Glacial Cooling in the Tropics: Exploring the Roles of Tropospheric Water Vapor, Surface Wind Speed, and Boundary Layer Processes

Abstract

This paper is a modeling study of possible roles for tropospheric water vapor, surface wind speed, and boundary layer processes in glacial cooling in the Tropics. The authors divide the Tropics into a region of persistent deep convection and a subtropical region with no deep convection. The regions are coupled via a radiatively driven Hadley cell and a wind-driven meridional overturning cell in the ocean. Radiation and the convective boundary layer (CBL) are treated in some detail. The amount of tropical cooling depends on the height of the tropospheric drying and on the extent to which cloud water in the CBL is converted into rainwater. In the most realistic case where the CBL clouds precipitate, variations in CBL depth are small, and the tropical SST becomes most sensitive to drying immediately above the CBL. Reducing the relative humidity of the entire troposphere above the subcloud layer by about 10%?20% cools the tropical SST by just over 2 K. It is shown that this climate sensitivity arises from a complex balance of processes that control the depth of the CBL, its greenhouse trapping, and the albedo of boundary layer clouds. An increase in surface wind speed, such as occurs in simulations of the last glacial maximum with coupled general circulation models, substantially reduces the SST although the change in surface air temperature is less. The Milankovitch cycles are expected to cause changes in atmosphere and ocean circulation. It appears that a circulation change that causes the lower midtroposphere to dry would be an effective way to induce strong cooling of tropical climate.