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Richard T. Wetherald

Abstract

This paper examines hydrological variability and its changes in two different versions of a coupled ocean–atmosphere general circulation model developed at the National Oceanic and Atmospheric Administration/Geophysical Fluid Dynamics Laboratory and forced with estimates of future increases of greenhouse gas and aerosol concentrations. This paper is the second part, documenting potential changes in variability as greenhouse gases increase. The variance changes are examined using an ensemble of 8 transient integrations for an older model version and 10 transient integrations for a newer model. Monthly and annual data are used to compute the mean and variance changes. Emphasis is placed on computing and analyzing the variance changes for the middle of the twenty-first century and compared with those found in the respective control integrations.

The hydrologic cycle intensifies because of the increase of greenhouse gases. In general, precipitation variance increases in most places. This is the case virtually everywhere the mean precipitation rate increases and many places where the precipitation decreases. The precipitation rate variance decreases in the subtropics, where the mean precipitation rate also decreases. The increased precipitation rate and variance, in middle to higher latitudes during late fall, winter, and early spring leads to increased runoff and its variance during that period.

On the other hand, the variance changes of soil moisture are more complicated, because soil moisture has both a lower and upper bound that tends to reduce its fluctuations. This is particularly true in middle to higher latitudes during winter and spring, when the soil moisture is close to its saturation value at many locations. Therefore, changes in its variance are limited. Soil moisture variance change is positive during the summer, when the mean soil moisture decreases and is close to the middle of its allowable range. In middle to high northern latitudes, an increase in runoff and its variance during late winter and spring plus the decrease in soil moisture and its variance during summer lend support to the hypothesis stated in other publications that a warmer climate can cause an increasing frequency of both excessive discharge and drier events, depending on season and latitude.

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Syukuro Manabe and Richard T. Wetherald

Abstract

Radiative convective equilibrium of the atmosphere with a given distribution of relative humidity is computed as the asymptotic state of an initial value problem.

The results show that it takes almost twice as long to reach the state of radiative convective equilibrium for the atmosphere with a given distribution of relative humidity than for the atmosphere with a given distribution of absolute humidity.

Also, the surface equilibrium temperature of the former is almost twice as sensitive to change of various factors such as solar constant, CO2 content, O3 content, and cloudiness, than that of the latter, due to the adjustment of water vapor content to the temperature variation of the atmosphere.

According to our estimate, a doubling of the CO2 content in the atmosphere has the effect of raising the temperature of the atmosphere (whose relative humidity is fixed) by about 2C. Our model does not have the extreme sensitivity of atmospheric temperature to changes of CO2 content which was adduced by Möller.

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Syukuro Manabe and Richard T. Wetherald

Abstract

An attempt is made to estimate the temperature changes resulting from doubling the present CO2 concentration by the use of a simplified three-dimensional general circulation model. This model contains the following simplications: a limited computational domain, an idealized topography, no beat transport by ocean currents, and fixed cloudiness. Despite these limitations, the results from this computation yield some indication of how the increase of CO2 concentration may affect the distribution of temperature in the atmosphere. It is shown that the CO2 increase raises the temperature of the model troposphere, whereas it lowers that of the model stratosphere. The tropospheric warming is somewhat larger than that expected from a radiative-convective equilibrium model. In particular, the increase of surface temperature in higher latitudes is magnified due to the recession of the snow boundary and the thermal stability of the lower troposphere which limits convective beating to the lowest layer. It is also shown that the doubling of carbon dioxide significantly increases the intensity of the hydrologic cycle of the model.

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Richard T. Wetherald and Syukuro Manabe

Abstract

A study is conducted to evaluate the response of a simplified three-dimensional model climate to changes of the solar constant. The model explicitly computes the heat transport by large-scale atmospheric disturbances. It contains the following simplifications: a limited computational domain, an idealized topography, no heat transport by ocean currents, no seasonal variation, and fixed cloudiness.

It is found that the temperature of the model troposphere increases with increasing solar radiation. The greatest increase occurs in the surface layer of higher latitudes due to the effects of the snow-cover feedback mechanism as well as the suppression of vertical mixing by a stable lower troposphere. This result is found to be qualitatively similar to that obtained from previous studies with one-dimensional zonal mean models.

One of the most interesting features of this investigation is the extreme sensitivity of the intensity of the computed hydrologic cycle to small changes of the solar constant. Current estimates indicate a 27% increase of the former as compared with a 6% increase of the latter. This large intensification of the hydrologic cycle in the model atmosphere results from the increase in the rate of evaporation which is caused by the following changes: 1) reduction of the Bowen ratio due to the nonlinear increase of saturation vapor pressure with increasing temperature at the earth's surface, and 2) decrease in the net upward terrestrial surface radiation resulting from the increase in the moisture content in air and from the reduction of the lapse rate (both of which increase the downward terrestrial radiation and increase the energy available for evaporation).

It is shown that the latitude of maximum snowfall retreats poleward as the solar constant is increased. Furthermore, the total amounts of snowfall and snow accumulation decrease markedly with increasing insolation due to the poleward shift of the region of subfreezing surface temperature away from the zone of maximum baroclinic instability.

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Richard T. Wetherald and Syukuro Manabe

Abstract

This study discusses how the sensitivity of climate may be affected by the variation of cloud cover based on the results from numerical experiments with a highly simplified, three-dimensional model of the atmospheric general circulation. The model explicitly computes the heat transport by large-scale atmospheric disturbances. It contains the following simplifications: a limited computational domain, an idealized geography, no heat transport by ocean currents and no seasonal variation. Two versions of the model are constructed. The first version includes prognostic schemes of cloud cover and its radiative influences, and the second version uses a prescribed distribution of cloud cover for the computation of radiative transfer. Two sets of equilibrium climates are obtained from the long-term integrations of both versions of the model for several values of the solar constant. Based on the comparison between the variable and the fixed cloud experiments, the influences of cloud cover variation on the response of a model climate to an increase of the solar constant are identified.

It is found that, in response to an increase of the solar constant, cloudiness diminishes in the upper and middle troposphere at most latitudes and increases near the earth's surface and the lower stratosphere of the model particularly in higher latitudes. Because of the changes described above, the total cloud amount diminishes in the region equatorward of 50° latitude with the exception of a narrow sub-tropical belt. However, it increases in the region poleward of this latitude. In both regions, the area mean change in the net incoming solar radiation, which is attributable to the cloud-cover change described above, is approximately compensated by the corresponding change in the outgoing terrestrial radiation at the top of the model atmosphere. For example, equatorward of 50° latitude, the reduction of both cloud amount and effective cloud-top height contributes to the increase in the area-mean flux of outgoing terrestrial radiation and compensates for the increase in the flux of net incoming solar radiation caused by the reduction of cloud amount. Poleward of 50° latitude, the increase of cloudiness contributes to the reduction of both net incoming solar and outgoing terrestrial fluxes at the top of the model atmosphere. Although the effective cloud-top height does not change as it does in lower latitudes, the changes of these fluxes approximately compensate each other because of the smallness of insolation in high latitudes. Owing to the compensations mentioned above, the changes of cloud cover have a relatively minor effect on the sensitivity of the area-mean climate of the model.

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Syukuro Manabe and Richard T. Wetherald

Abstract

A study of the climatic effect of doubling or quadrupling of CO2 in the atmosphere has been continued by the use of a simple general circulation model with a limited computational domain, highly idealized geography, no seasonal variation of insolation, and a simplified interaction between cloud and radiative transfer.

The results from the numerical experiments reveal that the response of the model climate to an increase of CO2 content in air is far from uniform geographically. For example, one can identify the high-latitude region of the continent where the runoff rate increases markedly, a zonal belt of decreasing soil moisture around 42° latitude, and a zone of enhanced wetness along the east coast of the subtropical portion of the model continent.

The general warming and the increase of moisture content of air, which results from a CO2 increase, contributes to the large reduction of the meridional temperature gradient in the lower model troposphere because of 1) poleward retreat of highly reflective snow cover and 2) large increase in the poleward transport of latent heat. The reduction of the meridional temperature gradient appears to reduce not only the eddy kinetic energy, but also the variance of temperature in the lower model troposphere. The penetration of moisture into higher latitudes in the CO2-rich warm climate is responsible for the large increase of the rates of precipitation and runoff in high latitudes of the model.

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RICHARD T. WETHERALD and SYUKURO MANABE

Abstract

The effect of the seasonal variation of solar radiation is incorporated into the joint ocean-atmosphere model developed at the Geophysical Fluid Dynamics Laboratory of the National Oceanic and Atmospheric Administration, and the resulting system is integrated for the 11/2-yr model time. The purpose of this study is to analyze the response of the joint air-sea model to seasonal changes in the solar zenith angle rather than to obtain a true equilibrium state. Comparisons are also made with results previously presented for the case of annual mean conditions.

The most important feature that emerges as a direct result of this seasonal variation is a significant warming of the lower troposphere in high latitudes. This warming is found to be caused by (1) the removal of the snowpack during the summer season, which decreases the earth's albedo there during this time, and (2) a net rise in the temperature of the ocean surface in high latitudes as a result of the seasonal variation of convective activity in the surface layer of the ocean. The present results indicate that the snow cover effect is the primary factor responsible for this warming trend whereas the ocean effect is of secondary importance.

The main consequences of this high latitude warming include a reduction of the mean atmospheric north–south temperature gradient (and, therefore, a reduction of baroclinic instability in middle latitudes), a reduction of the mean oceanic meridional circulation, and a reduction of the atmospheric and oceanic poleward heat energy transports.

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