Integrating Thermodynamic and Dynamic Views on the Control of the Top‐Heaviness of Convection in the Pacific ITCZ With Weak Temperature Gradient Simulations

Abstract

AbstractUnderstanding what controls vertical motion profile shape is fundamental to understanding tropical precipitation patterns. Two controls have been previously studied: the thermodynamic profiles of the environment and the dynamics imposed by sea surface temperature (SST) patterns. To fit these two perspectives together, we focus on two regions with distinctly top and bottom‐heavy vertical motion: The Western Pacific and the Central Eastern Pacific. These regions have roughly the same column‐integrated water, precipitation, and column‐integrated horizontal moisture advection, however the shape in the West is top heavy while the East is bottom heavy. The top‐heaviness angle is introduced to describe this difference. To study thermodynamic controls on vertical motion profile shape, we use weak temperature gradient simulations. We are able to simulate the shape differences between our two regions from the thermodynamics. We then show that the dry static stability and the underlying SST are the most important for the vertical motion shape differences between our two regions. We show that the qualitative shape differences can be explained using a simple entraining plume model. The entraining plume model accepts the temperature and moisture profiles as inputs and outputs the plume's buoyancy, which is directly related to the vertical motion profile shape. We find that increasing the dry static stability leads to bottom‐heaviness. We hypothesize that the SST gradients lead to a cooler equilibrium lower tropospheric temperature compared with no gradient, and this leads to a more conducive thermodynamic environment to bottom‐heaviness. Hence the dynamics control top‐heaviness through influencing the thermodynamic profiles.