The non-traditional Coriolis force drives zonal shear in the tropical atmosphere: A systematic multiple-scales exposition

We employ a multiple-scales asymptotic analysis to derive a simple model for the mesoscale tropical atmosphere interacting with a field of cloud-scale convective circulations. Most importantly, we take account of the fact that cloud-scale convection in the tropics experiences a westward tilt under the influence of the non-traditional Coriolis force. The systematic approach uncovers a two-way coupling between the mesoscale and cloud-scales and provides a physically consistent closure via the specification of averaged flux terms. This closure takes the form of a non-local vertical diffusion of absolute mesoscale horizontal momentum, with a diffusion kernel containing all details of the underlying convective circulations. Ultimately, it is shown that the westward tilt in convection creates up-scale fluxes of momentum and drives a self-regulating vertical shear of zonal wind at the mesoscale. In the tropics where vertical shear is typically weak, the effect of the non-traditional Coriolis terms (NCT) is maximized, and therefore has a significant effect on mesoscale dynamics. The mechanism for self-regulation uncovers an underlying tendency for the tropical atmosphere to adjust towards a state where the vertical shear of the mesoscale zonal wind balances the non-traditional component of Earth’s rotation. Ultimately, this study indicates that the NCT plays a significant role in tropical dynamics and suggests that we reassess the validity of omitting its effect in meteorological models.