A Simple Theory for Increasing Unevenness in Tropical Rainfall with Warming

Abstract:

Tropical rainfall on average increases with global warming. However, dry regions experience rainfall reductions, while wet regions receive even more rain. Here we present a simple theory to quantitatively capture this behavior. We first lay the foundation by showing that two idealized theories, namely convective quasi-equilibrium (QE) and weak temperature gradient (WTG), can form a zeroth-order picture for the relationship between rainfall and subcloud moist static energy (MSE) in the tropics. Then we show that subcloud MSE gradients are amplified in a warmer world, and thus the present‐day wet regions will become more competitive for convection. The central result of this work is a theoretical scaling that explains this amplification of subcloud MSE gradients based on the Clausius‐Clapeyron relationship, the assumption of uniform warming, and constant relative humidity.