The energy efficiency of tropical circulations

In 1987, Neelin & Held introduced the concept of the "gross moist stability" (GMS) to quantify how efficiently the tropical circulation transports energy. They constructed a simple model in which the spatial pattern of the GMS plays a leading role in determining the time-mean distribution of precipitation in the tropics. Since then, further work has revealed the importance of the GMS in theories of the Hadley Cell, the width of the intertropical convergence zone, and convectively coupled circulations, but a theory for the GMS itself remains elusive.

Here, I show that the atmospheric energy balance places strong constraints on the spatial distribution of the GMS, specifically, that the GMS must be uncorrelated with large-scale upward motion. This is contrary to the conventional view that convergence zones coincide with minima in the GMS. The importance of this result for convectively coupled circulations is explored using a series of convection-permitting simulations of a Mock-Walker cell in an idealised channel geometry. By varying an imposed radiative cooling profile, the vertical structure of the circulation is changed, allowing for large variations in the GMS. The results are then interpreted through a modified version of the theory of slow convectively coupled processes of Emanuel (2019).