The Mechanisms of Summer Dryness Induced by Greenhouse Warming

Abstract

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.