Wave-Driven Mean Tropical Upwelling in the Lower Stratosphere

The mean meridional circulation induced by zonal momentum forcing is considered in the context of (i) a wave–mean model, in which the momentum forcing arises naturally from the explicit representation of the stratospheric wave response to lower boundary forcing, and (ii) a zonally symmetric model, in which the momentum forcing is prescribed externally. Wave forcing is confined to one hemisphere, as a simple representation of the difference between the Northern and Southern Hemispheres. The main focus of the paper is on how the upwelling branch of the circulation in the Tropics depends on nonlinearity, model diffusion, and the transient effects of the seasonal cycle. Significant tropical upwelling can occur in the steady state if the zonal momentum force extends into the subtropics, provided the forcing is strong enough to redistribute angular momentum throughout the Tropics. The circulation in these strongly forced cases depends nonlinearly on the forcing amplitude. If the forcing is weak, so that the dynamics are approximately linear, significant tropical upwelling can occur in the steady state if (i) the radiative equilibrium temperature is such that angular momentum contours tilt poleward and upward, as at the winter solstice, or (ii) the model diffusion is large. In seasonally varying cases, weak subtropical forcing can also induce a time average tropical upwelling through the interaction of the transient angular momentum distribution and the transient forcing. When the forcing is strong, the effect of the seasonal cycle leads to an increase in the tropical upwelling beyond that of the steady-state response.