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.
The unphysical virtual salt flux (VSF) formulation widely used in the ocean component of climate models has the potential to cause systematic and significant biases in modeling the climate system and projecting its future evolution. Here a freshwater flux (FWF) and a virtual salt flux version of the Geophysical Fluid Dynamics Laboratory Climate Model version 2.1 (GFDL CM2.1) are used to evaluate and quantify the uncertainties induced by the VSF formulation. Both unforced and forced runs with the two model versions are performed and compared in detail. It is found that the differences between the two versions are generally small or statistically insignificant in the unforced control runs and in the runs with a small external forcing. In response to a large external forcing, however, some biases in the VSF version become significant, especially the responses of regional salinity and global sea level. However, many fundamental aspects of the responses differ only quantitatively between the two versions. An unexpected result is the distinctly different ENSO responses. Under a strong external freshwater forcing, the great enhancement of the ENSO variability simulated by the FWF version does not occur in the VSF version and is caused by the overexpansion of the top model layer. In summary, the principle assumption behind using virtual salt flux is not seriously violated and the VSF model has the ability to simulate the current climate and project near-term climate evolution. For some special studies such as a large hosing experiment, however, both the VSF formulation and the use of the FWF in the geopotential coordinate ocean model could have some deficiencies and one should be cautious to avoid them.
The formulation and simulation characteristics of two new global coupled climate models developed at NOAA's Geophysical Fluid Dynamics Laboratory (GFDL) are described. The models were designed to simulate atmospheric and oceanic climate and variability from the diurnal time scale through multicentury climate change, given our computational constraints. In particular, an important goal was to use the same model for both experimental seasonal to interannual forecasting and the study of multicentury global climate change, and this goal has been achieved.
Two versions of the coupled model are described, called CM2.0 and CM2.1. The versions differ primarily in the dynamical core used in the atmospheric component, along with the cloud tuning and some details of the land and ocean components. For both coupled models, the resolution of the land and atmospheric components is 2° latitude × 2.5° longitude; the atmospheric model has 24 vertical levels. The ocean resolution is 1° in latitude and longitude, with meridional resolution equatorward of 30° becoming progressively finer, such that the meridional resolution is 1/3° at the equator. There are 50 vertical levels in the ocean, with 22 evenly spaced levels within the top 220 m. The ocean component has poles over North America and Eurasia to avoid polar filtering. Neither coupled model employs flux adjustments.
The control simulations have stable, realistic climates when integrated over multiple centuries. Both models have simulations of ENSO that are substantially improved relative to previous GFDL coupled models. The CM2.0 model has been further evaluated as an ENSO forecast model and has good skill (CM2.1 has not been evaluated as an ENSO forecast model). Generally reduced temperature and salinity biases exist in CM2.1 relative to CM2.0. These reductions are associated with 1) improved simulations of surface wind stress in CM2.1 and associated changes in oceanic gyre circulations; 2) changes in cloud tuning and the land model, both of which act to increase the net surface shortwave radiation in CM2.1, thereby reducing an overall cold bias present in CM2.0; and 3) a reduction of ocean lateral viscosity in the extratropics in CM2.1, which reduces sea ice biases in the North Atlantic.
Both models have been used to conduct a suite of climate change simulations for the 2007 Intergovernmental Panel on Climate Change (IPCC) assessment report and are able to simulate the main features of the observed warming of the twentieth century. The climate sensitivities of the CM2.0 and CM2.1 models are 2.9 and 3.4 K, respectively. These sensitivities are defined by coupling the atmospheric components of CM2.0 and CM2.1 to a slab ocean model and allowing the model to come into equilibrium with a doubling of atmospheric CO2. The output from a suite of integrations conducted with these models is freely available online (see http://nomads.gfdl.noaa.gov/).
Manuscript received 8 December 2004, in final form 18 March 2005
The current generation of coupled climate models run at the Geophysical Fluid Dynamics Laboratory (GFDL) as part of the Climate Change Science Program contains ocean components that differ in almost every respect from those contained in previous generations of GFDL climate models. This paper summarizes the new physical features of the models and examines the simulations that they produce. Of the two new coupled climate model versions 2.1 (CM2.1) and 2.0 (CM2.0), the CM2.1 model represents a major improvement over CM2.0 in most of the major oceanic features examined, with strikingly lower drifts in hydrographic fields such as temperature and salinity, more realistic ventilation of the deep ocean, and currents that are closer to their observed values. Regional analysis of the differences between the models highlights the importance of wind stress in determining the circulation, particularly in the Southern Ocean. At present, major errors in both models are associated with Northern Hemisphere Mode Waters and outflows from overflows, particularly the Mediterranean Sea and Red Sea.
Impacts of mixing driven by barotropic tides in a coupled climate model are investigated by using an atmosphere–ocean–ice–land coupled climate model, the GFDL CM2.0. We focus on oceanic conditions of the Northern Atlantic. Barotropic tidal mixing effects increase the surface salinity and density in the Northern Atlantic and decrease the RMS error of the model surface salinity and temperature fields related to the observational data.
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).
Stouffer, Ronald J., Keith W Dixon, Michael J Spelman, William J Hurlin, Jianjun Yin, Jonathan M Gregory, A J Weaver, M Eby, G M Flato, D Y Robitaille, H Hasumi, A Oka, Aixue Hu, J H Jungclaus, I V Kamenkovich, A Levermann, M Montoya, S Murakami, S Nawrath, W R Peltier, G Vettoretti, A P Sokolov, and S L Weber, 2006: Investigating the Causes of the Response of the Thermohaline Circulation to Past and Future Climate Changes. Journal of Climate, 19(8), DOI:10.1175/JCLI3689.11. Abstract
The Atlantic thermohaline circulation (THC) is an important part of the earth's climate system. Previous research has shown large uncertainties in simulating future changes in this critical system. The simulated THC response to idealized freshwater perturbations and the associated climate changes have been intercompared as an activity of World Climate Research Program (WCRP) Coupled Model Intercomparison Project/Paleo-Modeling Intercomparison Project (CMIP/PMIP) committees. This intercomparison among models ranging from the earth system models of intermediate complexity (EMICs) to the fully coupled atmosphere–ocean general circulation models (AOGCMs) seeks to document and improve understanding of the causes of the wide variations in the modeled THC response. The robustness of particular simulation features has been evaluated across the model results. In response to 0.1-Sv (1 Sv 106 m3 s−1) freshwater input in the northern North Atlantic, the multimodel ensemble mean THC weakens by 30% after 100 yr. All models simulate some weakening of the THC, but no model simulates a complete shutdown of the THC. The multimodel ensemble indicates that the surface air temperature could present a complex anomaly pattern with cooling south of Greenland and warming over the Barents and Nordic Seas. The Atlantic ITCZ tends to shift southward. In response to 1.0-Sv freshwater input, the THC switches off rapidly in all model simulations. A large cooling occurs over the North Atlantic. The annual mean Atlantic ITCZ moves into the Southern Hemisphere. Models disagree in terms of the reversibility of the THC after its shutdown. In general, the EMICs and AOGCMs obtain similar THC responses and climate changes with more pronounced and sharper patterns in the AOGCMs.
The impact of the differences in the oceanic heat uptake and storage on the transient response to changes in radiative forcing is investigated using two newly developed coupled atmosphere-ocean models. In spite of its larger equilibrium climate sensitivity, one model (CM2.1) has smaller transient globally averaged surface air temperature (SAT) response than is found in the second model (CM2.0). The differences in the SAT response become larger as radiative forcing increases and the time scales become longer. The smaller transient SAT response in CM2.1 is due to its larger oceanic heat uptake. The heat storage differences between the two models also increase with time and larger rates of radiative forcing. The larger oceanic heat uptake in CM2.1 can be traced to differences in the Southern Ocean heat uptake and is related to a more realistic Southern Ocean simulation in the control integration.
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 transient responses of two versions of the Geophysical Fluid Dynamics Laboratory (GFDL) coupled climate model to a climate change forcing scenario are examined. The same computer codes were used to construct the atmosphere, ocean, sea ice and land surface components of the two models, and they employ the same types of sub-grid-scale parameterization schemes. The two model versions differ primarily, but not solely, in their spatial resolution. Comparisons are made of results from six coarse-resolution R15 climate change experiments and three medium-resolution R30 experiments in which levels of greenhouse gases (GHGs) and sulfate aerosols are specified to change over time. The two model versions yield similar global mean surface air temperature responses until the second half of the 21st century, after which the R15 model exhibits a somewhat larger response. Polar amplification of the Northern Hemisphere's warming signal is more pronounced in the R15 model, in part due to the R15's cooler control climate, which allows for larger snow and ice albedo positive feedbacks. Both models project a substantial weakening of the North Atlantic overturning circulation and a large reduction in the volume of Arctic sea ice to occur in the 21st century. Relative to their respective control integrations, there is a greater reduction of Arctic sea ice in the R15 experiments than in the R30 simulations as the climate system warms. The globally averaged annual mean precipitation rate is simulated to increase over time, with both model versions projecting an increase of about 8% to occur by the decade of the 2080s. While the global mean precipitation response is quite similar in the two models, regional differences exist, with the R30 model displaying larger increases in equatorial regions.
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.
The mechanism by which the model-simulated North Atlantic thermohaline circulation (THC) weakens in response to increasing greenhouse gas (GHG) forcing is investigated through the use of a set of five multi-century experiments. Using a coarse resolution version of the GFDL coupled climate model, the role of various surface fluxes in weakening the THC is assessed. Changes in net surface freshwater fluxes (precipitation, evaporation, and runoff from land) are found to be the dominant cause for the model's THC weakening. Surface heat flux changes brought about by rising GHG levels also contribute to THC weakening, but are of secondary importance. Wind stress variations have negligible impact on the THC's strength in the transient GHG experiment.
Manabe, Syukuro, Ronald J Stouffer, and Michael J Spelman, 1995: Interaction between polar climate and global warming In Fourth Conference on Polar Meteorology and Oceanography, Boston, MA, American Meteorological Society, J1-J9.
This study investigates the response of a climate model to a 1% per year increase of atmospheric carbon dioxide. The model is a general circulation model of the coupled ocean-atmosphere-land surface system, with a global computational domain, smoothed geography, and seasonal variation of insolation. The simulated increase of sea-surface temperature is very slow in the northern North Atlantic and the Circumpolar Ocean of the Southern Hemisphere where the vertical mixing of water penetrates very deeply and the rate of deep water formation is relatively fast. Extending this work, we investigated the transient responses of the coupled model to the doubling and quadrupling of atmospheric CO2, over the period of several centuries. During the entire 500-yr. period of the experiment, the global mean surface air temperature increases almost 3.5°C when CO2 is doubled, and 7°C when it is quadrupled. In the latter experiment, the thermal structure and dynamics of the model oceans undergo drastic changes, such as cessation of the thermohaline circulation in most of the model oceans, and substantial deepening of the thermocline, especially in the North Atlantic. These changes prevent the ventilation of the deeper layer of the oceans and, if they occurred in reality, could have a profound impact on the carbon cycle and biogeochemistry of the coupled ocean-atmosphere system.
Manabe, Syukuro, Ronald J Stouffer, Michael J Spelman, and Kirk Bryan, 1992: Response of a coupled ocean-atmosphere-land surface model to a gradual increase of atmospheric carbon dioxide In The Global Role of Tropical Rainfall, Hampton, Virginia, Deepak Publishing, 93-103. Abstract
This study investigates the response of a climate model to a gradual increase of atmospheric carbon dioxide. The model is a general circulation model of the coupled ocean-atmosphere-land surface system with a global computational domain, smoothed geography, and seasonal variation of insolation. It is found that the simulated warming of sea surface temperature is very slow over the northern North Atlantic and the circumpolar ocean of the Southern Hemisphere where the vertical mixing of water penetrates very deeply and the rate of deep water formation is relatively fast. With the exception of these two regions, the distribution of the change in surface temperature of the model is qualitatively similar to the equilibrium response of an atmospheric-mixed layer ocean model, which has been the subject of many previous studies.
The increase of atmospheric carbon dioxide affects not only the thermal structure of the coupled model, but also its hydrologic cycle. For example, the global mean rates of both precipitation and evaporation increase. The increase in evaporation rate is particularly large in low latitudes and decreases with increasing latitudes. On the other hand, the increase in the precipitation rate is substantial in high latitudes due to the increased penetration of warm, moisture-rich air into high latitudes. Thus, the rate of runoff in the subarctic basins is increased markedly.
In qualitative agreement with the results of equilibrium response studies, soil moisture is reduced in summer over extensive regions of the middle and high latitudes, such as the North American Great Plains, Western Europe, Northern Canada, and Siberia.
Manabe, Syukuro, Ronald J Stouffer, Michael J Spelman, and Kirk Bryan, 1992: Transient response of a coupled ocean-atmosphere-land surface model to increasing atmospheric carbon dioxide In Advances in Theoretical Hydrology: A Tribute to Jim Dooge, The Netherlands, Elsevier Science Publishers, 159-173. Abstract
This study investigates the response of a climate model to a gradual increase of atmospheric carbon dioxide. The model is a general circulation model of the coupled ocean-atmosphere-land surface system with a global computational domain, smoothed geography, and seasonal veriation of insolation. It is found that the simulated increase of sea surface temperature is very slow over the northern North Atlantic and the Circumpolar Ocean of the Southern Hemisphere where the vertical mixing of water penetrates very deeply and the rate of deep water formation is relatively fast. With the exception of these two regions identified above, the distribution of the change in surface temperature of the model is qualitatively similar to the equilibrium response of an atmospheric-mixed layer ocean model, which has been the subject of many previous studies. In most of the Northern Hemisphere, the seasonal dependence of surface air temperature change is also similar to the equilibrium response. For example, the temperature increase is at a maximum over the Arctic Ocean and its surroundings in the late fall and winter, whereas it is at a minimum in summer. However, the increase of surface air temperature and its seasonal variation is very small in the Circumpolar Ocean of the Southern Hemisphere and the northern North Atlantic.
The increase of atmospheric carbon dioxide affects not only the thermal structure of the coupled model but also its hydrologic cycle. For example, the global mean rates of both precipitation and evaporation increase. The increase in evaporation rate is particularly large in low latitudes and decreases with increasing latitudes. On the other hand, the increase in the precipitation rate is substantial in high latitudes due to the increased penetration of warm, moisture-rich air into high latitudes. Thus, the rate of runoff in the subarctic basin increases markedly.
In qualitative agreement with the results of equilibium response studies, soil moisture is reduced in summer over extensive regions of the middle and high latitudes, such as the North American Great Plains, Western Europe, Northern Canada, and Siberia.
This study investigates the seasonal variation of the transient response of a coupled ocean-atmosphere model
to a gradual increase (or decrease) of atmospheric carbon dioxide. The model is a general circulation model of
the coupled atmosphere-ocean-Iand surface system with a global computational domain, smoothed geography,
and seasonal variation of insolation.
It was found that the increase of surface air temperature in response to a gradual increase of atmospheric
carbon dioxide is at a maximum over the Arctic Ocean and its surroundings in the late fall and winter. On the
other hand, the Arctic warming is at a minimum in summer. In sharp contrast to the situation in the Arctic
Ocean, the increase of surface air temperature and its seasonal variation in the circumpolar ocean of the Southern
Hemisphere are very small because of the vertical mixing of heat over a deep water column.
In response to the gradual increase of atmospheric carbon dioxide, soil moisture is reduced during the June-July-
August period over most of the continents in the Northern Hemisphere with the notable exception of the
Indian subcontinent, where it increases. The summer reduction of soil moisture in the Northern Hemisphere
is relatively large over the region stretching from the northern United States to western Canada, eastern China,
southern Europe, Scandinavia, and most of the Russian Republic. During the December-January-February
period, soil moisture increases in middle and high latitudes of the Northern Hemisphere. The increase is relatively
large over the western portion of the Russian Republic and the central portion of Canada. On the other hand,
it is reduced in the subtropics, particularly over Southeast Asia and Mexico.
Because of the reduction (or delay) in the warming of the oceanic surface due to the thermal inertia of the
oceans, the increase of the moisture supply from the oceans to continents is reduced, thereby contributing to
the reduction of both soil moisture and runoff over the continents in middle and high latitudes of the Northern
Hemisphere. This mechanism enhances the summer reduction of soil moisture and lessens its increase during
winter in these latitudes.
The changes in surface air temperature and soil moisture in response to the gradual reduction of atmospheric
CO2 are opposite in sign but have seasonal and geographical distributions that are broadly similar to the response
to the gradual CO2 increase described above.
Manabe, Syukuro, Ronald J Stouffer, Michael J Spelman, and Kirk Bryan, 1991: Transient responses of a coupled ocean-atmosphere-land surface model to gradual changes of atmospheric CO2 In Global Change, Proceedings of the first Demetra meeting held at Chianciano Terme, Italy from 28 to 31 October 1991, Environment and Quality of Life, EUR 15158 EN, Directorate-General Science, Research and Development, European Commission, 82-93.
This study investigates the response of a climate model to a gradual increase or decrease of atmospheric carbon dioxide. The model is a general circulation model of the coupled atmosphere-ocean-land surface system with global geography and seasonal variation of insolation. To offset the bias of the coupled model toward settling into an unrealistic state, the fluxes of heat and water at the ocean-atmosphere interface are adjusted by amounts that vary with season and geography but do not change from one year to the next. Starting from a quasi-equilibrium climate, three numerical time integrations of the coupled model are performed with gradually increasing, constant, and gradually decreasing concentrations of atmospheric carbon dioxide.
It is noted that the simulated response of sea surface temperature is very slow over the northern North Atlantic and the Circumpolar Ocean of the Southern Hemisphere where vertical mixing of water penetrates very deply. However, in most of the Northern Hemisphere and low latitudes of the Southern Hemisphere, the distribution of the change in surface air temperature of the model at the time of doubling (or halving) of atmospheric carbon dioxide resembles the equilibrium response of an atmospheric-mixed layer ocean model to CO2 doubling (or halving). For example, the rise of annual mean surface air temperature in response to the gradual increase of atmospheric carbon dioxide increases with latitudes in the Northern Hemisphere and is larger over continents than oceans.
When time-dependent response of the model oceans to the increase of atmospheric carbon dioxide is compared with the corresponding response to the CO2 reduction at an identical rate, the penetration of the cold anomaly in the latter case is significantly deeper than that of the warm anomaly in the former case. The lack of symmetry in the penetration depth of a thermal anomaly between the two cases is associated with the difference in static stability, which is due mainly to the change in the vertical distribution of salinity in high latitudes and temperature changes in middle and low latitudes.
Despite the difference in penetration depth and accordingly, the effective thermal inertia of the oceans between two experiments, the time-dependent response of the global mean surface air temperature in the CO2 reduction experiment is similar in magnitude to the corresponding response in the CO2 growth experiment. In the former experiment with a colder climate, snow and sea ice with high surface albedo cover a much larger area, thereby enhancing their positive feedback effect upon surface air temperature. On the other hand, surface cooling is reduced due to the larger effective thermal inertia of the oceans. Because of the compensation between these two effects, the magnitude of surface air temperature response turned out to be similar between the two experiments.
The transient response of climate to an instantaneous increase in the atmospheric concentration of carbon dioxide has been investigated by a general circulation model of the coupled ocean-atmosphere-land system with global geography and annual mean insolation. An equilibrium climate of the coupled model climate during the 60-year period after the doubling is compared with the result from a control integration of the model without the doubling. The increase of surface air temperature in middle and high latitudes is slower in the Southern Hemisphere than the Northern Hemisphere. The large thermal inertia of the ocean-dominated hemisphere is partly responsible for this difference. The effective thermal inertia of the oceans becomes particularly large in high southern latitudes. Owing to the absence of meridional barriers at the latitudes of the Drake Passage, a wind-driven, deep cell of meridional circulation is maintained in the Circumpolar Ocean of the model. In addition, a deep reverse cell develops in the immediate vicinity of the Antarctic Continent. The thermal advection by these cells and associated convective overturning result in a very efficient mixing of heat in the 2-km thick upper layer and increase the effective thermal inertia of the ocean, thereby contributing to the slowdown of the CO2- induced warming of the near-surface layer of the Circumpolar Ocean of the model. It is surprising that, during the last 15 years of the 60-year experiment, sea surface temperatures in the Circumpolar Ocean actually reduce with time. Because of the increase in precipitation caused by the enhanced penetration of warm, moisture-rich air aloft into high latitudes, the surface halocline of the Circumpolar Ocean intensifies, thereby suppressing the convective mixing between the surface layer and the warmer underlying water. Thus, sea surface temperature is reduced in the Circumpolar Ocean towards the end of the experiment. In the Northern Hemisphere, the CO2-induced warming of the lower troposphere increases with increasing latitudes and is at a maximum near the North Pole due partly to the albedo feedback process involving sea ice and snow cover. The warming of the upper ocean layer also increases with increasing latitudes up to about 65 degrees N where the absorption of solar radiation increases markedly due to the poleward retreat of sea ice. Over the Arctic Ocean, the warming is very large in the surface layer of the model atmosphere, whereas it is very small in the underlying water. Both sea ice and a stable surface halocline act as thermal insulators and are responsible for the large air-sea contrast of the warming in this region. In short, the CO2- induced warming of the sea surface has a large interhemispheric asymmetry, in qualitative agreement with the results from a previous study conducted by use of a coupled model with a sector computational domain and an idealized geography. This asymmetry induces an atmospheric response which is quite different between the two hemispheres.
Bryan, Kirk, Syukuro Manabe, and Michael J Spelman, 1988: Interhemispheric asymmetry in the transient response of a coupled ocean-atmosphere model to a CO2 forcing. Journal of Physical Oceanography, 18(6), 851-867. Abstract PDF
Numerical experiments are carried out using a general circulation model of a coupled ocean-atmosphere system with idealized geography, exploring the transient response of climate to a rapid increase of atmospheric carbon dioxide. The computational domain of the model is bounded by meridians 120° apart, and includes two hemispheres. The ratio of land to sea at each latitude corresponds to the actual land-sea ratio for the present geography of the Earth. At the latitude of the Drake Passage the entire sector is occupied by ocean.
In the equivalent of the Northern Hemisphere the ocean delays the climate response to increased atmospheric carbon dioxide. The delay is of the order of several decades, a result corresponding to previous modeling studies. At high latitudes of the equivalent of the ocean-covered Southern Hemisphere, on the other hand, there is no warming at the sea surface, and even a slight cooling over the 50-year duration of the experiment. Two main factors appear to be involved. One is the very large ratio of ocean to land in the Southern Hemisphere. The other factor is the very deep penetration of a meridional overturning associated with the equatorward Ekman transport under the Southern Hemisphere westerlies. The deep cell delays the response to carbon-dioxide warming by upwelling unmodified waters from great depth. This deep cell disappears when the Drake Passage is removed from the model.
The climate response to a large increase in atmospheric CO2 was investigated in a numerical experiment with a coupled ocean-atmosphere model. The study is focused on one aspect of the experiment, the predicted response of the ocean to the warming episode. A fourfold increase in atmospheric CO2 causes a warming sufficiently intense to produce a partial collapse of the thermohaline circulation of the ocean. Surprisingly, the wind-driven circulation of the ocean is maintained without appreciable change. The global hydrological cycle intensifies without a major shift of the pattern of net precipitation over the model ocean. In the warming episode the downward pathways for heat, which include diffusion and model ocean. In the warming episode the downward pathways for heat, which include diffusion and Ekman pumping, remain open. The partial collapse of the thermohaline circulation closes the normal upward pathways associated with abyssal upwelling and high-latitude convection. As a result the thermocline is able to sequester almost twice as much heat than would be predicted from the behavior of a neutrally buoyant tracer introduced at the surface under normal climatic conditions. An enhanced sequestering of heat would produce a negative feedback for greenhouse warming. However, the partial collapse of the thermohaline circulation found in the numerical experiment would also affect the global carbon cycle, possibly producing a climatic feedback as strong as that caused by an enhanced uptake of heat from the atmosphere.
Spelman, Michael J., and Syukuro Manabe, 1984: Influence of oceanic heat transport upon the sensitivity of a model climate. Journal of Geophysical Research, 89(C1), 571-586. Abstract PDF
The influence of oceanic heat transport on the sensitivity of climate to an increase of the atmospheric CO2 concentration is studied by comparing the CO2-induced changes of two mathematical models. The first model is a general circulation model of the coupled ocean-atmosphere system which includes ocean currents. In the second model the oceanic component of the first model is replaced by a simple mixed layer without ocean currents. Both models have limited computational domain with idealized geography and annual mean insolation. For each model, the sensitivity of climate is evaluated from the difference between the equilibrium climates of the normal CO2 and 4 times the normal CO2 concentrations. The results indicate that the presence of ocean currents reduces the sensitivity of surface air temperature because of the difference in magnitude of the surface albedo feedback effect. The poleward transport of heat by ocean currents raises the surface temperature at high latitudes, shifts poleward the margins of snow and sea ice, decreases the contribution of the albedo feedback effect, and reduces the sensitivity of climate. The equilibrium response of climate is compared with the transient response of climate to a sudden increase of atmospheric CO2 content. According to this comparison, the latitudinal dependence of the equilibrium response of zonally averaged surface temperature is qualitatively similar to the transient response approximately 25 years after the time of the sudden CO2 increase. This result suggests that the distribution of the zonally averaged temperature change in response to a gradual increase of atmospheric carbon dioxide also resembles the distribution of the equilibrium response provided that the characteristic time scale of the CO2 increase is longer than 25 years.
The ocean's role in the delayed response of climate to increasing atmospheric carbon dioxide has been studied by means of a detailed three-dimensional climate model. A near-equilibrium state is perturbed by a fourfold, step-function increase in atmospheric carbon dioxide. The rise in the sea surface temperature was initially much more rapid in the tropics than at high latitudes. However, the fractional response, as normalized on the basis of the total difference between the high carbon dioxide and normal carbon dioxide climates, becomes almost uniform at all latitudes after 25 years. Because of the influence of a more rapid response over continents, the normalized response of the zonally averaged surface air temperature is faster and becomes nearly uniform with respect to latitude after only 10 years.
Manabe, Syukuro, Kirk Bryan, and Michael J Spelman, 1979: A global ocean-atmosphere climate model with seasonal variation for future studies of climate sensitivity. Dynamics of Atmospheres and Oceans, 3, 393-426.
A joint ocean-atmosphere model covering the entire globe has been constructed at the Geophysical Fluid Dynamics Laboratory (GFDL) of NOAA. This model differs from the earlier version of the joint model of Bryan and Manabe both in global domain and inclusion of realistic rather than idealized topography. This part of the paper describes the structure of the atmospheric portion of the joint model and discusses the atmospheric circulation and climate that emerges from the time integration of the model. The details of the oceanic part are given by Bryan et al. (1974), hereafter referred to as Part II.
The atmospheric part of the model incorporates the primitive equations of motion in a spherical coordinate system. The numerical problems associated with the treatment of mountains are minimized by using the "sigma" coordinate system in which pressure, normalized by surface pressure, is the vertical coordinate. For vertical finite differencing, nine levels are chosen so as to represent the planetary boundary layer and the stratosphere as well as the troposphere. For horizontal finite differencing, the regular latitude-longitude grid is used. To prevent linear computational instability in the time integration, Fourier filtering is applied in the longitudinal direction to all prognostic variables in higher latitudes such that the effective grid size of the model is approximately 500 km everywhere.
For the computation of radiative transfer, the distribution of water vapor, which is determined by the prognostic system of water vapor is used. However, the distribution of carbon dioxide, ozone and cloudiness are prescribed as a function of latitude and height and assumed to be constant with time. The temperature of the ground surface is determined such that it satisfies the condition of heat balance.
The prognostic system of water vapor includes the contribution of three-dimensional advection of water vapor and condensation in case of supersaturation. To simulate moist convection, a highly idealized procedure of moist convective adjustment is introduced. The prediction of soil moisture and snow depth is based upon the budget of water, snow and heat. Snow cover and sea ice are assumed to have much larger albedos than soil surface or open sea, and have a very significant effect upon the heat balance of the surface of the model.
Starting from the initial conditions of an isothermal and dry atmosphere at rest, the long-term integration of the joint model is conducted with the economical method adopted by Bryan and Manabe in their earlier study. The climate that emerges from this integration includes some of the basic features of the actual climate. However, it has many unrealistic features, which underscores the necessity of further increasing the computational resolution of horizontal finite differencing.
In order to identify the effect of the ocean currents upon climate, the joint model climate is compared with another climate obtained from the time integration of a so-called "A-model" in which oceanic regions are occupied by wet swampy surfaces without any heat capacity. Based upon the comparison between these two climates, the possible effects of oceanic heat transport on the climate are discussed. For example, the results show that the total poleward transport of energy is affected little by the oceanic heat transport. Although ocean currents significantly contribute to the transport, the atmospheric transport of energy in the presence of the latter decreases by approximately the same magnitude. Therefore, the total transport in the joint model differs little from that in the A-model. Further comparison between the two models indicates that ocean currents significantly affect not only the horizontal distribution of surface temperature of both oceans and continents but also the global distribution of precipitation.
Holloway, Jr, J L., Michael J Spelman, and Syukuro Manabe, 1973: Latitude-longitude grid suitable for numerical time integration of a global atmospheric model. Monthly Weather Review, 101(1), 69-78. Abstract PDF
A simple, free-surface, barotropic model and a nine-level, baroclinic model are numerically time integrated on both latitude-longitude grids and on Kurihara-type grids to compare the results obtained from the two grid systems. The prognostic variables are Fourier space filtered in the longitudinal direction on the latitude-longitude grids to permit the use of the same time-step length on both grids.
With respect to geopotential height and zonal wind distributions and to the phase speed of wave propagation, the results from the barotropic model, time-integrated on a sector latitude-longitude grid, agree better with a high-resolution control run than those computed on a modified Kurihara grid, particularly at high latitudes. The barotropic model is also time-integrated on a hemispheric, latitude-longitude grid, and the results compare well with a high-resolution control. The latter comparison is performed on initial data having strong cross-polar flow.
The mean sea-level pressure distribution obtained from a 64-day time integration of the baroclinic model on a global, latitude-longitude grid is better than that derived from a similar model using a Kurihara grid of comparable resolution. For example, the tendency for the Kurihara grid model to predict excessive pressures in the north polar region is for the most part corrected by use of the latitude-longitude grid.
