Glacial cooling in the tropics: Exploring the roles of tropospheric water vapor, surface wind speed, and boundary layer processes

This paper is a modeling study of possible roles for tropospheric water vap or, surface wind speed, and boundary layer processes in glacial cooling in the Tropics. The authors divide the Tropics into a region of persistent dee p convection and a subtropical region with no deep convection. The regions are coupled via a radiatively driven Hadley cell and a wind-driven meridion al overturning cell in the ocean. Radiation and the convective boundary lay er (CBL) are treated in some detail. The amount of tropical cooling depends on the height of the tropospheric dr ying and on the extent to which cloud water in the CBL is converted into ra inwater. In the most realistic case where the CBL clouds precipitate, varia tions in CBL depth are small, and the tropical SST becomes most sensitive t o drying immediately above the CBL. Reducing the relative humidity of the e ntire troposphere above the subcloud layer by about 10%-20% cools the tropi cal 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 i n surface wind speed, such as occurs in simulations of the last glacial max imum with coupled general circulation models, substantially reduces the SST although the change in surface air temperature is less. The Milankovitch c ycles 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.