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S. Manabe and R. J. Stouffer

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R. T. Wetherald and S. Manabe

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To improve understanding of the mechanisms responsible for CO2-induced, midcontinental summer dryness obtained by earlier modeling studies, several integrations were performed using a GCM with idealized geography. The simulated reduction of soil moisture in middle latitudes begins in late spring and is caused by the excess of evaporation over precipitation. The increase of carbon dioxide and the associated increase of atmospheric water vapor enhances the downward flux of terrestrial radiation at the continental surface at all latitudes. However, due mainly to the CO2-induced change in midtropospheric relative humidity, the increase in the downward flux of terrestrial radiation is larger in the equatorward side of the rain belt, making more energy available there for both sensible and latent heat. Since the saturation vapor pressure at the surface increases nonlinearly with surface temperature, a greater fraction of the additional radiative energy is realized as latent heat flux at the expense of sensible heat. Therefore, evaporation increases more than precipitation over the land surface in the equatorward side of the rain belt during spring and early summer and initiates the drying of the soil there. As the rain belt moves poleward from spring to summer, the soil moisture decreases in middle latitudes, reducing the rate of evaporation. This reduction of evaporation, in turn, causes a corresponding decreases of precipitation in middle latitudes. keeping the soil dry throughout the summer.

In high latitudes, there is also a tendency for increased summer dryness. As noted in our previous studies, this feature mainly results from the earlier removal of highly reflective snow cover in spring, which enhances the evaporation in the late spring. lengthening the period of drying during the summer season. A similar mechanism also operates in middle latitudes, but its contribution is relatively small. The drying of soil is also enhanced by the land surface-cloud interaction in both middle and high latitudes. Owing to the reduction of cloud cover that results from the decrease of relative humidity in the lower troposphere, solar radiation absorbed by the continental surface increases, thereby enhancing evaporation and further reducing the soil moisture in summer.

Although there is additional radiative energy available at the surface during winter. a greater fraction of it occurs as sensible heat rather than latent heat due to the colder surface temperature, thereby causing evaporation to increase less than precipitation. Because of the increased evaporation from the oceanic surface upstream whose temperature is warmer than the continental region in winter, precipitation over most of the continent increases substantially.

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S. Manabe and A. J. Broccoli

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An 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 C02-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 C02, 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 pateoclimatic data. Neverthless, the similarity between the LGM simulations and the available paleoclimatic data suggests that the estimates of C02-induced climate change obtained from these models may not be too far from reality.

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A. J. Broccoli and S. Manabe

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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 season 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.

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S. Manabe and R. J. Stouffer

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Two stable equilibria have been obtained from a global model of the coupled ocean-atmosphere system developed at the Geophysical Fluid Dynamics Laboratory of NOAA. The model used for this study consists of general circulation models of the atmosphere and the world oceans and a simple model of land surface. Starting from two different initial conditions, “asynchronous” time integrations of the coupled model, under identical boundary conditions, lead to two stable equilibria. In one equilibrium, the North Atlantic Oman has a vigorous thermohaline circulation and relatively saline and warm surface water. In the other equilibrium, there is no thermohaline circulation, and an intense halocline exists in the surface layer at high latitudes. In both integration the, air-sea exchange of water is adjusted to remove a systematic bias of the model that surpresses the thermohaline circulation in the North Atlantic. Nevertheless these results raise the intriguing possibility that the coupled system may have at least two equilibria. They also suggest that the themohaline overturning in the North Atlantic is mainly responsible for making the surface salinity of the northern North Atlantic higher than that of the northern North Pacific. Finally, a discussion is made on the paleoclimatic implications of these results for the large and abrupt transition between the Alleröd and Younger Dryas events which occurred about 11 000 years ago.

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R. T. Wetherald and S. Manabe

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The influence of the cloud feedback process upon the sensitivity of climate is investigated by comparing the behavior of two versions of a climate model with predicted and prescribed cloud cover. The model used for this study is a general circulation model of the atmosphere coupled with a mixed layer model of the oceans. The sensitivity of each version of the model is inferred from the equilibrium response of the model to a doubling of the atmospheric concentration of carbon dioxide.

It is found that the cloud feedback process in the present model enhances the sensitivity of the model climate. In response to the increase of atmospheric carbon dioxide, cloudiness increases around the tropopause and is reduced in the upper troposphere, thereby raising the height of the cloud layer in the upper troposphere. This rise of the high cloud layer implies a reduction of the temperature of the cloud top and, accordingly, of the upward terrestrial radiation from the top of the model atmosphere. Thus, the heat loss from the atmosphere-earth system of the model is reduced. As the high cloud layer rises, the vertical distribution of cloudiness changes, thereby affecting the absorption of solar radiation by the model atmosphere. At most latitudes the effect of reduced cloud amount in the upper troposphere overshadows that of increased cloudiness around the tropopause, thereby lowering the global mean planetary albedo and enhancing the CO2 induced warming.

On the other hand, the increase of low cloudiness in high latitudes raises the planetary albedo and thus decreases the CO2 induced warming of climate. However, the contribution of this negative feedback process is much smaller than the effect of the positive feedback process involving the change of high cloud.

The model used here does not take into consideration the possible change in the optical properties of clouds due to the change of their liquid water content. In view of the extreme idealization in the formulation of the cloud feedback process in the model, this study should be regarded as a study of the mechanisms involved in this process rather than the quantitative assessment of its influence on the sensitivity of climate.

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S. Manabe and R. T. Wetherald

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The change in soil wetness in response to an increase of atmospheric concentration of carbon dioxide is investigated by two versions of a climate model which consists of a general circulation model of the atmosphere and a static mixed layer ocean. In the first version of the model, the distribution of cloud cover is specified whereas it is computed in the second version incorporating the interaction among cloud cover, radiative transfer and the atmospheric circulation. The CO2-induced changes of climate and hydrology are evaluated based upon a comparison between two quasi-equilibrium climates of a model with a normal and an above normal concentration of atmospheric carbon dioxide.

It is shown that, in response to a doubling (or quadrupling) of atmospheric carbon dioxide, soil moisture is reduced in summer over extensive midcontinental regions of both North America and Eurasia in middle and high latitudes. Based upon the budget analysis of heat and water, the physical mechanisms responsible for the CO2-induced changes of soil moisture are determined for the following four regions: northern Canada, northern Siberia, the Great Plains of North America and southern Europe. It is found that, over northern Canada and northern Siberia, the CO2-induced reduction of soil moisture in summer results from the earlier occurrence of the snowmelt season followed by a period of intense evaporation. Over the Great Plains of North America, the earlier termination of the snowmelt season also contributes to the reduction of soil moisture during the summer season. In addition, the rainy period of late spring ends earlier, thus enhancing the CO2-induced reduction of soil moisture in summer. In the model with variable cloud cover, the summer dryness over the Great Plains is enhanced further by a reduction of cloud amount and precipitation in the lower model atmosphere. This reduction of cloud amount increases the solar energy reaching the continental surface and the rate of potential evaporation. Both the decrease of precipitation and the increase of potential evaporation further reduce the soil moisture during early summer and help to maintain it at a low level throughout the summer. Over Southern Europe, the CO2-induced reduction of soil wetness occurs in a qualitatively similar manner, although the relative magnitude of the contribution from the change in snowmelt is smaller.

During winter, soil moisture increases poleward of 30°N in response to an increase of atmospheric carbon dioxide. Because of the CO2-induced warming, a greater fraction of the total precipitation occurs as rainfall rather than snowfall. The warmer atmosphere also causes the accumulated snow cover to melt during winter. Both processes act to increase the soil moisture in all four regions during the winter season. The increase of soil moisture is enhanced further in high latitudes due to the increase of precipitation resulting from the penetration of warm, moisture-rich air into higher latitudes.

The CO2-induced warming of the lower model troposphere increases with increasing latitude. The present analysis suggests that the changes of soil wetness described in this investigation are controlled by the latitudinal profile of the warming and are very broad scale, mid-continental phenomena.

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B. G. HUNT and S. MANABE

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An investigation of tidal oscillations in the earth's atmosphere has been made using an 18 vertical level, hemispheric general circulation model. This approach permitted these tides to be investigated without resorting to linearization of the governing differential equations, as is required by the conventional approach. In addition, it allows the tides to be studied in relation to a realistic atmosphere, and thus in their actual roles as small perturbations, at least in the lower atmosphere, on the basic meteorological fields. Day-to-day surface pressure variations in good agreement with observation were produced by the model, the diurnal and semidiurnal pressure amplitudes and phases also being close to the observed values. An investigation into the excitation mechanism of the oscillation gave results supporting previous work in attributing the dominant cause of the tides to absorption of solar radiation by water vapor and ozone in the atmosphere. Contrary to previous studies, water vapor was found to be of primary importance in exciting both the diurnal and semidiurnal oscillations in the model atmosphere.

Generally speaking, the tidal wind and temperature variations obtained were also in agreement with observation and other theoretical work.

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T. Delworth, S. Manabe, and R. J. Stouffer

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A fully coupled ocean-atmosphere model is shown to have irregular oscillations of the thermohaline circulation in the North Atlantic Ocean with a time scale of approximately 50 years. The irregular oscillation appears to be driven by density anomalies in the sinking region of the thermohaline circulation (approximately 52°N to 72°N) combined with much smaller density anomalies of opposite sign in the broad, rising region. The spatial pattern of see surface temperature anomalies associated with this irregular oscillation bears an encouraging resemblance to a pattern of observed interdecadal variability in the North Atlantic. The anomalies of sea surface temperature induce model surface air temperature anomalies over the northern North Atlantic, Arctic, and northwestern Europe.

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K. Bryan, S. Manabe, and M. J. Spelman

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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 degrees 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.

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