Mechanisms of Global Warming Impacts on Regional Tropical Precipitation*

Mechanisms that determine the tropical precipitation anomalies under global warming are examined in an intermediate atmospheric model coupled with a simple land surface and a mixed layer ocean. To compensate for the warm tropospheric temperature, atmospheric boundary layer (ABL) moisture must increase to maintain positive convective available potential energy (CAPE) in convective regions. In nonconvective regions, ABL moisture is controlled by different balances and does not increase as much, creating a spatial gradient of ABL moisture anomalies. Associated with this spatial pattern of the ABL moisture anomalies are two main mechanisms responsible for the anomalous tropical precipitation. In the ''upped-ante mechanism,'' increases in ABL moisture are opposed by imported dry air wherever inflow from nonconvective regions over margins of convective regions occurs. The ABL moisture is not enough to meet the higher ''convective ante'' induced by the warmer tropospheric temperature, so precipitation is decreased. In the ''anomalous gross moist stability mechanism,'' gross moist stability is reduced due to increased ABL moisture. As a result, convection is enhanced and precipitation becomes heavier over convective regions. While the upped-ante mechanism induces negative precipitation anomalies over the margins of convective regions, the anomalous gross moist stability mechanism induces positive precipitation anomalies within convective regions. The importance of variation in gross moist stability, which is likely to differ among climate models, is suggested as a potential factor causing discrepancies in the predicted regional tropical precipitation changes.