Flattened Moisture Gradients, Trade Wind Weakening, and ITCZ Breakdown
Associated with Eddy Moisture Fluxes
Abstract
A moist static energy budget (MSE)-based transformed Eulerian mean
framework is applied to a two-layer model to understand how eddies
impact the mean tropical circulation during boreal summer. For fixed
sources and sinks of MSE, the mean overturning circulation weakens in
proportion to the eddy moisture flux divided by the normalized gross
moist stability (NGMS), a measure of the moist static stability. Since
the NGMS exhibits values between 0.1-0.2 in the rainy areas of the
tropics, the eddy moisture fluxes do not have to be large to flatten the
column moisture gradient, weaken the trade winds, and cause ITCZ break
down. Based on these findings, it is suggested that the
recently-documented convective aggregation-disaggregation cycles are
mediated by eddy activity. A comparison of days in which the mean
circulation is weak versus those that are strong in ERA5 data are
largely in agreement with the theory.