Sub-cloud moist entropy curvature as a predictor for changes in the seasonal cycle of tropical precipitation

Abstract

Convective Quasi-Equilibrium (CQE) may be a useful framework for understanding the precipitation minus evaporation (P − E) response to CO2-induced warming. To explore this proposition, a suite of aquaplanet simulations with a slab ocean from the Tropical Rain belts with an Annual cycle and a Continent Model Intercomparison Project (TRACMIP) is analyzed. A linear relationship between P − E and the curvature of sub-cloud moist entropy, a criterion for the onset of a tropical direct overturning circulation under CQE conditions, is shown to exist across many of the TRACMIP simulations. Furthermore, this linear relationship is a skillful predictor of changes in P − E in response to CO2-induced warming. The curvature metric also shows improvement in predicting P − E changes compared to the simpler method of relating P − E directly to the sub-cloud moist entropy field or a simple ‘wet-get-wetter’ type null hypothesis, especially on seasonal and shorter timescales. Using fixed relative humidity in the curvature metric and sub-cloud moist entropy degrades their ability to predict P − E changes, implying that both temperature and relative humidity changes in the boundary layer are important for characterizing future precipitation changes. To understand why the curvature metric is a skillful predictor of hydrological changes, a moist static energy (MSE) budget analysis is performed for a subset of the TRACMIP models. MSE divergence by transient eddies, which is well parameterized as a downgradient diffusive process, has a similar spatiotemporal structure to the curvature term, suggesting transient eddies are an important component to understanding the linear relationship between the curvature term and P − E.