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.
Oceanic heat uptake (OHU) is a significant source of uncertainty in both the transient and equilibrium responses to increasing the planetary radiative forcing. OHU differs among climate models and is related in part to their representation of vertical and lateral mixing. This study examines the role of ocean model formulation – specifically the choice of vertical coordinate and strength of background diapycnal diffusivity (Kd) – in the millennial-scale near-equilibrium climate response to a quadrupling of atmospheric CO2. Using two fully-coupled Earth System Models (ESMs) with nearly identical atmosphere, land, sea ice, and biogeochemical components, it is possible to independently configure their ocean model components with different formulations and produce similar near-equilibrium climate responses. The SST responses are similar between the two models (r2 = 0.75, global average ∼ 4.3 °C) despite their initial pre-industrial climate mean states differing by 0.4 °C globally. The surface and interior responses of temperature and salinity are also similar between the two models. However, the Atlantic Meridional Overturning Circulation (AMOC) responses are different between the two models, and the associated differences in ventilation and deep water formation have an impact on the accumulation of dissolved inorganic carbon in the ocean interior. A parameter sensitivity analysis demonstrates that increasing the amount of Kd produces very different near-equilibrium climate responses within a given model. These results suggest that the impact of the ocean vertical coordinate on the climate response is small relative to the representation of sub-gridscale mixing.
Greenland Ice Sheet (GIS) might have lost a large amount of its volume during the last interglacial and may do so again in the future due to climate warming. In this study, we test whether the climate response to the glacial meltwater is sensitive to its discharging location. Two fully coupled atmosphere–ocean general circulation models, CM2G and CM2M, which have completely different ocean components are employed to do the test. In each experiment, a prescribed freshwater flux of 0.1 Sv is discharged from one of the four locations around Greenland—Petermann, 79 North, Jacobshavn and Helheim glaciers. The results from both models show that the AMOC weakens more when the freshwater is discharged from the northern GIS (Petermann and 79 North) than when it is discharged from the southern GIS (Jacobshavn and Helheim), by 15% (CM2G) and 31% (CM2M) averaged over model year 50–300 (CM2G) and 70–300 (CM2M), respectively. This is due to easier access of the freshwater from northern GIS to the deepwater formation site in the Nordic Seas. In the long term (> 300 year), however, the AMOC change is nearly the same for freshwater discharged from any location of the GIS. The East Greenland current accelerates with time and eventually becomes significantly faster when the freshwater is discharged from the north than from the south. Therefore, freshwater from the north is transported efficiently towards the south first and then circulates back to the Nordic Seas, making its impact to the deepwater formation there similar to the freshwater discharged from the south. The results indicate that the details of the location of meltwater discharge matter if the short-term (< 300 years) climate response is concerned, but may not be critical if the long-term (> 300 years) climate response is focused upon.
Danabasoglu, Gokhan, Stephen G Yeager, W M Kim, E Behrens, M Bentsen, D Bi, A Biastoch, R Bleck, C Böning, A Bozec, V M Canuto, Christophe Cassou, Eric P Chassignet, A C Coward, S Danilov, N Diansky, H Drange, Riccardo Farneti, E Fernandez, P G Fogli, G Forget, Yosuke Fujii, Stephen M Griffies, A Gusev, P Heimbach, A Howard, M Ilicak, T Jung, Alicia R Karspeck, M Kelley, William G Large, A Leboissetier, J Lu, G Madec, S J Marsland, S Masina, A Navarra, A J George Nurser, Anna Pirani, Anastasia Romanou, D Salas y Mélia, and Bonita L Samuels, et al., January 2016: North Atlantic Simulations in Coordinated Ocean-ice Reference Experiments phase II (CORE-II). Part II: Inter-Annual to Decadal Variability. Ocean Modelling, 97, DOI:10.1016/j.ocemod.2015.11.007. Abstract
Simulated inter-annual to decadal variability and trends in the North Atlantic for the 1958−−2007 period from twenty global ocean – sea-ice coupled models are presented. These simulations are performed as contributions to the second phase of the Coordinated Ocean-ice Reference Experiments (CORE-II). The study is Part II of our companion paper (Danabasoglu et al., 2014) which documented the mean states in the North Atlantic from the same models. A major focus of the present study is the representation of Atlantic meridional overturning circulation (AMOC) variability in the participating models. Relationships between AMOC variability and those of some other related variables, such as subpolar mixed layer depths, the North Atlantic Oscillation (NAO), and the Labrador Sea upper-ocean hydrographic properties, are also investigated. In general, AMOC variability shows three distinct stages. During the first stage that lasts until the mid- to late-1970s, AMOC is relatively steady, remaining lower than its long-term (1958−−2007) mean. Thereafter, AMOC intensifies with maximum transports achieved in the mid- to late-1990s. This enhancement is then followed by a weakening trend until the end of our integration period. This sequence of low frequency AMOC variability is consistent with previous studies. Regarding strengthening of AMOC between about the mid-1970s and the mid-1990s, our results support a previously identified variability mechanism where AMOC intensification is connected to increased deep water formation in the subpolar North Atlantic, driven by NAO-related surface fluxes. The simulations tend to show general agreement in their representations of, for example, AMOC, sea surface temperature (SST), and subpolar mixed layer depth variabilities. In particular, the observed variability of the North Atlantic SSTs is captured well by all models. These findings indicate that simulated variability and trends are primarily dictated by the atmospheric datasets which include the influence of ocean dynamics from nature superimposed onto anthropogenic effects. Despite these general agreements, there are many differences among the model solutions, particularly in the spatial structures of variability patterns. For example, the location of the maximum AMOC variability differs among the models between Northern and Southern Hemispheres.
Model and observational studies have concluded that geothermal heating significantly alters the global overturning circulation and the properties of the widely-distributed Antarctic Bottom Waters. Here we test two distinct geothermal heat flux datasets under different experimental designs in a fully coupled model that mimics the control run of a typical Coupled Model Intercomparison Project (CMIP) climate model. Our regional analysis reveals that bottom temperature and transport changes, due to the inclusion of geothermal heating, are propagated throughout the water column, most prominently in the Southern Ocean, with the background density structure and major circulation pathways acting as drivers of these changes. Whilst geothermal heating enhances Southern Ocean abyssal overturning circulation by 20-50%, upwelling of warmer deep waters and cooling of upper ocean waters within the Antarctic Circumpolar Current (ACC) region decrease its transport by 3 to 5 Sv. The transient responses in regional bottom temperature increases exceed 0.1°C. The combination of large scale features that we show act to transport anomalies far from their geothermal source all exist in the Southern Ocean. Such features include steeply sloping isopycnals, weak abyssal stratification, voluminous southward flowing deep waters and exported bottom waters, the ACC, and the polar gyres. Recently the Southern Ocean has been identified as a prime region for deep ocean warming; geothermal heating should be included in climate models to ensure accurate representation of these abyssal temperature changes.
Ilicak, M, H Drange, Q Wang, R Gerdes, Y Aksenov, David A Bailey, M Bentsen, A Biastoch, A Bozec, C Böning, Christophe Cassou, Eric P Chassignet, A C Coward, B Curry, Gokhan Danabasoglu, S Danilov, E Fernandez, P G Fogli, Yosuke Fujii, Stephen M Griffies, Doroteaciro Iovino, Alexandra Jahn, T Jung, William G Large, Craig Lee, C Lique, J Lu, S Masina, A J George Nurser, C Roth, D Salas y Mélia, and Bonita L Samuels, et al., April 2016: An assessment of the Arctic Ocean in a suite of interannual CORE-II simulations. Part III: Hydrography and fluxes. Ocean Modelling, 100, DOI:10.1016/j.ocemod.2016.02.004. Abstract
In this paper we compare the simulated Arctic Ocean in fifteen global ocean-sea ice models in the framework of the Coordinated Ocean-ice Reference Experiments, phase II (CORE-II). Most of these models are the ocean and sea-ice components of the coupled climate models used in the Coupled Model Intercomparison Project Phase 5 (CMIP5) experiments. We mainly focus on the hydrography of the Arctic interior, the state of Atlantic Water layer and heat and volume transports at the gateways of the Davis Strait, the Bering Strait, the Fram Strait and the Barents Sea Opening. We found that there is a large spread in temperature in the Arctic Ocean between the models, and generally large differences compared to the observed temperature at intermediate depths. Warm bias models have a strong temperature anomaly of inflow of the Atlantic Water entering the Arctic Ocean through the Fram Strait. Another process that is not represented accurately in the CORE-II models is the formation of cold and dense water, originating on the eastern shelves. In the cold bias models, excessive cold water forms in the Barents Sea and spreads into the Arctic Ocean through the St. Anna Through. There is a large spread in the simulated mean heat and volume transports through the Fram Strait and the Barents Sea Opening. The models agree more on the decadal variability, to a large degree dictated by the common atmospheric forcing. We conclude that the CORE-II model study helps us to understand the crucial biases in the Arctic Ocean. The current coarse resolution state-of-the-art ocean models need to be improved in accurate representation of the Atlantic Water inflow into the Arctic and density currents coming from the shelves.
Tseng, Y-H, Hongyang Lin, Han-Ching Chen, K Thompson, M Bentsen, C Böning, A Bozec, Christophe Cassou, Eric P Chassignet, C H Chow, Gokhan Danabasoglu, S Danilov, Riccardo Farneti, P G Fogli, Yosuke Fujii, Stephen M Griffies, M Ilicak, T Jung, S Masina, A Navarra, L Patara, and Bonita L Samuels, et al., August 2016: North and Equatorial Pacific Ocean Circulation in the CORE-II Hindcast Simulations. Ocean Modelling, 104, DOI:10.1016/j.ocemod.2016.06.003. Abstract
We evaluate the mean circulation patterns, water mass distributions, and tropical dynamics of the North and Equatorial Pacific Ocean based on a suite of global ocean-sea ice simulations driven by the CORE-II atmospheric forcing from 1963-2007. The first three moments (mean, standard deviation and skewness) of sea surface height and surface temperature variability are assessed against observations. Large discrepancies are found in the variance and skewness of sea surface height and in the skewness of sea surface temperature. Comparing with the observation, most models underestimate the Kuroshio transport in the Asian Marginal seas due to the missing influence of the unresolved western boundary current and meso-scale eddies. In terms of the Mixed Layer Depths (MLDs) in the North Pacific, the two observed maxima associated with Subtropical Mode Water and Central Mode Water formation coalesce into a large pool of deep MLDs in all participating models, but another local maximum associated with the formation of Eastern Subtropical Mode Water can be found in all models with different magnitudes. The main model bias of deep MLDs results from excessive Subtropical Mode Water formation due to inaccurate representation of the Kuroshio separation and of the associated excessively warm and salty Kuroshio water. Further water mass analysis shows that the North Pacific Intermediate Water can penetrate southward in most models, but its distribution greatly varies among models depending not only on grid resolution and vertical coordinate but also on the model dynamics. All simulations show overall similar large scale tropical current system, but with differences in the structures of the Equatorial Undercurrent. We also confirm the key role of the meridional gradient of the wind stress curl in driving the equatorial transport, leading to a generally weak North Equatorial Counter Current in all models due to inaccurate CORE-II equatorial wind fields. Most models show a larger interior transport of Pacific subtropical cells than the observation due to the overestimated transport in the Northern Hemisphere likely resulting from the deep pycnocline.
Wang, Q, M Ilicak, R Gerdes, H Drange, Y Aksenov, David A Bailey, M Bentsen, A Biastoch, A Bozec, C Böning, Christophe Cassou, Eric P Chassignet, A C Coward, B Curry, Gokhan Danabasoglu, S Danilov, E Fernandez, P G Fogli, Yosuke Fujii, Stephen M Griffies, Doroteaciro Iovino, Alexandra Jahn, T Jung, William G Large, Craig Lee, C Lique, J Lu, S Masina, A J George Nurser, B Rabe, C Roth, D Salas y Mélia, and Bonita L Samuels, et al., March 2016: An assessment of the Arctic Ocean in a suite of interannual CORE-II simulations. Part I: Sea ice and solid freshwater. Ocean Modelling, 99, DOI:10.1016/j.ocemod.2015.12.008. Abstract
The Arctic Ocean simulated in fourteen global ocean-sea ice models in the framework of the Coordinated Ocean-ice Reference Experiments, phase II (CORE II) is analyzed. The focus is on the Arctic sea ice extent, the solid freshwater (FW) sources and solid freshwater content (FWC). Available observations are used for model evaluation. The variability of sea ice extent and solid FW budget is more consistently reproduced than their mean state in the models. The descending trend of September sea ice extent is well simulated in terms of the model ensemble mean. Models overestimating sea ice thickness tend to underestimate the descending trend of September sea ice extent. The models underestimate the observed sea ice thinning trend by a factor of two. When averaged on decadal time scales, the variation of Arctic solid FWC is contributed by those of both sea ice production and sea ice transport, which are out of phase in time. The solid FWC decreased in the recent decades, caused mainly by the reduction in sea ice thickness. The models did not simulate the acceleration of sea ice thickness decline, leading to an underestimation of solid FWC trend after 2000. The common model behaviour, including the tendency to underestimate the trend of sea ice thickness and March sea ice extent, remains to be improved.
Wang, Q, M Ilicak, R Gerdes, H Drange, Y Aksenov, David A Bailey, M Bentsen, A Biastoch, A Bozec, C Böning, Christophe Cassou, Eric P Chassignet, A C Coward, B Curry, Gokhan Danabasoglu, S Danilov, E Fernandez, P G Fogli, Yosuke Fujii, Stephen M Griffies, Doroteaciro Iovino, Alexandra Jahn, T Jung, William G Large, Craig Lee, C Lique, J Lu, S Masina, A J George Nurser, B Rabe, C Roth, D Salas y Mélia, and Bonita L Samuels, et al., March 2016: An assessment of the Arctic Ocean in a suite of interannual CORE-II simulations. Part II: Liquid freshwater. Ocean Modelling, 99, DOI:10.1016/j.ocemod.2015.12.009. Abstract
The Arctic Ocean simulated in fourteen global ocean-sea ice models in the framework of the Coordinated Ocean-ice Reference Experiments, phase II (CORE-II) is analyzed in this study. The focus is on the Arctic liquid freshwater (FW) sources and freshwater content (FWC). The models agree on the interannual variability of liquid FW transport at the gateways where the ocean volume transport determines the FW transport variability. The variation of liquid FWC is induced by both the surface FW flux (associated with sea ice production) and lateral liquid FW transport, which are in phase when averaged on decadal time scales. The liquid FWC shows an increase starting from the mid-1990s, caused by the reduction of both sea ice formation and liquid FW export, with the former being more significant in most of the models. The mean state of the FW budget is less consistently simulated than the temporal variability. The model ensemble means of liquid FW transport through the Arctic gateways compare well with observations. On average, the models have too high mean FWC, weaker upward trends of FWC in the recent decade than the observation, and low consistency in the temporal variation of FWC spatial distribution, which needs to be further explored for the purpose of model development.
Downes, S M., Riccardo Farneti, P Uotila, Stephen M Griffies, S J Marsland, David A Bailey, E Behrens, M Bentsen, D Bi, A Biastoch, C Böning, A Bozec, V M Canuto, Eric P Chassignet, Gokhan Danabasoglu, S Danilov, N Diansky, H Drange, P G Fogli, A Gusev, A Howard, M Ilicak, T Jung, M Kelley, William G Large, A Leboissetier, Matthew C Long, J Lu, S Masina, A Mishra, A Navarra, A J George Nurser, L Patara, and Bonita L Samuels, et al., October 2015: An assessment of Southern Ocean water masses and sea ice during 1988-2007 in a suite of inter-annual CORE-II simulations. Ocean Modelling, 94, DOI:10.1016/j.ocemod.2015.07.022. Abstract
We characterize the representation of the Southern Ocean water mass structure and sea ice within a suite of 15 global ocean-ice models run with the Coordinated Ocean-ice Reference Experiment Phase II (CORE-II) protocol. The main focus is the representation of the present (1988–2007) mode and intermediate waters, thus framing an analysis of winter and summer mixed layer depths; temperature, salinity, and potential vorticity structure; and temporal variability of sea ice distributions. We also consider the inter-annual variability over the same 20 year period. Comparisons are made between models as well as to observation-based analyses where available.
The CORE-II models exhibit several biases relative to Southern Ocean observations, including an underestimation of the model mean mixed layer depths of mode and intermediate water masses in March (associated with greater ocean surface heat gain), and an overestimation in September (associated with greater high latitude ocean heat loss and a more northward winter sea-ice extent). In addition, the models have cold and fresh/ warm and salty water column biases centered near 50°S. Over the 1988–2007 period, the CORE-II models consistently simulate spatially variable trends in sea-ice concentration, surface freshwater fluxes, mixed layer depths, and 200–700 m ocean heat content. In particular, sea-ice coverage around most of the Antarctic continental shelf is reduced, leading to a cooling and freshening of the near surface waters. The shoaling of the mixed layer is associated with increased surface buoyancy gain, except in the Pacific where sea ice is also influential. The models are in disagreement, despite the common CORE-II atmospheric state, in their spatial pattern of the 20-year trends in the mixed layer depth and sea-ice.
Farneti, Riccardo, S M Downes, Stephen M Griffies, S J Marsland, E Behrens, M Bentsen, D Bi, A Biastoch, C Böning, A Bozec, V M Canuto, Eric P Chassignet, Gokhan Danabasoglu, S Danilov, N Diansky, H Drange, P G Fogli, A Gusev, Robert Hallberg, A Howard, M Ilicak, T Jung, M Kelley, William G Large, A Leboissetier, Matthew C Long, J Lu, S Masina, A Mishra, A Navarra, A J George Nurser, L Patara, and Bonita L Samuels, et al., September 2015: An assessment of Antarctic Circumpolar Current and Southern Ocean Meridional Overturning Circulation during 1958–2007 in a suite of interannual CORE-II simulations. Ocean Modelling, 93, DOI:10.1016/j.ocemod.2015.07.009. Abstract
In the framework of the second phase of the Coordinated Ocean-ice Reference Experiments (CORE-II), we present an analysis of the representation of the Antarctic Circumpolar Current (ACC) and Southern Ocean Meridional Overturning Circulation (MOC) in a suite of seventeen global ocean-sea ice models. We focus on the mean, variability and trends of both the ACC and MOC over the 1958–2007 period, and discuss their relationship with the surface forcing. We aim to quantify the degree of eddy saturation and eddy compensation in the models participating in CORE-II, and compare our results with available observations, previous fine-resolution numerical studies and theoretical constraints. Most models show weak ACC transport sensitivity to changes in forcing during the past five decades, and they can be considered to be in an eddy saturated regime. Larger contrasts arise when considering MOC trends, with a majority of models exhibiting significant strengthening of the MOC during the late 20th and early 21st century. Only a few models show a relatively small sensitivity to forcing changes, responding with an intensified eddy-induced circulation that provides some degree of eddy compensation, while still showing considerable decadal trends. Both ACC and MOC interannual variability are largely controlled by the Southern Annular Mode (SAM). Based on these results, models are clustered into two groups. Models with constant or two-dimensional (horizontal) specification of the eddy-induced advection coefficient κ show larger ocean interior decadal trends, larger ACC transport decadal trends and no eddy compensation in the MOC. Eddy-permitting models or models with a three-dimensional time varying κ show smaller changes in isopycnal slopes and associated ACC trends, and partial eddy compensation. As previously argued, a constant in time or space κ is responsible for a poor representation of mesoscale eddy effects and cannot properly simulate the sensitivity of the ACC and MOC to changing surface forcing. Evidence is given for a larger sensitivity of the MOC as compared to the ACC transport, even when approaching eddy saturation. Future process studies designed for disentangling the role of momentum and buoyancy forcing in driving the ACC and MOC are proposed.
Simulation characteristics from eighteen global ocean–sea-ice coupled models are presented with a focus on the mean Atlantic meridional overturning circulation (AMOC) and other related fields in the North Atlantic. These experiments use inter-annually varying atmospheric forcing data sets for the 60-year period from 1948 to 2007 and are performed as contributions to the second phase of the Coordinated Ocean-ice Reference Experiments (CORE-II). The protocol for conducting such CORE-II experiments is summarized. Despite using the same atmospheric forcing, the solutions show significant differences. As most models also differ from available observations, biases in the Labrador Sea region in upper-ocean potential temperature and salinity distributions, mixed layer depths, and sea-ice cover are identified as contributors to differences in AMOC. These differences in the solutions do not suggest an obvious grouping of the models based on their ocean model lineage, their vertical coordinate representations, or surface salinity restoring strengths. Thus, the solution differences among the models are attributed primarily to use of different subgrid scale parameterizations and parameter choices as well as to differences in vertical and horizontal grid resolutions in the ocean models. Use of a wide variety of sea-ice models with diverse snow and sea-ice albedo treatments also contributes to these differences. Based on the diagnostics considered, the majority of the models appear suitable for use in studies involving the North Atlantic, but some models require dedicated development effort.
We provide an assessment of sea level simulated in a suite of global ocean-sea ice models using the interannual CORE atmospheric state to determine surface ocean boundary buoyancy and momentum fluxes. These CORE-II simulations are compared amongst themselves as well as to observation-based estimates. We focus on the final 15 years of the simulations (1993-2007), as this is a period where the CORE-II atmospheric state is well sampled, and it allows us to compare sea level related fields to both satellite and in situ analyses. The ensemble mean of the CORE-II simulations broadly agree with various global and regional observation-based analyses during this period, though with the global mean thermosteric sea level rise biased low relative to observation-based analyses. The simulations reveal a positive trend in dynamic sea level in the west Pacific and negative trend in the east, with this trend arising from wind shifts and regional changes in upper 700 m ocean heat content. The models also exhibit a thermosteric sea level rise in the subpolar North Atlantic associated with a transition around 1995/1996 of the North Atlantic Oscillation to its negative phase, and the advection of warm subtropical waters into the subpolar gyre. Sea level trends are predominantly associated with steric trends, with thermosteric effects generally far larger than halosteric effects, except in the Arctic and North Atlantic. There is a general anti-correlation between thermosteric and halosteric effects for much of the World Ocean, associated with density compensated changes.
Despite slow rates of ocean mixing, observational and modeling studies suggest that buoyancy is redistributed to all depths of the ocean on surprisingly short interannual to decadal time scales. The mechanisms responsible for this redistribution remain poorly understood. This work uses an Earth System Model to evaluate the global steady state ocean buoyancy (and related steric sea level) budget, its interannual variability, and its transient response to a doubling of CO2 over 70 years, with a focus on the deep ocean. At steady state, the simple view of vertical advective-diffusive balance for the deep ocean holds at low- to mid-latitudes. At higher latitudes, the balance depends on a myriad of additional terms, namely mesoscale and submesoscale advection, convection and overflows from marginal seas, and terms related to the nonlinear equation of state. These high-latitude processes rapidly communicate anomalies in surface buoyancy forcing to the deep ocean locally; the deep, high-latitude changes then influence the large-scale advection of buoyancy to create transient deep buoyancy anomalies at lower latitudes. Following a doubling of atmospheric carbon dioxide concentrations, the high latitude buoyancy sinks are suppressed by a slowdown in convection and reduced dense water formation. This change is accompanied by a slowing of both upper and lower cells of the global meridional overturning circulation, reducing the supply of dense water to low latitudes beneath the pycnocline and the commensurate flow of light waters to high latitudes above the pycnocline. By this mechanism, changes in high latitude buoyancy are communicated to the global deep ocean on relatively fast advective timescales.
The influence of changing ocean currents on climate change is evaluated by comparing an earth system model’s response to increased CO2 with and without an ocean circulation response. Inhibiting the ocean circulation response, by specifying a seasonally-varying preindustrial climatology of currents, has a much larger influence on the heat storage pattern than on the carbon storage pattern. The heat storage pattern without circulation changes resembles carbon storage (either with or without circulation changes) more than it resembles the heat storage when currents are allowed to respond. This is shown to be due to the larger magnitude of the redistribution transport – the change in transport due to circulation anomalies acting on control climate gradients – for heat than for carbon. The net ocean heat and carbon uptake are slightly reduced when currents are allowed to respond. Hence, ocean circulation changes potentially act to warm the surface climate. However, the impact of the reduced carbon uptake on radiative forcing is estimated to be small while the redistribution heat transport shifts ocean heat uptake from low to high latitudes increasing its cooling power. Consequently, global surface warming is significantly reduced by circulation changes. Circulation changes also shift the pattern of warming from broad northern hemisphere amplification to a more structured pattern with reduced warming at subpolar latitudes in both hemispheres and enhanced warming near the equator.
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.
A parameterization for the restratification by finite-amplitude, submesoscale, mixed layer eddies, formulated as an overturning streamfunction, has been recently proposed to approximate eddy fluxes of density and other tracers. Here, the technicalities of implementing the parameterization in the coarse-resolution ocean component of global climate models are made explicit, and the primary impacts on model solutions of implementing the parameterization are discussed. Three global ocean general circulation models including this parameterization are contrasted with control simulations lacking the parameterization. The MLE parameterization behaves as expected and fairly consistently in models differing in discretization, boundary layer mixing, resolution, and other parameterizations. The primary impact of the parameterization is a shoaling of the mixed layer, with the largest effect in polar winter regions. Secondary impacts include strengthening the Atlantic meridional overturning while reducing its variability, reducing CFC and tracer ventilation, modest changes to sea surface temperature and air-sea fluxes, and an apparent reduction of sea ice basal melting.
This paper documents time mean simulation characteristics from the ocean and sea ice components in a new coupled climate model developed at NOAA's Geophysical Fluid Dynamics Laboratory (GFDL). The climate model, known as CM3, is formulated with effectively the same ocean and sea ice components as the earlier GFDL climate model, CM2.1, yet with extensive developments made to the atmosphere and land model components. Both CM2.1 and CM3 show stable mean climate indices, such as large scale circulation and sea surface temperatures (SSTs). There are notable improvements in the CM3 climate simulation relative to CM2.1, including a modified SST bias pattern and reduced biases in the Arctic sea ice cover. We anticipate SST differences between CM2.1 and CM3 in lower latitudes through analysis of the atmospheric fluxes at the ocean surface in corresponding Atmospheric Model Intercomparison Project (AMIP) simulations. In contrast, SST changes in the high latitudes are dominated by ocean and sea ice effects absent in AMIP simulations. The ocean interior simulation in CM3 is generally warmer than CM2.1, which adversely impacts the interior biases.
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.
We overview problems and prospects in ocean circulation models, with emphasis on certain developments aiming to
enhance the physical integrity and flexibility of large-scale models used to study global climate. We also consider elements
of observational measures rendering information to help evaluate simulations and to guide development priorities.
http://www.oceanobs09.net/blog/?p=88
Coordinated Ocean-ice Reference Experiments (COREs) are presented as a tool to explore the behaviour of global ocean-ice models under forcing from a common atmospheric dataset. We highlight issues arising when designing coupled global ocean and sea ice experiments, such as difficulties formulating a consistent forcing methodology and experimental protocol. Particular focus is given to the hydrological forcing, the details of which are key to realizing simulations with stable meridional overturning circulations.
The atmospheric forcing from [Large, W., Yeager, S., 2004. Diurnal to decadal global forcing for ocean and sea-ice models: the data sets and flux climatologies. NCAR Technical Note: NCAR/TN-460+STR. CGD Division of the National Center for Atmospheric Research] was developed for coupled-ocean and sea ice models. We found it to be suitable for our purposes, even though its evaluation originally focussed more on the ocean than on the sea-ice. Simulations with this atmospheric forcing are presented from seven global ocean-ice models using the CORE-I design (repeating annual cycle of atmospheric forcing for 500 years). These simulations test the hypothesis that global ocean-ice models run under the same atmospheric state produce qualitatively similar simulations. The validity of this hypothesis is shown to depend on the chosen diagnostic. The CORE simulations provide feedback to the fidelity of the atmospheric forcing and model configuration, with identification of biases promoting avenues for forcing dataset and/or model development.
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.
In
many global ocean climate models, mesoscale eddies are parameterized as
along isopycnal diffusion and eddy-induced advection (or equivalently
skew-diffusion). The eddy-induced advection flattens isopycnals and acts as
a sink of available potential energy, whereas the isopycnal diffusion mixes
tracers along neutral directions. While much effort has gone into estimating
diffusivities associated with this closure, less attention has been paid to
the details of how this closure (which tries to flatten isopycnals)
interacts with the mixed layer (in which vertical mixing tries to drive the
isopycnals vertical). In order to maintain numerical stability, models often
stipulate a maximum slope Smax which in combination with
the thickness diffusivity Agm defines a maximum
eddy-induced advective transport Agm*Smax.
In this paper, we examine the impact of changing Smax
within the GFDL global coupled climate model. We show that this parameter
produces significant changes in wintertime mixed layer depth, with
implications for wintertime temperatures in key regions, the distribution of
precipitation, and the vertical structure of heat uptake. Smaller changes
are seen in details of ventilation and currents, and even smaller changes as
regards the overall hydrography. The results suggest that not only the value
of the coefficient, but the details of the tapering scheme, need to be
considered when comparing isopycnal mixing schemes in models.
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.
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.
The impact of changes in shortwave radiation penetration depth on the global ocean circulation and heat transport is studied using the GFDL Modular Ocean Model (MOM4) with two independent parameterizations that use ocean color to estimate the penetration depth of shortwave radiation. Ten to eighteen percent increases in the depth of 1% downwelling surface irradiance levels results in an increase in mixed layer depths of 3-20 m in the subtropical and tropical regions with no change at higher latitudes. While 1D models have predicted that sea surface temperatures at the equator would decrease with deeper penetration of solar irradiance, this study shows a warming, resulting in a 10% decrease in the required restoring heat flux needed to maintain climatological sea surface temperatures in the eastern equatorial Atlantic and Pacific Oceans. The decrease in the restoring heat flux is attributed to a slowdown in heat transport (5%) from the Tropics and an increase in the temperature of submixed layer waters being transported into the equatorial regions. Calculations were made using a simple relationship between mixed layer depth and meridional mass transport. When compared with model diagnostics, these calculations suggest that the slowdown in heat transport is primarily due to off-equatorial increases in mixed layer depths. At higher latitudes (5°-40°), higher restoring heat fluxes are needed to maintain sea surface temperatures because of deeper mixed layers and an increase in storage of heat below the mixed layer. This study offers a way to evaluate the changes in irradiance penetration depths in coupled ocean-atmosphere GCMs and the potential effect that large-scale changes in chlorophyll a concentrations will have on ocean circulation.
Gnanadesikan, Anand, Richard D Slater, and Bonita L Samuels, September 2003: Sensitivity of water mass transformation and heat transport to subgridscale mixing in coarse-resolution ocean models. Geophysical Research Letters, 30(18), 1967, DOI:10.1029/2003GL018036. Abstract
This paper considers the impact of the parameterization of subgridscale mixing on ocean heat transport in coarse-resolution ocean models of the type used in coupled climate models. Increasing the vertical diffusion increases poleward heat transport in both hemispheres. Increasing lateral diffusion associated with transient eddies increases poleward heat transport in the southern hemisphere while decreasing it in the northern hemisphere. The results are interpreted in the context of a simple analytical model.
Toggweiler, J R., and Bonita L Samuels, 1998: On the ocean's large-scale circulation near the limit of no vertical mixing. Journal of Physical Oceanography, 28(9), 1832-1852. Abstract PDF
By convention, the ocean's large-scale circulation is assumed to be a thermohaline overturning driven by the addition and extraction of buoyancy at the surface and vertical mixing in the interior. Previous work suggests that the overturning should die out as vertical mixing rates are reduced to zero. In this paper, a formal energy analysis is applied to a series of ocean general circulation models to evaluate changes in the large-scale circulation over a range of vertical mixing rates. Two different model configurations are used. One has an open zonal channel and an Antarctic Circumpolar Current (ACC). The other configuration does not. The authors find that a vigorous large-scale circulation persists at the limit of no mixing in the model with a wind-driven ACC. A wind-powered overturning circulation linked to the ACC can exist without vertical mixing and without much energy input from surface buoyancy forces.
The comment by Rahmstorf suggests that a numerical problem in Tziperman et al. (1994, TTFB) leads to a noisy E - P field that invalidates TTFB's conclusions. The authors eliminate the noise, caused by the Fourier filtering used in the model, and show that TTFB's conclusions are still valid. Rahmstorf questions whether a critical value in the freshwater forcing separates TTFB's stable and unstable runs. By TTFB's original definition, the unstable runs in both TTFB and in Rahmstorf's comment have most definitely crossed a stability transition point upon switching to mixed boundary conditions. Rahmstorf finally suggests that the instability mechanism active in TTFB is a fast convective mechanism, not the slow advective mechanism proposed in TTFB. The authors show that the timescale of the instability is, in fact, consistent with the advective mechanism
The Ekman divergence around Antarctica raises a large amount of deep water to the ocean's surface. The regional Ekman transport moves the upwelled deep water northward out of the circumpolar zone. The divergence and northward surface drift combine, in effect, to remove deep water from the interior of the ocean. This wind-driven removal process is facilitated by a unique dynamic constraint operating in the latitude band containing Drake Passage. Through a simple model sensitivity experiment we show that the upwelling and removal of deep water in the circumpolar belt may be quantitatively related to the formation of new deep water in the northern North Atlantic. These results show that stronger winds in the south can induce more deep water formation in the north and more deep outflow through the South Atlantic. The fact that winds in the southern hemisphere might influence the formation of deep water in the North Atlantic brings into question long-standing notions about the forces that drive the ocean's thermohaline circulation.
Brine rejection during the formation of Antarctic sea ice is known to enhance the salinity of dense shelf waters in the Weddell and Ross Seas. As these shelf waters flow off the shelves and descend to the bottom, they entrain ambient deep water to create new bottom water. It is not uncommon for ocean modelers to modify salinity boundary conditions around Antarctica in an attempt to include a "sea ice effect" in their models. However, the degree to which Antarctic salinities are enhanced is usually not quantified or defended.
In this paper, studies of shelf hydrography and delta18O are reviewed to assess the level of salinity enhancement appropriate for ocean general circulation models. The relevant quantities are 1) the salinity difference between the water masses modified on the shelves and the final offshelf flow and 2) the flux of salt (or freshwater) that gives rise to this salinity difference. Onshelf/offshelf salinity changes in the Weddell and Ross Seas appear to be fairly small, 0.15-0.20 salinity units. The quantity of brine needed to produce this salinification is equivalent to the salt drained from <0.50 m of new sea ice every year.
Salt fluxes and salinity distributions from three GCM simulations are then compared. The first model has its surface salinities simply restored to the Levitus observations. Levitus restoring produces a slight freshening in the area of the Weddell and Ross Sea shelves. The global-mean bottom-water salinity in this model is 34.57 psu, which 0.16 units less than observed. The second model includes a very modest salinity enhancement in the area of the Weddell and Ross Sea shelves. This produces a salt flux equivalent to the formation of ~ 0.50 m yr-1 of new sea ice. Even though this amount of salt input is close to the amount observed, global-average deep salinities in the second model are only 0.02 units greater than the deep salinities in the first model. The third model includes a large salinity enrichment, which is applied throughout the Weddell and Ross embayments without regard to water depth. Its deep salinities are 0.18 units higher than the deep salinities in the first model, but the amount of salt pumped into the model greatly exceeds the salt flux in nature.
The authors conclude that salt from sea ice is probably not a major influence on the salinity of Antarctic bottom waters. Predicted salinities in ocean GCMs are too fresh because of circulation deficiencies, not because of inadequate boundary conditions. Models that employ large salinity modifications near Antarctica run the risk of grossly distorting the processes of deep-water formation.
Toggweiler, J R., and Bonita L Samuels, 1993: New radiocarbon constraints on the upwelling of abyssal water to the ocean's surface In The Global Carbon Cycle, NATO ASI Series - Vol. 115. Heidelberg, Germany, Springer-Verlag, 333-366. Abstract
The output from seven different ocean model simulations is compared on the basis of the Δ 14C difference between North Pacific deep water and Antarctic surface water. This set of models produces a range of North Pacific-Antarctic Δ 14C differences between -173% and -108%, all but the smallest of which are substantially larger than the actual pre-bomb difference, -80 to -110%. Predicted values are highly correlated with the quantity of mid-depth water which flows out of the Pacific to the south. A circulation in which most of the Antarctic bottom water flows back out of the basin at mid-depth produces the smallest North Pacific-Antarctic Δ 14C differences, whereas a circulation in which all the inflow of bottom water upwells through the thermocline produces the largest and least realistic differences. According to the models, upwelled abyssal water becomes entrained into the wind-driven convergence of thermocline water toward the equator. When it reaches the surface it spreads to the north and south, producing a Δ 14C minimum along the equator. A detailed analysis of both pre-bomb and post-bomb Δ 14C data indicates that the oldest water in the tropical Pacific is actually found south of the equator and is associated with the upwelling off Peru, not the upwelling along the equator. Toggweiler et al. (1991) trace the low-Δ 14C signal in the Peru upwelling to deep water raised to the surface around Antarctica which is pushed northward into the thermocline by circumpolar winds. According to the models, even a small amount of abyssal water upwelling through the thermocline (~3x106 m3 s-1) leaves a characteristic signal in the surface Δ 14C distribution which is not observed. One is left with the general conclusion that there is very little upwelling associated with a top-to-bottom thermohaline circulation in the world ocean. Virtually all the upwelling of abyssal water to the ocean's surface occurs around Antarctica where it is mainly wind-forced. The implications of this conclusion for the carbon cycle are discussed.
Toggweiler, J R., and Bonita L Samuels, 1993: Is the magnitude of the deep outflow from the Atlantic Ocean actually governed by Southern Hemisphere winds? In The Global Carbon Cycle, NATO ASI Series - Vol. 115. Heidelberg, Germany, Springer-Verlag, 303-331. Abstract
The large-scale overturning in the Atlantic Ocean and its export of salty water to the other basins of the ocean is usually thought of as a thermohaline process driven by the formation of dense bottom water in the isolated basins of the North Atlantic. In this paper the output from several different runs of a global ocean GCM is used to show that the inflow of upper kilometer water in the South Atlantic and the outflow of deep water varies in direct proportion to the westerly wind stress in the circumpolar region of the southern hemisphere. According to the results presented here, the production of dense bottom water in the North Atlantic makes it possible for an Atlantic overturning to exist, but southern hemisphere winds appear to determine the magnitude of the inflow and outflow. The connection between southern hemisphere winds and the Atlantic overturning is due to a unique dynamic constraint which operates in the latitude band of Drake Passage. This constraint suggests the possibility of a very simple relationship between the magnitude of the northward wind drift at the latitude of the tip of South America and the magnitude of the inflow and outflow from the Atlantic basin.
