Impact Mechanisms of Shallow Cumulus Convection on Tropical Climate Dynamics

Abstract

Subtropical shallow cumulus convection is shown to play an important role in tropical climate dynamics, in which convective mixing between the atmospheric boundary layer and the free troposphere initiates a chain of large-scale feedbacks. It is found that the presence of shallow convection in the subtropics helps set the width and intensity of oceanic ITCZs, a mechanism here termed the shallow cumulus humidity throttle because of the control exerted on the moisture supply to the deep convection zones. These conclusions are reached after investigations based on a tropical climate model of intermediate complexity, with sufficient vertical degrees of freedom to capture (i) the effects of shallow convection on the boundary layer moisture budget and (ii) the dependency of deep convection on the free-tropospheric humidity. An explicit shallow cumulus mixing time scale in this simple parameterization is varied to assess sensitivity, with moist static energy budget analysis aiding to identify how the local effect of shallow convection is balanced globally. A reduction in the mixing efficiency of shallow convection leads to a more humid atmospheric mixed layer, and less surface evaporation, with a drier free troposphere outside of the convecting zones. Advection of drier free-tropospheric air from the subtropics by transients such as dry intrusions, as well as by mean inflow, causes a substantial narrowing of the convection zones by inhibition of deep convection at their margins. In the tropical mean, the reduction of convection by this narrowing more than compensates for the reduction in surface evaporation. Balance is established via a substantial decrease in tropospheric temperatures throughout the Tropics, associated with the reduction in convective heating. The temperature response?and associated radiative contribution to the net flux into the column?have broad spatial scales, while the reduction of surface evaporation is concentrated outside of the convecting zones. This results in differential net flux across the convecting zone, in a sense that acts to destabilize those areas that do convect. This results in stronger large-scale convergence and more intense convection within a narrower area. Finally, mixed layer ocean experiments show that in a coupled ocean?atmosphere system this indirect feedback mechanism can lead to SST differences up to +2 K between cases with different shallow cumulus mixing time, tending to counteract the direct radiative impact of low subtropical clouds on SST.