Climate models of varying complexity have been used for decades to investigate the impact of mountains on the atmosphere and surface climate. Here, the impact of removing the continental topography on the present-day ocean climate is investigated using three different climate models spanning multiple generations. An idealized study is performed where all present-day land surface topography is removed and the equilibrium change in the oceanic mean state with and without the mountains is studied. When the mountains are removed, changes found in all three models include a weakening of the Atlantic meridional overturning circulation and associated SST cooling in the subpolar North Atlantic. The SSTs also warm in all the models in the western North Pacific Ocean associated with a northward shift of the atmospheric jet and the Kuroshio. In the ocean interior, the magnitude of the temperature and salinity response to removing the mountains is relatively small and the sign and magnitude of the changes generally vary among the models. These different interior ocean responses are likely related to differences in the mean state of the control integrations due to differences in resolution and associated subgrid-scale mixing parameterizations. Compared to the results from 4xCO2 simulations, the interior ocean temperature changes caused by mountain removal are relatively small; however, the oceanic circulation response and Northern Hemisphere near-surface temperature changes are of a similar magnitude to the response to such radiative forcing changes.
Manabe, Syukuro, and Anthony J Broccoli, January 2020: Beyond Global Warming: How Numerical Models Revealed the Secrets of Climate Change, Princeton, NJ: Princeton University Press, 193pp.
Catalano, A J., Anthony J Broccoli, Sarah B Kapnick, and Tyler P Janoski, April 2019: High-Impact Extratropical Cyclones along the Northeast Coast of the United States in a Long Coupled Climate Model Simulation. Journal of Climate, 32(7), DOI:10.1175/JCLI-D-18-0376.1. Abstract
High-impact extratropical cyclones (ETCs) cause considerable damage along the Northeast coast of the United States through strong winds and inundation, but these relatively rare events are difficult to analyze owing to limited historical records. Using a 1505-year simulation from the GFDL FLOR coupled model, statistical analyses of extreme events are performed including exceedance probability computations to compare estimates from shorter segments to estimates that could be obtained from a record of considerable length. The most extreme events possess characteristics including exceptionally low central pressure, hurricane-force winds, and a large surge potential, which would greatly impact nearby regions. Return level estimates of metrics of ETC intensity using shorter, historical-length segments of the FLOR simulation are underestimated compared to levels determined using the full simulation. This indicates that if the underlying distributions of observed ETC metrics are similar to those of the 1505-year FLOR distributions, the actual frequency of extreme ETC events could also be underestimated.
Comparisons between FLOR and reanalysis products suggest that not all features of simulated high-impact ETCs are representative of observations. Spatial track densities are similar, but FLOR exhibits a negative bias in central pressure and a positive bias in wind speed, particularly for more intense events. Although the existence of these model biases precludes the quantitative use of model-derived return statistics as a substitute for those derived from shorter observational records, this work suggests that statistics from future models of higher fidelity could be used to better constrain the probability of extreme ETC events and their impacts.
Eastern North America contains densely populated, highly developed areas, making winter storms with strong winds and high snowfall among the costliest storm types. For this reason, it is important to determine how the frequency of high-impact winter storms, specifically those combining significant snowfall and winds, will change in this region under increasing greenhouse gas concentrations. This study uses a high-resolution coupled global climate model to simulate the changes in extreme winter conditions from the present climate to a future scenario with doubled-CO2 concentrations (2XC). In particular, this study focuses on changes in high snowfall, extreme wind (HSEW) events, which are defined as the occurrence of two-day snowfall and high winds exceeding thresholds based on extreme values from the control simulation where greenhouse gas concentrations remain fixed. Mean snowfall consistently decreases across the entire region, but extreme snowfall shows a more inconsistent pattern with some areas experiencing increases in the frequency of extreme snowfall events. Extreme wind events show relatively small changes in frequency with 2XC, with the exception of high-elevation areas where there are large decreases in frequency. As a result of combined changes in wind and snowfall, HSEW events decrease in frequency in the 2XC simulation for much of the eastern North America. Changes in the number of HSEW events in the 2XC environment are driven mainly by changes in the frequency of extreme snowfall events, with most of the region experiencing decreases in event frequency, except for certain inland areas at higher latitudes.
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.
The response of the equatorial Pacific Ocean’s seasonal cycle to orbital forcing is explored using idealized simulations with a coupled atmosphere-ocean GCM, in which eccentricity, obliquity, and longitude of the perihelion are altered while other boundary conditions are maintained at preindustrial levels. The importance of ocean dynamics in the climate response is investigated using additional simulations with a slab ocean version of the model. Precession is found to substantially influence the equatorial Pacific seasonal cycle through both thermodynamic and dynamic mechanisms while changes in obliquity have only a small effect. In the precession experiments, western equatorial Pacific SSTs respond in a direct thermodynamic manner to changes in insolation, while the eastern equatorial Pacific is first affected by the propagation of thermocline temperature anomalies from the west. These thermocline signals result from zonal wind anomalies associated with changes in the strength of subtropical anticyclones and shifts in the regions of convection in the western equatorial Pacific. The redistribution of heat from these thermocline signals, aided by the direct thermodynamic effect of insolation anomalies, results in large changes to the strength and timing of the eastern equatorial Pacific seasonal cycle. A comparison of 10 CMIP5 mid-Holocene experiments, in which the primary forcing is due to precession, shows that this response is relatively robust across models. Because equatorial Pacific SST anomalies have local climate impacts as well as non-local impacts through teleconnections, these results may be important to understanding paleoclimate variations both inside and outside of the tropical Pacific.
Using simulations performed with 18 coupled atmosphere-ocean global climate models from the CMIP5 project, projections of Northern Hemisphere snowfall under the RCP4.5 scenario are analyzed for the period 2006-2100. These models perform well in simulating 20th century snowfall, although there is a positive bias in many regions. Annual snowfall is projected to decrease across much of the Northern Hemisphere during the 21st century, with increases projected at higher latitudes. On a seasonal basis, the transition zone between negative and positive snowfall trends corresponds approximately to the -10 °C isotherm of the late 20th century mean surface air temperature such that positive trends prevail in winter over large regions of Eurasia and North America. Redistributions of snowfall throughout the entire snow season are projected to occur – even in locations where there is little change in annual snowfall. Changes in the fraction of precipitation falling as snow contribute to decreases in snowfall across most Northern Hemisphere regions, while changes in total precipitation typically contribute to increases in snowfall. A signal-to-noise analysis reveals that the projected changes in snowfall, based on the RCP4.5 scenario, are likely to become apparent during the 21st century for most locations in the Northern Hemisphere. The snowfall signal emerges more slowly than the temperature signal, suggesting that changes in snowfall are not likely to be early indicators of regional climate change.
Previdi, M, B G Liepert, D Peteet, James A Hansen, D J Beerling, Anthony J Broccoli, S Frolking, J Galloway, M Heimann, C Le Quéré, S Levitus, and V Ramaswamy, July 2013: Climate Sensitivity in the Anthropocene. Quarterly Journal of the Royal Meteorological Society, 139(674), DOI:10.1002/qj.2165. Abstract
Climate sensitivity in its most basic form is defined as the equilibrium change in global surface temperature that occurs in response to a climate forcing, or externally imposed perturbation of the planetary energy balance. Within this general definition, several specific forms of climate sensitivity exist that differ in terms of the types of climate feedbacks they include. Based on evidence from Earth’s history, we suggest here that the relevant form of climate sensitivity in the Anthropocene (e.g., from which to base future greenhouse gas (GHG) stabilization targets) is the Earth system sensitivity including fast feedbacks from changes in water vapor, natural aerosols, clouds and sea ice, slower surface albedo feedbacks from changes in continental ice sheets and vegetation, and climate-GHG feedbacks from changes in natural (land and ocean) carbon sinks. Traditionally, only fast feedbacks have been considered (with the other feedbacks either ignored or treated as forcing), which has led to estimates of the climate sensitivity for doubled CO2 concentrations of about 3°C. The 2×CO2 Earth system sensitivity is higher than this, being ~ 4-6°C if the ice sheet/vegetation albedo feedback is included in addition to the fast feedbacks, and higher still if climate-GHG feedbacks are also included. The inclusion of climate-GHG feedbacks due to changes in the natural carbon sinks has the advantage of more directly linking anthropogenic GHG emissions with the ensuing global temperature increase, thus providing a truer indication of the climate sensitivity to human perturbations. The Earth system climate sensitivity is difficult to quantify due to the lack of paleo-analogues for the present-day anthropogenic forcing, and the fact that ice sheet and climate-GHG feedbacks have yet to become
globally significant in the Anthropocene. Furthermore, current models are unable to adequately simulate the physics of ice sheet decay and certain aspects of the natural carbon and nitrogen cycles. Obtaining quantitative estimates of the Earth system sensitivity is therefore a high priority for future work.
The response of the Walker circulation to Last Glacial Maximum (LGM) forcing
is analyzed using an ensemble of six coordinated coupled climate model experiments.
The tropical atmospheric overturning circulation strengthens in all models in a manner
that is dictated by the response of the hydrological cycle to tropical cooling. This
response arises from the same mechanism that has been found to explain the weakening
of the tropical circulation in response to anthropogenic global warming, but with opposite
sign. Analysis of the model differences shows that the ascending branch of the Walker
circulation strengthens via this mechanism, but vertical motion also weakens over areas
of the Maritime Continent exposed due to lower sea level. Each model exhibits a
different balance between these two mechanisms, and the result is a Pacific Walker
circulation response that is not robust. Further, even those models that simulate a stronger
Walker circulation during the LGM do not simulate clear patterns of surface cooling,
such as La Niña-like cooling or enhanced equatorial cooling, as proposed by previous
studies. In contrast, the changes in the Walker circulation have a robust and distinctive
signature on the tilt of the equatorial thermocline, as expected from zonal momentum
balance. The changes in the Walker circulation also have a clear signature on the spatial
pattern of the precipitation changes. A reduction of the east-west salinity contrast in the
Indian Ocean is related to the precipitation changes resulting from a weakening of the
Indian Walker circulation. These results indicate that proxies of thermocline depth and
sea surface salinity can be used to detect actual LGM changes in the Pacific and Indian
Walker circulations, respectively and help constrain the sensitivity of the Walker
circulation to tropical cooling.
Understanding the plausible causes for the observed high dust concentrations in Antarctic ice cores during
the Last Glacial Maximum (LGM) is crucial for interpreting the Antarctic dust records in the past climates
and could provide insights into dust variability in future climates. Using the Geophysical Fluid Dynamics
Laboratory (GFDL) General Circulation Models, we conduct an investigation into the various factors
modulating dust emission, transport and deposition, with a view towards an improved quantification of the
LGM dust enhancements in the Antarctic ice cores. The model simulations show that the expansion of
source areas and changes in the Antarctic ice accumulation rates together can account for most of the
observed increase of dust concentrations in the Vostok, Dome C and Taylor Dome cores, but there is an
overestimate of the LGM-to-present ratio in the case of the Byrd core. The source expansion due to the
lowering of sea level yields a factor of 2–3 higher contribution than that due to the reduction of continental
vegetation. The changes in other climate parameters (e.g., SH precipitation change) are estimated to be
relatively less important within the context of this sensitivity study, while the model-simulated LGM
surface winds yield a 20–30 % reduction rather than an increase in dust deposition in Antarctica. This
research yields insights towards a fundamental understanding of the causes for the significant enhancement
of the dust deposition in the Antarctic ice cores during the LGM.
Came, R E., W B Curry, D W Oppo, Anthony J Broccoli, Ronald J Stouffer, and J Lynch-Stieglitz, 2007: North Atlantic intermediate depth variability during the Younger Dryas: Evidence from Benthic Foraminiferal Mg/Ca and the GFDL R30 Coupled Climate Model In Ocean Circulation: Mechanisms and Impacts, Geophysical Monograph Series 173, Washington, DC, American Geophysical Union, 247-263. Abstract
Two new records of paired benthic foraminiferal Mg/Ca and 18 O from two low latitude western Atlantic sediment cores—one taken from within the Florida Current and the other from the Little Bahama Bank — provide insights into the spatial distribution of intermediate depth temperature and salinity variability during the deglaciation. During the Younger Dryas cold event, both temperature and salinity increased at the Florida Current site and decreased at the Little Bahama Bank site. The temperature increase within the Florida Current is consistent with a reduction in the strength of the northward-moving surface return flow of the Atlantic meridional overturning circulation; the temperature decrease at the Little Bahama Bank is consistent with a cooling of high latitude North Atlantic surface waters. To test the possibility that a freshening of the surface North Atlantic caused the paleoceanographic changes during the Younger Dryas, the Geophysical Fluid Dynamics Laboratory (GFDL) R30 coupled ocean-atmosphere general circulation model was forced using a North Atlantic freshwater perturbation of 0.1 Sv for a period of 100 years. The freshwater flux causes an overall reduction in the Atlantic overturning from 25 Sv to 13 Sv. However, at ~1,100 m water depth, ventilation increases, causing decreases in both temperature and salinity throughout much of the intermediate depth, open-ocean North Atlantic. At the western boundary, intermediate depth temperatures and salinities increase due to weakened overturning, and also due to an increase in runoff from the Amazon River, which causes a surface stability and a decrease in the upwelling of colder, deeper waters.
Climate simulations, using models with different levels of complexity, indicate that the north-south position of the intertropical convergence zone (ITCZ) responds to changes in interhemispheric temperature contrast. Paleoclimate data on a variety of timescales suggest a similar behavior, with southward displacements of the ITCZ and associated changes in tropical atmospheric circulation during cold periods in the Northern Hemisphere. To identify a mechanism by which ITCZ displacements can be forced from the extratropics, we use a climate model with idealized geography and a simple slab ocean. We cool the northern extratropics and warm the southern extratropics to represent the asymmetric temperature changes associated with glacial-interglacial and millennial-scale climate variability. A southward shift in the ITCZ occurs, along with changes in the trade winds and an asymmetric response of the Hadley circulation. Changes in atmospheric heat exchange between the tropics and midlatitudes are the likely cause of this response, suggesting that this mechanism may play an important role in ITCZ displacements on timescales from decadal to glacial-interglacial.
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
Hewitt, C, Anthony J Broccoli, M Crucifix, Jonathan M Gregory, J F B Mitchell, and Ronald J Stouffer, 2006: The effect of a large freshwater perturbation on the Glacial North Atlantic Ocean using a Coupled General Circulation Model. Journal of Climate, 19(17), DOI:10.1175/JCLI3867.1. Abstract
The commonly held view of the condition in the North Atlantic at the last glacial maximum, based on the interpretation of proxy records, is of large-scale cooling compared to today, limited deep convection, and extensive sea ice, all associated with a southward displaced and weakened overturning thermohaline circulation (THC) in the North Atlantic. Not all studies support that view; in particular, the "strength of the overturning circulation" is contentious and is a quantity that is difficult to determine even for the present day. Quasi-equilibrium simulations with coupled climate models forced by glacial boundary conditions have produced differing results, as have inferences made from proxy records. Most studies suggest the weaker circulation, some suggest little or no change, and a few suggest a stronger circulation.
Here results are presented from a three-dimensional climate model, the Hadley Centre Coupled Model version 3 (HadCM3), of the coupled atmosphere–ocean–sea ice system suggesting, in a qualitative sense, that these diverging views could all have occurred at different times during the last glacial period, with different modes existing at different times. One mode might have been characterized by an active THC associated with moderate temperatures in the North Atlantic and a modest expanse of sea ice. The other mode, perhaps forced by large inputs of meltwater from the continental ice sheets into the northern North Atlantic, might have been characterized by a sluggish THC associated with very cold conditions around the North Atlantic and a large areal cover of sea ice. The authors' model simulation of such a mode, forced by a large input of freshwater, bears several of the characteristics of the Climate: Long-range Investigation, Mapping, and Prediction (CLIMAP) Project's reconstruction of glacial sea surface temperature and sea ice extent.
The climate response to idealized changes in the atmospheric CO2 concentration by the new GFDL climate model (CM2) is documented. This new model is very different from earlier GFDL models in its parameterizations of subgrid-scale physical processes, numerical algorithms, and resolution. The model was constructed to be useful for both seasonal-to-interannual predictions and climate change research. Unlike previous versions of the global coupled GFDL climate models, CM2 does not use flux adjustments to maintain a stable control climate. Results from two model versions, Climate Model versions 2.0 (CM2.0) and 2.1 (CM2.1), are presented.
Two atmosphere–mixed layer ocean or slab models, Slab Model versions 2.0 (SM2.0) and 2.1 (SM2.1), are constructed corresponding to CM2.0 and CM2.1. Using the SM2 models to estimate the climate sensitivity, it is found that the equilibrium globally averaged surface air temperature increases 2.9 (SM2.0) and 3.4 K (SM2.1) for a doubling of the atmospheric CO2 concentration. When forced by a 1% per year CO2 increase, the surface air temperature difference around the time of CO2 doubling [transient climate response (TCR)] is about 1.6 K for both coupled model versions (CM2.0 and CM2.1). The simulated warming is near the median of the responses documented for the climate models used in the 2001 Intergovernmental Panel on Climate Change (IPCC) Working Group I Third Assessment Report (TAR).
The thermohaline circulation (THC) weakened in response to increasing atmospheric CO2. By the time of CO2 doubling, the weakening in CM2.1 is larger than that found in CM2.0: 7 and 4 Sv (1 Sv 106 m3 s−1), respectively. However, the THC in the control integration of CM2.1 is stronger than in CM2.0, so that the percentage change in the THC between the two versions is more similar. The average THC change for the models presented in the TAR is about 3 or 4 Sv; however, the range across the model results is very large, varying from a slight increase (+2 Sv) to a large decrease (−10 Sv).
This study analyzes a three-member ensemble of experiments, in which 0.1 Sv of freshwater was applied to the North Atlantic for 100 years in order to address the potential for large freshwater inputs in the North Atlantic to drive abrupt climate change. The model used is the GFDL R30 coupled ocean–atmosphere general circulation model. We focus in particular on the effects of this forcing on the tropical Atlantic region, which has been studied extensively by paleoclimatologists. In response to the freshwater forcing, North Atlantic meridional overturning circulation is reduced to roughly 40% by the end of the 100 year freshwater pulse. Consequently, the North Atlantic region cools by up to 8°C. The extreme cooling of the North Atlantic increases the pole-to-equator temperature gradient and requires more heat be provided to the high latitude Atlantic from the tropical Atlantic. To accommodate the increased heat requirement, the ITCZ shifts southward to allow for greater heat transport across the equator. Accompanying this southward ITCZ shift, the Northeast trade winds strengthen and precipitation patterns throughout the tropical Atlantic are altered. Specifically, precipitation in Northeast Brazil increases, and precipitation in Africa decreases slightly. In addition, we find that surface air temperatures warm over the tropical Atlantic and over Africa, but cool over northern South America. Sea-surface temperatures in the tropical Atlantic warm slightly with larger warm anomalies developing in the thermocline. These responses are robust for each member of the ensemble, and have now been identified by a number of freshwater forcing studies using coupled OAGCMs. The model responses to freshwater forcing are generally smaller in magnitude, but have the same direction, as paleoclimate data from the Younger Dryas suggest. In certain cases, however, the model responses and the paleoclimate data directly contradict one another. Discrepancies between the model simulations and the paleoclimate data could be due to a number of factors, including inaccuracies in the freshwater forcing, inappropriate boundary conditions, and uncertainties in the interpretation of the paleoclimate data. Despite these discrepancies, it is clear from our results that abrupt climate changes in the high latitude North Atlantic have the potential to significantly impact tropical climate. This warrants further model experimentation into the role of freshwater forcing in driving climate change.
for climate research developed at the Geophysical Fluid Dynamics Laboratory (GFDL) are presented. The atmosphere model, known as AM2, includes a new gridpoint dynamical core, a prognostic cloud scheme, and a multispecies aerosol climatology, as well as components from previous models used at GFDL. The land model, known as LM2, includes soil sensible and latent heat storage, groundwater storage, and stomatal resistance. The performance of the coupled model AM2–LM2 is evaluated with a series of prescribed sea surface temperature (SST) simulations. Particular focus is given to the model's climatology and the characteristics of interannual variability related to E1 Niño– Southern Oscillation (ENSO).
One AM2–LM2 integration was performed according to the prescriptions of the second Atmospheric Model Intercomparison Project (AMIP II) and data were submitted to the Program for Climate Model Diagnosis and Intercomparison (PCMDI). Particular strengths of AM2–LM2, as judged by comparison to other models participating in AMIP II, include its circulation and distributions of precipitation. Prominent problems of AM2– LM2 include a cold bias to surface and tropospheric temperatures, weak tropical cyclone activity, and weak tropical intraseasonal activity associated with the Madden–Julian oscillation.
An ensemble of 10 AM2–LM2 integrations with observed SSTs for the second half of the twentieth century permits a statistically reliable assessment of the model's response to ENSO. In general, AM2–LM2 produces a realistic simulation of the anomalies in tropical precipitation and extratropical circulation that are associated with ENSO.
Uncertainty in cloud feedback is the leading cause of discrepancy in model predictions of climate change. The use of observed or model-simulated radiative fluxes to diagnose the effect of clouds on climate sensitivity requires an accurate understanding of the distinction between a change in cloud radiative forcing and a cloud feedback. This study compares simulations from different versions of the GFDL Atmospheric Model 2 (AM2) that have widely varying strengths of cloud feedback to illustrate the differences between the two and highlight the potential for changes in cloud radiative forcing to be misinterpreted.
We present results from a series of ensemble integrations of a global coupled atmosphere-ocean model for the period 1865-1997. Each ensemble consists of three integrations initialized from different points in a long-running GFDL R30 coupled model control simulation. The first ensemble includes time-varying forcing from greenhouse gases only. In the remaining three ensembles, forcings from anthropogenic sulfate aerosols, solar variability, and volcanic aerosols in the stratosphere are added progressively, such that the fourth ensemble uses all four of these forcings. The effects of anthropogenic sulfate aerosols are represented by changes in surface albedo, and the effects of volcanic aerosols are represented by latitude-dependent perturbations in incident solar radiation. Comparisons with observations reveal that the addition of the natural forcings (solar and volcanic) improves the simulation of global multidecadal trends in temperature, precipitation, and ocean heat content. Solar and volcanic forcings are important contributors to early twentieth century warming. Volcanic forcing reduces the warming simulated for the late twentieth century. Interdecadal variations in global mean surface air temperature from the ensemble of experiments with all four forcings are very similar to observed variations during most of the twentieth century. The improved agreement of simulated and observed temperature trends when natural climate forcings are included supports the climatic importance of variations in radiative forcing during the twentieth century.
Hewitt, C, Ronald J Stouffer, Anthony J Broccoli, J F B Mitchell, and P Valdes, 2003: The effect of ocean dynamics in a coupled GCM simulation of the Last Glacial Maximum. Climate Dynamics, 20(2/3), 203-218. Abstract PDF
General circulation models (GCMs) of the climate system are powerful tools for understanding and predicting climate change. The last glacial maximum (LGM) provides an extreme test of the model's ability to simulate a change of climate, and allows us to increase our understanding of mechanisms of climate change. We have used a coupled high resolution ocean-atmosphere GCM (HadCM3) to simulate the equilibrium climate at the LGM. The effect of ocean dynamics is investigated by carrying out a parallel experiment replacing the dynamic three-dimensional ocean GCM with a static thermodynamic mixed-layer ocean model. Changes to the ocean circulation, and feedbacks between the ocean, atmosphere and sea ice have an important influence on the surface response, and are discussed. The coupled model produces an intensified thermohaline circulation and an increase in the amount of heat transported northward by the Atlantic Ocean equatorward of 55°N, which is at odds with the interpretation of some proxy records. Such changes, which the thermodynamic mixed-layer ocean model cannot produce, have a large impact around the North Atlantic region, and are discussed in the study.
Jackson, C S., and Anthony J Broccoli, December 2003: Orbital forcing of Arctic climate: mechanisms of climate response and implications for continental glaciation. Climate Dynamics, 21(7-8), DOI:10.1007/s00382-003-0351-3. Abstract
Progress in understanding how terrestrial ice volume is linked to Earth’s orbital configuration has been impeded by the cost of simulating climate system processes relevant to glaciation over orbital time scales (103–105 years). A compromise is usually made to represent the climate system by models that are averaged over one or more spatial dimensions or by three-dimensional models that are limited to simulating particular “snapshots” in time. We take advantage of the short equilibration time (~10 years) of a climate model consisting of a three-dimensional atmosphere coupled to a simple slab ocean to derive the equilibrium climate response to accelerated variations in Earth’s orbital configuration over the past 165,000 years. Prominent decreases in ice melt and increases in snowfall are simulated during three time intervals near 26, 73, and 117 thousand years ago (ka) when aphelion was in late spring and obliquity was low. There were also significant decreases in ice melt and increases in snowfall near 97 and 142 ka when eccentricity was relatively large, aphelion was in late spring, and obliquity was high or near its long term mean. These ”glaciation-friendly” time intervals correspond to prominent and secondary phases of terrestrial ice growth seen within the marine 18O record. Both dynamical and thermal effects contribute to the increases in snowfall during these periods, through increases in storm activity and the fraction of precipitation falling as snow. The majority of the mid- to high latitude response to orbital forcing is organized by the properties of sea ice, through its influence on radiative feedbacks that nearly double the size of the orbital forcing as well as its influence on the seasonal evolution of the latitudinal temperature gradient.
Several indices of large-scale patterns of surface temperature variation were used to investigate climate change in North America over the 20th century. The observed variability of these indices was simulated well by a number of climate models. Comparison of index trends in observations and model simulations shows that North American temperature changes from 1950 to 1999 were unlikely to be due to natural climate variation alone. Observed trends over this period are consistent with simulations that include anthropogenic forcing from increasing atmospheric greenhouse gases and sulfate aerosols. However, most of the observed warming from 1900 to 1949 was likely due to natural climate variation.
A review is presented of the development and simulation characteristics of the most recent version of a global coupled model for climate variability and change studies at the Geophysical Fluid Dynamics Laboratory, as well as a review of the climate change experiments performed with the model. The atmospheric portion of the coupled model uses a spectral technique with rhomboidal 30 truncation, which corresponds to a transform grid with a resolution of approximately 3.75° longitude by 2.25° latitude. The ocean component has a resolution of approximately 1.875° longitude by 2.25° latitude. Relatively simple formulations of river routing, sea ice, and land surface processes are included. Two primary versions of the coupled model are described, differing in their initialization techniques and in the specification of sub-grid scale oceanic mixing of heat and salt. For each model a stable control integration of near milennial scale duration has been conducted, and the characteristics of both the time-mean and variability are described and compared to observations. A review is presented of a suite of climate change experiments conducted with these models using both idealized and realistic estimates of time-varying radiative forcing. Some experiments include estimates of forcing from past changes in volcanic aerosols and solar irradiance. The experiments performed are described, and some of the central findings are highlighted. In particular, the observed increase in global mean surface temperature is largely contained within the spread of simulated global mean temperatures from an ensemble of experiments using observationally-derived estimates of the changes in radiative forcing from increasing greenhouse gases and sulfate aerosols.
Andreasen, D H., A C Ravelo, and Anthony J Broccoli, 2001: Remote forcing at the Last Glacial Maximum in the tropical Pacific Ocean. Journal of Geophysical Research, 106(C1), 879-897. Abstract PDF
We present results of a Last Glacial Maximum (LGM) wind stress sensitivity experiment using a high-resolution ocean general circulation model of the tropical Pacific Ocean. LGM wind stress, used to drive the ocean model, was generated using an atmospheric general circulation model simulation forced by LGM boundary conditions as part of the Paleoclimate Modeling Intercomparison Project (PMIP) [Broccoli, 2000]. LGM wind stress anomalies were large in the western half of the basin, yet there was a significant hydrographic response in the eastern half. This ocean model experiment hind casts changes that are in close agreement with paleoceanographic data from the entire region, even without the explicit modeling of the air-sea interactions. Data and model both predict that the annual average thermocline tilt across the basin was enhanced. Data and model are consistent with a stronger equatorial undercurrent which shoaled to the west of where it does today, and stronger advection of water from the Peru Current into the east equatorial Pacific and across the equator. Paleoproductivity and sea surface temperature (SST) data are interpreted in light of the modeling results, indicating that paleoproductivity changes were related to wind-forced dynamical changes resulting from LGM boundary conditions, while SST changes were related to independent, possibly radiative, forcing. Overall, our results imply that much of the dynamic response of the tropical Pacific during the LGM can be explained by wind field changes resulting from global LGM boundary conditions.
The effect of changes in observational coverage on the association between the Arctic oscillation (AO) and extratropical Northern Hemisphere surface temperature is examined. A coupled atmosphere-ocean model, which produces a realistic simulation of the circulation and temperature patterns associated with the AO, is used as a surrogate for the real-climate system. The association between the AO and spatial mean-temperature, as quantified by regressing the latter on the AO index, is subject to a positive bias due to the incomplete spatial coverage of the observational network. The bias is largest during the early part of the twentieth century and decreases, but does not vanish, thereafter.
Hewitt, C, Anthony J Broccoli, J F B Mitchell, and Ronald J Stouffer, 2001: A coupled model study of the last glacial maximum: Was part of the North Atlantic relatively warm?Geophysical Research Letters, 28(8), 1571-1574. Abstract PDF
A coupled ocean-atmosphere general circulation model is used to simulate the climates of today and the last glacial maximum (LGM). The model, which does not require artificial flux adjustments, produces a pattern of cooling at the LGM that is broadly consistent with the findings from simpler models and paleoclimatic data. However, changes to the ocean circulation produce anomalously warm LGM surface conditions over parts of the North Atlantic, seemingly at odds with paleoceanographic data. The thermohaline circulation is intensified for several centuries, as is the northward heat transport in the Atlantic equatorward of 55°N, but this may be a transient result. Mechanisms that lead to this response are discussed.
We compared the temporal variability of the heat content of the world ocean, of the global atmosphere, and of components of Earth's cryosphere during the latter half of the 20th century. Each component has increased its heat content (the atmosphere and the ocean) or exhibited melting (the cryosphere). The estimated increase of observed global ocean heat content (over the depth range from 0 to 3000 meters) between the 1950s and 1990s is at least one order of magnitude larger than the increase in heat content of any other component. Simulation results using an atmosphere-ocean general circulation model that includes estimates of the radiative effects of observed temporal variations in greenhouse gases, sulfate aerosols, solar irradiance, and volcanic aerosols over the past century agree with our observation-based estimate of the increase in ocean heat content. The results we present suggest that the observed increase in ocean heat content may largely be due to the increase of anthropogenic gases in Earth's atmosphere.
McAveney, B, Anthony J Broccoli, Keith W Dixon, and Ronald J Stouffer, et al., 2001: Model evaluation In Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge, UK, Cambridge University Press, 472-523.
Broccoli, Anthony J., 2000: Extratropical influences on interhemispheric asymmetry of tropical climate In Paleoclimate Modelling Intercomparison Project (PMIP), Braconnot, P., Ed., WCRP-111, WMO/TD-No. 1007, Geneva, Switzerland, World Meteorological Organization, 233-234.
Broccoli, Anthony J., 2000: Tropical cooling at the Last Glacial Maximum: An atmosphere-mixed layer ocean model simulation. Journal of Climate, 13(5), 951-976. Abstract PDF
The sensitivity of tropical temperature to glacial forcing is examined by using an atmosphere-mixed layer ocean (A-MLO) model to simulate the climate of the last glacial maximum (LGL) following specifications established by the Paleoclimate Modeling Intercomparison Project. Changes in continental ice, orbital parameters, atmospheric CO2, and sea level constitute a global mean radiative forcing of -4.20 W m-2, with the vast majority of this forcing coming, in nearly equal portions, from the changes in continental ice and CO2. In response to this forcing, the global mean surface air temperature decreases by 4.0 K, with the largest cooling in the extratropical Northern Hemisphere. In the Tropics, a more modest cooling of 2.0 K (averaged from 30°N to 30°S) is simulated, but with considerable spatial variability resulting from the interhemispheric asymmetry in radiative forcing, contrast between oceanic and continental response, advective effects, and changes in soil moisture. Analysis of the tropical energy balance reveals that the decrease in top-of-atmosphere longwave emission associated with the tropical cooling is balanced primarily by the combination of increased reflection of shortwave radiation by clouds and increased atmospheric heat transport to the extratropics.
Comparisons with a variety of paleodata indicate that the overall tropical cooling is comparable to paleoceanographic reconstructions based on alkenones and species abundances of planktonic microorganisms, but smaller than the cooling inferred from noble gases in aquifiers, pollen, snow line depression, and the isotopic composition of corals. The differences in the magnitude of tropical cooling reconstructed from the different proxies preclude a definitive evaluation of the realism of the tropical sensitivity of the model. Nonetheless, the comparisons with paleodata suggest that it is unlikely that the A-MLO model exaggerates the actual climate sensitivity. The similarity between the sensitivity coefficients (i.e., the ratio of the change in global mean surface air temperature to the change in global mean radiative forcing) for the LGM simulation and a simulation of CO2 doubling suggests that similar climate feedbacks are involved in the responses to these two perturbations. More comprehensive simulation of the tropical temperature sensitivity to glacial forcing will require the use of coupled models, for which a number of technical obstacles remain.
Jackson, C S., and Anthony J Broccoli, 2000: The influence of orbital configuration on Arctic glaciation In Paleoclimate Modelling Intercomparison Project (PMIP), WCRP-111, WMO/TD No. 1007, Geneva, Switzerland, World Meteorological Organization, 177-178.
Taylor, Karl E., C Hewitt, P Braconnot, Anthony J Broccoli, Charles Doutriaux, and J F B Mitchell, et al., 2000: Analysis of forcing, response, and feedbacks in a Paleoclimate modeling experiment In Paleoclimate Modelling Intercomparison Project (PMIP), Braconnet, P., Ed., WCRP-111, WMO/TD No. 1007, Geneva, Switzerland, World Meteorological Organization, 43-49.
Joussaume, Sylvie, and Anthony J Broccoli, et al., 1999: Monsoon changes for 6000 years ago: Results of 18 simulations from the Paleoclimate Modeling Intercomparison Project (PMIP). Geophysical Research Letters, 26(7), 859-862. Abstract PDF
Amplification of the northern hemisphere seasonal cycle of insolation during the mid-Holocene causes a northward shift of the main regions of monsoon precipitation over Africa and India in all 18 simulations conducted for the Paleoclimate Modeling Intercomparison Project (PMIP). Differences among simulations are related to differences in model formulation. Despite qualitative agreement with paleoecological estimatesof biome shifts, the magnitude of the monsoon increases over northern Africa are underestimated by all the models.
Surface air temperatures from a 1000-yr integration of a coupled atmosphere-ocean model with constant forcing are analyzed by using a method that decomposes temperature variations into a component associated with a characteristic spatial structure and a residual. The structure function obtained from the coupled model output is almost identical to the so-called cold ocean-warm land (COWL) pattern based on observations, in which above-average spatial mean temperature is associated with anomalously cold oceans and anomalously warm land. This pattern features maxima over the high-latitude interiors of Eurasia and North America. The temperature fluctuations at the two continental centers exhibit almost no temporal correlation with each other. The temperature variations at the individual centers are related to telecommunication patterns in sea level pressure and 500-mb height that are similar to those identified in previous observational and modeling studies. As in observations, variations in the polarity and amplitude of this structure function are an important source of spatially averaged surface air temperature variability.
Results from parallel integrations of models with more simplified treatments of the ocean confirm that the contrast in thermal inertia between land and ocean is the primary factor for the existence of the COWL pattern, whereas dynamical air-sea interactions do not play a significant role. The internally generated variability in structure function amplitude in the coupled model integration is used to assess the importance of the upward trend in the amplitude of the observed structure function over the last 25 years. This trend, which has contributed to the accelerated warming of Northern Hemisphere temperature over recent decades, is unusually large compared with the trends generated internally by the coupled model. If the coupled model adequately estimates the internal variability of the real climate system, this would imply that the recent upturn in the observed structure function may not be purely a manifestation of unforced variability. A similar monotonic trend occurs when the same methodology is applied to a model integration with time-varying radiative forcing based on past and future CO2 and sulfate aerosol increases. This finding illustrates that this decomposition methodology yields ambiguous results when two distinct spatial patterns, the "natural" COWL pattern (i.e., that associated with internally generated variability) and the anthropogenic fingerprint, are present in the simulated climate record.
Broccoli, Anthony J., and Syukuro Manabe, 1997: Mountains and midlatitude aridity In Tectonic Uplift and Climate Change, New York, NY, Plenum Press, 89-121.
Broccoli, Anthony J., and E P Marciniak, 1996: Comparing simulated glacial climate and paleodata: A reexamination. Paleoceanography, 11(1), 3-14. Abstract PDF
Glacial sea surface temperatures (SSTs) simulated by an atmosphere-mixed layer ocean model are compared with those reconstructed by the Climate: Long-Range Investigation, Mapping, and Prediction (CLIMAP) Project using planktonic microfossils. Two methods of comparison are employed. The first is global and uses the subjectively analyzed (i.e., hand contoured, then digitized) data set published by CLIMAP. The second is restricted to only those discrete locations where the CLIMAP sediment cores were taken. Both methods indicate that many aspects of the reconstructed glacial SST changes are simulated reasonably well by the model, although there are areas of disagreement. The extent of the disagreement appears smaller when the SSTs are sampled at discrete locations, because the largest discrepancies occur in the subtropical Pacific where data are sparse. When examined separately for each ocean, the magnitude of the disagreement in low latitudes roughly corresponds to the magnitude of the uncertainties in SST estimation using planktonic microfossils, being largest in the Pacific and smallest in the Atlantic. Because the largest discrepancies occur where uncertainties in estimation are large, no clear determination of whether the climate model exaggerates or underestimates low-latitude climate sensitivity appears possible. Nonetheless, sampling at discrete locations may be the best procedure for evaluating climate model performance, because errors associated with extending analyses to data-void areas can be avoided and uncertainties associated with inadequate spatial sampling made more evident.
Lindberg, C, and Anthony J Broccoli, 1996: Representation of topography in spectral climate models and its effect on simulated precipitation. Journal of Climate, 9(11), 2641-2659. Abstract PDF
Spectral climate models are distinguished by their representation of variables as finite sums of spherical harmonics, with coefficients computed by an orthogonal projection of the variables onto the spherical harmonics. Representing the surface elevation in this manner results in its contamination by Gibbs-like truncation artifacts, which appear as spurious valleys and mountain chains in the topography. These "Gibbs ripples" are present in the surface topographies of spectral climate models from a number of research institutions. Integrations of the Geophysical Fluid Dynamics Laboratory (GFDL) climate model over a range of horizontal resolutions indicate that the Gibbs ripples lead to spurious, small-scale extrema in the spatial distribution of precipitation. This "cellular precipitation pathology" becomes more pronounced with increasing horizontal resolution, causing a deterioration in the fidelity of simulated precipitation in higher resolution models.
A method is described for reducing the Gibbs ripples that occur when making an incomplete spherical harmonic expansion of the topography. The new spherical harmonic representations of topography are formed by fitting a nonuniform spherical smoothing spline to geodetic data and found by solving a fixed-point problem. This regularization technique results in less distortion of features such as mountain height and continental boundaries than previous smoothing methods. These new expansions of the topography, when used as a lower boundary surface in the GFDL climate model, substantially diminish the cellular precipitation pathology and produce markedly more realistic simulations of precipitation. These developments make the prospect of using higher resolution spectral models for studies of regional hydrologic climate more attractive.
Broccoli, Anthony J., Syukuro Manabe, J F B Mitchell, and L Bengtsson, 1995: Comments on "Global climate change and tropical cyclones": Part II. Bulletin of the American Meteorological Society, 76(11), 2243-2245. PDF
Broccoli, Anthony J., 1994: Climate model sensitivity, paleoclimate and future climate change In Long-Term Climatic Variations, edited by J. C. Duplessy, and M. T. Spyridakis. NATO ASI Series I, Vol. 22, Berlin; Heidelberg, Springer-Verlag, 551-567. Abstract PDF
Radiatively active trace gases such as CO2, methane, nitrous oxide and chlorofluorocarbons warm the surface-troposphere system by increasing the infrared opacity of the atmosphere. Climate models have been used to estimate the warming associated with future increases in these gases. An important issue is the sensitivity, or the amount of climate response associated with a given amount of radiative forcing. Models currently used for climate change studies differ substantially in their sensitivity, and these differences contribute prominently to the uncertainty in estimating the magnitude and rate of future climate change. The observed climate record can be used to calibrate the sensitivity of climate models. Calibration based on the instrumental record (the last ~100 years) has the advantage of using direct observations of climatic parameters with relatively dense spatial coverage, but suffers from uncertainties about sources of climate forcing other than greenhouse gases and the possibility that natural climate variability may be comparable in magnitude to the greenhouse gas-induced change. In contrast, calibration using paleoclimate data from the late Quaternary involves larger changes in forcing, most of which are relatively well-documented, but is subject to the inherent uncertainties of reconstructing past climate from a variety of sometimes sparsely-distributed proxy data. While better monitoring of ongoing changes in climate forcing and response may be the best way to calibrate climate model sensitivity, paleoclimate information can provide much-needed insights into climate model performance, particularly as improvements occur in the quantity and quality of paleoclimate data.
Broccoli, Anthony J., and Syukuro Manabe, 1993: Climate model studies of interactions between ice sheets and the atmosphere-ocean system In Ice in the Climate System, W. R. Peltier, ed., NATO ASI Series I, Vol. 12, Berlin, Germany, Springer-Verlag, 271-290. Abstract PDF
A number of climate modeling studies have been conducted at the Geophysical Fluid Dynamics Laboratory to study the interaction of continental ice sheets with the climate system. This paper reviews some of the primary results from these studies. Substantial changes in the atmospheric circulation, location and intensity of storm tracks, precipitation distribution, sea surface temperature, sea ice extent, and soil moisture occur in response to the ice sheets of the last glacial maximum. Estimates of the mass budgets of these ice sheets suggest that they are not in equilibrium with the simulated LGM climate, although questions regarding the refreezing of surface meltwater make this result uncertain. Results from climate model experiments with and without orography suggest that orographic uplift could have produced a climate slightly more favorable for ice sheet initiation.
The role of mountains in maintaining extensive midlatitude arid regions in the Northern Hemisphere was investigated using simulations from the GFDL Global Climate Model with and without orography. In the integration with mountains, dry climates were simulated over central Asia and the interior of North America, in good agreement with the observed climate. In contrast, moist climates were simulated in the same regions in the integration without mountains. During all seasons but summer, large amplitude stationary waves occur in response to the Tibetan Plateau and Rocky Mountains. The midlatitude dry regions are located upstream of the troughs of these waves, where general subsidence and relatively infrequent storm development occur and precipitation is thus inhibited. In summer, this mechanism contributes to the dryness of interior North America as a stationary wave trough remains east of the Rockies, but is not effective in Eurasia due to seasonal changes in the atmospheric circulation. The dryness of interior Eurasia in summer results, in part, from the south Asian monsoon circulation induced by the Tibetan Plateau. Its rising branch is centered above the southeastern Tibetan Plateau, and its salient features are a cyclonic flow at low levels (the "south Asian low") and an anticyclonic flow in the upper troposphere. This circulation is associated with a northward displacement of the storm track and a flow of relatively dry, subsiding air across much of central Asia. In addition, land surface-atmosphere feedback contributes to the dryness of all midlatitude dry regions. Although the effect of this feedback is small in winter, it is responsible for more than half of the reduction in summer precipitation. Orography also substantially reduces the moisture transport across the continental interiors. The results from this experiment suggest that midlatitude dryness is largely due to the existence of orography. This is an alternative to the traditional explanation that distance from oceanic moisture sources, accentuated locally by the presence of mountain barriers upwind, is the major cause of midlatitude dry regions. Paleoclimatic evidence of less aridity during the late Tertiary, before substantial uplift of the Rocky Mountains and Tibetan Plateau is believed to have occurred, supports this possibility.
Broccoli, Anthony J., and Syukuro Manabe, 1991: Will global warming increase the frequency of tropical cyclones? In Fifth Conference on Climate Variations, Boston, MA, American Meteorological Society, 46.
Broccoli, Anthony J., and Syukuro Manabe, 1990: Can existing climate models be used to study anthropogenic changes in tropical cyclone climate?Geophysical Research Letters, 17(11), 1917-1920. Abstract PDF
The utility of current generation climate models for studying the influence of greenhouse warming on the tropical storm climatology is examined. A method developed to identify tropical cyclones is applied to a series of model integrations. The global distribution of tropical storms is simulated by these models in a generally realistic manner. While the model resolution is insufficient to reproduce the fine structure of tropical cyclones, the simulated storms become more realistic as resolution is increased. To obtain a preliminary estimate of the response of the tropical cyclone climatology, CO2 was doubled using models with varying cloud treatments and different horizontal resolutions. In the experiment with prescribed cloudiness, the number of storm‐days, a combined measure of the number and duration of tropical storms, undergoes a statistically significant increase in the doubled‐CO2 climate. In contrast, a smaller but significant reduction of the number of storm‐days is indicated in the experiment with cloud feedback. In both cases the response is independent of horizontal resolution. While the inconclusive nature of these experimeital results highlights the uncertainties that remain in examining the details of greenhouse gas induced climate change, the ability of the models to qualitatively simulate the tropical storm climatology suggests that they are appropriate tools for this problem.
Simulations from a global climate model with and without orography have been used to investigate the role of mountains in maintaining extensive arid climates in middle latitudes of the Northern Hemisphere. Dry climates similar to those observed were simulated over central Asia and western interior North America in the experiment with mountains, whereas relatively moist climates were simulated in these areas in the absence of orography. The experiments suggest that these interior regions are dry because general subsidence and relatively infrequent storm development occur upstream of orographically induced stationary wave troughs. Downstream of these troughs, precipitation-bearing storms develop frequently in association with strong jet streams. In contrast, both atmospheric circulation and precipitation were more zonally symmetric in the experiment without mountains. In addition, orography reduces the moisture transport into the continental interiors from nearby oceanic sources. The relative soil wetness of these regions in the experiment without mountains is consistent with paleoclimatic evidence of less aridity during the late Tertiary, before substantial uplift of the Rocky Mountains and Tibetan Plateau is believed to have occurred.
Broccoli, Anthony J., and Syukuro Manabe, 1987: The effects of the Laurentide ice sheet on North American climate during the last glacial maximum. Géographie Physique et Quaternaire, XLI(2), 291-299. Abstract PDF
A climate model, consisting of an atmospheric general circulation model coupled with a simple model of the oceanic mixed layer, is used to investigate the effects of the continental ice distribution of the last glacial maximum (LGM) on North American climate. This model has previously been used to simulate the LGM climate, producing temperature changes reasonably in agreement with paleoclimatic data. The LGM distribution of continental ice according to the maximum reconstruction of HUGHES et al. (1981) is used as input to the model. In response to the incorporation of the expanded continental ice of the LGM, the model produces major changes in the climate of North America. The ice sheet exerts an orographic effect on the tropospheric flow, resulting in a splitting of the midlatitude westerlies in all seasons but summer. Winter temperatures are greatly reduced over a wide region south of the Laurentide ice sheet, although summer cooling is less extensive. An area of reduced soil moisture develops in the interior of North America just south of the ice margin. At the same time, precipitation increases in a belt extending from the extreme southeastern portion of the ice sheet eastward into the North Atlantic. Some of these findings are similar to paleoclimatic inferences based on geological evidence.
Broccoli, Anthony J., and Syukuro Manabe, 1987: The influence of continental ice, atmospheric CO2, and land albedo on the climate of the last glacial maximum. Climate Dynamics, 1, 87-99. Abstract PDF
The contributions of expanded continental ice, reduced atmospheric CO2, and changes in land albedo to the maintenance of the climate of the last glascial maximum (LGM) are examined. A series of experiments is performed using an atmosphere-mixed layer ocean model in which these changes in boundary conditions are incorporated either singly or in combination. The model used has been shown to produce a reasonably realistic simulation of the reduced temperature of the LGM (Manabe and Broccoli 1985b). By comparing the results from pairs of experiments, the effects of each of these environmental changes can be determined.The contributions of expanded continental ice, reduced atmospheric CO2, and changes in land albedo to the maintenance of the climate of the last glacial maximum (LGM) are examined. A series of experiments is performed using an atmosphere-mixed layer ocean model in which these changes in boundary conditions are incorporated either singly or in combination. The model used has been shown to produce a reasonably realistic simulation of the reduced temperature of the LGM (Manabe and Broccoli 1985b). By comparing the results from pairs of experiments, the effects of each of these environmental changes can be determined.
Expanded continental ice and reduced atmospheric CO2 are found to have a substantial impact on global mean temperature. The ice sheet effect is confined almost exclusively to the Northern Hemisphere, while lowered CO2 cools both hemispheres. Changes in land albedo over ice-free areas have only a minor thermal effect on a global basis. The reduction of CO2 content in the atmosphere is the primary contributor to the cooling of the Southern Hemisphere. The model sensitivity to both the ice sheet and CO2 effects is characterized by a high latitude amplification and a late autumn and early winter maximum. Expanded continental ice and reduced atmospheric CO2 are found to have a substantial impact on global mean temperature. The ice sheet effect is confined almost exclusively to the Northern Hemisphere, while lowered CO2 cools both hemispheres. Changes in land albedo over ice-free areas have only a minor thermal effect on a global basis. The reduction of CO2 content in the atmosphere is the primary contributor to the cooling of the Southern Hemisphere. The model sensitivity to both the ice sheet and CO2effects is characterized by a high latitude amplification and a late autumn and early winter maximum.
Substantial changes in Northern Hemisphere tropospheric circulation are found in response to LGM boundary contitions during winter. An amplified flow pattern and enhanced westerlies occur in the vicinity of the North American and Eurasian ice sheets. These alterations of the troposopheric circulation are primarily the result of the ice sheet effect, with reduced CO2 contributing only a slight amplification of the ice sheet-induced pattern.
Broccoli, Anthony J., 1986: Characteristics of seasonal snow cover as simulated by GFDL climate models In SNOW WATCH '85, Boulder, CO, World Data Center A for Glaciology (Snow and Ice), 241-248. Abstract PDF
Two climate simulations were performed using an atmospheric general circulation model developed at the Geophysical Fluid Dynamics Laboratory. The model employed for these simulations uses the spectral method, in which the horizontal distributions of atmospheric variables are represented by a limited number of spherical harmonics. In this study, the seasonally-varying distribution of insolation at the top of the atmosphere was prescribed, along with the climatological distributions of sea surface temperature and sea ice. The snow cover distributions produced in these simulations were compared with satellite observations. Both versions of the model generate snow cover very similar in extent to the observed snow cover.Two climate simulations were performed using an atmospheric general circulation model developed at the Geophysical Fluid Dynamics Laboratory. The model employed for these simulations uses the spectral method, in which the horizontal distributions of atmospheric variables are represented by a limited number of spherical harmonics. In this study, the seasonally-varying distribution of insolation at the top of the atmosphere was prescribed, along with the climatological distributions of sea surface temperature and sea ice. The snow cover distributions produced in these simulations were compared with satellite observations. Both versions of the model generate snow cover very similar in extent to the observed snow cover.
Manabe, Syukuro, and Anthony J Broccoli, 1985: A comparison of climate model sensitivity with data from the last glacial maximum. Journal of the Atmospheric Sciences, 42(23), 2643-2651. Abstract PDF
attempt has been made to use paleoclimatic data from the last glacial maximum to evaluate the sensitivity of two versions of an atmosphere/mixed-layer ocean model. Each of these models has been used to study the CO2-induced changes in climate. The models differ in their treatment of cloudiness, with one using a fixed cloud distribution and the other using a simple parameterization to predict clouds. The models also differ in the magnitude of their response to a doubling of atmospheric CO2, with the variable cloud model being nearly twice as sensitive as the fixed cloud version. Given the distributions of continental ice sheets, surface albedo, and the reduced carbon dioxide concentration of the ice age, the climate of the last glacial maximum (LGM) is simulated by each model and compared with the corresponding simulation of the present climate. Both models generate differences in sea surface temperature and surface air temperature which compare favorably with estimates of the actual differences in temperature between the LGM and the present. However, it is difficult to determine which version of the model is more realistic in simulating the ice age climate for two reasons: 1) the differences between the two models are relatively small; and 2) there are substantial uncertainties in the paleoclimatic data. Nevertheless, the similarity between the LGM simulations and the available paleoclimatic data suggests that the estimates of CO2-induced climate change obtained from these models may not be too far from reality.
The climate influence of the land ice that existed 18,000 years before present (18K B.P.) is investigated by use of a general circulation model of the atmosphere coupled with a static mixed layer ocean. Simulated climates are obtained from two versions of the model: one with the land ice distribution of the present and the other with that of 18K B.P. In the northern hemisphere the tropospheric flow field is strongly influenced by the Laurentide ice sheet and features a split flow straddling the ice sheet, with a strong jet stream forming the southern branch. The northern branch of the flow brings very cold air over the North Atlantic Ocean, where thick sea ice is maintained. The distribution of sea surface temperature (SST) difference between the two experiments in the northern hemisphere resembles the difference between the SST at 18K B.P. and at present, as estimated by the CLIMAP Project (1981). The 18K B.P. ice sheets have very little influence upon atmospheric temperature and SST in the southern hemisphere. This is because the interhemispheric heat transport hardly changes as the loss of heat energy due to the reflection of solar radiation by continental ice sheets in the northern hemisphere is almost completely counterbalanced by the in situ reduction of upward terrestrial radiation. Hydrologic changes in the model climate are also found, with statistically significant decreases in soil moisture occurring in a zone located to the south of the ice sheets in North American and Eurasia. These findings are consistent with some geological evidence of regionally drier climates from the last glacial maximum.
Manabe, Syukuro, and Anthony J Broccoli, 1984: Ice-age climate and continental ice sheets: some experiments with a general circulation model. Annals of Glaciology, 5, 100-105. Abstract PDF
The climatic influence of the land ice which existed 18 ka BP is investigated using a climate model developed at the Geophysical Fluid Dynamics Laboratory of the National Oceanic and Atmospheric Administration. The model consists of an atmospheric general circulation model coupled with a static mixed layer ocean model. Simulated climates are obtained from each of two versions of the model: one with the land-ice distribution of the present and the other with that of 18 ka BP.
In the northern hemisphere, the difference in the distribution of sea surface temperature (SST) between the two experiments resembles the difference between the SST at 18 ka BP and at present as estimated by CLIMAP Project Members (1981). In the northern hemisphere a substantial lowering of air temperature also occurs in winter, with a less pronounced cooling during summer. The mid-tropospheric flow field is influenced by the Laurentide ice sheet and features a split jet stream straddling the ice sheet and a long wave trough along the east coast of North America. In the southern hemisphere of 18 ka BP, the ice sheet has little influence on temperature. An examination of hemispheric heat balances indicates that this is because only a small change in interhemispheric heat transport exists, as the in situ radiative compensation in the northern hemisphere counterbalances the effective reflection of solar radiation by continental ice sheets.
Hydrologic changes in the model climate are also found, with statistically significant decreases in soil moisture occurring in a zone located to the south of the ice sheets in North America and Eurasia. These findings are consistent with some geological evidence of regionally drier climates from the last glacial maximum.
Manabe, Syukuro, and Anthony J Broccoli, 1984: Influence of the Climap ice sheet on the climate of a general circulation model: implications for the Milankovitch theory In Milankovitch and Climate, Part 2, Amsterdam, The Netherlands, Reidel Publishing Co, 789-799. PDF
