The role of shallow heating in driving tropical atmospheric circulations

One of the remaining mysteries related to the tropical circulations is the feedback between the large-scale convective heating and the circulation driven by such heating. In the past two decades, significant amount of modeling evidence has accumulated to show that the Conditional Instability of the Second Kind(CISK)can not effectively explain that feedback since elevated deep heating can not drive strong enough convergent low-level winds required to maintain the heating. On the other hand, many observational results have shown that there exists strong correlation between the outgoing langwave radiation (OLR), that is an index of the strength of the deep convection, and the convergent surface winds. The seemingly contradictory results imply that there is an important component missing in the current theory on tropical atmospheric dynamics.

In this study, that missing component in the tropical heating-circulation system is revealed. We propose, based on our theoretical studies, that the missing piece in the feedback is the warm precipitation associated with the shallow convection that has been ignored in most investigations for many decades. The surface winds driven by the latent heat released in the shallow convective system can not only provide enough moisture convergence to support shallow convection but also serve as a moisture source for the deep convection. To verify our theoretical results, a dry primitive equation model is deployed to simulate the circulation driven by a shallow heat source. The modeling results agree well with our theoretical arguments.

Based on these results, a new paradigm of the tropical heating-circulation system is drawn. The potential implications are also stated.