ocean/sea‐ice model. OM4 serves as the ocean/sea‐ice component for the GFDL climate and Earth system models. It is also used for climate science research and is contributing to the Coupled Model Intercomparison Project version 6 Ocean Model Intercomparison Project (CMIP6/OMIP). The ocean component of OM4 uses version 6 of the Modular Ocean Model (MOM6) and the sea‐ice component uses version 2 of the Sea Ice Simulator (SIS2), which have identical horizontal grid layouts (Arakawa C‐grid). We follow the Coordinated Ocean‐sea ice Reference Experiments (CORE) protocol to assess simulation quality across a broad suite of climate relevant features. We present results from two versions differing by horizontal grid spacing and physical parameterizations: OM4p5 has nominal 0.5° spacing and includes mesoscale eddy parameterizations and OM4p25 has nominal 0.25° spacing with no mesoscale eddy parameterization.
MOM6 makes use of a vertical Lagrangian‐remap algorithm that enables general vertical coordinates. We show that use of a hybrid depth‐isopycnal coordinate reduces the mid‐depth ocean warming drift commonly found in pure z* vertical coordinate ocean models. To test the need for the mesoscale eddy parameterization used in OM4p5, we examine the results from a simulation that removes the eddy parameterization. The water mass structure and model drift are physically degraded relative to OM4p5, thus supporting the key role for a mesoscale closure at this resolution.
Warming‐driven expansion of the oxygen minimum zone (OMZ) in the equatorial Pacific would bring very low oxygen waters closer to the ocean surface and possibly impact global carbon/nutrient cycles and local ecosystems. Global coarse Earth System Models (ESMs) show, however, disparate trends that poorly constrain these future changes in the upper OMZ. Using an ESM with a high‐resolution ocean (1/10°), we show that a realistic representation of the Equatorial Undercurrent (EUC) dynamics is crucial to represent the upper OMZ structure and its temporal variability. We demonstrate that coarser ESMs commonly misrepresent the EUC, leading to an unrealistic “tilt” of the OMZ (e.g., shallowing toward the east) and an exaggerated sensitivity to EUC changes overwhelming other important processes like diffusion and biology. This shortcoming compromises the ability to reproduce the OMZ variability and could explain the disparate trends in ESMs projections.
Chiodi, A M., John P Dunne, and D E Harrison, March 2019: Estimating Air-Sea Carbon Flux Uncertainty over the Tropical Pacific: Importance of Winds and Wind Analysis Uncertainty. Global Biogeochemical Cycles, 33(3), DOI:10.1029/2018GB006047. Abstract
The tropical Pacific is a major natural source of CO2 to the atmosphere and contributor to global air‐sea carbon flux variability. High time‐resolution wind and CO2 measurements from equatorial Pacific moorings reveal the primary factor controlling mooring‐observed flux variability to be near‐surface wind variability, above CO2 variability, in this region over the last 10 years. The analysis product winds used most widely in previous calculations of basin‐scale carbon flux are compared with mooring winds and found to exhibit significant differences in mean, variability, and trend. Earth system model calculations are in basic agreement with the mooring results and used to estimate effects of wind uncertainty on our knowledge of regional air‐sea carbon exchange. Results show that NCEP1 and NCEP2 winds contain biases large enough to obscure the interannual variability of CO2 flux (RMSE ≈ σ) and cause spurious 25‐year (1992–2016) trend components in equatorial Pacific carbon flux of 0.038–0.039 and 0.016–0.021 Pg C yr−1 per decade, respectively. These spurious trends act to reduce by up to 50% the 25‐year trend in equatorial Pacific carbon flux simulated by the Earth system model under increasing atmospheric CO2 concentration. The Cross‐Calibrated‐Multi‐Platform wind product tracks observed variability of equatorial Pacific wind better (interannual RMSE ≈ 0.4σ) than the NCEP reanalyses when site sampled at mooring locations yet still causes a spurious regional trend (0.03 Pg C yr−1 per decade) that masks 40% of the simulated 25‐year trend in carbon flux. The mooring observations are fundamental to identifying the limitations of current wind products to characterizing long‐term trends and understanding air‐sea carbon exchange.
Fennel, K, S Alin, L Barbero, W Evans, T Bourgeois, S Cooley, and John P Dunne, et al., March 2019: Carbon cycling in the North American coastal ocean: A synthesis. Biogeosciences, 16(6), DOI:10.5194/bg-16-1281-2019. Abstract
A quantification of carbon fluxes in the coastal ocean and across its boundaries, specifically the air-sea, land-to-coastal-ocean and coastal-to-open-ocean interfaces, is important for assessing the current state and projecting future trends in ocean carbon uptake and coastal ocean acidification, but is currently a missing component of global carbon budgeting. This synthesis reviews recent progress in characterizing these carbon fluxes with focus on the North American coastal ocean. Several observing networks and high-resolution regional models are now available. Recent efforts have focused primarily on quantifying net air-sea exchange of carbon dioxide (CO2). Some studies have estimated other key fluxes, such as the exchange of organic and inorganic carbon between shelves and the open ocean. Available estimates of air-sea CO2 flux, informed by more than a decade of observations, indicate that the North American margins act as a net sink for atmospheric CO2. This net uptake is driven primarily by the high-latitude regions. The estimated magnitude of the net flux is 160±80TgC/y for the North American Exclusive Economic Zone, a number that is not well constrained. The increasing concentration of inorganic carbon in coastal and open-ocean waters leads to ocean acidification. As a result conditions favouring dissolution of calcium carbonate occur regularly in subsurface coastal waters in the Arctic, which are naturally prone to low pH, and the North Pacific, where upwelling of deep, carbon-rich waters has intensified and, in combination with the uptake of anthropogenic carbon, leads to low seawater pH and aragonite saturation states during the upwelling season. Expanded monitoring and extension of existing model capabilities are required to provide more reliable coastal carbon budgets, projections of future states of the coastal ocean, and quantification of anthropogenic carbon contributions.
We describe GFDL's CM4.0 physical climate model, with emphasis on those aspects that may be of particular importance to users of this model and its simulations. The model is built with the AM4.0/LM4.0 atmosphere/land model and OM4.0 ocean model. Topics include the rationale for key choices made in the model formulation, the stability as well as drift of the pre‐industrial control simulation, and comparison of key aspects of the historical simulations with observations from recent decades. Notable achievements include the relatively small biases in seasonal spatial patterns of top‐of‐atmosphere fluxes, surface temperature, and precipitation; reduced double Intertropical Convergence Zone bias; dramatically improved representation of ocean boundary currents; a high quality simulation of climatological Arctic sea ice extent and its recent decline; and excellent simulation of the El Niño‐Southern Oscillation spectrum and structure. Areas of concern include inadequate deep convection in the Nordic Seas; an inaccurate Antarctic sea ice simulation; precipitation and wind composites still affected by the equatorial cold tongue bias; muted variability in the Atlantic Meridional Overturning Circulation; strong 100 year quasi‐periodicity in Southern Ocean ventilation; and a lack of historical warming before 1990 and too rapid warming thereafter due to high climate sensitivity and strong aerosol forcing, in contrast to the observational record. Overall, CM4.0 scores very well in its fidelity against observations compared to the Coupled Model Intercomparison Project Phase 5 generation in terms of both mean state and modes of variability and should prove a valuable new addition for analysis across a broad array of applications.
Exchanges between coastal and oceanic waters shape both coastal ecosystem processes and signatures that they impart on global biogeochemical cycles. The time‐scales of these exchanges, however, are poorly represented in current‐generation, coarse‐grid climate models. Here we provide a novel global perspective on coastal residence time (CRT) and its spatio‐temporal variability using a new age tracer implemented in global ocean models. Simulated CRTs range widely from several days in narrow boundary currents to multiple years on broader shelves and in semi‐enclosed seas, in agreement with available observations. Overall, CRT is better characterized in high‐resolution models (1/8° and 1/4°) than the coarser (1° and 1/2°) versions. This is in large part because coastal and open ocean grid cells are more directly connected in coarse models, prone to erroneous coastal flushing and an underestimated CRT. Additionally, we find that geometric enclosure of a coastal system places an important constraint on CRT.
Lotze, Heike K., Derek P Tittensor, Andrea Bryndum-Buchholz, Tyler D Eddy, William W L Cheung, Eric D Galbraith, M Barange, Nicolas Barrier, Daniele Bianchi, Julia L Blanchard, Laurent Bopp, Matthias Büchner, C Bulman, D A Carozza, Villy Christensen, Marta Coll, John P Dunne, Elizabeth A Fulton, S Jennings, M C Jones, S Mackinson, Olivier Maury, S Niranen, R Oliveros-Ramos, Tilla Roy, J A Fernandes, Jacob Schewe, Yunne-Jai Shin, T Silva, Jeroen Steenbeek, and Charles A Stock, et al., June 2019: Global ensemble projections reveal trophic amplification of ocean biomass declines with climate change. Proceedings of the National Academy of Sciences, 116(26), DOI:10.1073/pnas.1900194116. Abstract
While the physical dimensions of climate change are now routinely assessed through multimodel intercomparisons, projected impacts on the global ocean ecosystem generally rely on individual models with a specific set of assumptions. To address these single-model limitations, we present standardized ensemble projections from six global marine ecosystem models forced with two Earth system models and four emission scenarios with and without fishing. We derive average biomass trends and associated uncertainties across the marine food web. Without fishing, mean global animal biomass decreased by 5% (±4% SD) under low emissions and 17% (±11% SD) under high emissions by 2100, with an average 5% decline for every 1 °C of warming. Projected biomass declines were primarily driven by increasing temperature and decreasing primary production, and were more pronounced at higher trophic levels, a process known as trophic amplification. Fishing did not substantially alter the effects of climate change. Considerable regional variation featured strong biomass increases at high latitudes and decreases at middle to low latitudes, with good model agreement on the direction of change but variable magnitude. Uncertainties due to variations in marine ecosystem and Earth system models were similar. Ensemble projections performed well compared with empirical data, emphasizing the benefits of multimodel inference to project future outcomes. Our results indicate that global ocean animal biomass consistently declines with climate change, and that these impacts are amplified at higher trophic levels. Next steps for model development include dynamic scenarios of fishing, cumulative human impacts, and the effects of management measures on future ocean biomass trends.
Mariotti, Annarita, and John P Dunne, et al., April 2019: NOAA General Modeling Meeting and Fair Brings Together Its Modeling Enterprise. Bulletin of the American Meteorological Society, 100(4), DOI:10.1175/BAMS-D-18-0318.1.
Ocean calcium carbonate (CaCO3) production and preservation play a key role in the global carbon cycle. Coastal and continental shelf (neritic) environments account for more than half of global CaCO3 accumulation. Previous neritic CaCO3 budgets have been limited in both spatial resolution and ability to project responses to environmental change. Here, a 1° spatially explicit budget for neritic CaCO3 accumulation is developed. Globally gridded satellite and benthic community area data are used to estimate community CaCO3 production. Accumulation rates (PgC yr−1) of four neritic environments are calculated: coral reefs/banks (0.084), seagrass-dominated embayments (0.043), and carbonate rich (0.037) and poor (0.0002) shelves. This analysis refines previous neritic CaCO3 accumulation estimates (~0.16) and shows almost all coastal carbonate accumulation occurs in the tropics, >50% of coral reef accumulation occurs in the Western Pacific Ocean, and 80% of coral reef, 63% of carbonate shelf, and 58% of bay accumulation occur within three global carbonate hot spots: the Western Pacific Ocean, Eastern Indian Ocean, and Caribbean Sea. These algorithms are amenable for incorporation into Earth System Models that represent open ocean pelagic CaCO3 production and deep-sea preservation and assess impacts and feedbacks of environmental change.
Climate variations have a profound impact on marine ecosystems and the communities that depend upon them. Anticipating ecosystem shifts using global Earth system models (ESMs) could enable communities to adapt to climate fluctuations and contribute to long-term ecosystem resilience. We show that newly developed ESM-based marine biogeochemical predictions can skillfully predict satellite-derived seasonal to multiannual chlorophyll fluctuations in many regions. Prediction skill arises primarily from successfully simulating the chlorophyll response to the El Niño–Southern Oscillation and capturing the winter reemergence of subsurface nutrient anomalies in the extratropics, which subsequently affect spring and summer chlorophyll concentrations. Further investigations suggest that interannual fish-catch variations in selected large marine ecosystems can be anticipated from predicted chlorophyll and sea surface temperature anomalies. This result, together with high predictability for other marine-resource–relevant biogeochemical properties (e.g., oxygen, primary production), suggests a role for ESM-based marine biogeochemical predictions in dynamic marine resource management efforts.
Resplandy, Laure, Ralph F Keeling, Yassir A Eddebbar, M K Brooks, R Wang, Laurent Bopp, Matthew C Long, and John P Dunne, et al., December 2019: Quantification of ocean heat uptake from changes in atmospheric O2 and CO2 composition. Scientific Reports, 9, 20244, DOI:10.1038/s41598-019-56490-z. Abstract
The ocean is the main source of thermal inertia in the climate system. Ocean heat uptake during recent decades has been quantified using ocean temperature measurements. However, these estimates all use the same imperfect ocean dataset and share additional uncertainty due to sparse coverage, especially before 2007. Here, we provide an independent estimate by using measurements of atmospheric oxygen (O2) and carbon dioxide (CO2) – levels of which increase as the ocean warms and releases gases – as a whole ocean thermometer. We show that the ocean gained 1.29 ± 0.79 × 1022 Joules of heat per year between 1991 and 2016, equivalent to a planetary energy imbalance of 0.80 ± 0.49 W watts per square metre of Earth’s surface. We also find that the ocean-warming effect that led to the outgassing of O2 and CO2 can be isolated from the direct effects of anthropogenic emissions and CO2 sinks. Our result – which relies on high-precision O2 atmospheric measurements dating back to 1991 – leverages an integrative Earth system approach and provides much needed independent confirmation of heat uptake estimated from ocean data.
The attribution of anthropogenically forced trends in the climate system requires an understanding of when and how such signals emerge from natural variability. We applied time-of-emergence diagnostics to a large ensemble of an Earth system model, which provides both a conceptual framework for interpreting the detectability of anthropogenic impacts in the ocean carbon cycle and observational sampling strategies required to achieve detection. We found emergence timescales that ranged from less than a decade to more than a century, a consequence of the time lag between the chemical and radiative impacts of rising atmospheric CO2 on the ocean. Processes sensitive to carbonate chemical changes emerge rapidly, such as the impacts of acidification on the calcium carbonate pump (10 years for the globally integrated signal and 9–18 years for regionally integrated signals) and the invasion flux of anthropogenic CO2 into the ocean (14 years globally and 13–26 years regionally). Processes sensitive to the ocean’s physical state, such as the soft-tissue pump, which depends on nutrients supplied through circulation, emerge decades later (23 years globally and 27–85 years regionally).
Sulpis, O, Carolina O Dufour, D S Trossman, Andrea J Fassbender, Brian K Arbic, B P Boudreau, John P Dunne, and A Mucci, December 2019: Reduced CaCO3 flux to the seafloor and weaker bottom current speeds curtail benthic CaCO3 dissolution over the 21st century. Global Biogeochemical Cycles, 33(12), DOI:10.1029/2019GB006230. Abstract
Results from a range of Earth System and climate models of various resolution run under high‐CO2 emission scenarios challenge the paradigm that seafloor CaCO3 dissolution will grow in extent and intensify as ocean acidification develops over the next century. Under the “business as usual”, RCP8.5 scenario, CaCO3 dissolution increases in some areas of the deep ocean, such as the eastern central Pacific Ocean, but is projected to decrease in the Northern Pacific and abyssal Atlantic Ocean by the year 2100. The flux of CaCO3 to the seafloor and bottom‐current speeds, both of which are expected to decrease globally through the 21st century, govern changes in benthic CaCO3 dissolution rates over 53 and 31% of the dissolving seafloor, respectively. Below the calcite compensation depth (CCD), a reduced CaCO3 flux to the CaCO3‐free seabed modulates the amount of CaCO3 material dissolved at the sediment‐water interface. Slower bottom‐water circulation leads to thicker diffusive boundary layers above the sediment bed and a consequent stronger transport barrier to CaCO3 dissolution. While all investigated models predict a weakening of bottom current speeds over most of the seafloor by the end of the 21st century, strong discrepancies exist in the magnitude of the predicted speeds. Overall, the poor performance of most models in reproducing modern bottom‐water velocities and CaCO3 rain rates coupled with the existence of large disparities in predicted bottom‐water chemistry across models, hampers our ability to robustly estimate the magnitude and temporal evolution of anthropogenic CaCO3 dissolution rates and the associated anthropogenic CO2 neutralization.
Seasonal to interannual predictions of ecosystem dynamics have the potential to improve the management of living marine resources. Prediction of oceanic net primary production (NPP), the foundation of marine food webs and the biological carbon pump, is particularly promising, with recent analysis suggesting that ecosystem feedback processes may lead to higher predictability of NPP at interannual scales than for physical variables like sea surface temperature (SST). Here, we assessed the potential predictability of oceanic NPP and SST across seasonal to interannual lead times using reduced dimension, linear dynamical spatio-temporal models (rDSTM). This approach combines empirical orthogonal function (EOF) analysis with vector autoregressive (VAR) modeling to simplify the analysis of spatio-temporal data. The rDSTMs were fitted to monthly NPP and SST anomalies derived from 20 years of remote sensing data (1997-2017), considering two alternative algorithms commonly used to estimate NPP (VGPM and Eppley-VGPM) and optimally analyzed SST fields (AVHRR OISST). The local decay of anomalies provided high predictability up to three months, and subsequent interactions with remote forcing significantly extended predictability beyond the initial anomaly decay. Indeed, interactions among spatial modes associated with the propagation of major climate modes, particularly the El Niño-Southern Oscillation (ENSO), extended the predictability horizon above one year in some regions. Patterns of enhanced NPP predictability matched the location of oligotrophic gyres and transition regions between ocean biomes, where fluctuations in biome boundaries generate large biogeochemical perturbations that leave lasting imprints on NPP. In these areas, the predictability horizon for NPP was longer than for SST, although SST was more predictable over large areas of the equatorial and northeast Pacific. Our results support the potential for extending seasonal to interannual physical climate predictions to predict ocean productivity.
Ocean surface winds determine energy, material and momentum fluxes through the air-sea interface. Accounting for wind variability in time and space is thus essential to reliably analyze and simulate ocean circulation and the dynamics of marine ecosystems. Here, we present an assessment of surface winds from three widely used atmospheric reanalysis products (NCEP/NCAR, ERA-Interim and JRA-55) and their corresponding ocean forcing data sets (CORE v2.1, DFS v5.2 and JRA55-do), which include corrections for use in ocean simulations. We compared wind patterns most relevant to ocean circulation (surface wind stress, its curl and estimates of induced vertical upwelling velocity) across global and regional scales, with added emphasis on the main Eastern Boundary Upwelling Ecosystems (EBUEs). All products provided consistent large-scale patterns in surface winds and wind stress, although agreement was reduced for indices involving the calculation of spatial derivatives, like wind stress curl and Ekman pumping. Fidelity with respect to a reference reanalysis based on blended satellite and buoy observations (CCMP v2.0) improved in more recent, higher resolution products like JRA-55 and ERA-Interim. Adjustments applied when deriving ocean forcing data sets from atmospheric reanalysis robustly improved wind speed and wind stress vectors, but degraded wind stress curl (and implied Ekman upwelling) in two of the three ocean forcing products considered (DFS v5.2 and CORE v2.1).
At regional scales, we found significant inconsistencies in equatorial and polar regions, as well as in coastal areas. In EBUEs, upwelling favorable winds were weaker in atmospheric reanalysis products and ocean forcing data sets than estimates based on CCMP v2.0 and QuikSCAT. All reanalysis products featured lower amplitude seasonal cycles and contrasting patterns of low frequency variability within each EBUE, including the presence of sudden changes in mean upwelling only for some products.
Taken together, our results highlight the importance of incorporating uncertainties in wind forcing into ocean simulation experiments and retrospective analysis, and of correcting reanalysis products for ocean forcing data sets. Despite the continued improvement in the quality of wind data sets, prevailing limitations in reanalysis models demonstrate the need to confirm global products against regional measurements whenever possible and improve correction strategies across multiple ocean-relevant wind properties.
Claret, M, Eric D Galbraith, J B Palter, Daniele Bianchi, K Fennel, D Gilbert, and John P Dunne, October 2018: Rapid coastal deoxygenation due to ocean circulation shift in the northwest Atlantic. Nature Climate Change, 8(10), DOI:10.1038/s41558-018-0263-1. Abstract
Global observations show that the ocean lost approximately 2% of its oxygen inventory over the past five decades1,2,3, with important implications for marine ecosystems4,5. The rate of change varies regionally, with northwest Atlantic coastal waters showing a long-term drop6,7 that vastly outpaces the global and North Atlantic basin mean deoxygenation rates5,8. However, past work has been unable to differentiate the role of large-scale climate forcing from that of local processes. Here, we use hydrographic evidence to show that a Labrador Current retreat is playing a key role in the deoxygenation on the northwest Atlantic shelf. A high-resolution global coupled climate–biogeochemistry model9 reproduces the observed decline of saturation oxygen concentrations in the region, driven by a retreat of the equatorward-flowing Labrador Current and an associated shift towards more oxygen-poor subtropical waters on the shelf. The dynamical changes underlying the shift in shelf water properties are correlated with a slowdown in the simulated Atlantic Meridional Overturning Circulation (AMOC)10. Our results provide strong evidence that a major, centennial-scale change of the Labrador Current is underway, and highlight the potential for ocean dynamics to impact coastal deoxygenation over the coming century.
Fennel, K, S Alin, L Barbero, W Evans, T Bourgeois, S Cooley, and John P Dunne, et al., November 2018: Chapter 16: Coastal ocean and continental shelves. In Second State of the Carbon Cycle Report (SOCCR2): A Sustained Assessment Report [Cavallaro, N., G. Shrestha, R. Birdsey, M. A. Mayes, R. G. Najjar, S. C. Reed, P. Romero-Lankao, and Z. Zhu (eds.)], Washington, DC, U.S. Global Change Research Program, 649-688. Abstract
https://carbon2018.globalchange.gov/
Frenger, I, Daniele Bianchi, C Stührenberg, , John P Dunne, Curtis A Deutsch, Eric D Galbraith, and F Schütte, February 2018: Biogeochemical role of subsurface coherent eddies in the ocean: Tracer cannonballs, hypoxic storms, and microbial stewpots?Global Biogeochemical Cycles, 32(2), DOI:10.1002/2017GB005743. Abstract
Subsurface coherent eddies are well-known features of ocean circulation, but the sparsity of observations prevents an assessment of their importance for biogeochemistry. Here, we use a global eddying (0.1° ) ocean-biogeochemical model to carry out a census of subsurface coherent eddies originating from eastern boundary upwelling systems (EBUS), and quantify their biogeochemical effects as they propagate westward into the subtropical gyres. While most eddies exist for a few months, moving over distances of 100s of km, a small fraction (< 5%) of long-lived eddies propagates over distances greater than 1000km, carrying the oxygen-poor and nutrient-rich signature of EBUS into the gyre interiors. In the Pacific, transport by subsurface coherent eddies accounts for roughly 10% of the offshore transport of oxygen and nutrients in pycnocline waters. This "leakage" of subsurface waters can be a significant fraction of the transport by nutrient-rich poleward undercurrents, and may contribute to the well-known reduction of productivity by eddies in EBUS. Furthermore, at the density layer of their cores, eddies decrease climatological oxygen locally by close to 10%, thereby expanding oxygen minimum zones. Finally, eddies represent low-oxygen extreme events in otherwise oxygenated waters, increasing the area of hypoxic waters by several percent and producing dramatic short-term changes that may play an important ecological role. Capturing these non-local effects in global climate models, which typically include non-eddying oceans, would require dedicated parameterizations.
Huntzinger, D N., A Chatterjee, D J P Moore, S Ohrel, T O West, Ben Poulter, A P Walker, and John P Dunne, et al., November 2018: Chapter 19: Future of the North American carbon cycle In Second State of the Carbon Cycle Report (SOCCR2): A Sustained Assessment Report [Cavallaro, N., G. Shrestha, R. Birdsey, M. A. Mayes, R. G. Najjar, S. C. Reed, P. Romero-Lankao, and Z. Zhu (eds.)], Washington, DC, U.S. Global Change Research Program, 760- 809. Abstract
Icebergs and glacial meltwater have been observed to significantly affect chlorophyll concentrations, primary production and particle export locally, yet the quantitative influence of glacial iron on the carbon cycle of the Southern Ocean remains unknown. We analyse the impact of icebergs and glacial meltwater on the Southern Ocean carbon cycle in a global Earth System Model. We consider several simulations spanning low and high bounds of current estimates of glacial iron concentration. We find that a high glacial iron input produces the best agreement with observed iron and chlorophyll distributions. These high glacial iron input results indicate that about 30% of the Southern Ocean particle export production, i.e., the flux of particulate organic matter through the 100 m depth level, is driven by glacial iron sources. This export production is associated with an uptake of 0.14 Pg carbon per year, which reduces carbon outgassing in the Southern Ocean by 30%.
Muller-Karger, F E., E Hestir, C Ade, K Turpie, D A Roberts, D A Siegel, R J Miller, D Humm, N Izenberg, M Keller, F Morgan, R Frouin, A G Dekker, R Gardner, B Schaeffer, B A Franz, N Pahlevan, A G Mannino, J A Concha, S G Ackleson, K C Cavanaugh, Anastasia Romanou, M Tzortziou, E S Boss, R Pavlick, A Freeman, Cecile S Rousseaux, and John P Dunne, et al., April 2018: Satellite sensor requirements for monitoring essential biodiversity variables of coastal ecosystems. Ecological Applications, 28(3), DOI:10.1002/eap.1682. Abstract
The biodiversity and high productivity of coastal terrestrial and aquatic habitats are the foundation for important benefits to human societies around the world. These globally distributed habitats need frequent and broad systematic assessments, but field surveys only cover a small fraction of these areas. Satellite-based sensors can repeatedly record the visible and near-infrared reflectance spectra that contain the absorption, scattering, and fluorescence signatures of functional phytoplankton groups, colored dissolved matter, and particulate matter near the surface ocean, and of biologically structured habitats (floating and emergent vegetation, benthic habitats like coral, seagrass, and algae). These measures can be incorporated into Essential Biodiversity Variables (EBVs), including the distribution, abundance, and traits of groups of species populations, and used to evaluate habitat fragmentation. However, current and planned satellites are not designed to observe the EBVs that change rapidly with extreme tides, salinity, temperatures, storms, pollution, or physical habitat destruction over scales relevant to human activity. Making these observations requires a new generation of satellite sensors able to sample with these combined characteristics: (1) spatial resolution on the order of 30 to 100-m pixels or smaller; (2) spectral resolution on the order of 5 nm in the visible and 10 nm in the short-wave infrared spectrum (or at least two or more bands at 1,030, 1,240, 1,630, 2,125, and/or 2,260 nm) for atmospheric correction and aquatic and vegetation assessments; (3) radiometric quality with signal to noise ratios (SNR) above 800 (relative to signal levels typical of the open ocean), 14-bit digitization, absolute radiometric calibration <2%, relative calibration of 0.2%, polarization sensitivity <1%, high radiometric stability and linearity, and operations designed to minimize sunglint; and (4) temporal resolution of hours to days. We refer to these combined specifications as H4 imaging. Enabling H4 imaging is vital for the conservation and management of global biodiversity and ecosystem services, including food provisioning and water security. An agile satellite in a 3-d repeat low-Earth orbit could sample 30-km swath images of several hundred coastal habitats daily. Nine H4 satellites would provide weekly coverage of global coastal zones. Such satellite constellations are now feasible and are used in various applications.
Ocean chlorophyll concentration, a proxy for phytoplankton, is strongly influenced by internal ocean dynamics such as those associated with El Niño–Southern Oscillation (ENSO). Observations show that ocean chlorophyll responses to ENSO generally lead sea surface temperature (SST) responses in the equatorial Pacific. A long-term global earth system model simulation incorporating marine biogeochemical processes also exhibits a preceding chlorophyll response. In contrast to simulated SST anomalies which significantly lag the wind-driven subsurface heat response to ENSO, chlorophyll anomalies respond rapidly. Iron was found to be the key factor connecting the simulated surface chlorophyll anomalies to the subsurface ocean response. Westerly wind bursts decrease central Pacific chlorophyll by reducing iron supply through wind-driven thermocline deepening, but increase western Pacific chlorophyll by enhancing the influx of coastal iron from the maritime continent. Our results mechanistically support the potential for chlorophyll-based indices to inform seasonal ENSO forecasts beyond previously identified SST-based indices.
Reliable estimates of historical and current biogeochemistry are essential for understanding past ecosystem variability and predicting future changes. Efforts to translate improved physical ocean state estimates into improved biogeochemical estimates, however, are hindered by high biogeochemical sensitivity to transient momentum imbalances that arise during physical data assimilation. Most notably, the breakdown of geostrophic constraints on data assimilation in equatorial regions can lead to spurious upwelling, resulting in excessive equatorial productivity and biogeochemical fluxes. This hampers efforts to understand and predict the biogeochemical consequences of El Niño and La Niña. We develop a strategy to robustly integrate an ocean biogeochemical model with an ensemble coupled-climate data assimilation system used for seasonal to decadal global climate prediction. Addressing spurious vertical velocities requires two steps. First, we find that tightening constraints on atmospheric data assimilation maintains a better equatorial wind stress and pressure gradient balance. This reduces spurious vertical velocities, but those remaining still produce substantial biogeochemical biases. The remainder is addressed by imposing stricter fidelity to model dynamics over data constraints near the equator. We determine an optimal choice of model-data weights that removed spurious biogeochemical signals while benefitting from off-equatorial constraints that still substantially improve equatorial physical ocean simulations. Compared to the unconstrained control run, the optimally constrained model reduces equatorial biogeochemical biases and markedly improves the equatorial subsurface nitrate concentrations and hypoxic area. The pragmatic approach described herein offers a means of advancing earth system prediction in parallel with continued data assimilation advances aimed at fully considering equatorial data constraints.
Resplandy, Laure, Ralph F Keeling, Yassir A Eddebbar, M K Brooks, R Wang, Laurent Bopp, Matthew C Long, John P Dunne, W Koeve, and A Oschlies, November 2018: Quantification of ocean heat uptake from changes in atmospheric O2 and CO2 composition. Nature, 563(7729), DOI:10.1038/s41586-018-0651-8. Abstract
The ocean is the main source of thermal inertia in the climate system1. During recent decades, ocean heat uptake has been quantified by using hydrographic temperature measurements and data from the Argo float program, which expanded its coverage after 20072,3. However, these estimates all use the same imperfect ocean dataset and share additional uncertainties resulting from sparse coverage, especially before 20074,5. Here we provide an independent estimate by using measurements of atmospheric oxygen (O2) and carbon dioxide (CO2)—levels of which increase as the ocean warms and releases gases—as a whole-ocean thermometer. We show that the ocean gained 1.33 ± 0.20 × 1022 joules of heat per year between 1991 and 2016, equivalent to a planetary energy imbalance of 0.83 ± 0.11 watts per square metre of Earth’s surface. We also find that the ocean-warming effect that led to the outgassing of O2 and CO2 can be isolated from the direct effects of anthropogenic emissions and CO2 sinks. Our result—which relies on high-precision O2 measurements dating back to 19916—suggests that ocean warming is at the high end of previous estimates, with implications for policy-relevant measurements of the Earth response to climate change, such as climate sensitivity to greenhouse gases7 and the thermal component of sea-level rise.
To explore the mechanisms involved in the global ocean circulation response to the shoaling and closure of the Central American Seaway (CAS), we performed a suite of sensitivity experiments using the Geophysical Fluid Dynamics Laboratory Earth System Model (ESM), GFDL‐ESM 2G, varying only the seaway widths and sill depths. Changes in large‐scale transport, global ocean mean state, and deep‐ocean circulation in all simulations are driven by the direct impacts of the seaway on global mass, heat and salt transports. Net mass transport through the seaway into the Caribbean is 20.5‐23.1 Sv with a deep CAS, but only 14.1 Sv for the wide, shallow CAS. Seaway transport originates from the Antarctic Circumpolar Current in the Pacific and rejoins it in the South Atlantic, reducing the Indonesian Throughflow and transporting heat and salt southward into the South Atlantic, in contrast to present‐day and previous CAS simulations. The increased southward salt transport increases the large‐scale upper ocean density, and the freshening and warming from the changing ocean transports decreases the intermediate and deep‐water density. The new ocean circulation pathway traps heat in the Southern Hemisphere oceans and reduces the northern extent of Antarctic Bottom Water penetration in the Atlantic, strengthening and deepening Atlantic meridional overturning, in contrast to previous studies. In all simulations, the seaway has a profound effect on the global ocean mean state and alters deep‐water mass properties and circulation in the Atlantic, Indian and Pacific basins, with implications for changing deep‐water circulation as a possible driver for changes in long‐term climate.
Tittensor, Derek P., Tyler D Eddy, Heike K Lotze, Eric D Galbraith, William W L Cheung, M Barange, Julia L Blanchard, Laurent Bopp, Andrea Bryndum-Buchholz, Matthias Büchner, C Bulman, D A Carozza, Villy Christensen, Marta Coll, John P Dunne, J A Fernandes, Elizabeth A Fulton, A J Hobday, V Huber, S Jennings, M Jones, P Lehodey, Jason S Link, S Mackinson, Olivier Maury, S Niiranen, R Oliveros-Ramos, Tilla Roy, Jacob Schewe, Yunne-Jai Shin, T Silva, and Charles A Stock, et al., April 2018: A protocol for the intercomparison of marine fishery and ecosystem models: Fish-MIP v1.0. Geoscientific Model Development, 11(4), DOI:10.5194/gmd-11-1421-2018. Abstract
Model intercomparison studies in the climate and Earth sciences communities have been crucial to building credibility and coherence for future projections. They have quantified variability among models, spurred model development, contrasted within- and among-model uncertainty, assessed model fits to historical data, and provided ensemble projections of future change under specified scenarios. Given the speed and magnitude of anthropogenic change in the marine environment and the consequent effects on food security, biodiversity, marine industries, and society, the time is ripe for similar comparisons among models of fisheries and marine ecosystems. Here, we describe the Fisheries and Marine Ecosystem Model Intercomparison Project protocol version 1.0 (Fish-MIP v1.0), part of the Inter-Sectoral Impact Model Intercomparison Project (ISIMIP), which is a cross-sectoral network of climate impact modellers. Given the complexity of the marine ecosystem, this class of models has substantial heterogeneity of purpose, scope, theoretical underpinning, processes considered, parameterizations, resolution (grain size), and spatial extent. This heterogeneity reflects the lack of a unified understanding of the marine ecosystem and implies that the assemblage of all models is more likely to include a greater number of relevant processes than any single model. The current Fish-MIP protocol is designed to allow these heterogeneous models to be forced with common Earth System Model (ESM) Coupled Model Intercomparison Project Phase 5 (CMIP5) outputs under prescribed scenarios for historic (from the 1950s) and future (to 2100) time periods; it will be adapted to CMIP phase 6 (CMIP6) in future iterations. It also describes a standardized set of outputs for each participating Fish-MIP model to produce. This enables the broad characterization of differences between and uncertainties within models and projections when assessing climate and fisheries impacts on marine ecosystems and the services they provide. The systematic generation, collation, and comparison of results from Fish-MIP will inform an understanding of the range of plausible changes in marine ecosystems and improve our capacity to define and convey the strengths and weaknesses of model-based advice on future states of marine ecosystems and fisheries. Ultimately, Fish-MIP represents a step towards bringing together the marine ecosystem modelling community to produce consistent ensemble medium- and long-term projections of marine ecosystems.
Turi, G, M J Alexander, Nicole S Lovenduski, Antonietta Capotondi, J Scott, Charles A Stock, John P Dunne, Jasmin G John, and Michael G Jacox, February 2018: Response of O2 and pH to ENSO in the California Current System in a high resolution global climate model. Ocean Science, 14(1), DOI:10.5194/os-14-69-2018. Abstract
We use a novel, high-resolution global climate model (GFDL-ESM2.6) to investigate the influence of warm and cold El Niño/Southern Oscillation (ENSO) events on the physics and biogeochemistry of the California Current System (CalCS). We focus on the effect of ENSO on variations in the O2 concentration and the pH of the coastal waters of the CalCS. An assessment of the CalCS response to six El Niño and seven La Niña events in ESM2.6 reveals significant variations in the response between events. However, these variations overlay a consistent physical and biogeochemical (O2 and pH) response in the composite mean. Focusing on the mean response, our results demonstrate that O2 and pH are affected rather differently in the euphotic zone above ~100 m. The strongest O2 response reaches up to several 100 km offshore, whereas the pH signal occurs only within a ~100 km-wide band along the coast. By splitting the changes in O2 and pH into individual physical and biogeochemical components that are affected by ENSO variability, we found that O2 variability in the surface ocean is primarily driven by changes in surface temperature that affect the O2 solubility. In contrast, surface pH changes are predominantly driven by changes in dissolved inorganic carbon (DIC), which in turn is affected by upwelling, explaining the confined nature of the pH signal close to the coast. Below ~100 m, we find conditions with anomalously low O2 and pH, and by extension also anomalously low aragonite saturation, during La Niña. This result is consistent with findings from previous studies and highlights the stress that the CalCS ecosystem could periodically undergo in addition to impacts due to climate change.
The measured concentration of chlorophyll a in the surface ocean spans four orders of magnitude, from ∼0.01 mg m-3 in the oligotrophic gyres to >10 mg m-3 in coastal zones. Productive regions encompass only a small fraction of the global ocean area yet they contribute disproportionately to marine resources and biogeochemical processes, such as fish catch and coastal hypoxia. These regions and/or the full observed range of chlorophyll concentration, however, are often poorly represented in global earth system models (ESMs) used to project climate change impacts on marine ecosystems. Furthermore, recent high resolution (∼10 km) global earth system simulations suggest that this shortfall is not solely due to coarse resolution (∼100 km) of most global ESMs. By integrating a global biogeochemical model that includes two phytoplankton size classes (typical of many ESMs) into a regional simulation of the California Current System (CCS) we test the hypothesis that a combination of higher spatial resolution and enhanced resolution of phytoplankton size classes and grazer linkages may enable global ESMs to better capture the full range of observed chlorophyll. The CCS is notable for encompassing both oligotrophic (<0.1 mg m-3) and productive (>10 mg m-3) endpoints of the global chlorophyll distribution. As was the case for global high-resolution simulations, the regional high-resolution implementation with two size classes fails to capture the productive endpoint. The addition of a third phytoplankton size class representing a chain-forming coastal diatom enables such models to capture the full range of chlorophyll concentration along a nutrient supply gradient, from highly productive coastal upwelling systems to oligotrophic gyres. Weaker ‘top-down’ control on coastal diatoms results in stronger trophic decoupling and increased phytoplankton biomass, following the introduction of new nutrients to the photic zone. The enhanced representation of near-shore chlorophyll maxima allows the model to better capture coastal hypoxia along the continental shelf of the North American west coast and may improve the representation of living marine resources.
Yamamoto, A, J B Palter, Carolina O Dufour, Stephen M Griffies, Daniele Bianchi, M Claret, John P Dunne, I Frenger, and Eric D Galbraith, October 2018: Roles of the ocean mesoscale in the horizontal supply of mass, heat, carbon and nutrients to the Northern Hemisphere subtropical gyres. Journal of Geophysical Research: Oceans, 123(10), DOI:10.1029/2018JC013969. Abstract
Horizontal transport at the boundaries of the subtropical gyres plays a crucial role in providing the nutrients that fuel gyre primary productivity, the heat that helps restratify the surface mixed layer, and the dissolved inorganic carbon (DIC) that influences air‐sea carbon exchange. Mesoscale eddies may be an important component of these horizontal transports; however, previous studies have not quantified the horizontal tracer transport due to eddies across the subtropical gyre boundaries. Here we assess the physical mechanisms that control the horizontal transport of mass, heat, nutrients and carbon across the North Pacific and North Atlantic subtropical gyre boundaries using the eddy‐rich ocean component of a climate model (GFDL's CM2.6) coupled to a simple biogeochemical model (mini‐BLING). Our results suggest that horizontal transport across the gyre boundaries supplies a substantial amount of mass and tracers to the ventilated layer of both Northern Hemisphere subtropical gyres, with the Kuroshio and Gulf Stream acting as main exchange gateways. Mass, heat, and DIC supply is principally driven by the time‐mean circulation, whereas nutrient transport differs markedly from the other tracers, as nutrients are mainly supplied to both subtropical gyres by down‐gradient eddy mixing across gyre boundaries. A budget analysis further reveals that the horizontal nutrient transport, combining the roles of both mean and eddy components, is responsible for more than three quarters of the total nutrient supply into the subtropical gyres, surpassing a recent estimate based on a coarse resolution model and thus further highlighting the importance of horizontal nutrient transport.
In this two-part paper, a description is provided of a version of the AM4.0/LM4.0 atmosphere/land model that will serve as a base for a new set of climate and Earth system models (CM4 and ESM4) under development at NOAA's Geophysical Fluid Dynamics Laboratory (GFDL). This version, with roughly 100km horizontal resolution and 33 levels in the vertical, contains an aerosol model that generates aerosol fields from emissions and a “light” chemistry mechanism designed to support the aerosol model but with prescribed ozone. In Part I, the quality of the simulation in AMIP (Atmospheric Model Intercomparison Project) mode – with prescribed sea surface temperatures (SSTs) and sea ice distribution – is described and compared with previous GFDL models and with the CMIP5 archive of AMIP simulations. The model's Cess sensitivity (response in the top-of-atmosphere radiative flux to uniform warming of SSTs) and effective radiative forcing are also presented. In Part II, the model formulation is described more fully and key sensitivities to aspects of the model formulation are discussed, along with the approach to model tuning.
In Part II of this two-part paper, documentation is provided of key aspects of a version of the AM4.0/LM4.0 atmosphere/land model that will serve as a base for a new set of climate and Earth system models (CM4 and ESM4) under development at NOAA's Geophysical Fluid Dynamics Laboratory (GFDL). The quality of the simulation in AMIP (Atmospheric Model Intercomparison Project) mode has been provided in Part I. Part II provides documentation of key components and some sensitivities to choices of model formulation and values of parameters, highlighting the convection parameterization and orographic gravity wave drag. The approach taken to tune the model's clouds to observations is a particular focal point. Care is taken to describe the extent to which aerosol effective forcing and Cess sensitivity have been tuned through the model development process, both of which are relevant to the ability of the model to simulate the evolution of temperatures over the last century when coupled to an ocean model.
Blanchard, Julia L., Reg A Watson, Elizabeth A Fulton, R S Cottrell, K L Nash, Andrea Bryndum-Buchholz, Matthias Büchner, D A Carozza, William W L Cheung, J Elliott, L N K Davidson, N K Dulvy, and John P Dunne, et al., August 2017: Linked sustainability challenges and trade-offs among fisheries, aquaculture and agriculture. Nature Ecology & Evolution, 1(9), DOI:10.1038/s41559-017-0258-8. Abstract
Fisheries and aquaculture make a crucial contribution to global food security, nutrition and livelihoods. However, the UN Sustainable Development Goals separate marine and terrestrial food production sectors and ecosystems. To sustainably meet increasing global demands for fish, the interlinkages among goals within and across fisheries, aquaculture and agriculture sectors must be recognized and addressed along with their changing nature. Here, we assess and highlight development challenges for fisheries-dependent countries based on analyses of interactions and trade-offs between goals focusing on food, biodiversity and climate change. We demonstrate that some countries are likely to face double jeopardies in both fisheries and agriculture sectors under climate change. The strategies to mitigate these risks will be context-dependent, and will need to directly address the trade-offs among Sustainable Development Goals, such as halting biodiversity loss and reducing poverty. Countries with low adaptive capacity but increasing demand for food require greater support and capacity building to transition towards reconciling trade-offs. Necessary actions are context-dependent and include effective governance, improved management and conservation, maximizing societal and environmental benefits from trade, increased equitability of distribution and innovation in food production, including continued development of low input and low impact aquaculture.
Johnson, K S., J N Plant, John P Dunne, Lynne D Talley, and Jorge L Sarmiento, August 2017: Annual nitrate drawdown observed by SOCCOM profiling floats and the relationship to annual net community production. Journal of Geophysical Research: Oceans, 122(8), DOI:10.1002/2017JC012839. Abstract
Annual nitrate cycles have been measured throughout the pelagic waters of the Southern Ocean, including regions with seasonal ice cover and southern hemisphere subtropical zones. Vertically resolved nitrate measurements were made using in situ ultraviolet spectrophotometer (ISUS) and submersible ultraviolet nitrate analyzer (SUNA) optical nitrate sensors deployed on profiling floats. Thirty-one floats returned forty complete annual cycles. The mean nitrate profile from the month with the highest winter nitrate minus the mean profile from the month with the lowest nitrate yields the annual nitrate drawdown. This quantity was integrated to 200 m depth and converted to carbon using the Redfield Ratio to estimate Annual Net Community Production (ANCP) throughout the Southern Ocean south of 30° S. A well-defined, zonal mean distribution is found with highest values (3 to 4 mol C m−2 y−1) from 40 to 50° S. Lowest values are found in the subtropics and in the seasonal ice zone. The area weighted mean was 2.9 mol C m−2 y−1 for all regions south of 40° S. Cumulative ANCP south of 50° S is 1.3 Pg C y−1. This represents about 13% of global ANCP in about 14% of the global ocean area.
Accurate representation of the remineralization of sinking organic matter is crucial for reliable projections of the marine carbon cycle. Both water temperature and oxygen concentration are thought to influence remineralization rates, but limited data constraints have caused disagreement concerning the degree of these influences. We analyse a compilation of POC flux measurements from 19 globally distributed sites. Our results indicate that the attenuation of the flux of particulate organic matter depends on temperature with a Q10 between 1.5 and 2.01, and on oxygen described by a half saturation constant between 4 and 12 μmol/L. We assess the impact of the temperature and oxygen dependence in the biogeochemistry model COBALT, coupled to GFDL's Earth System Model ESM2M. The new remineralization parameterization results in shallower remineralization in the low latitudes but deeper remineralization in the high latitudes, redistributing POC flux towards the poles. It also decreases the volume of the oxygen minimum zones, partly addressing a long-standing bias in global climate models. Extrapolating temperature-dependent remineralization rates to the surface (i.e., beyond the depth range of POC flux data) resulted in rapid recycling and excessive surface nutrients. Surface nutrients could be ameliorated by reducing near surface rates in a manner consistent with bacterial colonization, suggesting the need for improved remineralization constraints within the euphotic zone. The temperature and oxygen dependence cause an additional 10% decrease in global POC flux at 500m depth, but no significant change in global POC flux at 2000m under the RCP8.5 future projection.
Mislan, K A., Curtis A Deutsch, R W Brill, John P Dunne, and Jorge L Sarmiento, October 2017: Projections of climate driven changes in tuna vertical habitat based on species-specific differences in blood oxygen affinity. Global Change Biology, 23(10), DOI:10.1111/gcb.13799. Abstract
Oxygen concentrations are hypothesized to decrease in many areas of the ocean as a result of anthropogenically-driven climate change, resulting in habitat compression for pelagic animals. The oxygen partial pressure, pO2, at which blood is 50% saturated (P50) is a measure of blood oxygen affinity and a gauge of the tolerance of animals for low ambient oxygen. Tuna species display a wide range of blood oxygen affinities (i.e., P50 values) and therefore may be differentially impacted by habitat compression as they make extensive vertical movements to forage on sub-daily time scales. To project the effects of end-of-the-century climate change on tuna habitat, we calculate tuna P50 depths (i.e., the vertical position in the water column at which ambient pO2 is equal to species-specific blood P50 values) from 21st century Earth System Model (ESM) projections included in the fifth phase of the Climate Model Intercomparison Project (CMIP5). Overall, we project P50 depths to shoal, indicating likely habitat compression for tuna species due to climate change. Tunas that will be most impacted by shoaling are Pacific and southern bluefin tunas – habitat compression is projected for the entire geographic range of Pacific bluefin tuna and for the spawning region of southern bluefin tuna. Vertical shifts in P50 depths will potentially influence resource partitioning among Pacific bluefin, bigeye, yellowfin, and skipjack tunas in the northern subtropical and eastern tropical Pacific Ocean, the Arabian Sea, and the Bay of Bengal. By establishing linkages between tuna physiology and environmental conditions, we provide a mechanistic basis to project the effects of anthropogenic climate change on tuna habitats.
Neuer, S, H M Benway, N Bates, C A Carlson, M Church, M DeGrandpre, and John P Dunne, et al., September 2017: Monitoring Ocean Change in the 21st Century. EOS, 98, DOI:10.1029/2017EO080045.
Orr, James C., R G Najjar, Olivier Aumont, Laurent Bopp, J L Bullister, Gokhan Danabasoglu, Scott C Doney, John P Dunne, J-C Dutay, H D Graven, Stephen M Griffies, and Jasmin G John, et al., June 2017: Biogeochemical protocols and diagnostics for the CMIP6 Ocean Model Intercomparison Project (OMIP). Geoscientific Model Development, 10(6), DOI:10.5194/gmd-10-2169-2017. Abstract
The Ocean Model Intercomparison Project (OMIP) focuses on the physics and biogeochemistry of the ocean component of Earth System Models participating in the sixth phase of the Coupled Model Intercomparison Project (CMIP6). OMIP aims to provide standard protocols and diagnostics for ocean models, while offering a forum to promote their common assessment and improvement. It also offers to compare solutions of the same ocean models when forced with reanalysis data (OMIP simulations) versus when integrated within fully coupled Earth System Models (CMIP6). Here we detail simulation protocols and diagnostics for OMIP's biogeochemical and inert chemical tracers. These passive-tracer simulations will be coupled online to ocean circulation models, initialized with observational data or output from a model spin up, and forced by repeating the 1948–2009 surface fluxes of heat, fresh water, and momentum. These so-called OMIP-BGC simulations include three inert chemical tracers (CFC-11, CFC-12, SF6 and biogeochemical tracers (e.g., dissolved inorganic carbon, carbon isotopes, alkalinity, nutrients, and oxygen). Modelers will use their preferred prognostic BGC model but should follow common guidelines for gas exchange and carbonate chemistry. Simulations include both natural and total carbon tracers. The required forced simulation (omip1) will be initialized with gridded observational climatologies. An optional forced simulation (omip1-spunup) will be initialized instead with BGC fields from a long model spin up, preferably for 2000 years or more and forced by repeating the same 62-year meteorological forcing. That optional run will also include abiotic tracers of total dissolved inorganic carbon and radiocarbon, CTabio and 14CTabio, to assess deep-ocean ventilation and distinguish the role of physics vs. biology. These simulations will be forced by observed atmospheric histories of the three inert gases and CO2 as well as carbon isotope ratios of CO2. OMIP-BGC simulation protocols are founded on those from previous phases of the Ocean Carbon-Cycle Model Intercomparison Project. They have been merged and updated to reflect improvements concerning gas exchange, carbonate chemistry, and new data for initial conditions and atmospheric gas histories. Code is provided to facililtate their implementation.
Photosynthesis fuels marine food webs, yet differences in fish catch across globally distributed marine ecosystems far exceed differences in net primary production (NPP). We consider the hypothesis that ecosystem-level variations in pelagic and benthic energy flows from phytoplankton to fish, trophic transfer efficiencies, and fishing effort can quantitatively reconcile this contrast in an energetically consistent manner. To test this hypothesis, we enlist global fish catch data that include previously neglected contributions from small-scale fisheries, a synthesis of global fishing effort, and plankton food web energy flux estimates from a prototype high-resolution global earth system model (ESM). After removing a small number of lightly fished ecosystems, stark interregional differences in fish catch per unit area can be explained (r = 0.79) with an energy-based model that (i) considers dynamic interregional differences in benthic and pelagic energy pathways connecting phytoplankton and fish, (ii) depresses trophic transfer efficiencies in the tropics and, less critically, (iii) associates elevated trophic transfer efficiencies with benthic-predominant systems. Model catch estimates are generally within a factor of 2 of values spanning two orders of magnitude. Climate change projections show that the same macroecological patterns explaining dramatic regional catch differences in the contemporary ocean amplify catch trends, producing changes that may exceed 50% in some regions by the end of the 21st century under high-emissions scenarios. Models failing to resolve these trophodynamic patterns may significantly underestimate regional fisheries catch trends and hinder adaptation to climate change.
We use a large initial condition suite of simulations (30 runs) with an Earth system model to assess the detectability of biogeochemical impacts of ocean acidification (OA) on the marine alkalinity distribution from decadally repeated hydrographic measurements such as those produced by the Global Ship-Based Hydrographic Investigations Program (GO-SHIP). Detection of these impacts is complicated by alkalinity changes from variability and long-term trends in freshwater and organic matter cycling and ocean circulation. In our ensemble simulation, variability in freshwater cycling generates large changes in alkalinity that obscure the changes of interest and prevent the attribution of observed alkalinity redistribution to OA. These complications from freshwater cycling can be mostly avoided through salinity normalization of alkalinity. With the salinity-normalized alkalinity, modeled OA impacts are broadly detectable in the surface of the subtropical gyres by 2030. Discrepancies between this finding and the finding of an earlier analysis suggest that these estimates are strongly sensitive to the patterns of calcium carbonate export simulated by the model. OA impacts are detectable later in the subpolar and equatorial regions due to slower responses of alkalinity to OA in these regions and greater seasonal equatorial alkalinity variability. OA impacts are detectable later at depth despite lower variability due to smaller rates of change and consistent measurement uncertainty.
Attributing uncertainty in ocean carbon uptake between societal trajectory (scenarios), earth system model construction (structure), and inherent natural variation in climate (internal), is critical to make progress in identifying, understanding and reducing those uncertainties. In the present issue of Global Biogeochemical Cycles, Lovenduski et al. (2016) disentangle these drivers of uncertainty in ocean carbon uptake over time and space and assess the resulting implications for the emergence timescales of structural and scenario uncertainty over internal variability. Such efforts are critical for establishing realizable and efficient monitoring goals and prioritizing areas of continued model development. Under recently proposed climate stabilization targets, such efforts to partition uncertainty also become increasingly critical to societal decision-making in the context of carbon stabilization.
Jones, C, Vivek Arora, Pierre Friedlingstein, Laurent Bopp, Victor Brovkin, John P Dunne, H D Graven, F Hoffman, Tatiana Ilyina, and Jasmin G John, et al., August 2016: C4MIP – The Coupled Climate–Carbon Cycle Model Intercomparison Project: experimental protocol for CMIP6. Geoscientific Model Development, 9(8), DOI:10.5194/gmd-9-2853-2016. Abstract
Coordinated experimental design and implementation has become a cornerstone of global climate modelling. So-called Model Intercomparison Projects (MIPs) enable systematic and robust analysis of results across many models to identify common signals and understand model similarities and differences without being hindered by ad-hoc differences in model set-up or experimental boundary conditions. The activity known as the Coupled Model Intercomparison Project (CMIP) has thus grown significantly in scope and as it enters its 6th phase, CMIP6, the design and documentation of individual simulations has been devolved to individual climate science communities.
The Coupled Climate-Carbon Cycle Model Intercomparison Project (C4MIP) takes responsibility for design, documentation and analysis of carbon cycle feedbacks and interactions in climate simulations. These feedbacks are potentially large and play a leading order contribution in determining the atmospheric composition in response to human emissions of CO2 and in the setting of emissions targets to stabilise climate or avoid dangerous climate change. For over a decade C4MIP has coordinated coupled climate-carbon cycle simulations and in this paper we describe the C4MIP simulations that will be formally part of CMIP6. While the climate-carbon cycle community has formed this experimental design the simulations also fit into the wider CMIP activity and conform to some common standards such as documentation and diagnostic requests and are designed to complement the CMIP core experiments known as the DECK.
C4MIP has 3 key strands of scientific motivation and the requested simulations are designed to satisfy their needs: (1) pre-industrial and historical simulations (formally part of the common set of CMIP6 experiments) to enable model evaluation; (2) idealised coupled and partially-coupled simulations with 1 % per year increases in CO2 to enable diagnosis of feedback strength and its components; (3) future scenario simulations to project how the Earth System will respond over the 21st century and beyond to anthropogenic activity.
This paper documents in detail these simulations, explains their rationale and planned analysis, and describes how to set-up and run the simulations. Particular attention is paid to boundary conditions and input data required, and also the output diagnostics requested. It is important that modelling groups participating in C4MIP adhere as closely as possible to this experimental design.
Thermal expansion of the ocean in response to warming is an important component of historical sea-level rise1. Observational studies show that the Atlantic and Southern oceans are warming faster than the Pacific Ocean2, 3, 4, 5. Here we present simulations using a numerical atmospheric-ocean general circulation model with an interactive carbon cycle to evaluate the impact of carbon emission rates, ranging from 2 to 25 GtC yr−1, on basin-scale ocean heat uptake and sea level. For simulations with emission rates greater than 5 GtC yr−1, sea-level rise is larger in the Atlantic than Pacific Ocean on centennial timescales. This basin-scale asymmetry is related to the shorter flushing timescales and weakening of the overturning circulation in the Atlantic. These factors lead to warmer Atlantic interior waters and greater thermal expansion. In contrast, low emission rates of 2 and 3 GtC yr−1 will cause relatively larger sea-level rise in the Pacific on millennial timescales. For a given level of cumulative emissions, sea-level rise is largest at low emission rates. We conclude that Atlantic coastal areas may be particularly vulnerable to near-future sea-level rise from present-day high greenhouse gas emission rates.
Kwon, Eun Young, Y H Kim, Y-G Park, Y-H Park, John P Dunne, and K-I Chang, November 2016: Multi-decadal Wind-Driven Shifts in Northwest Pacific Temperature, Salinity, O2 and PO4. Global Biogeochemical Cycles, 30(11), DOI:10.1002/2016GB005442. Abstract
The North Pacific gyre boundaries are characterized by stark contrasts in physical and biogeochemical properties. Meridional movement of gyre boundaries, influenced by climate change, can therefore exert a large influence on not only marine ecosystems but also on climate. We examine the evidence for wind-driven southward shifts in subsurface temperature, salinity, PO4, and O2 within the Northwest Pacific from the 1950s to the 2000s. Gyre boundary shifts can explain 30 ~ 60% of temperature and salinity trends zonally averaged in the Northwest Pacific, and observed PO4 and O2 trends along the 137°E and 144°E meridians. The close tie between the wind-driven shifts in gyre boundaries and the tracer distributions is further supported by results from an eddy-resolving (0.1° × 0.1°) GFDL climate model, suggesting that the physical and biogeochemical properties averaged within the Northwest Pacific gyre boundaries closely follow the latitude changes of the zero Sverdrup stream function with lags of zero to three years. The gyre shift effect on tracer distribution is poorly represented in a coarse resolution (1° × 1°) model due partly to poor representations of fronts and eddies. This study suggests that future changes in Northwest Pacific PO4 and O2 content may depend not only on ocean temperature and stratification, but also on the ocean gyre response to winds.
Laufkötter, Charlotte, M Vogt, Nicolas Gruber, Olivier Aumont, Laurent Bopp, Scott C Doney, John P Dunne, Judith Hauck, and Jasmin G John, et al., July 2016: Projected decreases in future marine export production: the role of the carbon flux through the upper ocean ecosystem. Biogeosciences, 13(13), DOI:10.5194/bg-13-4023-2016. Abstract
Accurate projections of marine particle export production (EP) are crucial for predicting the response of the marine carbon cycle to climate change, yet models show a wide range in both global EP and their responses to climate change. This is, in part, due to EP being the net result of a series of processes, starting with net primary production (NPP) in the sunlit upper ocean, followed by the formation of particulate organic matter and the subsequent sinking and remineralization of these particles, with each of these processes responding differently to changes in environmental conditions. Here, we compare future projections in EP over the 21st century, generated by four marine ecosystem models under IPCC's high emission scenario RCP8.5, and determine the processes driving these changes. The models simulate small to modest decreases in global EP between −1 and −12 %. Models differ greatly with regard to the drivers causing these changes. Among them, the formation of particles is the most uncertain process with models not agreeing on either magnitude or the direction of change. The removal of the sinking particles by remineralization is simulated to increase in the low and intermediate latitudes in three models, driven by either warming-induced increases in remineralization or slower particle sinking, and show insignificant changes in the remaining model. Changes in ecosystem structure, particularly the relative role of diatoms matters as well, as diatoms produce larger and denser particles that sink faster and are partly protected from remineralization. Also this controlling factor is afflicted with high uncertainties, particularly since the models differ already substantially with regard to both the initial (present-day) distribution of diatoms (between 11–94 % in the Southern Ocean) and the diatom contribution to particle formation (0.6–3.8 times lower/higher than their contribution to biomass). As a consequence, changes in diatom concentration are a strong driver for EP changes in some models but of low significance in others. Observational and experimental constraints on ecosystem structure and how the fixed carbon is routed through the ecosystem to produce export production are urgently needed in order to improve current generation ecosystem models and their ability to project future changes.
Lee, Y J., P A Matrai, Marjorie A M Friedrichs, Vincent S Saba, Olivier Aumont, M Babin, Erik T Buitenhuis, M Chevallier, L de Mora, M Dessert, John P Dunne, I H Ellingsen, Daniel Feldman, R Frouin, Marion Gehlen, T Gorgues, Tatiana Ilyina, M Jin, Jasmin G John, J Lawrence, Manfredi Manizza, C Menkes, C Perruche, V Le Fouest, E E Popova, Anastasia Romanou, A Samuelsen, Jörg Schwinger, Roland Séférian, and Charles A Stock, et al., December 2016: Net primary productivity estimates and environmental variables in the Arctic Ocean: An assessment of coupled physical-biogeochemical models. Journal of Geophysical Research: Oceans, 121(12), DOI:10.1002/2016JC011993. Abstract
The relative skill of 21 regional and global biogeochemical models was assessed in terms of how well the models reproduced observed net primary productivity (NPP) and environmental variables such as nitrate concentration (NO3), mixed layer depth (MLD), euphotic layer depth (Zeu), and sea ice concentration, by comparing results against a newly updated, quality-controlled in situ NPP database for the Arctic Ocean (1959–2011). The models broadly captured the spatial features of integrated NPP (iNPP) on a pan-Arctic scale. Most models underestimated iNPP by varying degrees in spite of overestimating surface NO3, MLD, and Zeu throughout the regions. Among the models, iNPP exhibited little difference over sea ice condition (ice-free versus ice-influenced) and bottom depth (shelf versus deep ocean). The models performed relatively well for the most recent decade and toward the end of Arctic summer. In the Barents and Greenland Seas, regional model skill of surface NO3 was best associated with how well MLD was reproduced. Regionally, iNPP was relatively well simulated in the Beaufort Sea and the central Arctic Basin, where in situ NPP is low and nutrients are mostly depleted. Models performed less well at simulating iNPP in the Greenland and Chukchi Seas, despite the higher model skill in MLD and sea ice concentration, respectively. iNPP model skill was constrained by different factors in different Arctic Ocean regions. Our study suggests that better parameterization of biological and ecological microbial rates (phytoplankton growth and zooplankton grazing) are needed for improved Arctic Ocean biogeochemical modeling.
Marine species ranging in size from microscopic zooplankton to large predatory fish move vertically in the ocean water column to forage for food and avoid predators. Oxygen and temperature decrease, often rapidly, from shallow to deeper depths, restricting the ability of species to use the vertical habitat. One physiological trait that determines the tolerance of organisms to low oxygen is the oxygen affinity of oxygen carrier proteins, hemoglobin and hemocyanin, in the blood. To quantify the range of oxygen affinities for marine organisms, we surveyed the literature for measurements of oxygen binding to blood at multiple temperatures to account for its temperature sensitivity. Oxygen affinity is mapped within the ocean environment using the depth at which oxygen pressure decreases to the point at which the blood is 50% oxygenated (P50 depth) as organisms move from the surface to depth in the ocean water column. We find that vertical gradients in both temperature and oxygen impact the vertical position and areal extent of P50 depths. Shifts in P50 due to temperature cause physiological types with the same P50 in the surface ocean to have different P50 depths and physiological types with different P50’s in the surface ocean to have the same P50 depth. The vertical distances between P50 depths are spatially variable, which may determine the frequency of ecological interactions, such as competition and predation. In summary, P50 depth, which represents a key physiological transition point between dexoxygenated and oxygenated blood, provides mechanistic insight into organism function within the water column of the global ocean.
Nevison, Cynthia D., Manfredi Manizza, Ralph F Keeling, B B Stephens, Jonathan D Bent, and John P Dunne, et al., March 2016: Evaluating CMIP5 ocean biogeochemistry and Southern Ocean carbon uptake using atmospheric potential oxygen (APO): Present day performance and future projection. Geophysical Research Letters, 43(5), DOI:10.1002/2015GL067584. Abstract
Observed seasonal cycles in atmospheric potential oxygen (APO ~ O2 + 1.1 CO2) were used to evaluate 8 ocean biogeochemistry models from the Coupled Model Intercomparison Project (CMIP5). Model APO seasonal cycles were computed from the CMIP5 air-sea O2 and CO2 fluxes and compared to observations at 3 Southern Hemisphere monitoring sites. Four of the models captured either the observed APO seasonal amplitude or phasing relatively well, while the other four did not. Many models had an unrealistic seasonal phasing or amplitude of the CO2 flux, which in turn influenced APO. By 2100 under RCP8.5, the models projected little change in the O2 component of APO but large changes in the seasonality of the CO2 component associated with ocean acidification. The models with poorer performance on present-day APO tended to project larger net carbon uptake in the Southern Ocean, both today and in 2100.
Sedigh Marvasti, S, Anand Gnanadesikan, A A Bidokhti, John P Dunne, and S Ghader, February 2016: Challenges in modelling spatiotemporally varying phytoplankton blooms in the Northwestern Arabian Sea and Gulf of Oman. Biogeosciences, 13(4), DOI:10.5194/bg-13-1049-2016. Abstract
We examine interannual variability of phytoplankton blooms in northwestern Arabian Sea and Gulf of Oman. Satellite data (SeaWIFS ocean color) shows two climatological blooms in this region, a wintertime bloom peaking in February and a summertime bloom peaking in September. A pronounced anti-correlation between the AVISO sea surface height anomaly (SSHA) and chlorophyll is found during the wintertime bloom. On a regional scale, interannual variability of the wintertime bloom is thus dominated by cyclonic eddies which vary in location from one year to another. These results were compared against the outputs from three different 3-D Earth System models. We show that two coarse (1°) models with the relatively complex biogeochemistry (TOPAZ) capture the annual cycle but neither eddies nor the interannual variability. An eddy-resolving model (GFDL CM2.6) with a simpler biogeochemistry (miniBLING) displays larger interannual variability, but overestimates the wintertime bloom and captures eddy-bloom coupling in the south but not in the north. The southern part of the domain is a region with a much sharper thermocline and nutricline relatively close to the surface, in which eddies modulate diffusive nutrient supply to the surface (a mechanism not previously emphasized in the literature). We suggest that for the model to simulate the observed wintertime blooms within cyclones, it will be necessary to represent this relatively unusual nutrient structure as well as the cyclonic eddies. This is a challenge in the Northern Arabian Sea as it requires capturing the details of the outflow from the Persian Gulf.
Séférian, Roland, Marion Gehlen, Laurent Bopp, Laure Resplandy, James C Orr, O Marti, and John P Dunne, et al., May 2016: Inconsistent strategies to spin up models in CMIP5: implications for ocean biogeochemical model performance assessment. Geoscientific Model Development, 9(5), DOI:10.5194/gmd-9-1827-2016. Abstract
During the fifth phase of the Coupled Model Intercomparison Project (CMIP5) substantial efforts were carried out on the systematic assessment of the skill of Earth system models. One goal was to check how realistically representative marine biogeochemical tracer distributions could be reproduced by models. Mean-state assessments routinely compared model hindcasts to available modern biogeochemical observations. However, these assessments considered neither the extent of equilibrium in modeled biogeochemical reservoirs nor the sensitivity of model performance to initial conditions or to the spin-up protocols. Here, we explore how the large diversity in spin-up protocols used for marine biogeochemistry in CMIP5 Earth system models (ESM) contribute to model-to-model differences in the simulated fields. We take advantage of a 500 year spin-up simulation of IPSL-CM5A-LR to quantify the influence of the spin-up protocol on model ability to reproduce relevant data fields. Amplification of biases in selected biogeochemical fields (O2, NO3, Alk-DIC) is assessed as a function of spin-up duration. We demonstrate that a relationship between spin-up duration and assessment metrics emerges from our model results and is consistent when confronted against a larger ensemble of CMIP5 models. This shows that drift has implications on their performance assessment in addition to possibly aliasing estimates of climate change impact. Our study suggests that differences in spin-up protocols could explain a substantial part of model disparities, constituting a source of model-to-model uncertainty. This requires more attention in future model intercomparison exercices in order to provide realistic ESM results on marine biogeochemistry and carbon cycle feedbacks.
Tagliabue, Alessandro, Olivier Aumont, R DeAth, John P Dunne, Stephanie Dutkiewicz, Eric D Galbraith, K Misumi, J Keith Moore, A Ridgwell, E Sherman, and Charles A Stock, et al., February 2016: How well do global ocean biogeochemistry models simulate dissolved iron distributions?Global Biogeochemical Cycles, 30(2), DOI:10.1002/2015GB005289. Abstract
Numerical models of ocean biogeochemistry are relied upon to make projections about the impact of climate change on marine resources and test hypotheses regarding the drivers of past changes in climate and ecosystems. In large areas of the ocean, iron availability regulates the functioning of marine ecosystems and hence the ocean carbon cycle. Accordingly, our ability to quantify the drivers and impacts of fluctuations in ocean ecosystems and carbon cycling in space and time relies on first achieving an appropriate representation of the modern marine iron cycle in models. When the iron distributions from thirteen global ocean biogeochemistry models are compared against the latest oceanic sections from the GEOTRACES programme we find that all models struggle to reproduce many aspects of the observed spatial patterns. Models that reflect the emerging evidence for multiple iron sources or subtleties of its internal cycling perform much better in capturing observed features than their simpler contemporaries, particularly in the ocean interior. We show that the substantial uncertainty in the input fluxes of iron results in a very wide range of residence times across models, which has implications for the response of ecosystems and global carbon cycling to perturbations. Given this large uncertainty, iron-fertilisation experiments based on any single current generation model should be interpreted with caution. Improvements to how such models represent iron scavenging and also biological cycling are needed to raise confidence in their projections of global biogeochemical change in the ocean.
Westberry, T, Patrick Schultz, John P Dunne, M R Hiscock, S Maritorena, Jorge L Sarmiento, D A Siegel, and M J Behrenfeld, February 2016: Annual cycles of phytoplankton biomass in the Subarctic Atlantic and Pacific Ocean. Global Biogeochemical Cycles, 30(2), DOI:10.1002/2015GB005276. Abstract
High latitude phytoplankton blooms support productive fisheries and play an important role in oceanic uptake of atmospheric carbon dioxide. In the subarctic North Atlantic Ocean, blooms are a recurrent feature each year, while in the eastern subarctic Pacific only small changes in chlorophyll (Chl) are seen over the annual cycle. Here, we show that when evaluated using phytoplankton carbon biomass (Cphyto) rather than Chl, an annual bloom in the North Pacific is evident and can even rival blooms observed in the North Atlantic. The annual increase in subarctic Pacific phytoplankton biomass is not readily observed in the Chl record because it is paralleled by light- and nutrient-driven decreases in cellular pigment levels (Cphyto:Chl). Specifically, photoacclimation and iron stress effects on Cphyto:Chl oppose the biomass increase, leading to only modest changes in bulk Chl. The magnitude of the photoacclimation effect is quantified using descriptors of the near-surface light environment and a photophysiological model. Iron-stress effects are diagnosed from satellite chlorophyll fluorescence data. Last, we show that biomass accumulation in the Pacific is slower than the Atlantic, but is closely tied to similar levels of seasonal nutrient uptake in both basins. Annual cycles of satellite-derived Chl and Cphyto are reproduced by in situ autonomous profiling floats. These results contradict the long-standing paradigm that environmental conditions prevent phytoplankton accumulation in the subarctic Northeast Pacific and suggest a greater seasonal decoupling between phytoplankton growth and losses than traditionally implied. Further, our results highlight the role of physiological processes in shaping bulk properties, such as Chl, and their interpretation in studies of ocean ecosystem dynamics and climate change.
Anderegg, W, A P Ballantyne, W Kolby Smith, J D Majkut, S Rabin, C Beaulieu, R A Birdsey, John P Dunne, R A Houghton, R B Myneni, Yude Pan, Jorge L Sarmiento, N Serota, and Elena Shevliakova, et al., December 2015: Tropical nighttime warming as a dominant driver of variability in the terrestrial carbon sink. Proceedings of the National Academy of Sciences, 112(51), DOI:10.1073/pnas.1521479112. Abstract
The terrestrial biosphere is currently a strong carbon (C) sink but may switch to a source in the 21st century as climate-driven losses exceed CO2-driven C gains, thereby accelerating global warming. Although it has long been recognized that tropical climate plays a critical role in regulating interannual climate variability, the causal link between changes in temperature and precipitation and terrestrial processes remains uncertain. Here, we combine atmospheric mass balance, remote sensing-modeled datasets of vegetation C uptake, and climate datasets to characterize the temporal variability of the terrestrial C sink and determine the dominant climate drivers of this variability. We show that the interannual variability of global land C sink has grown by 50–100% over the past 50 y. We further find that interannual land C sink variability is most strongly linked to tropical nighttime warming, likely through respiration. This apparent sensitivity of respiration to nighttime temperatures, which are projected to increase faster than global average temperatures, suggests that C stored in tropical forests may be vulnerable to future warming.
This study examines the role of processes transporting tracers across the Polar Front (PF) in the depth interval between the surface and major topographic sills, which we refer to as the “PF core”. A preindustrial control simulation of an eddying climate model coupled to a biogeochemical model (CM2.6-miniBLING, 0.1° ocean model) is used to investigate the transport of heat, carbon, oxygen and phosphate across the PF core, with a particular focus on the role of mesoscale eddies. We find that the total transport across the PF core results from an ubiquitous Ekman transport that drives the upwelled tracers to the north, and a localized opposing eddy transport that induces tracer leakages to the south at major topographic obstacles. In the Ekman layer, the southward eddy transport only partially compensates the northward Ekman transport, while below the Ekman layer, the southward eddy transport dominates the total transport but remains much smaller in magnitude than the near-surface northward transport. Most of the southward branch of the total transport is achieved below the PF core, mainly through geostrophic currents. We find that the eddy diffusive transport reinforces the southward eddy advective transport for carbon and heat, and opposes it for oxygen and phosphate. Eddy advective transport is likely to be the leading-order component of eddy-induced transport for all four tracers. However, eddy diffusive transport may provide a significant contribution to the southward eddy heat transport due to strong along-isopycnal temperature gradients.
The world’s major Eastern Boundary Currents (EBC)
are critically important areas for global fisheries. Computational
limitations have divided past EBC modeling
into two types: high-resolution regional approaches
that resolve the strong mesoscale structures involved;
and coarse global approaches that represent the largescale
context for EBCs but crudely resolve only the
largest scales of their local manifestation. These latter
global studies have illustrated the complex mechanisms
involved in the climate change and acidification response
in these regions, with the EBC response dominated not
by local adjustments but large-scale reorganization of
ocean circulation through remote forcing of water mass
supply pathways. While qualitatively illustrating the limitations
of regional high-resolution studies in long-term
projections, these studies lack the ability to robustly
quantify change because of the inability of these models
to represent the baseline mesoscale structures of EBCs.
In the present work, we compare current generation
coarse resolution (1˚) and a prototype next generation
high-resolution (1/10˚) Earth System Models (ESMs)
from NOAA ’s Geophysical Fluid Dynamics Laboratory
in representing the four major EBCs. We review the
long-known temperature biases that the coarse models
suffer in being unable to represent the timing and intensity
of upwelling-favorable winds. In promising contrast,
we show that the high-resolution prototype is capable
of representing not only the overall mesoscale structure
in physical and biogeochemical fields, but also the
appropriate offshore extent of temperature anomalies
and other EBC characteristics. In terms of representation
of large-scale circulation, results were mixed, with the
high-resolution prototype addressing some, but not all,
of the biases in the coarse-resolution ESM. The ability
to simulate EBCs in the global context at high resolution
in global ESMs represents a fundamental milestone
towards both seasonal to interannual ecological forecasting
and long-term projection of climate, ecosystem, and
acidification baselines and sensitivity.
http://www.calcofi.org/publications/calcofireports/v56/Vol56-Dunne.web.72-75.pdf
We assess the uptake, transport and storage of oceanic anthropogenic carbon and
heat over the period 1861 to 2005 in a new set of coupled carbon-climate Earth
System models conducted for the fifth Coupled Model Intercomparison Project
(CMIP5), with a particular focus on the Southern Ocean. Simulations show the
Southern Ocean south of 30°S, occupying 30% of global surface ocean area, accounts
for 43 ± 3% (42 ± 5 Pg C) of anthropogenic CO2 and 75 ± 22% (23 ± 9 *1022J) of heat
uptake by the ocean over the historical period. Northward transport out of the Southern
Ocean is vigorous, reducing the storage to 33 ± 6 Pg anthropogenic carbon and 12 ± 7
*1022J heat in the region. The CMIP5 models as a class tend to underestimate the
observational-based global anthropogenic carbon storage, but simulate trends in global
ocean heat storage over the last fifty years within uncertainties of observation-based
estimates. CMIP5 models suggest global and Southern Ocean CO2 uptake have been
largely unaffected by recent climate variability and change. Anthropogenic carbon and
heat storage show a common broad-scale pattern of change, but ocean heat storage is
more structured than ocean carbon storage. Our results highlight the significance of
the Southern Ocean for the global climate and as the region where models differ the
most in representation of anthropogenic CO2 and in particular heat uptake.
Galbraith, Eric D., John P Dunne, Anand Gnanadesikan, Richard D Slater, Jorge L Sarmiento, Carolina O Dufour, Gregory F de Souza, Daniele Bianchi, M Claret, Keith B Rodgers, and S Sedigh Marvasti, December 2015: Complex functionality with minimal computation: Promise and pitfalls of reduced-tracer ocean biogeochemistry models. Journal of Advances in Modeling Earth Systems, 7(4), DOI:10.1002/2015MS000463. Abstract
Earth System Models increasingly include ocean biogeochemistry models in order to predict changes in ocean carbon storage, hypoxia and biological productivity under climate change. However, state-of-the-art ocean biogeochemical models include many advected tracers, that significantly increase the computational resources required, forcing a tradeoff with spatial resolution. Here, we compare a state-of-the art model with 30 prognostic tracers (TOPAZ) with two reduced-tracer models, one with 6 tracers (BLING), the other with 3 tracers (miniBLING). The reduced-tracer models employ parameterized, implicit biological functions, that nonetheless capture many of the most important processes resolved by TOPAZ. All three are embedded in the same coupled climate model. Despite the large difference in tracer number, the absence of tracers for living organic matter is shown to have a minimal impact on the transport of nutrient elements, and the three models produce similar mean annual pre-industrial distributions of macronutrients, oxygen and carbon. Significant differences do exist amongst the models, in particular the seasonal cycle of biomass and export production, but it does not appear that these are necessary consequences of the reduced tracer number. With increasing CO2, changes in dissolved oxygen and anthropogenic carbon uptake are very similar across the different models. Thus, while the reduced-tracer models do not explicitly resolve the diversity and internal dynamics of marine ecosystems, we demonstrate that such models are applicable to a broad suite of major biogeochemical concerns, including anthropogenic change. These results are very promising for the further development and application of reduced-tracer biogeochemical models that incorporate ‘sub-ecosystem-scale' parameterizations.
We characterize impacts on heat in the ocean climate system from transient ocean mesoscale eddies. Our tool is a suite of centennial-scale 1990 radiatively forced numerical climate simulations from three GFDL coupled models comprising the CM2-O model suite. CM2-O models differ in their ocean resolution: CM2.6 uses a 0.1° ocean grid, CM2.5 uses an intermediate grid with 0.25° spacing, and CM2-1deg uses a nominally 1.0° grid.
Analysis of the ocean heat budget reveals that mesoscale eddies act to transport heat upward in a manner that partially compensates (or offsets) for the downward heat transport from the time mean currents. Stronger vertical eddy heat transport in CM2.6 relative to CM2.5 accounts for the significantly smaller temperature drift in CM2.6. The mesoscale eddy parameterization used in CM2-1deg also imparts an upward heat transport, yet it differs systematically from that found in CM2.6. This analysis points to the fundamental role that ocean mesoscale features play in transient ocean heat uptake. In general, the more accurate simulation found in CM2.6 provides an argument for either including a rich representation of the ocean mesoscale in model simulations of the mean and transient climate, or for employing parameterizations that faithfully reflect the role of eddies in both lateral and vertical heat transport.
Hauck, Judith, C Volker, D A Wolf-Gladrow, Charlotte Laufkötter, M Vogt, Olivier Aumont, Laurent Bopp, Erik T Buitenhuis, Scott C Doney, John P Dunne, Nicolas Gruber, T Hashioka, Jasmin G John, C Le Quéré, Ivan D Lima, Hideyuki Nakano, Roland Séférian, and I J Totterdell, September 2015: On the Southern Ocean CO2 uptake and the role of the biological carbon pump in the 21st century. Global Biogeochemical Cycles, 29(9), DOI:10.1002/2015GB005140. Abstract
We use a suite of eight ocean biogeochemical/ecological general circulation models from the MAREMIP and CMIP5 archives to explore the relative roles of changes in winds (positive trend of Southern Annular Mode, SAM) and in warming- and freshening-driven trends of upper ocean stratification in altering export production and CO2 uptake in the Southern Ocean at the end of the 21st century. The investigated models simulate a broad range of responses to climate change, with no agreement on a dominance of either the SAM or the warming signal south of 44 ∘ S. In the southernmost zone, i.e., south of 58∘ S, they concur on an increase of biological export production, while between 44 and 58∘ S the models lack consensus on the sign of change in export. Yet, in both regions, the models show an enhanced CO2 uptake during spring and summer. This is due to a larger CO2 (aq) drawdown by the same amount of summer export production at a higher Revelle factor at the end of the 21st century. This strongly increases the importance of the biological carbon pump in the entire Southern Ocean. In the temperate zone, between 30 and 44∘ S all models show a predominance of the warming signal and a nutrient-driven reduction of export production. As a consequence, the share of the regions south of 44∘ S to the total uptake of the Southern Ocean south of 30∘ S is projected to increase at the end of the 21st century from 47 to 66% with a commensurable decrease to the north. Despite this major reorganization of the meridional distribution of the major regions of uptake, the total uptake increases largely in line with the rising atmospheric CO2. Simulations with the MITgcm-REcoM2 model show that this is mostly driven by the strong increase of atmospheric CO2, with the climate-driven changes of natural CO2 exchange offsetting that trend only to a limited degree (∼10%) and with negligible impact of climate effects on anthropogenic CO2 uptake when integrated over a full annual cycle south of 30∘S.
Reversibility studies suggest a lagged recovery of global mean sea surface temperatures after mitigation, raising the question of whether a similar lag is likely for marine net primary production (NPP). Here we assess NPP reversibility with a mitigation scenario in which projected Representative Concentration Pathway (RCP8.5) forcings are applied out to 2100, and then reversed over the course of the following century in a fully coupled carbon-climate earth system model. In contrast to the temperature lag, we find a rapid increase in global mean NPP, including an overshoot to values above contemporary means. The enhanced NPP arises from a transient imbalance between the cooling surface ocean and continued warming in subsurface waters, which weakens upper ocean density gradients, resulting in deeper mixing and enhanced surface nitrate. We also find a marine ecosystem regime shift as persistent silicate depletion results in increased prevalence of large, non-diatom phytoplankton.
Jonsson, B F., Scott C Doney, John P Dunne, and M Bender, February 2015: Evaluating Southern Ocean biological production in two ocean biogeochemical models on daily to seasonal time-scales using satellite surface chlorophyll and O2/Ar observations. Biogeosciences, 12(3), DOI:10.5194/bg-12-681-2015. Abstract
We assess the ability of ocean biogeochemical models to represent seasonal structures in biomass and net community production (NCP) in the Southern Ocean. Two models are compared to observations on daily to seasonal time scales in four different sections of the region. We use daily satellite fields of Chlorophyll (Chl) as a proxy for biomass, and in-situ observations of O2 and Ar supersaturation (ΔO2Ar) to estimate NCP. ΔO2Ar is converted to the flux of biologically generated O2 from sea to air ("O2 bioflux"). All data are aggregated to a climatological year with a daily resolution. To account for potential regional differences within the Southern Ocean, we conduct separate analyses of sections south of South Africa, around the Drake Passage, south of Australia, and south of New Zealand.
We find that the models simulate the upper range of Chl concentrations well, underestimate spring levels significantly, and show differences in skill between early and late parts of the growing season. While there is a great deal of scatter in the bioflux observations in general, the four sectors each have distinct patterns that the models pick up. Neither model exhibit a significant distinction between the Australian and New Zealand sectors, and between the Drake Passage and African sectors. South of 60° S, the models fail to predict the observed extent of biological O2 undersaturation. We suggest that this shortcoming may be due either to problems with the ecosystem dynamics or problems with the vertical transport of oxygen.
Overall, the bioflux observations are in general agreement with the seasonal structures in satellite chlorophyll, suggesting that this seasonality represent changes in carbon biomass and not Chl : C ratios. This agreement is shared in the models and allows us to interpret the seasonal structure of satellite chlorophyll as qualitatively reflecting the integral of biological production over time for the purposes of model assessment.
Laufkötter, Charlotte, M Vogt, Nicolas Gruber, M Aita-Noguchi, Olivier Aumont, Laurent Bopp, Erik T Buitenhuis, Scott C Doney, John P Dunne, T Hashioka, Judith Hauck, T Hirata, and Jasmin G John, et al., December 2015: Drivers and uncertainties of future global marine primary production in marine ecosystem models. Biogeosciences, 12(23), DOI:10.5194/bg-12-6955-2015. Abstract
Past model studies have projected a global decrease in marine net primary production (NPP) over the 21st century, but these studies focused on the multi-model mean and mostly ignored the large inter-model differences. Here, we analyze model simulated changes of NPP for the 21st century under IPCC's high emission scenario RCP8.5 using a suite of nine coupled carbon–climate Earth System Models with embedded marine ecosystem models with a focus on the spread between the different models and the underlying reasons. Globally, five out of the nine models show a decrease in NPP over the course of the 21st century, while three show no significant trend and one even simulates an increase. The largest model spread occurs in the low latitudes (between 30° S and 30° N), with individual models simulating relative changes between −25 and +40%. In this region, the inter-quartile range of the differences between the 2012–2031 average and the 2081–2100 average is up to 3 mol C m-2 yr-1. These large differences in future change mirror large differences in present day NPP. Of the seven models diagnosing a net decrease in NPP in the low latitudes, only three simulate this to be a consequence of the classical interpretation, i.e., a stronger nutrient limitation due to increased stratification and reduced upwelling. In the other four, warming-induced increases in phytoplankton growth outbalance the stronger nutrient limitation. However, temperature-driven increases in grazing and other loss processes cause a net decrease in phytoplankton biomass and reduces NPP despite higher growth rates. One model projects a strong increase in NPP in the low latitudes, caused by an intensification of the microbial loop, while the remaining model simulates changes of less than 0.5%. While there is more consistency in the modeled increase in NPP in the Southern Ocean, the regional inter-model range is also very substantial. In most models, this increase in NPP is driven by temperature, but is also modulated by changes in light, macronutrients and iron as well as grazing. Overall, current projections of future changes in global marine NPP are subject to large uncertainties and necessitate a dedicated and sustained effort to improve the models and the concepts and data that guide their development.
Nevison, Cynthia D., Manfredi Manizza, Ralph F Keeling, M Kahru, Laurent Bopp, and John P Dunne, et al., January 2015: Evaluating the ocean biogeochemical components of Earth system models using atmospheric potential oxygen and ocean color data. Biogeosciences, 12(1), DOI:10.5194/bg-12-193-2015. Abstract
The observed seasonal cycles in atmospheric potential oxygen (APO) at a range of mid to high latitude surface monitoring sites are compared to those inferred from the output of 6 Earth System Models participating in the fifth phase of the Coupled Model Intercomparison Project (CMIP5). The simulated air–sea O2 fluxes are translated into APO seasonal cycles using a matrix method that takes into account atmospheric transport model (ATM) uncertainty among 13 different ATMs. Half of the ocean biogeochemistry models tested are able to reproduce the observed APO cycles at most sites, to within the current large ATM uncertainty, while the other half generally are not. Net Primary Production (NPP) and net community production (NCP), as estimated from satellite ocean color data, provide additional constraints, albeit more with respect to the seasonal phasing of ocean model productivity than the overall magnitude. The present analysis suggests that, of the tested ocean biogeochemistry models, CESM and GFDL ESM2M are best able to capture the observed APO seasonal cycle at both Northern and Southern Hemisphere sites. In the northern oceans, the comparison to observed APO suggests that most models tend to underestimate NPP or deep ventilation or both.
Rykaczewski, Ryan R., and John P Dunne, et al., August 2015: Poleward displacement of coastal upwelling-favorable winds in the ocean's eastern boundary currents through the 21st century. Geophysical Research Letters, 42(15), DOI:10.1002/2015GL064694. Abstract
Upwelling is critical to the biological production, acidification, and deoxygenation of the ocean's major eastern boundary current ecosystems. A leading conceptual hypothesis projects that the winds that induce coastal upwelling will intensify in response to increased land-sea temperature differences associated with anthropogenic global warming. We examine this hypothesis using an ensemble of coupled, ocean–atmosphere models and find limited evidence for intensification of upwelling-favorable winds or atmospheric pressure gradients in response to increasing land-sea temperature differences. However, our analyses reveal consistent latitudinal and seasonal dependence of projected changes in wind intensity associated with poleward migration of major atmospheric high-pressure cells. Summertime winds near poleward boundaries of climatological upwelling zones are projected to intensify, while winds near equatorward boundaries are projected to weaken. Developing a better understanding future changes in upwelling winds is essential to identifying portions of the oceans susceptible to increased hypoxia, ocean acidification, and eutrophication under climate change.
Saba, Vincent S., Charles A Stock, and John P Dunne, October 2015: Relation of Marine Primary Productivity to Leatherback Biology and Behavior In The Leatherback Turtle: Biology and Conservation, Baltimore, MD, John Hopkins University Press, 173-184.
Van Oostende, N, John P Dunne, S E Fawcett, and B B Ward, August 2015: Phytoplankton succession explains size-partitioning of new production following upwelling-induced blooms. Journal of Marine Systems, 148, DOI:10.1016/j.jmarsys.2015.01.009. Abstract
Large and chain-forming diatoms typically dominate the phytoplankton biomass after initiation of coastal upwelling. The ability of these diatoms to accelerate and maintain elevated nitrate uptake rates has been proposed to explain the dominance of diatoms over all other phytoplankton groups. Moreover, the observed delay in biomass accumulation following nitrate supply after initiation of upwelling events has been hypothesised to result from changes in the diatom community structure or from physiological acclimation. To investigate these mechanisms, we used both numerical modelling and experimental incubations that reproduced the characteristic succession from small to large species in phytoplankton community composition and size structure. Using the Tracers Of Phytoplankton with Allometric Zooplankton (TOPAZ) ecosystem model as a framework, we find that variations in functional group-specific traits must be taken into account, through adjustments of group-dependent maximum production rates (PCmax, s− 1), in order to accurately reproduce the observed patterns and timescales of size-partitioned new production in a non-steady state environment. Representation of neither nutrient acclimation, nor diatom diversity in the model was necessary as long as lower than theoretical maximum production rates were implemented. We conclude that this physiological feature, PCmax, is critical in representing the early, relatively higher specific nitrate uptake rate of large diatoms, and explains the differential success of small and large phytoplankton communities in response to nitrate supply during upwelling.
Willis-Norton, E, Elliot L Hazen, S Fossette, G Shillinger, Ryan R Rykaczewski, D G Foley, John P Dunne, and Steven J Bograd, March 2015: Climate change impacts on leatherback turtle pelagic habitat in the southeast Pacific. Deep-Sea Research, Part II, 113, DOI:10.1016/j.dsr2.2013.12.019. Abstract
Eastern Pacific populations of the leatherback turtle (Dermochelys coriacea) have declined by over 90% during the past three decades. The decline is primarily attributed to human pressures, including unsustainable egg harvest, development on nesting beaches, and by-catch mortality. In particular, the effects of climate change may impose additional stress upon already threatened leatherback populations. This study analyzes how the pelagic habitat of Eastern Pacific leatherbacks may be affected by climate change over the next century. This population adheres to a persistent migration pattern; following nesting at Playa Grande, Costa Rica, individuals move rapidly through equatorial currents and into foraging habitat within the oligotrophic South Pacific Gyre. Forty-six nesting females were fitted with satellite tags. Based on the turtle positions, ten environmental variables were sampled along the tracks. Presence/absence habitat models were created to determine the oceanographic characteristics of the preferred turtle habitat. Core pelagic habitat was characterized by relatively low sea surface temperatures and chlorophyll-a. Based on these habitat models, we predicted habitat change using output from the Geophysical Fluid Dynamics Laboratory prototype Earth System Model under the Special Report on Emissions Scenario’s A2 (business-as-usual) scenario. Although the model predicted both habitat losses and gains throughout the region, we estimated that overall the core pelagic habitat of the Eastern Pacific leatherback population will decline by approximately 15 percent within the next century. This habitat modification might increase pressure on a critically endangered population, possibly forcing distributional shifts, behavioral changes, or even extinction.
We trace the marine biogeochemical silicon (Si) cycle using the stable isotope composition of Si dissolved in seawater (expressed as image). Open ocean image observations indicate a surprisingly strong influence of the physical circulation on the large-scale marine Si distribution. Here, we present an ocean general circulation model simulation that deconvolves the physical and biogeochemical controls on the image distribution in the deep oceanic interior. By parsing dissolved Si into its preformed and regenerated components, we separate the influence of deep water formation and circulation from the effects of biogeochemical cycling related to opal dissolution at depth. We show that the systematic meridional image gradient observed in the deep Atlantic Ocean is primarily determined by the preformed component of Si, whose distribution in the interior is controlled solely by the circulation. We also demonstrate that the image value of the regenerated component of Si in the global deep ocean is dominantly set by oceanic regions where opal export fluxes to the deep ocean are large, i.e. primarily in the Southern Ocean's opal belt. The global importance of this regionally dynamic Si cycling helps explain the observed strong physical control on the oceanic image distribution, since most of the regenerated Si present within the deep Atlantic and Indo-Pacific Oceans is in fact transported into these basins by deep waters flowing northward from the Southern Ocean. Our results thus provide a mechanistic explanation for the observed image distribution that emphasizes the dominant importance of the Southern Ocean in the marine Si cycle.
Gehlen, Marion, and John P Dunne, et al., December 2014: Projected pH reductions by 2100 might put deep North Atlantic biodiversity at risk. Biogeosciences, 11(23), DOI:10.5194/bg-11-6955-2014. Abstract
This study aims at evaluating the potential for impacts of ocean acidification on North Atlantic deep-sea ecosystems in response to IPCC AR5 Representative Concentration Pathways (RCP). Deep-sea biota is likely highly vulnerable to changes in seawater chemistry and sensitive to moderate excursions in pH. Here we show, from seven fully-coupled Earth system models, that for three out of four RCPs over 17% of the seafloor area below 500 m depth in the North Atlantic sector will experience pH reductions exceeding −0.2 units by 2100. Increased stratification in response to climate change partially alleviates the impact of ocean acidification on deep benthic environment. We report major potential consequences of pH reductions for deep-sea biodiversity hotspots, such as seamounts and canyons. By 2100 and under the high CO2 scenario RCP8.5 pH reductions exceeding −0.2, (respectively −0.3) units are projected in close to 23% (~ 15%) of North Atlantic deep-sea canyons and ~ 8% (3%) of seamounts – including seamounts proposed as sites of marine protected areas. The spatial pattern of impacts reflects the depth of the pH perturbation and does not scale linearly with atmospheric CO2 concentration. Impacts may cause negative changes of the same magnitude or exceeding the current target of 10% of preservation of marine biomes set by the convention on biological diversity implying that ocean acidification may offset benefits from conservation/management strategies relying on the regulation of resource exploitation.
Gnanadesikan, Anand, John P Dunne, and Rym Msadek, May 2014: Connecting Atlantic Temperature Variability and Biological Cycling in two Earth System Models. Journal of Marine Systems, 133, DOI:10.1016/j.jmarsys.2013.10.003. Abstract
Connections between the interdecadal variability in North Atlantic temperatures and biological cycling have been widely hypothesized. However, it is unclear whether such connections are due to small changes in basin-averaged temperatures indicated by the Atlantic Multidecadal Oscillation (AMO) Index, or whether both biological cycling and the AMO index are causally linked to changes in the Atlantic Meridional Overturning Circulation (AMOC). We examine interdecadal variability in the annual and month-by-month diatom biomass in two Earth System Models with the same formulations of atmospheric, land, sea ice and ocean biogeochemical dynamics but different formulations of ocean physics and thus different AMOC structure and variability. In the isopycnal-layered ESM2G, strong interdecadal changes in surface salinity associated with changes in AMOC produce spatially heterogeneous variability in convection, nutrient supply and thus diatom biomass. These changes also produce changes in ice cover, shortwave absorption and temperature and hence the AMO Index. Off West Greenland, these changes are consistent with observed changes in fisheries and support climate as a causal driver.. In the level-coordinate ESM2M, nutrient supply is much higher and interdecadal changes in diatom biomass are much smaller in amplitude and not strongly linked to the AMO index.
Ishii, Masao, and John P Dunne, et al., February 2014: Air–sea CO2 flux in the Pacific Ocean for the period 1990–2009. Biogeosciences, 11(3), DOI:10.5194/bg-11-709-2014. Abstract
Air–sea CO2 fluxes over the Pacific Ocean are known to be characterized by coherent large-scale structures that reflect not only ocean subduction and upwelling patterns, but also the combined effects of wind-driven gas exchange and biology. On the largest scales, a large net CO2 influx into the extratropics is associated with a robust seasonal cycle, and a large net CO2 efflux from the tropics is associated with substantial interannual variability. In this work, we have synthesized estimates of the net air–sea CO2 flux from a variety of products, drawing upon a variety of approaches in three sub-basins of the Pacific Ocean, i.e., the North Pacific extratropics (18–66° N), the tropical Pacific (18° S–18° N), and the South Pacific extratropics (44.5–18° S). These approaches include those based on the measurements of CO2 partial pressure in surface seawater (pCO2sw), inversions of ocean-interior CO2 data, forward ocean biogeochemistry models embedded in the ocean general circulation models (OBGCMs), a model with assimilation of pCO2sw data, and inversions of atmospheric CO2 measurements. Long-term means, interannual variations and mean seasonal variations of the regionally integrated fluxes were compared in each of the sub-basins over the last two decades, spanning the period from 1990 through 2009. A simple average of the long-term mean fluxes obtained with surface water pCO2 diagnostics and those obtained with ocean-interior CO2 inversions are −0.47 ± 0.13 Pg C yr−1 in the North Pacific extratropics, +0.44 ± 0.14 Pg C yr−1 in the tropical Pacific, and −0.37 ± 0.08 Pg C yr−1 in the South Pacific extratropics, where positive fluxes are into the atmosphere. This suggests that approximately half of the CO2 taken up over the North and South Pacific extratropics is released back to the atmosphere from the tropical Pacific. These estimates of the regional fluxes are also supported by the estimates from OBGCMs after adding the riverine CO2 flux, i.e., −0.49 ± 0.02 Pg C yr−1 in the North Pacific extratropics, +0.41 ± 0.05 Pg C yr−1 in the tropical Pacific, and −0.39 ± 0.11 Pg C yr−1 in the South Pacific extratropics. The estimates from the atmospheric CO2 inversions show large variations amongst different inversion systems, but their median fluxes are consistent with the estimates from climatological pCO2sw data and pCO2sw diagnostics. In the South Pacific extratropics, where CO2 variations in the surface and ocean interior are severely undersampled, the difference in the air–sea CO2 flux estimates between the diagnostic models and ocean-interior CO2 inversions is larger (0.18 Pg C yr−1). The range of estimates from forward OBGCMs is also large (−0.19 to −0.72 Pg C yr−1). Regarding interannual variability of air–sea CO2 fluxes, positive and negative anomalies are evident in the tropical Pacific during the cold and warm events of the El Niño–Southern Oscillation in the estimates from pCO2sw diagnostic models and from OBGCMs. They are consistent in phase with the Southern Oscillation Index, but the peak-to-peak amplitudes tend to be higher in OBGCMs (0.40 ± 0.09 Pg C yr−1) than in the diagnostic models (0.27 ± 0.07 Pg C yr−1).
The robustness of Transient Climate Response to cumulative Emissions (TCRE) is tested using an Earth System Model (Geophysical Fluid Dynamics Laboratory-ESM2G) forced with seven different constant rates of carbon emissions (2 GtC/yr to 25 GtC/yr), including low emission rates that have been largely unexplored in previous studies. We find the range of TCRE resulting from varying emission pathways to be 0.76 to 1.04°C/TtC. This range, however, is small compared to the uncertainty resulting from varying model physics across the Fifth Coupled Model Intercomparison Project ensemble. TCRE has a complex relationship with emission rates; TCRE is largest for both low (2 GtC/yr) and high (25 GtC/yr) emissions and smallest for present-day emissions (5–10 GtC/yr). Unforced climate variability hinders precise estimates of TCRE for periods shorter than 50 years for emission rates near or smaller than present day values. Even if carbon emissions would stop, the prior emissions pathways will affect the future climate responses.
Logan, Cheryl A., John P Dunne, C M Eakin, and Simon D Donner, January 2014: Incorporating adaptive responses into future projections of coral bleaching. Global Change Biology, 20(1), DOI:10.1111/gcb.12390. Abstract
Climate warming threatens to increase mass coral bleaching events, and several studies have projected the demise of tropical coral reefs this century. However, recent evidence indicates corals may be able to respond to thermal stress though adaptive processes (e.g., genetic adaptation, acclimatization, and symbiont shuffling). How these mechanisms might influence warming-induced bleaching remains largely unknown. This study compared how different adaptive processes could affect coral bleaching projections. We used the latest bias-corrected global sea surface temperature (SST) output from the NOAA/GFDL Earth System Model 2 (ESM2M) for the preindustrial period through 2100 to project coral bleaching trajectories. Initial results showed that, in the absence of adaptive processes, application of a preindustrial climatology to the NOAA Coral Reef Watch bleaching prediction method overpredicts the present-day bleaching frequency. This suggests that corals may have already responded adaptively to some warming over the industrial period. We then modified the prediction method so that the bleaching threshold either permanently increased in response to thermal history (e.g., simulating directional genetic selection) or temporarily increased for 2–10 years in response to a bleaching event (e.g., simulating symbiont shuffling). A bleaching threshold that changes relative to the preceding 60 years of thermal history reduced the frequency of mass bleaching events by 20–80% compared with the ‘no adaptive response’ prediction model by 2100, depending on the emissions scenario. When both types of adaptive responses were applied, up to 14% more reef cells avoided high-frequency bleaching by 2100. However, temporary increases in bleaching thresholds alone only delayed the occurrence of high-frequency bleaching by ca. 10 years in all but the lowest emissions scenario. Future research should test the rate and limit of different adaptive responses for coral species across latitudes and ocean basins to determine if and how much corals can respond to increasing thermal stress.
Organic particles sinking from the sunlit surface are oases of food for heterotrophic bacteria living
in the deep ocean. Particle-attached bacteria need to solubilize particles, so they produce exoenzymes
that cleave bonds to make molecules small enough to be transported through bacterial cell walls.
Releasing exoenzymes, which have an energetic cost, to the external environment is risky because there
is no guarantee that products of exoenzyme activity, called hydrolysate, will diffuse to the particleattached
bacterium that produced the exoenzymes. Strategies used by particle-attached bacteria to
counteract diffusive losses of exoenzymes and hydrolysate are investigated in a water column model.
We find that production of exoenzymes by particle-attached bacteria is only energetically worthwhile
at high bacterial abundances. Quorum sensing provides the means to determine local abundances,
and thus the model results support lab and field studies which found that particle-attached bacteria
have the ability to use quorum sensing. Additional model results are that particle-attached bacterial
production is sensitive to diffusion of hydrolysate from the particle and is enhanced by as much as
15 times when diffusion of exoenzymes and hydrolysate from particles is reduced by barriers of
biofilms and particle-attached bacteria. Bacterial colonization rates and activities on particles in both
the euphotic and mesopelagic zones impact remineralization length scales. Shoaling or deepening of
the remineralization depth has been shown to exert significant influence on the residence time and
concentration of carbon in the atmosphere and ocean. By linking variability in remineralization depths
to mechanisms governing bacterial colonization of particles and group coordination of exoenzyme
production using a model, we quantitatively connect microscale bacteria-particle interactions to the
carbon cycle and provide new insights for future observations.
The Carbon, Ocean Biogeochemistry and Lower Trophics (COBALT) marine ecosystem model robustly captures large-scale observed patterns in the flow of carbon through the planktonic food web when embedded within a global ocean-ice simulation. The simulation offers holistic, quantitative, and self-consistent estimates of carbon and energy flows across ocean biomes. Results emphasize the importance of small phytoplankton to global productivity. This leads to widespread carnivorous feeding by mesozooplankton and muted cross-biome differences in annual mean mesozooplankton trophic level. Results also support highly distributed respiration across the planktonic food web. In oceanic upwelling regions, shortened food webs, elevated growth efficiencies, and tight consumer-phytoplankton coupling supports 47% of pelagic mesozooplankton production despite these areas accounting for only 21% of ocean area and 33% of net primary production (NPP). In seasonally stratified regions (40% of ocean area and 36% of NPP), weakened phytoplankton-consumer coupling reduces mesozooplankton production to 39% and enhances export such that it accounts for 55% of the global total. In oligotrophic systems (39% of ocean area and 27% of NPP), the dominance of small phytoplankton and low consumer growth efficiencies support only 15% of mesozooplankton production and 14% of export globally. Bacterial production, in contrast, is maintained in constant proportion to primary production across ecosystems. Further diagnosis of simulations elucidates the mechanisms underlying these cross biome contrasts and regularities. Results herein represent a baseline for further exploration of global-scale planktonic food web dynamics within an increasingly mechanistic dynamic global physical-biological framework.
Pronounced projected 21st century trends in regional oceanic net primary production (NPP) raise the prospect of significant redistributions of marine resources. Recent results further suggest that NPP changes may be amplified at higher trophic levels. Here, we elucidate the role of planktonic food web dynamics in driving projected changes in mesozooplankton production (MESOZP) found to be, on average, twice as large as projected changes in NPP by the latter half of the 21st century under a high emissions scenario. Globally, MESOZP was projected to decline by 7.9% but regional MESOZP changes sometimes exceeded 50%. Changes in three planktonic food web properties – zooplankton growth efficiency (ZGE), the trophic level of mesozooplankton (MESOTL), and the fraction of NPP consumed by zooplankton (zooplankton-phytoplankton coupling, ZPC), were demonstrated to be responsible for the projected amplification. Zooplankton growth efficiencies (ZGE) changed with NPP, amplifying both NPP increases and decreases. Negative amplification (i.e., exacerbation) of projected subtropical NPP declines via this mechanism was particularly strong since consumers in the subtropics already have limited surplus energy above basal metabolic costs. Increased mesozooplankton trophic level (MESOTL) resulted from projected declines in large phytoplankton production, the primary target of herbivorous mesozooplankton. This further amplified negative subtropical NPP declines but was secondary to ZGE and, at higher latitudes, was often offset by increased ZPC. Marked ZPC increases were projected for high latitude regions experiencing shoaling of deep winter mixing or decreased winter sea ice – both tending to increase winter zooplankton biomass and enhance grazer control of spring blooms. Increased ZPC amplified projected NPP increases associated with declining sea ice in the Artic and damped projected NPP declines associated with decreased mixing in the Northwest Atlantic and Southern Ocean. Improved understanding of the complex interactions governing these food web properties is essential to further refine estimates of climate-driven productivity changes across trophic levels.
Global climate change is expected to affect the ocean's biological productivity. The most comprehensive information available about the global distribution of contemporary ocean primary productivity is derived from satellite data. Large spatial patchiness and interannual to multidecadal variability in chlorophyll a concentration challenges efforts to distinguish a global, secular trend given satellite records which are limited in duration and continuity. The longest ocean color satellite record comes from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS), which failed in December 2010. The Moderate Resolution Imaging Spectroradiometer (MODIS) ocean color sensors are beyond their originally planned operational lifetime. Successful retrieval of a quality signal from the current Visible Infrared Imager Radiometer Suite (VIIRS) instrument, or successful launch of the Ocean and Land Colour Instrument (OLCI) expected in 2014 will hopefully extend the ocean color time series and increase the potential for detecting trends in ocean productivity in the future. Alternatively, a potential discontinuity in the time series of ocean chlorophyll a, introduced by a change of instrument without overlap and opportunity for cross-calibration, would make trend detection even more challenging. In this paper, we demonstrate that there are a few regions with statistically significant trends over the ten years of SeaWiFS data, but at a global scale the trend is not large enough to be distinguished from noise. We quantify the degree to which red noise (autocorrelation) especially challenges trend detection in these observational time series. We further demonstrate how discontinuities in the time series at various points would affect our ability to detect trends in ocean chlorophyll a. We highlight the importance of maintaining continuous, climate-quality satellite data records for climate-change detection and attribution studies.
Bopp, Laurent, and John P Dunne, et al., October 2013: Multiple stressors of ocean ecosystems in the 21st century: projections with CMIP5 models. Biogeosciences, 10(10), DOI:10.5194/bg-10-6225-2013. Abstract
Ocean ecosystems are increasingly stressed by human-induced changes of their physical, chemical and biological environment. Among these changes, warming, acidification, deoxygenation and changes in primary productivity by marine phytoplankton can be considered as four of the major stressors of open ocean ecosystems. Due to rising atmospheric CO2 in the coming decades, these changes will be amplified. Here, we use the most recent simulations performed in the framework of the Coupled Model Intercomparison Project 5 to assess how these stressors may evolve over the course of the 21st century. The 10 Earth System Models used here project similar trends in ocean warming, acidification, deoxygenation and reduced primary productivity for each of the IPCC's representative concentration parthways (RCP) over the 21st century. For the "business-as-usual" scenario RCP8.5, the model-mean changes in 2090s (compared to 1990s) for sea surface temperature, sea surface pH, global O2 content and integrated primary productivity amount to +2.73 °C, −0.33 pH unit, −3.45% and −8.6%, respectively. For the high mitigation scenario RCP2.6, corresponding changes are +0.71 °C, −0.07 pH unit, −1.81% and −2.0% respectively, illustrating the effectiveness of extreme mitigation strategies. Although these stressors operate globally, they display distinct regional patterns. Large decreases in O2 and in pH are simulated in global ocean intermediate and mode waters, whereas large reductions in primary production are simulated in the tropics and in the North Atlantic. Although temperature and pH projections are robust across models, the same does not hold for projections of sub-surface O2 concentrations in the tropics and global and regional changes in net primary productivity.
Brody, S R., S Lozier, and John P Dunne, June 2013: A comparison of methods to determine phytoplankton bloom initiation. Journal of Geophysical Research: Oceans, 118(5), DOI:10.1002/jgrc.20167. Abstract
Phytoplankton bloom phenology has important consequences for marine ecosystems and fisheries. Recent studies have used remotely-sensed ocean color data to calculate metrics associated with the phenological cycle, such as the phytoplankton bloom initiation date, on the regional and global scale. These metrics are often linked to physical or biological forcings. Most studies choose one of several common methods for calculating bloom initiation, leading to questions about whether bloom initiation datescalculated with different methods yield comparable results. Here, we compare three methods for finding the date of phytoplankton bloom initiation in the North Atlantic: a biomass-based threshold method, a rate of change method, and a cumulative biomass-based threshold method. We use these methods to examine whether the onset of positive ocean-atmosphere heat fluxes coincides with subpolar bloom initiation. In several coherent locations, we find differences in the patterns of bloom initiation created by each method and differences in the synchrony between bloom initiation and positive heat fluxes, that likely indicate various physical processes at play in the study region. We also assess the effect of missing data on the chosen methods.
Cheung, William W., Jorge L Sarmiento, John P Dunne, and Thomas L Frölicher, et al., March 2013: Shrinking of fishes exacerbates impacts of global ocean changes on marine ecosystems. Nature Climate Change, 3(3), DOI:10.1038/NCLIMATE1691. Abstract
Changes in temperature, oxygen content and other ocean
biogeochemical properties directly affect the ecophysiology
of marine water-breathing organisms1–3. Previous studies
suggest that the most prominent biological responses are
changes in distribution4–6, phenology7,8 and productivity9.
Both theory and empirical observations also support the
hypothesis that warming and reduced oxygen will reduce
body size of marine fishes10–12. However, the extent to
which such changes would exacerbate the impacts of climate
and ocean changes on global marine ecosystems remains
unexplored. Here,we employ a model to examine the integrated
biological responses of over 600 species of marine fishes
due to changes in distribution, abundance and body size.
The model has an explicit representation of ecophysiology,
dispersal, distribution, and population dynamics3. We show
that assemblage-averaged maximum body weight is expected
to shrink by 14–24% globally from 2000 to 2050 under
a high-emission scenario. About half of this shrinkage is
due to change in distribution and abundance, the remainder
to changes in physiology. The tropical and intermediate
latitudinal areas will be heavily impacted, with an average
reduction of more than 20%. Our results provide a new
dimension to understanding the integrated impacts of climate
change on marine ecosystems.
Cocco, V, Fortunat Joos, M Steinacher, Thomas L Frölicher, Laurent Bopp, and John P Dunne, et al., March 2013: Oxygen and indicators of stress for marine life in multi-model global warming projections. Biogeosciences, 10(3), DOI:10.5194/bg-10-1849-2013. Abstract
Decadal-to-century scale trends for a range of marine environmental variables are investigated using results from seven Earth System Models forced by a high greenhouse gas emission scenario. The models as a class represent the observation-based distribution of the fugacity of oxygen (fO2) and carbon dioxide (fO2, and the logarithm of their ratio, i.e. the Respiration Index (RI), albeit major mismatches between observation-based and simulated values remain for individual models. All models project an increase in SST between 2 °C and 3 °C by year 2100, a decrease in upper ocean pH and in the saturation state of water with respect to calcium carbonate minerals, and a decrease in the total ocean inventory of dissolved oxygen by 2% to 4%. Projected fO2 changes in the thermocline show a complex pattern with both increasing and decreasing trends reflecting the subtle balance of different competing factors such as circulation, production, remineralisation, and temperature changes. Projected changes in the total volume of hypoxic and suboxic waters remain relatively small in all models. A widespread increase of fO2 in the thermocline is projected. The median of the thermocline fO2 distribution shifts from 350 μatm in year 1990 to 700–800 μatm in year 2100, primarily as a result of the invasion of anthropogenic carbon from the atmosphere and is responsible for the widespread decrease in the RI outside low oxygen regions. The co-occurrence of changes in a range of environmental variables indicates the need to further investigate their synergistic impacts on marine ecosystems and Earth System feedbacks.
We describe carbon system formulation and simulation characteristics of two new global coupled carbon-climate Earth System Models, ESM2M and ESM2G. These models demonstrate good climate fidelity as described in Part I while incorporating explicit and consistent carbon dynamics. The two models differ almost exclusively in the physical ocean component; ESM2M uses Modular Ocean Model version 4.1 with vertical pressure layers while ESM2G uses Generalized Ocean Layer Dynamics with a bulk mixed layer and interior isopycnal layers. On land, both ESMs include a revised land model to simulate competitive vegetation distributions and functioning, including carbon cycling among vegetation, soil and atmosphere. In the ocean, both models include new biogeochemical algorithms including phytoplankton functional group dynamics with flexible stoichiometry. Preindustrial simulations are spun up to give stable, realistic carbon cycle means and variability. Significant differences in simulation characteristics of these two models are described. Due to differences in oceanic ventilation rates (Part I) ESM2M has a stronger biological carbon pump but weaker northward implied atmospheric CO2 transport than ESM2G. The major advantages of ESM2G over ESM2M are: improved representation of surface chlorophyll in the Atlantic and Indian Oceans and thermocline nutrients and oxygen in the North Pacific. Improved tree mortality parameters in ESM2G produced more realistic carbon accumulation in vegetation pools. The major advantages of ESM2M over ESM2G are reduced nutrient and oxygen biases in the Southern and Tropical Oceans.
A fundamental aspect of greenhouse-gas-induced warming is a
global-scale increase in absolute humidity
1,2
. Under continued
warming, this response has been shown to pose increasingly
severe limitations on human activity in tropical and midlatitudes during peak months of heat stress
3
. One heat-stress
metric with broad occupational health applications
4–6
is wetbulb globe temperature. We combine wet-bulb globe temperatures from global climate historical reanalysis
7
and Earth
System Model (ESM2M) projections
8–10
with industrial
4
and
military
5
guidelines for an acclimated individual’s occupational
capacity to safely perform sustained labour under environmental heat stress (labour capacity)—here defined as a global
population-weighted metric temporally fixed at the 2010 distribution. We estimate that environmental heat stress has reduced labour capacity to 90% in peak months over the past few
decades. ESM2M projects labour capacity reduction to 80% in
peak months by 2050. Under the highest scenario considered
(Representative Concentration Pathway 8.5), ESM2M projects
labour capacity reduction to less than 40% by 2200 in peak
months, with most tropical and mid-latitudes experiencing
extreme climatological heat stress. Uncertainties and caveats
associated with these projections include climate sensitivity,
climate warming patterns, CO2 emissions, future population
distributions, and technological and societal change.
Two comprehensive Earth System Models, identical apart from their oceanic components, are used to estimate the uncertainty in projections of 21st century sea level rise due to representational choices in ocean physical formulation. Most prominent among the formulation differences is that one (ESM2M) uses a traditional z-coordinate ocean model, while the other (ESM2G) uses an isopycnal-coordinate ocean. As evidence of model fidelity, differences in 20th century global-mean steric sea level rise are not statistically significant between either model and observed trends. However, differences between the two models’ 21st century projections are systematic and both statistically and climatically significant. By 2100, ESM2M exhibits 18% higher global steric sea level rise than ESM2G for all four radiative forcing scenarios (28 to 49 mm higher), despite having similar changes between the models in the near-surface ocean for several scenarios. These differences arise primarily from the vertical extent over which heat is taken up and the total heat uptake by the models (9% more in ESM2M than ESM2G). The fact that the spun-up control state of ESM2M is warmer than ESM2G also contributes, by giving thermal expansion coefficients that are about 7% larger in ESM2M than ESM2G. The differences between these models provide a direct estimate of the sensitivity of 21st century sea level rise to ocean model formulation, and, given the span of these models across the observed volume of the ventilated thermocline, may also approximate the sensitivities expected from uncertainties in the characterization of interior ocean physical processes.
Hazen, Elliot L., Ryan R Rykaczewski, and John P Dunne, et al., March 2013: Predicted habitat shifts of Pacific top predators in a changing climate. Nature Climate Change, 3(3), DOI:10.1038/nclimate1686. Abstract
To manage marine ecosystems proactively, it is important to identify species at risk and habitats critical for conservation. Climate change scenarios have predicted an average sea surface temperature (SST) rise of 1–6 °C by 2100 (refs 1, 2), which could affect the distribution and habitat of many marine species. Here we examine top predator distribution and diversity in the light of climate change using a database of 4,300 electronic tags deployed on 23 marine species from the Tagging of Pacific Predators project, and output from a global climate model to 2100. On the basis of models of observed species distribution as a function of SST, chlorophyll a and bathymetry, we project changes in species-specific core habitat and basin-scale patterns of biodiversity. We predict up to a 35% change in core habitat for some species, significant differences in rates and patterns of habitat change across guilds, and a substantial northward displacement of biodiversity across the North Pacific. For already stressed species, increased migration times and loss of pelagic habitat could exacerbate population declines or inhibit recovery. The impending effects of climate change stress the urgency of adaptively managing ecosystems facing multiple threats.
Howell, E A., Colette C C Wabnitz, John P Dunne, and J J Polovina, July 2013: Climate-induced primary productivity change and fishing impacts on the Central North Pacific ecosystem and Hawaii-based pelagic longline fishery. Climatic Change, 119(1), DOI:10.1007/s10584-012-0597-z. Abstract
An existing Ecopath with Ecosim (EwE) model for the Central North Pacific was updated and modified to focus on the area used by the Hawaii-based pelagic longline fishery. The EwE model was combined with output from a coupled NOAA Geophysical Fluid Dynamics Laboratory climate and biogeochemical model to investigate the likely ecosystem impacts of fishing and climate-induced primary productivity changes. Four simulations were conducted based on 2 fishing effort and climate scenarios from 2010 to 2100. Modeled small and large phytoplankton biomass decreased by 10 % and 20 % respectively, resulting in a 10 % decline in the total biomass of all higher trophic level groups combined. Climate impacts also affected the Hawaii longline fishery, with a 25–29 % reduction in modeled target species yield. Climate impacts on the ecosystem and the fishery were partially mitigated by a drop in fishing effort. Scenarios with a 50 % reduction in fishing effort partially restored longline target species yield to current levels, and decreased longline non-target species yield. These model results suggest that a further reduction in fishery landings mortality over time than the 2010 level may be necessary to mitigate climate impacts and help sustain yields of commercially preferred fish species targeted by the Hawaii longline fishery through the 21st century.
Jonsson, B F., Scott C Doney, John P Dunne, and M Bender, June 2013: Evaluation of Southern Ocean O2/Ar-based NCP estimates in a model framework. Journal of Geophysical Research, 118(2), DOI:10.1002/jgrg.20032. Abstract
The sea-air biological O2 flux assessed from measurements of surface O2 supersaturation in excess of Ar supersaturation (“O2 bioflux”) is increasingly being used to constrain net community production (NCP) in the upper ocean mixed layer. In making these calculations, one generally assumes that NCP is at steady state, mixed-layer depth is constant, and there is no O2 exchange across the base of the mixed layer. The object of this paper is to evaluate the magnitude of errors introduced by violations of these assumptions. Therefore, we examine the differences between the sea-air biological O2 flux and NCP in the Southern Ocean mixed layer as calculated using two ocean biogeochemistry general circulation models. In this approach, NCP is considered a known entity in the prognostic model, whereas O2 bioflux is estimated using the model-predicted O2/Ar ratio to compute the mixed-layer biological O2 saturation and the gas transfer velocity to calculate flux. We find that the simulated biological O2 flux gives an accurate picture of the regional-scale patterns and trends in model NCP. However, on local scales, violations of the assumptions behind the O2/Ar method lead to significant, non-uniform differences between model NCP and biological O2 flux. These errors arise from two main sources. First, venting of biological O2 to the atmosphere can be misaligned from NCP in both time and space. Second, vertical fluxes of oxygen across the base of the mixed layer complicate the relationship between NCP and the biological O2 flux. Our calculations show that low values of O2 bioflux correctly register that NCP is also low (< 10 mmol m-2 day-1), but fractional errors are large when rates are this low. Values between 10 and 40 mmol m-2 day-1 in areas with intermediate mixed-layer depths of 30 to 50 meters have the smallest absolute and relative errors. Areas with O2 bioflux higher than 30 mmol m-2 d-1 and mixed layers deeper than 40 meters tend to underestimate NCP by up to 20 mmol m-2 d-1. Excluding time periods when mixed-layer biological O2 is undersaturated, O2 bioflux underestimates time-averaged NCP by 5% - 15%. If these time periods are included, O2 bioflux underestimates mixed-layer NCP by 20% - 35% in the Southern Ocean. The higher error estimate is relevant if one wants to estimate seasonal NCP, since a significant amount of biological production takes place when mixed layer biological O2 is undersaturated.
Pastor, M V., J B Palter, J L Pelegri, and John P Dunne, August 2013: Physical drivers of interannual chlorophyll variability in the eastern subtropical North Atlantic. Journal of Geophysical Research: Oceans, 118, DOI:10.1002/jgrc.20254. Abstract
Interannual chlorophyll variability and its driving mechanisms are evaluated in the eastern subtropical North Atlantic, where elevated surface chlorophyll concentrations regularly extend more than 1500 km into the central subtropical North Atlantic and modulate the areal extent of the North Atlantic's lowest chlorophyll waters. We first characterize the considerable interannual variability in the size of the high chlorophyll region using SeaWiFS satellite data. We then evaluate the relationship between satellite chlorophyll and Sea Surface Height (SSH), which are anticorrelated in the study region, most likely as a result of the inverse relationship between SSH and nutricline depth. To put these results in a longer temporal context, we study a hindcast simulation of a global ocean model with biogeochemistry (GFDL's MOM4.1 with TOPAZ biogeochemistry), after evaluating the model's skill at simulating chlorophyll and SSH relative to observations. In the simulation, the variability seen during the satellite era appears to be imbedded in a much larger multidecadal modulation. The drivers of such variability are assessed by evaluating all the terms in the nutrient budget of the euphotic zone. Because diffusive processes are not a dominant control on nutrient supply, stratification is not a good indicator of nutrient supply. Rather, vertical advection of nutrients, strongly tied to Ekman pumping, is the leading driver of variability in the size of the high chlorophyll region and the productivity within the study area.
Resplandy, Laure, Laurent Bopp, James C Orr, and John P Dunne, June 2013: Role of mode and intermediate waters in future ocean acidification: analysis of CMIP5 models. Geophysical Research Letters, 40(12), DOI:10.1002/grl.50414. Abstract
Consistently with the past decades observations, CMIP5 Earth System Models project highest acidification rates in subsurface waters. Using 7 ESMs, we find that high acidification rates in mode and intermediate waters (MIW) on centennial timescales (-0.0008 ± 4 × 10–5 yr–1 to -0.0023 ± 0.0001 yr–1 depending on the scenario) are predominantly explained by the geochemical effect of increasing atmospheric CO2, whereas physical and biological climate change feedbacks explain less than 10% of the simulated changes. MIW are characterized by a larger surface area to volume ratio than deep and bottom waters leading to 5 to 10 times larger carbon uptake. In addition, MIW geochemical properties result in a sensitivity to increasing carbon concentration twice largerthan surface waters (Δ[H+] of +1.2 mmol.m–3 for every mmol.m–3 of dissolved carbon in MIW vs. +0.6 in surface waters). Low pH transported by mode and intermediate waters are likely to influence surface pH in upwelling regions decades after their isolation from the atmosphere.
Vancoppenolle, M, Laurent Bopp, G Madec, John P Dunne, Tatiana Ilyina, P R Halloran, and N Steiner, July 2013: Future Arctic Ocean Primary Productivity from CMIP5 Simulations: Uncertain Outcome, but Consistent Mechanisms. Global Biogeochemical Cycles, 27, DOI:10.1002/gbc.20055. Abstract
Net Arctic Ocean primary production (PP) is expected to
increase over this century, due to less perennial sea ice and more
available light, but could decrease depending on changes in nitrate
(NO3) supply. Here, CMIP5 simulations performed with 11 Earth System
Models are analyzed in terms of PP, surface NO3 and sea ice coverage
over 1900-2100. Whereas the mean model simulates reasonably well
Arctic-integrated PP (511 TgC/yr, 1998-2005) and projects a mild 58
TgC/yr increase by 2080-2099 for the strongest climate change
scenario, models do not agree on the sign of future PP change.
However, similar mechanisms operate in all models. The perennial ice
loss-driven increase in PP is in most models NO3-limited. The Arctic
surface NO3 is decreasing over the 21st century (-2.3 1 mmol/m3),
associated with shoaling mixed layer and with decreasing NO3 in the
nearby North Atlantic and Pacific waters. However, the inter-model
spread in the degree of NO3 limitation is initially high, resulting
from >1000 yr spin-up simulations. This initial NO3 spread, combined
with the trend, causes a large variation in the timing of oligotrophy
onset – which directly controls the sign of future PP change.
Virtually all models agree in the open ocean zones on more spatially
integrated PP and less PP per unit area. The source of model
uncertainty is located in the sea ice zone, where a subtle balance
between light and nutrient limitations determines the PP change.
Hence, it is argued that reducing uncertainty on present Arctic NO3 in
the sea ice zone would render Arctic PP projections much more
consistent.
Woodworth, P A., J J Polovina, John P Dunne, and Julia L Blanchard, March 2013: Ecosystem size structure response to 21st century climate projection: large fish abundance decreases in the central North Pacific and increases in the California Current. Global Change Biology, 19(3), DOI:10.1111/gcb.12076. Abstract
Output from an earth system model is paired with a size-based food web model to investigate the effects of climate change on the abundance of large fish over the 21st century. The earth system model, forced by the Intergovernmental Panel on Climate Change (IPCC) Special report on emission scenario A2, combines a coupled climate model with a biogeochemical model including major nutrients, three phytoplankton functional groups, and zooplankton grazing. The size-based food web model includes linkages between two size-structured pelagic communities: primary producers and consumers. Our investigation focuses on seven sites in the North Pacific, each highlighting a specific aspect of projected climate change, and includes top-down ecosystem depletion through fishing. We project declines in large fish abundance ranging from 0 to 75.8% in the central North Pacific and increases of up to 43.0% in the California Current (CC) region over the 21st century in response to change in phytoplankton size structure and direct physiological effects. We find that fish abundance is especially sensitive to projected changes in large phytoplankton density and our model projects changes in the abundance of large fish being of the same order of magnitude as changes in the abundance of large phytoplankton. Thus, studies that address only climate-induced impacts to primary production without including changes to phytoplankton size structure may not adequately project ecosystem responses.
Bianchi, Daniele, John P Dunne, Jorge L Sarmiento, and Eric D Galbraith, May 2012: Data-based estimates of suboxia, denitrification and N2O production in the ocean, and their sensitivities to dissolved O2. Global Biogeochemical Cycles, 26, GB2009, DOI:10.1029/2011GB004209. Abstract
Oxygen minimum zones (OMZs) are major sites of fixed nitrogen removal from the open ocean. However, commonly-used gridded data sets such as the World Ocean Atlas (WOA) tend to overestimate the concentration of O2 compared to measurements in grids where O2 falls in the suboxic range (O2 < 2 - 10 mmol/m3), thereby underestimating the extent of O2 depletion in OMZs. We evaluate the distribution of the OMZs by (1) mapping high-quality oxygen measurements from the WOCE program, and (2) by applying an empirical correction to the gridded WOA based on in situ observations. The resulting suboxic volumes are a factor 3 larger than in the uncorrected gridded WOA. We combine the new oxygen data sets with estimates of global export and simple models of remineralization to estimate global denitrification and N2O production. We obtain a removal of fixed nitrogen of 70 {plus minus} 50 Tg/year in the open ocean and 198 {plus minus} 64 Tg/year in the sediments, and a global N2O production of 6.2 {plus minus} 3.2 Tg/year. Our results (1) reconcile water column denitrification rates based on global oxygen distributions with previous estimates based on nitrogen isotopes, (2) revise existing estimates of sediment denitrification down by one-third through the use of spatially-explicit fluxes, and (3) provide independent evidence supporting the idea of a historically-balanced oceanic nitrogen cycle. These estimates are most sensitive to uncertainties in the global export production, the oxygen threshold for suboxic processes, and the efficiency of particle respiration under suboxic conditions. Ocean deoxygenation, an expected response to anthropogenic climate change, could increase denitrification by 14 Tg/year of nitrogen per 1 mmol/m3 of oxygen reduction if uniformly distributed, while leaving N2O production relatively unchanged.
We describe the physical climate formulation and simulation characteristics of two new global coupled carbon-climate Earth System Models, ESM2M and ESM2G. These models demonstrate similar climate fidelity as the Geophysical Fluid Dynamics Laboratory’s previous CM2.1 climate model while incorporating explicit and consistent carbon dynamics. The two models differ exclusively in the physical ocean component; ESM2M uses Modular Ocean Model version 4.1 with vertical pressure layers while ESM2G uses Generalized Ocean Layer Dynamics with a bulk mixed layer and interior isopycnal layers. Differences in the ocean mean state include the thermocline depth being relatively deep in ESM2M and relatively shallow in ESM2G compared to observations. The crucial role of ocean dynamics on climate variability is highlighted in the El Niño-Southern Oscillation being overly strong in ESM2M and overly weak ESM2G relative to observations. Thus, while ESM2G might better represent climate changes relating to: total heat content variability given its lack of long term drift, gyre circulation and ventilation in the North Pacific, tropical Atlantic and Indian Oceans, and depth structure in the overturning and abyssal flows, ESM2M might better represent climate changes relating to: surface circulation given its superior surface temperature, salinity and height patterns, tropical Pacific circulation and variability, and Southern Ocean dynamics. Our overall assessment is that neither model is fundamentally superior to the other, and that both models achieve sufficient fidelity to allow meaningful climate and earth system modeling applications. This affords us the ability to assess the role of ocean configuration on earth system interactions in the context of two state-of-the-art coupled carbon-climate models.
We assess the global balance of calcite export through the water column and burial in sediments as it varies regionally. We first drive a comprehensive 1-D model for sediment calcite preservation with globally gridded field observations and satellite-based syntheses. We then reformulate this model into a simpler five-parameter box model, and combine it with algorithms for surface calcite export and water column dissolution for a single expression for the vertical calcite balance. The resulting metamodel is optimized to fit the observed distributions of calcite burial flux. We quantify the degree to which calcite export, saturation state, organic carbon respiration, and lithogenic sedimentation modulate the burial of calcite. We find that 46% of burial and 88% of dissolution occurs in sediments overlain by undersaturated bottom water with sediment calcite burial strongly modulated by surface export. Relative to organic carbon export, we find surface calcite export skewed geographically toward relatively warm, oligotrophic areas dominated by small, prokaryotic phytoplankton. We assess century-scale projected impacts of warming and acidification on calcite export, finding high sensitive to inferred saturation state controls. With respect to long term glacial cycling, our analysis supports the hypothesis that strong glacial abyssal stratification drives the lysocline towards much closer correspondence with the saturation horizon. Our analysis suggests that, over the transition from interglacial to glacial ocean, a resulting ~0.029 PgC a-1 decrease in deep Atlantic, Indian and Southern Ocean calcite burial leads to slow increase in ocean alkalinity until Pacific mid-depth calcite burial increases to compensate.
Gnanadesikan, Anand, John P Dunne, and Jasmin G John, March 2012: Understanding why the volume of suboxic waters does not increase over centuries of global warming in an Earth System Model. Biogeosciences, 9(3), DOI:10.5194/bg-9-1159-2012. Abstract
Global warming is expected to reduce oxygen solubility
and vertical exchange in the ocean, changes which
would be expected to result in an increase in the volume of
hypoxic waters. A simulation made with a full Earth System
model with dynamical atmosphere, ocean, sea ice and biogeochemical
cycling (the Geophysical Fluid Dynamics Laboratory’s
Earth System Model 2.1) shows that this holds true
if the condition for hypoxia is set relatively high. However,
the volume of the most hypoxic (i.e., suboxic) waters does
not increase under global warming, as these waters actually
become more oxygenated. We show that the rise in dissolved
oxygen in the tropical Pacific is associated with a drop in
ventilation time. A term-by-term analysis within the least
oxygenated waters shows an increased supply of dissolved
oxygen due to lateral diffusion compensating an increase in
remineralization within these highly hypoxic waters. This
lateral diffusive flux is the result of an increase of ventilation
along the Chilean coast, as a drying of the region under
global warming opens up a region of wintertime convection
in our model. The results highlight the potential sensitivity of
suboxic waters to changes in subtropical ventilation as well
as the importance of constraining lateral eddy transport of
dissolved oxygen in such waters.
With a more-than-doubling in the atmospheric abundance of the potent greenhouse gas methane (CH4) since preindustrial times, and indications of renewed growth following a leveling off in recent years, questions arise as to future trends and resulting climate and public health impacts from continued growth without mitigation. Changes in atmospheric methane lifetime are determined by factors which regulate the abundance of OH, the primary methane removal mechanism, including changes in CH4 itself. We investigate the role of emissions of short-lived species and climate in determining the evolution of tropospheric methane lifetime in a suite of historical (1860�2005) and Representative Concentration Pathway (RCP) simulations (2006�2100), conducted with the Geophysical Fluid Dynamics Laboratory (GFDL) fully coupled chemistry-climate model (CM3). From preindustrial to present, CM3 simulates an overall 5% increase in CH4 lifetime due to a doubling of the methane burden which offsets coincident increases in nitrogen oxide (NOx) emissions. Over the last two decades, however, the methane lifetime declines steadily, coinciding with the most rapid climate warming and observed slow-down in CH4 growth rates, reflecting a possible negative feedback through the CH4 sink. The aerosol indirect effect plays a significant role in the CM3 climate and thus in the future evolution of the methane lifetime, due to the rapid projected decline of aerosols under all four RCPs. In all scenarios, the methane lifetime decreases (by 5�13%) except for the most extreme warming case (RCP8.5), where it increases by 4% due to the near-doubling of the CH4 abundance, reflecting a positive feedback on the climate system. In the RCP4.5 scenario changes in short-lived climate forcing agents reinforce climate warming and enhance OH, leading to a more-than-doubling of the decrease in methane lifetime from 2006 to 2100 relative to a simulation in which only well-mixed greenhouse gases are allowed to change along the RCP4.5 scenario (13% vs. 5%) Future work should include process-based studies to better understand and elucidate the individual mechanisms controlling methane lifetime.
Keller, K M., Thomas L Frölicher, and John P Dunne, et al., December 2012: Variability of the ocean carbon cycle in response to the North Atlantic Oscillation. Tellus B, DOI:10.3402/tellusb.v64i0.18738. Abstract
Climate modes such as the North Atlantic Oscillation (NAO), representing internal variability of the climate
system, influence the ocean carbon cycle and may mask trends in the sink of anthropogenic carbon.
Here, utilising control runs of six fully coupled Earth System Models, the response of the ocean carbon cycle to
the NAO is quantified. The dominating response, a seesaw pattern between the subtropical gyre and the
subpolar Northern Atlantic, is instantaneous (B3 months) and dynamically consistent over all models and
with observations for a range of physical and biogeochemical variables. All models show asymmetric responses
to NAO and NAO forcing, implying non-linearity in the connection between NAO and the ocean carbon
cycle. However, model differences in regional expression and magnitude and conflicting results with regard to
airsea flux and CO2 partial pressure remain. Typical NAO-driven variations are 910 mmol/m3 in the surface
concentration of dissolved inorganic carbon and alkalinity and 98 ppm in the airsea partial pressure
difference. The effect on the basin-wide airsea CO2 flux is small due to compensating fluxes on the sub-basin
scale. Two models show a reduced carbon sink in the north-eastern North Atlantic during negative NAO
phases, qualitatively in accordance with the observed decline during a phase of predominantly negative NAO.
The results indicate that wind-driven dynamics are the main driver of the response to the NAO, which via
vertical mixing, upwelling and the associated entrainment of dissolved inorganic carbon and nutrients leave
an imprint on surface pCO2 and the airsea CO2 flux as well as on biological export production, pH and the
calcium carbonate saturation state. The biogeochemical response to the NAO is predominantly governed by
vertical exchange between the surface and the thermocline; large-scale horizontal transport mechanisms are of
minor importance.
Logan, Cheryl A., John P Dunne, C M Eakin, and Simon D Donner, July 2012: A framework for comparing coral bleaching thresholds In Proceedings of the 12th International Coral Reef Symposium, Cairns, Australia, 9-13 July 2012, 1-5. Abstract
Coral reefs are highly vulnerable to bleaching under elevated temperature. Since 2002, NOAA Coral
Reef Watch has used a bleaching threshold based on global sea surface temperatures to provide operational
bleaching warnings. Recent studies suggest that modifications to the current global bleaching prediction method
may result in higher predictive power. Here, we present a method for comparing four bleaching prediction
methods at different spatial and temporal resolutions, each calibrated against the global bleaching observational
dataset from ReefBase between 1985 and 2005. We identify one method (“MMMmax”) that consistently gives
the highest predictive power at all spatial and temporal resolutions examined. An improved bleaching threshold
will refine future bleaching projections under climate change and provide more reliable real-time bleaching
alerts to international coral reef managers.
Ainsworth, C H., Jameal F Samhouri, D S Busch, William W L Cheung, John P Dunne, and T A Okey, July 2011: Potential impacts of climate change on Northeast Pacific marine foodwebs and fisheries. ICES Journal of Marine Science, 68(6), DOI:10.1093/icesjms/fsr043. Abstract
Although there has been considerable research on the impacts of individual changes in water temperature, carbonate chemistry, and
other variables on species, cumulative impacts of these effects have rarely been studied. Here, we simulate changes in (i) primary productivity,
(ii) species range shifts, (iii) zooplankton community size structure, (iv) ocean acidification, and (v) ocean deoxygenation
both individually and together using five Ecopath with Ecosim models of the northeast Pacific Ocean. We used a standardized
method to represent climate effects that relied on time-series forcing functions: annual multipliers of species productivity. We
focused on changes in fisheries landings, biomass, and ecosystem characteristics (diversity and trophic indices). Fisheries landings generally
declined in response to cumulative effects and often to a greater degree than would have been predicted based on individual
climate effects, indicating possible synergies. Total biomass of fished and unfished functional groups displayed a decline, though unfished
groups were affected less negatively. Some functional groups (e.g. pelagic and demersal invertebrates) were predicted to respond
favourably under cumulative effects in some regions. The challenge of predicting climate change impacts must be met if we are to
adapt and manage rapidly changing marine ecosystems in the 21st century.
Cheung, William W., John P Dunne, Jorge L Sarmiento, and D J Pauly, July 2011: Integrating ecophysiology and plankton dynamics into projected maximum fisheries catch potential under climate change in the Northeast Atlantic. ICES Journal of Marine Science, 68(6), DOI:10.1093/icesjms/fsr012. Abstract
Previous global analyses projected shifts in species distributions and maximum fisheries catch potential across ocean basins by 2050 under the Special Report on Emission Scenarios (SRES) A1B. However, these studies did not account for the effects of changes in ocean biogeochemistry and phytoplankton community structure that affect fish and invertebrate distribution and productivity. This paper uses a dynamic bioclimatic envelope model that incorporates these factors to project distribution and maximum catch potential of 120 species of exploited demersal fish and invertebrates in the Northeast Atlantic. Using projections from the US National Oceanic and Atmospheric Administration's (NOAA) Geophysical Fluid Dynamics Laboratory Earth System Model (ESM2.1) under the SRES A1B, we project an average rate of distribution-centroid shift of 52 km decade−1 northwards and 5.1 m decade−1 deeper from 2005 to 2050. Ocean acidification and reduction in oxygen content reduce growth performance, increase the rate of range shift, and lower the estimated catch potentials (10-year average of 2050 relative to 2005) by 20–30% relative to simulations without considering these factors. Consideration of phytoplankton community structure may further reduce projected catch potentials by ∼10%. These results highlight the sensitivity of marine ecosystems to biogeochemical changes and the need to incorporate likely hypotheses of their biological and ecological effects in assessing climate change impacts.
Surface ocean iron speciation is simulated using a time-dependent box-model of lightmediated
redox cycling over a range of aeolian inputs of soluble iron in the stratified epipelagic ocean.
At steady-state, Dissolved iron (DFe) concentration increases with aeolian input of soluble iron up to
0.1 μmol m-2 d-1, and is limited by the solubility of ferric hydroxide at higher fluxes which causes the
formation of colloidal iron. We demonstrate that even in the presence of ample excess ligand, rapid
conversion of dissolved iron between oxidized and reduced forms in the tropical surface ocean exposes
DFe to colloid formation and scavenging. This result provides an explanation for the much smaller
range of interregional variability in DFe measurements (0.05-0.4 nM) than soluble Fe fluxes (0.01-1
μmol m-2 d-1) and dust fluxes (0.1-10 g m-2 d-1) predicted by atmospheric models. We incorporate the
critical behavior of the full chemical speciation model into a reduced, computationally efficient model
suitable for large scale calculations.
The distribution of radiocarbon (14C) in the ocean and atmosphere has fluctuated on timescales ranging from seasons to millennia. It is thought that these fluctuations partly reflect variability in the climate system, offering a rich potential source of information to help understand mechanisms of past climate change. Here, a long simulation with a new, coupled model is used to explore the mechanisms that redistribute 14C within the Earth system on inter-annual to centennial timescales. The model, CM2Mc, is a lower-resolution version of the Geophysical Fluid Dynamics Laboratory's CM2M model, uses no flux adjustments, and incorporates a simple prognostic ocean biogeochemistry model including 14C. The atmospheric 14C and radiative boundary conditions are held constant, so that the oceanic distribution of 14C is only a function of internal climate variability. The simulation displays previously-described relationships between tropical sea surface 14C and the model-equivalents of the El Niño Southern Oscillation and Indonesian Throughflow. Sea surface 14C variability also arises from fluctuations in the circulations of the subarctic Pacific and Southern Ocean, including North Pacific decadal variability, and episodic ventilation events in the Weddell Sea that are reminiscent of the Weddell Polynya of 1974–1976. Interannual variability in the air-sea balance of 14C is dominated by exchange within the belt of intense Southern Westerly winds, rather than at the convective locations where the surface 14C is most variable. Despite significant interannual variability, the simulated impact on air-sea exchange is an order of magnitude smaller than the recorded atmospheric 14C variability of the past millennium. This result partly reflects the importance of variability in the production rate of 14C in determining atmospheric 14C, but may also reflect an underestimate of natural climate variability, particularly in the Southern Westerly winds.
Gnanadesikan, Anand, John P Dunne, and Jasmin G John, July 2011: What ocean biogeochemical models can tell us about bottom-up control of ecosystem variability. ICES Journal of Marine Science, 68(6), DOI:10.1093/icesjms/fsr068. Abstract
Processes included in earth system models amplify the impact of climate variability on phytoplankton biomass and, therefore, on
upper trophic levels. Models predict much larger relative interannual variability in large phytoplankton biomass compared with
total phytoplankton biomass, supporting the goal of better constraining size-structured primary production and biomass from
remote sensing. The largest modelled variability in annually averaged large phytoplankton biomass is associated with changes in
the areal extent of relatively productive regions. Near the equator, changes in the areal extent of the high-productivity zone are
driven by large-scale shifts in nutrient fields, as well as changes in currents. Along the poleward edge of the Subtropical Gyres,
changes in physical mixing dominate. Finally, models indicate that high-latitude interannual variability in large phytoplankton
biomass is highest during spring. Mechanisms for producing such variability differ across biomes with internal ocean processes,
such as convection complicating efforts to link ecosystem variability to climate modes defined using sea surface temperature
alone. In salinity-stratified subpolar regions, changes in bloom timing driven by salinity can produce correlations between low
surface temperatures and high productivity, supporting the potential importance of using coupled atmosphere–ocean reanalyses,
rather than simple forced ocean reanalyses, for attributing past ecosystem shifts.
Polovina, J J., John P Dunne, P A Woodworth, and E A Howell, July 2011: Projected expansion of the subtropical biome and contraction of the temperate and equatorial upwelling biomes in the North Pacific under global warming. ICES Journal of Marine Science, 68(6), DOI:10.1093/icesjms/fsq198. Abstract
A climate model that includes a coupled ocean biogeochemistry model is used to define large oceanic biomes in the North Pacific
Ocean and describe their changes over the 21st century in response to the IPCC Special Report on Emission Scenario A2 future atmospheric
CO2 emissions scenario. Driven by enhanced stratification and a northward shift in the mid-latitude westerlies under climate
change, model projections demonstrated that between 2000 and 2100, the area of the subtropical biome expands by ~30% by 2100,
whereas the area of temperate and equatorial upwelling (EU) biomes decreases by ~34 and 28%, respectively, by 2100. Over the
century, the total biome primary production and fish catch is projected to increase by 26% in the subtropical biome and decrease
by 38 and 15% in the temperate and the equatorial biomes, respectively. Although the primary production per unit area declines
slightly in the subtropical and the temperate biomes, it increases 17% in the EU biome. Two areas where the subtropical biome boundary
exhibits the greatest movement is in the northeast Pacific, where it moves northwards by as much as 1000 km per 100 years and at
the equator in the central Pacific, where it moves eastwards by 2000 km per 100 years. Lastly, by the end of the century, there are
projected to be more than 25 million km2 of water with a mean sea surface temperature of 31°C in the subtropical and EU
biomes, representing a new thermal habitat. The projected trends in biome carrying capacity and fish catch suggest resource managers
might have to address long-term trends in fishing capacity and quota levels.
Rykaczewski, Ryan R., and John P Dunne, April 2011: A measured look at ocean chlorophyll trends. Nature, 472(7342), DOI:10.1038/nature09952.
The study of climate impacts on Living Marine Resources (LMRs) has increased rapidly in recent years with the availability of climate model simulations contributed to the assessment reports of the Intergovernmental Panel on Climate Change (IPCC). Collaboration between climate and LMR scientists and shared understanding of critical challenges for such applications are essential for developing robust projections of climate impacts on LMRs. This paper assesses present approaches for generating projections of climate impacts on LMRs using IPCC-class climate models, recommends practices that should be followed for these applications, and identifies priority developments that could improve current projections. Understanding of the climate system and its representation within climate models has progressed to a point where many climate model outputs can now be used effectively to make LMR projections. However, uncertainty in climate model projections (particularly biases and inter-model spread at regional to local scales), coarse climate model resolution, and the uncertainty and potential complexity of the mechanisms underlying the response of LMRs to climate limit the robustness and precision of LMR projections. A variety of techniques including the analysis of multi-model ensembles, bias corrections, and statistical and dynamical downscaling can ameliorate some limitations, though the assumptions underlying these approaches and the sensitivity of results to their application must be assessed for each application. Developments in LMR science that could improve current projections of climate impacts on LMRs include improved understanding of the multi-scale mechanisms that link climate and LMRs and better representations of these mechanisms within more holistic LMR models. These developments require a strong baseline of field and laboratory observations including long time-series and measurements over the broad range of spatial and temporal scales over which LMRs and climate interact. Priority developments for IPCC-class climate models include improved model accuracy (particularly at regional and local scales), inter-annual to decadal-scale predictions, and the continued development of earth system models capable of simulating the evolution of both the physical climate system and biosphere. Efforts to address these issues should occur in parallel and be informed by the continued application of existing climate and LMR models.
A simple model of the temperature-dependent biological decay of dissolved oil is embedded in
an ocean climate circulation model and used to simulate underwater plumes of dissolved and
suspended oil originating from a point source in the northern Gulf of Mexico. Plumes at different
source depths are considered and the behavior at each depth is found to be determined by the
combination of sheared current strength and vertical profile of decay rate. An upper bound on the
supply rate of dissolved and suspended oil is estimated for the interior water column from
contemporary analysis of the Deepwater Horizon blowout. For all plume scenarios, toxic levels
of dissolved oil are found to remain confined to the northern Gulf of Mexico, and abate within a
few weeks after the spill stops. An estimate of oxygen consumption due to microbial oxidation of
oil suggests that the presence of oil alone will not lead to hypoxia, but a deep plume of oil and
methane (which dissolves readily in water) does lead to localized regions of persistent hypoxia
and anoxia in the vicinity of the source.
Galbraith, Eric D., Anand Gnanadesikan, John P Dunne, and M R Hiscock, March 2010: Regional impacts of iron-light colimitation in a global biogeochemical model. Biogeosciences, 7(3), DOI:10.5194/bg-7-1043-2010. Abstract
Laboratory and field studies have revealed that
iron has multiple roles in phytoplankton physiology, with
particular importance for light-harvesting cellular machinery.
However, although iron-limitation is explicitly included
in numerous biogeochemical/ecosystem models, its implementation
varies, and its effect on the efficiency of light harvesting
is often ignored. Given the complexity of the ocean
environment, it is difficult to predict the consequences of applying
different iron limitation schemes. Here we explore
the interaction of iron and nutrient cycles in an ocean general
circulation model using a new, streamlined model of
ocean biogeochemistry. Building on previously published
parameterizations of photoadaptation and export production,
the Biogeochemistry with Light Iron Nutrients and Gasses
(BLING) model is constructed with only three explicit tracers
but including macronutrient and micronutrient limitation,
light limitation, and an implicit treatment of community
structure. The structural simplicity of this computationally inexpensive
model allows us to clearly isolate the global effect
that iron availability has on maximum light-saturated
photosynthesis rates vs. the effect iron has on photosynthetic
efficiency. We find that the effect on light-saturated photosynthesis
rates is dominant, negating the importance of photosynthetic
efficiency in most regions, especially the cold
waters of the Southern Ocean. The primary exceptions to
this occur in iron-rich regions of the Northern Hemisphere,
where high light-saturated photosynthesis rates allow photosynthetic
efficiency to play a more important role. In other
words, the ability to efficiently harvest photons has little effect
in regions where light-saturated growth rates are low.
Additionally, we speculate that the small phytoplankton cells
dominating iron-limited regions tend to have relatively high
photosynthetic efficiency, due to reduced packaging effects.
If this speculation is correct, it would imply that natural communities
of iron-stressed phytoplankton may tend to harvest
photons more efficiently than would be inferred from iron limitation
experiments with larger phytoplankton. We suggest
that iron limitation of photosynthetic efficiency has a
relatively small impact on global biogeochemistry, though it
is expected to impact the seasonal cycle of plankton as well
as the vertical structure of primary production.
Henson, Stephanie A., Jorge L Sarmiento, John P Dunne, Laurent Bopp, Ivan D Lima, Scott C Doney, Jasmin G John, and C Beaulieu, February 2010: Detection of anthropogenic climate change in satellite records of ocean chlorophyll and productivity. Biogeosciences, 7(2), DOI:10.5194/bg-7-621-2010. Abstract
Global climate change is predicted to alter the ocean's biological productivity. But how will we recognise the impacts of climate change on ocean productivity? The most comprehensive information available on its global distribution comes from satellite ocean colour data. Now that over ten years of satellite-derived chlorophyll and productivity data have accumulated, can we begin to detect and attribute climate change-driven trends in productivity? Here we compare recent trends in satellite ocean colour data to longer-term time series from three biogeochemical models (GFDL, IPSL and NCAR). We find that detection of climate change-driven trends in the satellite data is confounded by the relatively short time series and large interannual and decadal variability in productivity. Thus, recent observed changes in chlorophyll, primary production and the size of the oligotrophic gyres cannot be unequivocally attributed to the impact of global climate change. Instead, our analyses suggest that a time series of similar to 40 years length is needed to distinguish a global warming trend from natural variability. In some regions, notably equatorial regions, detection times are predicted to be shorter (similar to 20-30 years). Analysis of modelled chlorophyll and primary production from 2001-2100 suggests that, on average, the climate change-driven trend will not be unambiguously separable from decadal variability until similar to 2055. Because the magnitude of natural variability in chlorophyll and primary production is larger than, or similar to, the global warming trend, a consistent, decades-long data record must be established if the impact of climate change on ocean productivity is to be definitively detected.
Rykaczewski, Ryan R., and John P Dunne, November 2010: Enhanced nutrient supply to the California Current Ecosystem with global warming and increased stratification in an earth system model. Geophysical Research Letters, 37, L21606, DOI:10.1029/2010GL045019. Abstract
A leading hypothesis relating productivity with climate variability in the California
Current Ecosystem (CCE) describes an alternation between warmer, well-stratified periods of
low productivity and cooler periods of high productivity. This empirical relationship suggests
that productivity will decline with global warming. Here, we explore the response of
productivity to future climate change in the CCE using an earth system model. This model
projects increases in nitrate supply and productivity in the CCE during the 21st century despite
increases in stratification and limited change in wind-driven upwelling. We attribute the
increased nitrate supply to enrichment of deep source waters entering the CCE resulting from
decreased ventilation of the North Pacific. Decreases in dissolved-oxygen concentration and
increasing acidification accompany projected increases in nitrate. This analysis illustrates that
anthropogenic climate change may be unlike past variability; empirical relationships based on
historical observations may be inappropriate for projecting ecosystem responses to future climate
change.
Saba, Vincent S., and John P Dunne, et al., September 2010: Challenges of modeling depth-integrated marine primary productivity over multiple decades: A case study at BATS and HOT. Global Biogeochemical Cycles, 24, GB3020, DOI:10.1029/2009GB003655. Abstract
The performance of 36 models (22 ocean color models and 14 biogeochemical ocean circulation models (BOGCMs)) that estimate depth-integrated marine net primary productivity (NPP) was assessed by comparing their output to in situ 14C data at the Bermuda Atlantic Time series Study (BATS) and the Hawaii Ocean Time series (HOT) over nearly two decades. Specifically, skill was assessed based on the models' ability to estimate the observed mean, variability, and trends of NPP. At both sites, more than 90% of the models underestimated mean NPP, with the average bias of the BOGCMs being nearly twice that of the ocean color models. However, the difference in overall skill between the best BOGCM and the best ocean color model at each site was not significant. Between 1989 and 2007, in situ NPP at BATS and HOT increased by an average of nearly 2% per year and was positively correlated to the North Pacific Gyre Oscillation index. The majority of ocean color models produced in situ NPP trends that were closer to the observed trends when chlorophyll-a was derived from high-performance liquid chromatography (HPLC), rather than fluorometric or SeaWiFS data. However, this was a function of time such that average trend magnitude was more accurately estimated over longer time periods. Among BOGCMs, only two individual models successfully produced an increasing NPP trend (one model at each site). We caution against the use of models to assess multiannual changes in NPP over short time periods. Ocean color model estimates of NPP trends could improve if more high quality HPLC chlorophyll-a time series were available.
While nutrient depletion scenarios have long shown that the high-latitude High Nutrient Low Chlorophyll (HNLC) regions are the most effective for sequestering atmospheric carbon dioxide, recent simulations with prognostic biogeochemical models have suggested that only a fraction of the potential drawdown can be realized. We use a global ocean biogeochemical general circulation model developed at GFDL and Princeton to examine this and related issues. We fertilize two patches in the North and Equatorial Pacific, and two additional patches in the Southern Ocean HNLC region north of the biogeochemical divide and in the Ross Sea south of the biogeochemical divide. We evaluate the simulations using observations from both artificial and natural iron fertilization experiments at nearby locations. We obtain by far the greatest response to iron fertilization at the Ross Sea site, where sea ice prevents escape of sequestered CO2 during the wintertime, and the CO2 removed from the surface ocean by the biological pump is carried into the deep ocean by the circulation. As a consequence, CO2 remains sequestered on century time-scales and the efficiency of fertilization remains almost constant no matter how frequently iron is applied as long as it is confined to the growing season. The second most efficient site is in the Southern Ocean. The North Pacific site has lower initial nutrients and thus a lower efficiency. Fertilization of the Equatorial Pacific leads to an expansion of the suboxic zone and a striking increase in denitrification that causes a sharp reduction in overall surface biological export production and CO2 uptake. The impacts on the oxygen distribution and surface biological export are less prominent at other sites, but nevertheless still a source of concern. The century time scale retention of iron in this model greatly increases the long-term biological response to iron addition as compared with simulations in which the added iron is rapidly scavenged from the ocean.
An ecosystem model was used to (1) determine the extent to which global trends in the ratio of mesozooplankton production to primary production (referred to herein as the “z-ratio”) can be explained by nutrient enrichment, temperature, and euphotic zone depth, and (2) quantitatively diagnose the mechanisms driving these trends. Equilibrium model solutions were calibrated to observed and empirically derived patterns in phytoplankton biomass and growth rates, mesozooplankton biomass and growth rates, and the fraction of phytoplankton that are large (>5 μm ESD). This constrained several otherwise highly uncertain model parameters. Most notably, half-saturation constants for zooplankton feeding were constrained by the biomass and growth rates of their prey populations, and low zooplankton basal metabolic rates were required to match observations from oligotrophic ecosystems. Calibrated model solutions had no major biases and produced median z-ratios and ranges consistent with estimates. However, much of the variability around the median values in the calibration dataset (72 points) could not be explained. Model results were then compared with an extended global compilation of z-ratio estimates (>10 000 points). This revealed a modest yet significant (r=0.40) increasing trend in z-ratios from values 0.01–0.04 to 0.1–0.2 with increasing primary productivity, with the transition from low to high z-ratios occurring at lower primary productivity in cold-water ecosystems. Two mechanisms, both linked to increasing phytoplankton biomass, were responsible: (1) zooplankton gross growth efficiencies increased as their ingestion rates became much greater than basal metabolic rates and (2) the trophic distance between primary producers and mesozooplankton shortened as primary production shifted toward large phytoplankton. Mechanism (1) was most important during the transition from low to moderate productivity ecosystems and mechanism (2) was responsible for a relatively abrupt transition to values >0.1 in high productivity ecosystems. Substantial z-ratio variations overlying these mean trends remained unexplained by these mechanisms. Potential sources of this variability include zooplankton patchiness, unresolved effects of advection and unsteady dynamics, unresolved shifts in mesozooplankton sizes and species, and unresolved aspects of zooplankton bioenergetics. Comparison of the modeled z-ratio patterns and mechanisms diagnosed herein with those obtained using models with expanded biological dynamics embedded in global circulation models will help further elucidate the causes of this variation.
We present a new methodology for database-driven ecosystem model generation and apply the methodology to the world's 66 currently defined Large Marine Ecosystems. The method relies on a large number of spatial and temporal databases, including FishBase, SeaLifeBase, as well as several other databases developed notably as part of the Sea Around Us project. The models are formulated using the freely available Ecopath with Ecosim (EwE) modeling approach and software. We tune the models by fitting to available time series data, but recognize that the models represent only a first-generation of database-driven ecosystem models. We use the models to obtain a first estimate of fish biomass in the world's LMEs. The biggest hurdles at present to further model development and validation are insufficient time series trend information, and data on spatial fishing effort.
Friedrichs, Marjorie A., M-E Carr, R T Barber, M Scardi, D Antoine, R A Armstrong, I Asanuma, M J Behrenfeld, Erik T Buitenhuis, Fei Chai, James R Christian, A M Ciotti, Scott C Doney, M Dowell, John P Dunne, B Gentili, W Gregg, N Hoepffner, J Ishizaka, T Kameda, Ivan D Lima, J Marra, F Melin, J Keith Moore, A Morel, R T O'Malley, J E O'Reilly, and Vincent S Saba, et al., February 2009: Assessing the uncertainties of model estimates of primary productivity in the tropical Pacific Ocean. Journal of Marine Systems, 76(1-2), DOI:10.1016/j.jmarsys.2008.05.010. Abstract
Depth-integrated primary productivity (PP) estimates obtained from satellite
ocean color-based
models (SatPPMs) and those generated from biogeochemical ocean general
circulation models
(BOGCMs) represent a key resource for biogeochemical and ecological studies
at global as well as
regional scales. Calibration and validation of these PP models are not
straightforward, however,
and comparative studies show large differences between model estimates. The
goal of this paper is
to compare PP estimates obtained from 30 different models (21 SatPPMs and 9
BOGCMs) to a
tropical Paci fic PP database consisting of ~1000
14C
measurements spanning more than a decade
(1983–1996). Primary findings include: skill varied significantly
between models, but performance
was not a function of model complexity or type (i.e. SatPPM vs. BOGCM);
nearly all models underestimated the observed variance of PP, specifically yielding too
few low PP (<0.2 g Cm-2
d-1)
values; more than half of the total root-mean-squared model–data differences
associated with the
satellite-based PP models might be accounted for by uncertainties in the
input variables and/or the
PP data; and the tropical Pacific database captures a broad scale shift from
low biomass normalized
productivity in the 1980s to higher biomass-normalized productivity in the
1990s,
which was not successfully captured by any of the models. This latter result
suggests that
interdecadal and global changes will be a significant challenge for both
SatPPMs and BOGCMs.
Finally, average root-mean-squared differences between in situ PP data on
the equator at 140°W and PP estimates from the satellite-based productivity models were 58% lower
than analogous
values computed in a previous PP model comparison 6 years ago. The success
of these types of
comparison exercises is illustrated by the continual modification and
improvement of the
participating models and the resulting increase in model skill.
The interannual to decadal variability in the timing and magnitude of the North Atlantic phytoplankton bloom is examined using a combination of satellite data and output from an ocean biogeochemistry general circulation model. The timing of the bloom as estimated from satellite chlorophyll data is used as a novel metric for validating the model's skill. Maps of bloom timing reveal that the subtropical bloom begins in winter and progresses northward starting in May in subpolar regions. A transition zone, which experiences substantial interannual variability in bloom timing, separates the two regions. Time series of the modeled decadal (1959–2004) variability in bloom timing show no long‐term trend toward earlier or delayed blooms in any of the three regions considered here. However, the timing of the subpolar bloom does show distinct decadal‐scale periodicity, which is found to be correlated with the North Atlantic Oscillation (NAO) index. The mechanism underpinning the relationship is identified as anomalous wind‐driven mixing conditions associated with the NAO. In positive NAO phases, stronger westerly winds result in deeper mixed layers, delaying the start of the subpolar spring bloom by 2–3 weeks. The subpolar region also expands during positive phases, pushing the transition zone further south in the central North Atlantic. The magnitude of the bloom is found to be only weakly dependent on bloom timing, but is more strongly correlated with mixed layer depth. The extensive interannual variability in the timing of the bloom, particularly in the transition region, is expected to strongly impact the availability of food to higher trophic levels.
Henson, Stephanie A., D Raitsos, John P Dunne, and A McQuatters-Gollop, November 2009: Decadal variability in biogeochemical models: Comparison with a 50-year ocean colour dataset. Geophysical Research Letters, 36(L21601), DOI:10.1029/2009GL040874. Abstract
Assessing the skill of biogeochemical models to hindcast past variability is challenging, yet vital in order to assess their ability to predict biogeochemical change. However, the validation of decadal variability is limited by the sparsity of consistent, long-term biological datasets. The Phytoplankton Colour Index (PCI) product from the Continuous Plankton Recorder survey, which has been sampling the North Atlantic since 1948, is an example of such a dataset. Converting the PCI to chlorophyll values using SeaWiFS data allows a direct comparison with model output. Here we validate decadal variability in chlorophyll from the GFDL TOPAZ model. The model demonstrates skill at reproducing interannual variability, but cannot simulate the regime shifts evident in the PCI data. Comparison of the model output, data and climate indices highlights under-represented processes that it may be necessary to include in future biogeochemical models in order to accurately simulate decadal variability in ocean ecosystems.
Rodgers, Keith B., Robert M Key, Anand Gnanadesikan, Jorge L Sarmiento, John P Dunne, and A R Jacobson, et al., September 2009: Using altimetry to help explain patchy changes in hydrographic carbon measurements. Journal of Geophysical Research, C09013, DOI:10.1029/2008JC005183. Abstract
Here we use observations and ocean models to identify mechanisms driving large seasonal to interannual variations in dissolved inorganic carbon (DIC) and dissolved oxygen (O2) in the upper ocean. We begin with observations linking variations in upper ocean DIC and O2 inventories with changes in the physical state of the ocean. Models are subsequently used to address the extent to which the relationships derived from short-timescale (6 months to 2 years) repeat measurements are representative of variations over larger spatial and temporal scales. The main new result is that convergence and divergence (column stretching) attributed to baroclinic Rossby waves can make a first-order contribution to DIC and O2 variability in the upper ocean. This results in a close correspondence between natural variations in DIC and O2 column inventory variations and sea surface height (SSH) variations over much of the ocean. Oceanic Rossby wave activity is an intrinsic part of the natural variability in the climate system and is elevated even in the absence of significant interannual variability in climate mode indices. The close correspondence between SSH and both DIC and O2 column inventories for many regions suggests that SSH changes (inferred from satellite altimetry) may prove useful in reducing uncertainty in separating natural and anthropogenic DIC signals (using measurements from Climate Variability and Predictability's CO2/Repeat Hydrography program).
Correction: 10.1029/2009JC005835
The chemical characteristics of aggregating material in the marine environment are largely unknown. We investigated neutral aldose (NA) abundance and composition in aggregation of marine snow and other organic matter (OM) size fractions in the field. Four sample sets were fractionated using membrane filtration and ultrafiltration into the following size fractions: particulate material, high-molecular-weight (HMW) material, and low-molecular-weight (LMW) material. We also collected three sample sets of marine-snow aggregates. Each sample set contained small, medium, and large aggregate size fractions and each size fraction consisted of 25–50 aggregates. For 7 marine-snow samples and for each water-sample size fraction, we determined monomeric and polymeric NA concentration, NA yield (amount of NA-C normalized to organic carbon), and composition; total organic carbon (TOC) concentration; transparent exopolymer particles (TEP) concentration, and TEP propensity (TEP concentration after inducing TEP formation in filtered samples). This is the first study to include compound-specific NA determinations on these four marine OM size fractions.
The mass balances of organic carbon and NA indicated that there were no serious contamination or loss problems. Concentrations, yields, and NA mol fractions in water samples were similar to results from other studies. Glucose and galactose had the highest relative abundance in all size fractions. The NA yield increased with increasing molecular weight or particle size for all fractions except marine snow. The NA yield increased in the order: LMW< marine snow< HMW< particles. Marine snow had a higher average NA yield than the LMW fraction, but lower than particle and HMW-fractions. This indicates that OM in marine snow could have been diagenetically derived from particulate and HMW-fractions, that is, marine snow may include material from the particulate and the colloidal phase.
TEP concentration or TEP propensity was positively correlated with concentrations of all individual NAs as well as the sum NA concentrations, indicating that TEP contains neutral sugars in addition to the acidic polysaccharides stained in the determination of TEP concentrations.
Despite the relatively low NA yield in marine snow, marine snow was enriched in NA when compared with seawater, with enrichment factors of 34–225 (average 125). By combining data from this study with data from other studies, we estimate that < 10% of carbohydrates in marine snow comprise NAs.
There was no clear correlation between marine-snow aggregate size and NA yield, that is, there appears to be no general age difference between small and large marine-snow aggregates. NA composition was similar among different marine-snow size fracions collected during the same day, indicating that aggregation/disaggregation reactions resulted in homogenizing NA composition in marine-snow aggregates of all sizes. The NA composition of marine snow was different from that of other OM size fractions, indicating either that bacterial degradation has modified the composition of marine snow to a larger extent than other OM size fractions or that marine snow is formed through the aggregation of selected subcomponents of OM.
The impact of the penetration length scale of shortwave radiation into the surface ocean is investigated with a fully coupled ocean, atmosphere, land and ice model. Oceanic shortwave radiation penetration is assumed to depend on the chlorophyll concentration. As chlorophyll concentrations increase the distribution of shortwave heating becomes shallower. This change in heat distribution impacts mixed-layer depth. This study shows that removing all chlorophyll from the ocean results in a system that tends strongly towards an El Niño state—suggesting that chlorophyll is implicated in maintenance of the Pacific cold tongue. The regions most responsible for this response are located off-equator and correspond to the oligotrophic gyres. Results from a suite of surface chlorophyll perturbation experiments suggest a potential positive feedback between chlorophyll concentration and a non-local coupled response in the fully coupled ocean-atmosphere system.
Nitrogen fixation is crucial for maintaining biological productivity in the oceans, because it replaces the biologically available nitrogen that is lost through denitrification. But, owing to its temporal and spatial variability, the global distribution of marine nitrogen fixation is difficult to determine from direct shipboard measurements. This uncertainty limits our understanding of the factors that influence nitrogen fixation, which may include iron, nitrogen-to-phosphorus ratios, and physical conditions such as temperature. Here we determine nitrogen fixation rates in the world's oceans through their impact on nitrate and phosphate concentrations in surface waters, using an ocean circulation model. Our results indicate that nitrogen fixation rates are highest in the Pacific Ocean, where water column denitrification rates are high but the rate of atmospheric iron deposition is low. We conclude that oceanic nitrogen fixation is closely tied to the generation of nitrogen-deficient waters in denitrification zones, supporting the view that nitrogen fixation stabilizes the oceanic inventory of fixed nitrogen over time.
Dunne, John P., Jorge L Sarmiento, and Anand Gnanadesikan, December 2007: A synthesis of global particle export from the surface ocean and cycling through the ocean interior and on the seafloor. Global Biogeochemical Cycles, 21, GB4006, DOI:10.1029/2006GB002907. Abstract
We present a new synthesis of the oceanic
cycles of organic carbon, silicon, and calcium carbonate. Our calculations
are based on a series of algorithms starting with satellite-based primary
production and continuing with conversion of primary production to sinking
particle flux, penetration of particle flux to the deep sea, and
accumulation in sediments. Regional and global budgets from this synthesis
highlight the potential importance of shelves and near-shelf regions for
carbon burial. While a high degree of uncertainty remains, this analysis
suggests that shelves, less than 50 m water depths accounting for 2% of the
total ocean area, may account for 48% of the global flux of organic carbon
to the seafloor. Our estimates of organic carbon and nitrogen flux are in
generally good agreement with previous work while our estimates for CaCO3
and SiO2 fluxes are lower than recent work. Interannual
variability in particle export fluxes is found to be relatively small
compared to intra-annual variability over large domains with the single
exception of the dominating role of El Niño-Southern Oscillation variability
in the central tropical Pacific. Comparison with available sediment-based
syntheses of benthic remineralization and burial support the recent theory
of mineral protection of organic carbon flux through the deep ocean,
pointing to lithogenic material as an important carrier phase of organic
carbon to the deep seafloor. This work suggests that models which exclude
the role of lithogenic material would underestimate the penetration of POC
to the deep seafloor by approximately 16–51% globally, and by a much larger
fraction in areas with low productivity. Interestingly, atmospheric dust can
only account for 31% of the total lithogenic flux and 42% of the
lithogenically associated POC flux, implying that a majority of this
material is riverine or directly erosional in origin.
Friedrichs, Marjorie A., J A Dusenberry, L A Anderson, R A Armstrong, Fei Chai, James R Christian, Scott C Doney, John P Dunne, M Fujii, Raleigh Hood, Dennis J McGillicuddy, Jr, J Keith Moore, M Schartau, Y H Spitz, and J D Wiggert, 2007: Assessment of skill and portability in regional marine biogeochemical models: Role of multiple planktonic groups. Journal of Geophysical Research, 112, C08001, DOI:10.1029/2006JC003852. Abstract
Application of biogeochemical models to the study of marine ecosystems is pervasive, yet objective quantification of these models' performance is rare. Here, 12 lower trophic level models of varying complexity are objectively assessed in two distinct regions (equatorial Pacific and Arabian Sea). Each model was run within an identical one-dimensional physical framework. A consistent variational adjoint implementation assimilating chlorophyll-a, nitrate, export, and primary productivity was applied and the same metrics were used to assess model skill. Experiments were performed in which data were assimilated from each site individually and from both sites simultaneously. A cross-validation experiment was also conducted whereby data were assimilated from one site and the resulting optimal parameters were used to generate a simulation for the second site. When a single pelagic regime is considered, the simplest models fit the data as well as those with multiple phytoplankton functional groups. However, those with multiple phytoplankton functional groups produced lower misfits when the models are required to simulate both regimes using identical parameter values. The cross-validation experiments revealed that as long as only a few key biogeochemical parameters were optimized, the models with greater phytoplankton complexity were generally more portable. Furthermore, models with multiple zooplankton compartments did not necessarily outperform models with single zooplankton compartments, even when zooplankton biomass data are assimilated. Finally, even when different models produced similar least squares model-data misfits, they often did so via very different element flow pathways, highlighting the need for more comprehensive data sets that uniquely constrain these pathways.
Carr, M-E, and John P Dunne, et al., March 2006: A comparison of global estimates of marine primary production from ocean color. Deep-Sea Research, Part II, 53(5-7), DOI:10.1016/j.dsr2.2006.01.028. Abstract
The third primary production algorithm round robin (PPARR3) compares output from 24 models that estimate depthintegrated
primary production from satellite measurements of ocean color, as well as seven general circulation models
(GCMs) coupled with ecosystem or biogeochemical models. Here we compare the global primary production fields
corresponding to eight months of 1998 and 1999 as estimated from common input fields of photosynthetically-available
radiation (PAR), sea-surface temperature (SST), mixed-layer depth, and chlorophyll concentration. We also quantify the
sensitivity of the ocean-color-based models to perturbations in their input variables. The pair-wise correlation between
ocean-color models was used to cluster them into groups or related output, which reflect the regions and environmental
conditions under which they respond differently. The groups do not follow model complexity with regards to wavelength or
depth dependence, though they are related to the manner in which temperature is used to parameterize photosynthesis.
Global average PP varies by a factor of two between models. The models diverged the most for the Southern Ocean, SST
under 10 C, and chlorophyll concentration exceeding 1mg Chlm3. Based on the conditions under which the model results
diverge most, we conclude that current ocean-color-based models are challenged by high-nutrient low-chlorophyll
conditions, and extreme temperatures or chlorophyll concentrations. The GCM-based models predict comparable primary
production to those based on ocean color: they estimate higher values in the Southern Ocean, at low SST, and in the
equatorial band, while they estimate lower values in eutrophic regions (probably because the area of high chlorophyll
concentrations is smaller in the GCMs). Further progress in primary production modeling requires improved understanding
of the effect of temperature on photosynthesis and better parameterization of the maximum photosynthetic rate.
The formulation and simulation characteristics of two new global coupled climate models developed at NOAA's Geophysical Fluid Dynamics Laboratory (GFDL) are described. The models were designed to simulate atmospheric and oceanic climate and variability from the diurnal time scale through multicentury climate change, given our computational constraints. In particular, an important goal was to use the same model for both experimental seasonal to interannual forecasting and the study of multicentury global climate change, and this goal has been achieved.
Two versions of the coupled model are described, called CM2.0 and CM2.1. The versions differ primarily in the dynamical core used in the atmospheric component, along with the cloud tuning and some details of the land and ocean components. For both coupled models, the resolution of the land and atmospheric components is 2° latitude × 2.5° longitude; the atmospheric model has 24 vertical levels. The ocean resolution is 1° in latitude and longitude, with meridional resolution equatorward of 30° becoming progressively finer, such that the meridional resolution is 1/3° at the equator. There are 50 vertical levels in the ocean, with 22 evenly spaced levels within the top 220 m. The ocean component has poles over North America and Eurasia to avoid polar filtering. Neither coupled model employs flux adjustments.
The control simulations have stable, realistic climates when integrated over multiple centuries. Both models have simulations of ENSO that are substantially improved relative to previous GFDL coupled models. The CM2.0 model has been further evaluated as an ENSO forecast model and has good skill (CM2.1 has not been evaluated as an ENSO forecast model). Generally reduced temperature and salinity biases exist in CM2.1 relative to CM2.0. These reductions are associated with 1) improved simulations of surface wind stress in CM2.1 and associated changes in oceanic gyre circulations; 2) changes in cloud tuning and the land model, both of which act to increase the net surface shortwave radiation in CM2.1, thereby reducing an overall cold bias present in CM2.0; and 3) a reduction of ocean lateral viscosity in the extratropics in CM2.1, which reduces sea ice biases in the North Atlantic.
Both models have been used to conduct a suite of climate change simulations for the 2007 Intergovernmental Panel on Climate Change (IPCC) assessment report and are able to simulate the main features of the observed warming of the twentieth century. The climate sensitivities of the CM2.0 and CM2.1 models are 2.9 and 3.4 K, respectively. These sensitivities are defined by coupling the atmospheric components of CM2.0 and CM2.1 to a slab ocean model and allowing the model to come into equilibrium with a doubling of atmospheric CO2. The output from a suite of integrations conducted with these models is freely available online (see http://nomads.gfdl.noaa.gov/).
Manuscript received 8 December 2004, in final form 18 March 2005
The current generation of coupled climate models run at the Geophysical Fluid Dynamics Laboratory (GFDL) as part of the Climate Change Science Program contains ocean components that differ in almost every respect from those contained in previous generations of GFDL climate models. This paper summarizes the new physical features of the models and examines the simulations that they produce. Of the two new coupled climate model versions 2.1 (CM2.1) and 2.0 (CM2.0), the CM2.1 model represents a major improvement over CM2.0 in most of the major oceanic features examined, with strikingly lower drifts in hydrographic fields such as temperature and salinity, more realistic ventilation of the deep ocean, and currents that are closer to their observed values. Regional analysis of the differences between the models highlights the importance of wind stress in determining the circulation, particularly in the Southern Ocean. At present, major errors in both models are associated with Northern Hemisphere Mode Waters and outflows from overflows, particularly the Mediterranean Sea and Red Sea.
Jin, X, Nicolas Gruber, John P Dunne, Jorge L Sarmiento, and R A Armstrong, June 2006: Diagnosing the contribution of phytoplankton functional groups to the production and export of particulate organic carbon, CaCO3, and opal from global nutrient and alkalinity distributions. Global Biogeochemical Cycles, 20, GB2015, DOI:10.1029/2005GB002532. Abstract
We diagnose the contribution of four main phytoplankton functional groups to the production and export of particulate organic carbon (POC), CaCO3, and opal by combining in a restoring approach global oceanic observations of nitrate, silicic acid, and alkalinity with a simple size-dependent ecological/biogeochemical model. In order to determine the robustness of our results, we employ three different variants of the ocean general circulation model (OGCM) required to transport and mix the nutrients and alkalinity into the upper ocean. In our standard model, the global export of CaCO3 is diagnosed as 1.1 PgC yr−1 (range of sensitivity cases 0.8 to 1.2 PgC yr−1) and that of opal as 180 Tmol Si yr−1 (range 160 to 180 Tmol Si yr−1). CaCO3 export is found to have three maxima at approximately 40¡ÆS, the equator, and around 40¡ÆN. In contrast, the opal export is dominated by the Southern Ocean with a single maximum at around 60¡ÆS. The molar export ratio of inorganic to organic carbon is diagnosed in our standard model to be about 0.09 (range 0.07 to 0.10) and found to be remarkably uniform spatially. The molar export ratio of opal to organic nitrogen varies substantially from values around 2 to 3 in the Southern Ocean south of 45¡ÆS to values below 0.5 throughout most of the rest of the ocean, except for the North Pacific. Irrespective of which OGCM is used, large phytoplankton dominate the export of POC, with diatoms alone accounting for 40% of this export, while the contribution of coccolithophorids is only about 10%. Small phytoplankton dominate net primary production (NPP) with a fraction of ¡70%. Diatoms and coccolithophorids account for about 15% and less than 2% of NPP, respectively. These diagnosed contributions of the main phytoplankton functional groups to NPP are also robust across all OGCMs investigated. Correlation and regression analyses reveal that the variations in the relative contributions of diatoms and coccolithophorids to NPP can be predicted reasonably well on the basis of a few key parameters.
Passow, U, John P Dunne, J W Murray, L Balistrieri, and A L Alldredge, August 2006: Organic carbon to 234Th ratios of marine organic matter. Marine Chemistry, 100(3-4), DOI:10.1016/j.marchem.2005.10.020. Abstract
disequilibrium between 234Th and its mother isotope uranium depend largely on the determination of the organic carbon to 234thorium (OC : 234Th) ratio. The variability of the OC : 234Th ratio in different size fractions of suspended matter, ranging from the truly dissolved (< 3 or 10 kDa) fraction to several millimeter sized marine snow, as well as from sediment trap material was assessed during an eight-day cruise off the coast of California in Spring 1997. The affinity of polysaccharide particles called TEP (transparent exopolymer particles) and inorganic clays to 234Th was investigated through correlations. The observed decrease in the OC : 234Th ratio with size, within the truly dissolved to small particle size range, is consistent with concepts of irreversible colloidal aggregation of non-porous nano-aggregates. No consistent trend in the OC : 234Th ratio was observed for particles between 1 or 10 to 6000 μm. Origin and fate of marine particles belonging to this size range are diverse and interactions with 234Th too complex to expect a consistent relationship between OC : 234Th ratio and size, if all categories of particles are included. The relationship between OC and 234Th was significant when data from the truly dissolved fraction were excluded. However, variability was very large, implying that OC flux calculations using different collection methods (e.g. sediment trap, Niskin bottles or pumps) would differ significantly. Therefore a large uncertainty in OC flux calculations based on the 234Th method exist due to individual decisions as to which types or size classes of particles best represent sinking material in a specific area. Preferential binding of 234Th to specific substance classes could explain the high variability in the relationship between OC and 234Th. At 15 m, in the absence of lithogenic material, the OC : 234Th ratio was a function of the fraction of TEP or TEP-precursors in OC, confirming that acidic polysaccharides have a high affinity for 234Th and that TEP carry a ligand for 234Th. Preferential binding to TEP might change distribution patterns of 234Th considerably, as TEP may sink when included in large aggregates, or remain suspended or even ascend when existing as individual particles or microaggregates. In the presence of lithogenic matter, at depths below 30 m, the ratio between 234Th and OC was linearly related to the ratio between alumino silicates and C. The affinity of inorganic substances to 234Th is known to be relatively low, suggesting that a coating of acidic polysaccharides was responsible for the apparently high affinity between 234Th and lithogenic material. Overall, OC : 234Th ratios of all material collected during this investigation can best be explained by differential binding of 234Th to both TEP and TEP-precursors, as well as to lithogenic minerals, which were very abundant in an intermediate nepheloid layer between 50 and 90 m.
We present new empirical and mechanistic models for predicting the export of organic carbon out of the surface ocean by sinking particles. To calibrate these models, we have compiled a synthesis of field observations related to ecosystem size structure, primary production and particle export from around the globe. The empirical model captures 61% of the observed variance in the ratio of particle export to primary production (the pe ratio) using sea-surface temperature and chlorophyll concentrations (or primary productivity) as predictor variables. To describe the mechanisms responsible for pe-ratio variability, we present size-based formulations of phytoplankton grazing and sinking particle export, combining them into an alternative, mechanistic model. The formulation of grazing dynamics, using simple power laws as closure terms for small and large phytoplankton, reproduces 74% of the observed variability in phytoplankton community composition wherein large phytoplankton augment small ones as production increases. The formulation for sinking particle export partitions a temperature-dependent fraction of small and large phytoplankton grazing into sinking detritus. The mechanistic model also captures 61% of the observed variance in pe ratio, with large phytoplankton in high biomass and relatively cold regions leading to more efficient export. In this model, variability in primary productivity results in a biomass-modulated switch between small and large phytoplankton pathways.
This paper summarizes the formulation of the ocean component to the Geophysical Fluid Dynamics Laboratory's (GFDL) climate model used for the 4th IPCC Assessment (AR4) of global climate change. In particular, it reviews the numerical schemes and physical parameterizations that make up an ocean climate model and how these schemes are pieced together for use in a state-of-the-art climate model. Features of the model described here include the following: (1) tripolar grid to resolve the Arctic Ocean without polar filtering, (2) partial bottom step representation of topography to better represent topographically influenced advective and wave processes, (3) more accurate equation of state, (4) three-dimensional flux limited tracer advection to reduce overshoots and undershoots, (5) incorporation of regional climatological variability in shortwave penetration, (6) neutral physics parameterization for representation of the pathways of tracer transport, (7) staggered time stepping for tracer conservation and numerical efficiency, (8) anisotropic horizontal viscosities for representation of equatorial currents, (9) parameterization of exchange with marginal seas, (10) incorporation of a free surface that accommodates a dynamic ice model and wave propagation, (11) transport of water across the ocean free surface to eliminate unphysical "virtual tracer flux" methods, (12) parameterization of tidal mixing on continental shelves. We also present preliminary analyses of two particularly important sensitivities isolated during the development process, namely the details of how parameterized subgridscale eddies transport momentum and tracers.
Murray, B, John P Dunne, and T Chapin, November 2005: 234Th, 210Pb, 210Po and stable Pb in the central equatorial Pacific: Tracers for particle cycling. Deep-Sea Research, Part I, 52(11), DOI:10.1016/j.dsr.2005.06.016. Abstract
Samples were collected during the 1992 US JGOFS EqPac Survey I and II cruises from 12°N to 12°S at 140°W in the central equatorial Pacific for water column profiles of dissolved, particulate and total 234Th, 210Pb and 210Po and total acid soluble stable Pb and sediment trap fluxes of 234Th, 210Pb and 210Po. Survey I occurred in February/March with moderate El Nino conditions while Survey II was conducted in September/October when there was a well developed cold-tongue. 234Th, 210Pb and 210Po are all particle reactive yet they partition differently between dissolved and particulate phases. Fractionation factors (the ratios of the distribution coefficients) show that the selectivity for suspended and sediment trap particles follows Th>Po>Pb. Scavenging residence times (τ) for 234Th, 210Pb and 210Po ranged from 25 to 100 d, 3 to 8 years and 100 to 500 d, respectively. These particle reactive tracers have very different distributions in the water column, which reflect differences in their sources and sinks. The deficiency of 234Th relative to 238U was fairly uniformly distributed meridionally, though deficiencies were higher during Survey II when there was higher new production. Excess 210Pb relative to 226Ra was very asymmetrical with much higher excess values north of the equator. The distributions were similar for Surveys I and II. The deficiency of 210Po relative to210Pb had a symmetrical distribution about the equator for both Survey I and II but the deficiencies were larger during Survey I when upwelling was smaller. Stable Pb was generally higher at the surface than at 250 m and there was no meridional trend from 12°N to 12°S. A mass balance for 210Pb was used to determine the atmospheric input of 210Pb. The average values for Surveys I and II were 0.12 and 0.32 dpm cm−2 year−1, respectively. There was no general increase in atmospheric input of 210Pb north of the equator but there was a strong maximum at 2–3°N during Survey I coincident with the location of the intertropical convergence zone (ITCZ), suggesting a large role for wet deposition. A mass balance for stable Pb was used to determine the atmospheric input of stable Pb. Results ranged from 110 to 140 pmol cm−2 year−1. This flux was low in the southern hemisphere and increased steadily north of the equator. We evaluated use of 210Po as a tracer for export of particulate organic matter during Survey I. Organic carbon and 210Po were highly correlated in suspended matter and sediment trap samples. Average values of organic carbon fluxes determined from the deficiencies of 210Po times the orgC/210Po ratio agreed well with those determined from the deficiencies of 234Th times the organic carbon/234Th ratio and 15N-new production, but had a much larger variability because of the more variable advection corrections.
Gnanadesikan, Anand, John P Dunne, Robert M Key, K Matsumoto, Jorge L Sarmiento, Richard D Slater, and P S Swathi, December 2004: Oceanic ventilation and biogeochemical cycling: Understanding the physical mechanisms that produce realistic distributions of tracers and productivity. Global Biogeochemical Cycles, 18(4), GB4010, DOI:10.1029/2003GB002097. Abstract
Differing models of the ocean circulation support different rates of ventilation, which in turn produce different distributions of radiocarbon, oxygen, and export production. We examine these fields within a suite of general circulation models run to examine the sensitivity of the circulation to the parameterization of subgridscale mixing and surface forcing. We find that different models can explain relatively high fractions of the spatial variance in some fields such as radiocarbon, and that newer estimates of the rate of biological cycling are in better agreement with the models than previously published estimates. We consider how different models achieve such agreement and show that they can accomplish this in different ways. For example, models with high vertical diffusion move young surface waters into the Southern Ocean, while models with high winds move more young North Atlantic water into this region. The dependence on parameter values is not simple. Changes in the vertical diffusion coefficient, for example, can produce major changes in advective fluxes. In the coarse-resolution models studied here, lateral diffusion plays a major role in the tracer budget of the deep ocean, a somewhat worrisome fact as it is poorly constrained both observationally and theoretically.
Sarmiento, Jorge L., Nicolas Gruber, M Brzezinski, and John P Dunne, January 2004: High-latitude controls of thermocline nutrients and low latitude biological productivity. Nature, 427, 56-60. Abstract PDF
The ocean's biological pump strips nutrients out of the surface waters and exports them into the thermocline and deep waters. If there were no return path of nutrients from deep waters, the biological pump would eventually deplete the surface waters and thermocline of nutrients; surface biological productivity would plummet. Here we make use of the combined distributions of silicic acid and nitrate to trace the main nutrient return path from deep waters by upwelling in the Southern Ocean and subsequent entrainment into subantarctic mode water. We show that the subantarctic mode water, which spreads throughout the entire Southern Hemisphere and North Atlantic Ocean, is the main source of nutrients for the thermocline. We also find that an additional return path exists in the northwest corner of the Pacific Ocean, where enhanced vertical mixing, perhaps driven by tides, brings abyssal nutrients to the surface and supplies them to the thermocline of the North Pacific. Our analysis has important implications for our understanding of large-scale controls on the nature and magnitude of low-latitude biological productivity and its sensitivity to climate change.
Sarmiento, Jorge L., John P Dunne, and R A Armstrong, 2004: Do We Now Understand The Ocean’s Biological Pump?U.S. JGOFS News, 12, 1-5.
Aufdenkampe, A K., J J McCarthy, C Navarette, M Rodier, John P Dunne, and J W Murray, 2002: Biogeochemical controls on new production in the tropical Pacific. Deep-Sea Research, Part II, 49(13-14), DOI:10.1016/S0967-0645(02)00051-6. Abstract
Sources of variability in new production (NP) measured during nine cruises in the tropical Pacific Ocean are examined with respect to other biological and chemical properties. NP measured along the equator during the Zonal Flux and Flupac cruises using 15NO3 incubation methods is presented in this paper and compared to similar data from seven previously published cruises to the tropical Pacific. The Zonal Flux cruise found a strong zonal gradient of increasing NP to the east that followed increasing nitrate inventories. NP values ranged from 0.8 and 3.8 mmol N m−2 d−1 from 165°E to 150°W, respectively. During the 7-day Flupac Time Series II at 150°W, NP measurements also showed strong variability, ranging from 1.9 to 3.6 mmol N m−2 d−1, despite relatively uniform nitrate. Both cruises observed a previously measured but seldom discussed trend for f-ratios to increase substantially at the limits of the euphotic zone (0.1% E0).
Multiple linear regression (MLR) analyses of areal, depth-integrated data from 121 stations in the tropical Pacific previously have showed that variability in primary production (or chlorophyll), ammonium, nitrate and temperature together could “explain” 79% of the variability in NP (Aufdenkampe et al., Global Biogeochem. Cycles 15 (2001) 101). In the present study, the MLR method was extended to depth specific data, where the same variables were shown to explain 77% of nitrate uptake variability. MLR was then used to investigate differences between individual cruises in the relationships of NP to these variables. Similar to MLR results with combined data from all cruises, MLR of individual cruises also found primary production (or chlorophyll), ammonium and nitrate to be consistently the best variables to explain variability in areal NP, exhibiting R2 values from 0.45 to 0.92. However, nitrate is consistently a much stronger predictor of NP within cruises than between cruises. Other lines of evidence—including plots of each property vs. NP and vs. standard residuals of the all-cruise MLR, and differences in MLR partial slopes for individual cruises—together demonstrate that the relationship of NP to nitrate exhibits subtle but real differences from one cruise to the next. Zonal Flux and Flupac sampled the two extremes of this observed NP-to-nitrate variability.
Dunne, John P., A H Devol, and S Emerson, 2002: The Oceanic Remote Chemical/Optical Analyzer (ORCA)--An autonomous moored profiler. Journal of Atmospheric and Oceanic Technology, 19(10), 1709-1721. Abstract PDF
An autonomous, moored profiler [the Oceanic Remote Chemical/Optical Analyzer (ORCA)] was developed to sense a variety of chemical and optical properties in the upper water column. It is presently used to monitor water quality parameters in South Puget Sound--a largely undeveloped area subject to extensive future urbanization. ORCA has three main components: 1) a three-point moored Autonomous Temperature Line Acquisition System (ATLAS) toroidal float; 2) a profiling assembly on the float with computer, winch, cellular system, meteorological sensors (wind, temperature, humidity, irradiance), solar panels, and batteries; and 3) an underwater sensor package consisting of a Seabird CTD profiler, YSI dissolved oxygen electrode, Wetlabs transmissometer, and Wetlabs chlorophyll fluorometer. At regular sampling intervals, ORCA profiles the water column using the winch and pressure information from the CTD. The data are recorded on the computer and transmitted to the lab automatically via cellular communications. Data are presented from a 1-day deployment in May 2000 and from a long-term, 7-month deployment. The dataset reveals the combination of intermittent stratification mixing and strong seasonal forcing in this estuarine system.
We use an ocean biogeochemical-transport box model of the top 100 m of the water column to estimate the CaCO3 to organic carbon export ratio from observations of the vertical gradients of potential alkalinity and nitrate. We find a global average molar export ratio of 0.06 ± 0.03. This is substantially smaller than earlier estimates of 0.25 on which a majority of ocean biogeochemical models had based their parameterization of CaCO3 production. Contrary to the pattern of coccolithophore blooms determined from satellite observations, which show high latitude predominance, we find maximum export ratios in the equatorial region and generally smaller ratios in the subtropical and subpolar gyres. Our results suggest a dominant contribution to global calcification by low-latitude nonbloom forming coccolithophores or other organisms such as foraminifera and pteropods.
Aufdenkampe, A K., J J McCarthy, M Rodier, C Navarette, John P Dunne, and J W Murray, 2001: Estimation of new production in the tropical Pacific. Global Biogeochemical Cycles, 15(1), 101-112. Abstract
A synthesis of field data from nine cruises and 121 stations in the tropical Pacific (15°N–16°S by 135°W–167°E) was used to develop a statistical model relating areal new production rates (based on 15NO3 uptake incubations) to other measured biological and chemical water properties. The large dynamic range of f ratios (new to primary production) measured in the region (0.01−0.46, with a mean of 0.16 ± 0.08) could not be described by any simple function of any of the more than three dozen measured variables tested. Thus the commonly used approach of extrapolating new production using mean f ratios is likely to lead to large uncertainties when used in the tropical Pacific. An alternative approach is examined in which new production is estimated directly by multiple linear regression (MLR) of measured properties. Nearly 80% of variability in new production could be explained with a MLR of four variables together (rates of primary production (or chlorophyll inventories), inventories of ammonium and nitrate, and temperature) better than any single variable alone or any other combination of variables. Each of these variables exhibited effective linearity with respect to new production for this data set, and the robustness of this MLR method to predict new production for other data sets was confirmed by cross validation. These results thus provide a robust, simple tool to extend new production estimates to locations and times where it is not measured directly, using ship-based measurements and potentially remotely sensed data from moorings and satellites.
Dunne, John P., J W Murray, M Rodier, and D Hansell, May 2000: Export flux in the western and central equatorial Pacific: zonal and temporal variability. Deep-Sea Research, Part I, 47(5), DOI:10.1016/S0967-0637(99)00089-8. Abstract
Particulate organic carbon export fluxes were measured along the equator to resolve the zonal extent of high productivity in the equatorial Pacific during two cruises: the French JGOFS FLUPAC study aboard the R/V l'Atalante in October 1994 and the Zonal Flux study aboard the R/V Thomas G. Thompson in April 1996. Both cruise tracks went along the equator from 165°E to 150°W. The cruises took place under different seasonal and El Niño-Southern Oscillation (ENSO) conditions: FLUPAC during a strong El Niño in the boreal fall and Zonal Flux during a mild La Niña in the boreal spring. Drifting sediment traps were deployed at the base of the euphotic zone and calibrated using 234Th. These traps showed over-trapping by 2.7±1.5 times during FLUPAC and 1.5±0.7 times during Zonal Flux. During the FLUPAC time-series at 167°E, the upper euphotic zone was devoid of nitrate, and particulate organic carbon export was low (6±1 mmol m−2 d−1). The FLUPAC time-series at 150°W had abundant nitrate and much higher particulate organic carbon export (12±1 mmol m−2 d−1). Similarly high levels of particulate organic carbon export were observed all along the equator during the Zonal Flux cruise (10±2 mmol m−2 d−1), when cold tongue, high nitrate conditions extended west of 165°E.
Synthesis of this data with results from the US Joint Global Ocean Flux Study (JGOFS) equatorial Pacific (EqPac) program allowed a detailed evaluation of equatorial production variability. Data from the TOGA-TAO array illustrated that both Kelvin Waves and tropical instability waves (TIW) were present during the FLUPAC cruise, while neither wave type was present during Zonal Flux. Comparison with results from the US JGOFS EqPac cruises suggested that the ubiquity of super-μM nitrate was the major forcing for new production and particle export near the equator, accounting for a doubling of production over areas with only subsurface nitrate. Within the high nitrate zone, new production and particle export were both found to be enhanced during TIW activity and diminished during Kelvin Wave activity. While the geographical extent of surface nutrients and associated enhanced production is clearly a strong function of season and ENSO, we suggest that equatorially trapped waves — rather than long-term variability in upwelling velocity — are the dominant sources of variability within the equatorial upwelling zone. Comparison of new production and particle export and regressions between nitrate and total organic carbon (TOC) suggest that accumulation and transport of TOC accounts for 17–27% of new production.
Dunne, John P., J W Murray, A K Aufdenkampe, S Blain, and M Rodier, 1999: Silicon-nitrogen coupling in the equatorial Pacific upwelling zone. Global Biogeochemical Cycles, 13(3), 715-726. Abstract
We describe the role of diatoms on nitrogen and silicon cycling in the equatorial Pacific upwelling zone (EUZ) using water column nutrient data from 19 equatorial cruises and particle concentration, new production, and sediment trap data from the U.S. Joint Global Ocean Flux Study (JGOFS) equatorial Pacific (EqPac), France JGOFS fluxes in the Pacific (FLUPAC), and U.S. Zonal Flux cruises. Our results suggest that production and sinking of diatoms dominate particulate nitrogen export at silicate concentrations above 4 μM. Below this level, silicate is preferentially retained; while inorganic nitrogen is completely utilized, silicate remains at concentrations of 1-2 μM and is completely exhausted only under nonsteady state conditions. This lower nutrient condition accounts for a majority of particulate nitrogen export in the EUZ with minor loss of particulate silicon. Retention of silicon relative to nitrogen appears due to a combination of new production by nondiatoms, dissolution of silica frustules after grazing, iron limitation, and steady state upwelling. This synthesis supports the argument that diatom production was tightly coupled to new production during the U.S. JGOFS EqPac survey II cruise [Dugdale and Wilkerson, 1998]. However, this compilation suggests EqPac survey II cruise took place during a period of atypically high subsurface nutrients. We conclude that silicon and nitrogen are tightly coupled only at periods of very high nutrient concentration and nonsteady state. In addition, nutrient cycling in the EUZ is consistent at all times with a mechanism of combined iron and grazing control of phytoplankton size classes [Landry et al., 1997].
We determined the sensitivity of the calculated sinking flux of 234Th in the central equatorial Pacific to physical processes and scavenging mechanisms by imposing a meridional and vertical advection and diffusion field on a simple dissolved and particulate 234Th cycle. We used the model to estimate the efficiency with which the 234Th deficiency relative to 238U reflected the predicted sinking flux of 234Th on particles and compared our results with 234Th data taken during the JGOFS-EqPac 1992 Survey II Cruise. 234Th deficiencies near the equator were strongly affected by both vertical advection and horizontal diffusion. The model 234Th deficiency at the equator underestimated the model 234Th sinking flux by 144% in neglecting advection and diffusion in the presence of strong upwelling at the equator. The model 234Th deficiency at the equator corrected for advection overestimated the sinking flux of 234Th by 33% in neglecting horizontal diffusion. Analysis of the scavenging mechanism suggests that, during situations of export governed by rapidly sinking particles, 234Th-based estimates of particle export are only half as sensitive to advection compared to situations of export governed by slowly sinking particles. Given that results using the mechanism of slowly sinking particles compare better with the observed 234Th deficiency and calculated meridional 234Th fluxes at the equator than the mechanism of rapidly sinking particles, we consider the mechanism of slowly sinking particle more appropriate for this region. In agreement with previous studies based on observed 234Th gradients, this study supports the incorporation of vertical advection terms in the 234Th balance to estimate particulate carbon export at the equator but suggests that this method may have overestimated the sinking flux at the equator during EqPac Survey II by 0–63% due to the role of horizontal diffusion.
Dunne, John P., 1999: Measured and Modeled Particle Export in Equatorial and Coastal Upwelling Regions, Ph.D. Thesis, Seattle, WA: University of Washington, 167pp.
Archer, D, R C Barber, and John P Dunne, et al., 1997: A meeting place of great ocean currents: shipboard observations of a convergent front at 2°N in the Pacific. Deep-Sea Research, Part II, 44(9-10), DOI:10.1016/S0967-0645(97)00031-3. Abstract
We present a synthesis of physical, chemical and biological shipboard observations of a convergent front at 2°N, 140°W and its surrounding environment. The front was a component of a tropical instability wave generated by shear between westward-flowing equatorial waters to the south and warmer equatorial counter current water to the north. Surface waters on the cold side were undersaturated with oxygen, which suggests that the water had only been exposed at the sea surface for a period of a few weeks. Although the atmospheric exposure time was short, the effects of biological activity could be detected in enhanced concentrations of total (dissolved plus suspended particulate) organic carbon concentration, proving that TOC can be produced in the top centimeters of the changing environmental conditions. The front itself was dominated by the accumulation of a “patch” of buoyant diatoms Rhizosolenia castracanei concentrated in the top centimeters of the warm surface water north of the front, and elevated chlorophyll concentrations were observed from the air over a spatial scale of order 10–20 km northward from the front. The nitrogen budget and thorium data suggest that a significant fraction of the elevated POC, and virtually all of the PON, arrived in the patch waters as imported particles rather than in situ photosynthesis. Photosynthetic uptake of carbon appears to have occurred in patch waters, but without corresponding uptake of fixed nitrogen (an uncoupling of the usual Redfield stoichiometry). Solute chemistry of the patch appears to be controlled by turbulent mixing, which flushes out patch waters on a time scale of days
Dunne, John P., J W Murray, J Young, L Balistrieri, and J Bishop, 1997: 234Th and particle cycling in the central equatorial Pacific. Deep-Sea Research, Part II, 44(9-10), DOI:10.1016/S0967-0645(97)00063-5. Abstract
US JGOFS-EgPac234Th data sets for 1992 boreal spring (Survey I, TT007) and fall (Survey 11, TT011) cruises from 12°N to 12°S along 140°W were used to determine rates of 234Th and particle cycling using a thorium sorption model and three coupled particle-thorium models. Sampling methodology had a large impact on model results — estimates of particulate organic carbon varied by a factor of 3 between bottle and in-situ filtration techniques. Adsorption rate constants and residence times from the thorium sorption model showed strong depth, latitudinal and seasonal variability which we were able to attribute to changes in particle concentration. A reevaluation of the `particle concentration effect' on the adsorption rate constant, k′, showed that our values of k′ increased with particle concentration and were consistent with other study sites with similar particle concentrations. Recycling of particulate organic carbon in the euphotic zone of the central equatorial Pacific was 2–10 times faster than sites previously studied. Calculations of adsorption rate constants from the thorium sorption, coupled particle-234Th and phytoplankton models were extremely dependent on the model treatment of remineralization. Results from the coupled particle-234Th model, where particles have a constant ]ability, suggested that 234Th recycled three to four times between the dissolved and paticulate phases before being removed from the euphotic zone. Aggregation rate constants and sinking rates in the central equatorial system were compared with other sites using the size-fractionated model developed by Clegg and Whitfield (1991,Deep-Sea Research,38, 91–120). Removal of particles by sinking from the equatorial euphotic zone depended on a mechanism of differential recycling of organic matter in the euphotic zone in which only a fraction of the particles are remineralized and the more refractory particles sink.
Murray, J W., J Young, J Newton, John P Dunne, T Chapin, and B Paul, 1996: Export flux of particulate organic carbon from the central equatorial Pacific determined using a combined drifting trap-234Th approach. Deep-Sea Research, Part II, 43(4-6), DOI:10.1016/0967-0645(96)00036-7. Abstract
The export flux of particulate organic carbon from the euphotic zone in the central equatorial Pacific was measured using an approach that utilizes 234Th and organic carbon analyses on water column and drifting sediment trap samples. This study was conducted as part of the U.S. Joint Global Ocean Flux Study (U.S. JGOFS) EqPac process study from 12°N to 12°S at 140°W. Samples were collected during the Survey I (February–March 1992) and Survey II (August–September 1992) cruises. The accuracy of drifting sediment traps was evaluated by comparing the measured flux of 234Th with the flux calculated from the deficiency of 234Th relative to 238U in the water column. Calculated 234Th fluxes were corrected for the effects of horizontal and vertical advection. The uncertainties on these 234Th fluxes averaged 39% for Survey I and 20% for Survey II. Comparison of measured and calculated 234Th fluxes revealed evidence for overtrapping, especially in the shallow traps (≤ 100 m). Measured and calculated 234Th fluxes agreed to within 50% for traps at 150–250 m. Good correlation was obtained between measured fluxes of organic carbon and 234Th except for some shallow samples high in organic carbon, suggesting that 234Th was a good tracer for organic carbon. The flux of particulate organic carbon (POC) was calculated as the product of the calculated flux of 234Th times the organic carbon/234Th ratio in trap samples. Assuming that the organic carbon/234Th ratio in trap samples was representative of sinking particles, we used an average value for the organic carbon/234Th ratio for each station. The variability in the station-averaged POC/234Th ratio ranged from 10% to 30%. The POC fluxes calculated using our combined 234Th-trap approach ranged from 1 to 6 mmol C m−2 day−1 during Survey I, and from 2 to 30 mmol C m−2 day−1 during Survey II. The average uncertainty for the POC fluxes was ±60%. Primary and new production integrated to the depth of the 0.1 % light level varied by factors of 2–3 for Survey I and Survey II, respectively. The export of particulate organic carbon from the euphotic zone also increased by a factor of 3. The corresponding e-ratios (POC export/primary production) ranged from 0.03 to 0.11 for Survey I, and 0.04 to 0.23 for Survey II. Annual average regional rates (10°N–10°S; 90°W–180°E) of new (0.47 Gt C year−1) and particulate export (0.42 Gt C year−1) production were in good agreement, suggesting that, on an annual basis, significant export of DOC need not be invoked to balance new and export production in this region.
