Regulation of tropical deep convection by boundary layer momentum dynamics and free tropospheric thermodynamics

Theories for the position and intensity of precipitation over tropical oceans on climatic time scales fall broadly in two categories. Some, Lindzen and Nigam (1987) being a paradigmatic example, focus on the momentum budget of the planetary boundary layer (PBL), driven by forcings other than those directly associated with deep convection itself, while others focus on local thermodynamic factors. In the latter, horizontal structure is largely irrelevant, except in a mean field sense (i.e., is the SST large or small compared to some very large-scale mean value), while in the former the horizontal structure provided by the lower boundary condition (e.g. the SST gradient) is essential. Particularly in the case of narrow intertropical convergence zones (ITCZs), there is some evidence for both classes of theories. This is perplexing, since the two are so completely different.

We develop a simple model to help us think about this problem. The model includes a mixed layer representation of PBL, coupled to a free troposphere whose vertical structure is that of the quasi-equilibrium tropical circulation model developed by Neelin and Zeng, except that the vertical structure functions now begin at PBL top instead of the surface. In this model, the interaction of boundary layer momentum dynamics and free-tropospheric thermodynamics is relatively transparent and controlled by a small number of parameters. The model is used to understand this interaction in the cases of convectively coupled gravity waves and steady, axisymmetric ITCZs.