Abstract

A coupled atmosphere–ocean general circulation model (AOGCM) is integrated to a near-equilibrium state with the normal, half-normal, and twice-normal amounts of carbon dioxide in the atmosphere. Most of the ocean below the surface layers achieves 70% of the total response almost twice as fast when the changes in radiative forcing are cooling as compared to the case when they are warming the climate system. In the cooling case, the time to achieve 70% of the equilibrium response in the midoceanic depths is about 500–1000 yr. In the warming case, this response time is 1300–1700 yr. In the Pacific Ocean and the bottom half of the Atlantic Ocean basins, the response is similar to the global response in that the cooling case results in a shorter response time scale. In the upper half of the Atlantic basin, the cooling response time scales are somewhat longer than in the warming case due to changes in the oceanic thermohaline circulation. In the oceanic surface mixed layer and atmosphere, the response time scale is closely coupled. In the Southern Hemisphere, the near-surface response time is slightly faster in the cooling case. However in the Northern Hemisphere, the near-surface response times are faster in the warming case by more than 500 yr at times during the integrations. In the Northern Hemisphere, both the cooling and warming cases have much shorter response time scales than found in the Southern Hemisphere. Oceanic mixing of heat is the key in determining these time scales. It is shown that the model's simulation of present-day radiocarbon and chlorofluorocarbon (CFC) distributions compares favorably to the observations indicating that the quantitative time scales may be realistic.

Abstract

The influence of differing rates of increase of the atmospheric CO₂ concentration on the climatic response is investigated using a coupled ocean–atmosphere model. Five transient integrations are performed each using a different constant exponential rate of CO₂ increase ranging from 4% yr⁻¹ to 0.25% yr⁻¹. By the time of CO₂ doubling, the surface air temperature response in all the transient integrations is locally more than 50% and globally more than 35% of the equilibrium response. The land–sea contrast in the warming, which is evident in the equilibrium results, is larger in all the transient experiments. The land–sea difference in the response increases with the rate of increase in atmospheric CO₂ concentration. The thermohaline circulation (THC) weakens in response to increasing atmospheric CO₂ concentration in all the transient integrations, confirming earlier work. The results also indicate that the slower the rate of increase, the larger the weakening of the THC by the time of doubling. Two of the transient experiments are continued beyond the time of CO₂ doubling with the CO₂ concentration maintained at that level. The amount of weakening of the THC after the CO₂ stops increasing is smaller in the experiment with the slower rate of CO₂ increase, indicating that the coupled system has more time to adjust to the forcing when the rate of CO₂ increase is slower. After a period of slow overturning, the THC gradually recovers and eventually regains the intensity found in the control integration, so that the equilibrium THC is very similar in the control and doubled CO₂ integrations. Considering only the sea level changes due to the thermal expansion of seawater, the integration with the slowest rate of increase in CO₂ concentration (i.e., 0.25% yr⁻¹) has the largest globally averaged sea level rise by the time of CO₂ doubling (about 42 cm). However, only a relatively small fraction of the equilibrium sea level rise of 1.9 m is realized by the time of doubling in all the transient integrations. This implies that sea level continues to rise long after the CO₂ concentration stops increasing, as the warm anomaly penetrates deeper into the ocean.

Abstract

To speculate on the future change of climate over several centuries, three 500-year integrations of a coupled ocean-atmosphere model were performed. In addition to the standard integration in which the atmospheric concentration of carbon dioxide remains unchanged, two integrations are conducted. In one integration, the C0₂ concentration increases by 1% yr⁻¹ (compounded) until it reaches four times the initial value at the 140th year and remains unchanged thereafter. In another integration, the C0₂ concentration also increases at the rate of 1% yr⁻¹ until it reaches twice the initial value at the 70th year and remains unchanged thereafter.

One of the most notable features of the C0₂-quadrupling integration is the gradual disappearance of thermohaline circulations in most of the model oceans during the first 250-year period, leaving behind wind-driven cells. For example, thermohaline circulation nearly vanishes in the North Atlantic during the fist 200 years of the integration. In the Weddell and Ross seas, thermohaline circulation becomes weaker and shallower, thereby reducing the rate of bottom water formation and weakening the northward flow of bottom water in the Pacific and Atlantic oceans. The weakening or near disappearance of thermohaline circulation described above is attributable mainly to the capping of the model oceans by relatively fresh water in high latitudes where the excess of precipitation over evaporation increases markedly due to the enhanced poleward moisture transport in the warmer model troposphere.

In the C0₂-doubling integration, the thermohaline circulation weakens by a factor of more than 2 in the North Atlantic during the first 150 years but almost recovers its original intensity by the 500th year. The increase and downward penetration of positive heat and temperature anomaly in low and middle latitudes of the North Atlantic helps to increase the density contrast between the sinking and rising regions, contributing to this slow recovery. The recovery is aided by the gradual increase in surface salinity that accompanies the intensification of the thermohaline circulation.

During the 500-year period of the doubling and quadrupling experiments, the global mean surface air temperature increases by about 3.5°C and 7°C, respectively. The rise of sea level due to the thermal expansion of sea water is about 1 and 1.8 m, respectively, and could be much larger if the contribution of meltwater from continental ice sheets were included. It is speculated that the two experiments described above provide a probable range of future climate change.

Abstract

Two coupled atmosphere–ocean general circulation models developed at GFDL show differing stability properties of the Atlantic thermohaline circulation (THC) in the Coupled Model Intercomparison Project/Paleoclimate Modeling Intercomparison Project (CMIP/PMIP) coordinated “water-hosing” experiment. In contrast to the R30 model in which the “off” state of the THC is stable, it is unstable in the CM2.1. This discrepancy has also been found among other climate models. Here a comprehensive analysis is performed to investigate the causes for the differing behaviors of the THC. In agreement with previous work, it is found that the different stability of the THC is closely related to the simulation of a reversed thermohaline circulation (RTHC) and the atmospheric feedback. After the shutdown of the THC, the RTHC is well developed and stable in R30. It transports freshwater into the subtropical North Atlantic, preventing the recovery of the salinity and stabilizing the off mode of the THC. The flux adjustment is a large term in the water budget of the Atlantic Ocean. In contrast, the RTHC is weak and unstable in CM2.1. The atmospheric feedback associated with the southward shift of the Atlantic ITCZ is much more significant. The oceanic freshwater convergence into the subtropical North Atlantic cannot completely compensate for the evaporation, leading to the recovery of the THC in CM2.1. The rapid salinity recovery in the subtropical North Atlantic excites large-scale baroclinic eddies, which propagate northward into the Nordic seas and Irminger Sea. As the large-scale eddies reach the high latitudes of the North Atlantic, the oceanic deep convection restarts. The differences in the southward propagation of the salinity and temperature anomalies from the hosing perturbation region in R30 and CM2.1, and associated different development of a reversed meridional density gradient in the upper South Atlantic, are the cause of the differences in the behavior of the RTHC. The present study sheds light on important physical and dynamical processes in simulating the dynamical behavior of the THC.

Abstract

The study analyzes the variability of surface air temperature (SAT) and sea surface temperature (SST) obtained from a 1000-yr integration of a coupled atmosphere-ocean-land surface model, which consists of general circulation models of the atmosphere and oceans and a heat and water budget model of land surface.

It also explores the role of oceans in maintaining the variability of SAT by comparing the long-term integration of the coupled model with those of two simpler models. They are 1 ) a “mixed layer model,” that is, the general circulation model of the atmosphere combined with a simple slab model of the mixed layer ocean, and 2) a “fixed SST model,” that is, the same atmosphere model overlying seasonally varying, prescribed SST.

With the exception of the tropical Pacific, both the coupled and mixed layer models are capable of approximately simulating the standard deviations of observed annual and 5-yr-mean anomalies of local SAT. The standard deviation tends to be larger over continents than over oceans, in agreement with the observations. Over most continental regions, the standard deviations of annual, 5-yr- and 25-yr-mean SATs in the fixed SST model are slightly less than but comparable to the corresponding standard deviations in the coupled mode1, suggesting that a major fraction of low-frequency local SAT variability over continents of the coupled model is generated in situ.

Over the continents of both the coupled and the mixed layer models, the spectral density of local SAT is nearly independent of frequency. On the other hand, the spectral density of local SAT over most of the oceans of both models increases very gradually with decreasing frequency apparently influenced by the thermal inertia of mixed layer oceans. However, both SST and SAT spectra in the coupled model are substantially different from those in the mixed layer model near the Denmark Strait and in some regions in the circumpolar ocean of the Southern Hemisphere where water mixes very deeply. In these regions, both SST and SAT are much more persistent in the coupled than in the mixed layer models, and their spectral densities are much larger at multidecadal and/or centennial timescales.

It appears significant that not only the coupled model but also the mixed layer model without ocean currents can approximately simulate the power spectrum of observed, global mean SAT at decadal to interdecadal timescales. However, neither model generates a sustained, long-term warming trend of significant magnitude such as that observed since the end of the last century.

Abstract

This study compares the variability of surface air temperature in three long coupled ocean–atmosphere general circulation model integrations. It is shown that the annual mean climatology of the surface air temperatures (SAT) in all three models is realistic and the linear trends over the 1000-yr integrations are small over most areas of the globe. Second, although there are notable differences among the models, the models’ SAT variability is fairly realistic on annual to decadal timescales, both in terms of the geographical distribution and of the global mean values. A notable exception is the poor simulation of observed tropical Pacific variability. In the HadCM2 model, the tropical variability is overestimated, while in the GFDL and HAM3L models, it is underestimated. Also, the ENSO-related spectral peak in the globally averaged observed SAT differs from that in any of the models. The relatively low resolution required to integrate models for long time periods inhibits the successful simulation of the variability in this region. On timescales longer than a few decades, the largest variance in the models is generally located near sea ice margins in high latitudes, which are also regions of deep oceanic convection and variability related to variations in the thermohaline circulation. However, the exact geographical location of these maxima varies from model to model. The preferred patterns of interdecadal variability that are common to all three coupled models can be isolated by computing empirical orthogonal functions (EOFs) of all model data simultaneously using the common EOF technique. A comparison of the variance each model associated with these common EOF patterns shows that the models generally agree on the most prominent patterns of variability. However, the amplitudes of the dominant modes of variability differ to some extent between the models and between the models and observations. For example, two of the models have a mode with relatively large values of the same sign over most of the Northern Hemisphere midlatitudes. This mode has been shown to be relevant for the separation of the temperature response pattern due to sulfate aerosol forcing from the response to greenhouse gas forcing. This indicates that the results of the detection of climate change and its attribution to different external forcings may differ when unperturbed climate variability in surface air temperature is estimated using different coupled models. Assuming that the simulation of variability of the global mean SAT is as realistic on longer timescales as it is for the shorter timescales, then the observed warming of more than 0.5 K of the SAT in the last 110 yr is not likely to be due to internally generated variability of the coupled atmosphere–ocean–sea ice system. Instead, the warming is likely to be due to changes in the radiative forcing of the climate system, such as the forcing associated with increases in greenhouse gases.

Abstract

Two possible interpretations of forced climate change view it as projecting, either linearly or nonlinearly, onto the dominant modes of variability of the climate system. An evaluation of these two interpretations is performed using annual mean sea level pressure (SLP) and surface air temperature (SAT) fields obtained from integrations of the Geophysical Fluid Dynamics Laboratory coupled general circulation model forced with varying concentrations of greenhouse gases.

The dominant modes of SLP both represent much of the total variability and remain important in warmer climates. With SAT, however, the dominant modes are often related to variations in the sea-ice edge and so do not remain important once the ice has retreated; those unrelated to sea ice remain dominant in the warmer climates but represent smaller fractions of the total variability.

In general, climate change tends to project most strongly onto the more dominant modes. The change in SLP projects partially onto the top two modes in the Northern Hemisphere, reflecting both an overall decrease in hemispheric SLP as well as the pattern of change. In the Southern Hemisphere the change projects negligibly onto the dominant patterns between equilibrium climates but very strongly onto the Antarctic oscillation–like mode in the transient integrations. Changes in SAT project partially onto the dominant modes but relate more to the mean warming rather than the pattern of change. In general, the change projects most strongly onto the more dominant modes.

In all SLP domains, the projection of climate change overwhelmingly manifests itself as a linear translation in the mode, consistent with the linear interpretation. In SAT domains related to sea-ice variability, the projection reflects an increased tendency toward ice-free regimes, consistent with the nonlinear perspective; however this nonlinear projection represents only a small portion of the overall climate change.

Abstract

The response of an atmosphere–ocean general circulation model (AOGCM) to perturbations of freshwater fluxes across the sea surface in the North Atlantic and Southern Ocean is investigated. The purpose of this study is to investigate aspects of the so-called bipolar seesaw where one hemisphere warms and the other cools and vice versa due to changes in the ocean meridional overturning. The experimental design is idealized where 1 Sv (1 Sv ≡ 10⁶ m³ s⁻¹) of freshwater is added to the ocean surface for 100 model years and then removed. In one case, the freshwater perturbation is located in the Atlantic Ocean from 50° to 70°N. In the second case, it is located south of 60°S in the Southern Ocean.

In the case where the North Atlantic surface waters are freshened, the Atlantic thermohaline circulation (THC) and associated northward oceanic heat transport weaken. In the Antarctic surface freshening case, the Atlantic THC is mainly unchanged with a slight weakening toward the end of the integration. This weakening is associated with the spreading of the fresh sea surface anomaly from the Southern Ocean into the rest of the World Ocean. There are two mechanisms that may be responsible for such weakening of the Atlantic THC. First is that the sea surface salinity (SSS) contrast between the North Atlantic and North Pacific is reduced. And, second, when freshwater from the Southern Ocean reaches the high latitudes of the North Atlantic Ocean, it hinders the sinking of the surface waters, leading to the weakening of the THC.

The spreading of the fresh SSS anomaly from the Southern Ocean into the surface waters worldwide was not seen in earlier experiments. Given the geography and climatology of the Southern Hemisphere where the climatological surface winds push the surface waters northward away from the Antarctic continent, it seems likely that the spreading of the fresh surface water anomaly could occur in the real world.

A remarkable symmetry between the two freshwater perturbation experiments in the surface air temperature (SAT) response can be seen. In both cases, the hemisphere with the freshwater perturbation cools, while the opposite hemisphere warms slightly. In the zonally averaged SAT figures, both the magnitude and the pattern of the anomalies look similar between the two cases. The oceanic response, on the other hand, is very different for the two freshwater cases, as noted above for the spreading of the SSS anomaly and the associated THC response.

If the differences between the atmospheric and oceanic responses apply to the real world, then the interpretation of paleodata may need to be revisited. To arrive at a correct interpretation, it matters whether or not the evidence is mainly of atmospheric or oceanic origin. Also, given the sensitivity of the results to the exact details of the freshwater perturbation locations, especially in the Southern Hemisphere, a more realistic scenario must be constructed to explore these questions.

Abstract

A set of state-of-the-science climate models are used to investigate global sea level rise (SLR) patterns induced by ocean dynamics in twenty-first-century climate projections. The identified robust features include bipolar and bihemisphere seesaws in the basin-wide SLR, dipole patterns in the North Atlantic and North Pacific, and a beltlike pattern in the Southern Ocean. The physical and dynamical mechanisms that cause these patterns are investigated in detail using version 2.1 of the Geophysical Fluid Dynamics Laboratory (GFDL) Coupled Model (CM2.1). Under the Intergovernmental Panel on Climate Change’s (IPCC) Special Report on Emissions Scenarios (SRES) A1B scenario, the steric sea level changes relative to the global mean (the local part) in different ocean basins are attributed to differential heating and salinity changes of various ocean layers and associated physical processes. As a result of these changes, water tends to move from the ocean interior to continental shelves. In the North Atlantic, sea level rises north of the Gulf Stream but falls to the south. The dipole pattern is induced by a weakening of the meridional overturning circulation. This weakening leads to a local steric SLR east of North America, which drives more waters toward the shelf, directly impacting northeastern North America. An opposite dipole occurs in the North Pacific. The dynamic SLR east of Japan is linked to a strong steric effect in the upper ocean and a poleward expansion of the subtropical gyre. In the Southern Ocean, the beltlike pattern is dominated by the baroclinic process during the twenty-first century, while the barotropic response of sea level to wind stress anomalies is significantly delayed.

Abstract

The effects of land-use and land-cover change (LULCC) on surface climate using two ensembles of numerical experiments with the Geophysical Fluid Dynamics Laboratory (GFDL) comprehensive Earth System Model ESM2Mb are investigated in this study. The experiments simulate historical climate with two different assumptions about LULCC: 1) no land-use change with potential vegetation (PV) and 2) with the CMIP5 historical reconstruction of LULCC (LU). Two different approaches were used in the analysis: 1) the authors compare differences in LU and PV climates to evaluate the regional and global effects of LULCC and 2) the authors characterize subgrid climate differences among different land-use tiles within each grid cell in the LU experiment. Using the first method, the authors estimate the magnitude of LULCC effect to be similar to some previous studies. Using the second method, the authors found a pronounced subgrid signal of LULCC in near-surface temperature over majority of areas affected by LULCC. The signal is strongest on croplands, where it is detectable with 95% confidence over 68.5% of all nonglaciated land grid cells in June–July–August, compared to 8.3% in the first method. In agricultural areas, the subgrid signal tends to be stronger than LU–PV signal by a factor of 1.3 in tropics in both summer and winter and by 1.5 in extratropics in winter. This analysis for the first time demonstrates and quantifies the local, subgrid-scale LULCC effects with a comprehensive ESM and compares it to previous global and regional approaches.