Mechanism for Increasing Tropical Rainfall Unevenness With Global Warming

Global climate models predict that tropical rainfall will be distributed more unevenly with global warming; that is, dry regions or months will get drier and wet regions or months will get wetter. Previous mechanisms such as “dry-get-drier, wet-get-wetter”; “rich-get-richer”; or “upped-ante” focus on the spatial pattern of rainfall changes rather than the changes in probability distribution. Here, we present a quantitative explanation of the warming-induced probability distribution change of rainfall: Subcloud moist static energy (MSE) gradients are amplified by Clausius-Clapeyron relationship given roughly uniform warming and constant relative humidity. Therefore, the present-day wet regions will become more competitive for convection in a warmer world. Though changes in the atmospheric circulation pattern can enhance rainfall in one place and suppress rainfall in another, our results show that the total effect should be a decrease in the area of active convection even with uniform warming.