Convective Circulations and the Coriolis Force: A Mechanism for Upscale Momentum Fluxes in the Tropics

Abstract In the study of subgrid-scale tropical convection, the importance of retaining the frequently omitted “nontraditional” component of the Coriolis force is increasingly being recognized. A number of recent papers have developed linear theories examining the behavior of a diabatic heat-source-driven convective circulation in the presence of the full Coriolis force, and it was shown that the nontraditional Coriolis terms drive vertical shears on the large scales through upscale fluxes of momentum. In the present work, we generalize these results to the nonlinear regime, using a formal asymptotic theory based upon the fact that rotation is a second-order effect compared with advection by the vertical component of velocity at subgrid scales. Ultimately, we demonstrate that the same basic flow structures persist, with a particular emphasis on the counterrotating vortex pair induced by the nontraditional Coriolis terms which drive a westward tilt in convection. We compute the form of the upscale momentum flux convergence in the nonlinear regime, greatly extending the regimes of validity provided by the simple analytical expressions previously given in the linear case. This study constitutes an important step toward being able to accurately and consistently parameterize the large-scale vertical shear driven by nonlinear, subgrid convective processes under the influence of the nontraditional Coriolis force terms.