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5. Summary

We have used a simple mechanistic model of the stratosphere to investigate the mean meridional circulation response of an atmosphere subjected to zonal momentum and radiative forcing, with particular emphasis on the low-latitude ``tropical upwelling''. Both a zonal wavenumber-one model with self-consistent waves and a zonally symmetric model in which the momentum forcing was derived from the wave-one model, produce upwelling responses that are in good agreement with each other, as well as with vertical velocities observed in the real stratosphere.

The zonally symmetric model was subsequently used with a compact momentum forcing to investigate more cleanly the effects of model diffusion, the seasonal cycle, and nonlinearity. It was found that, while diffusion was necessary to produce an upwelling response that penetrated deep into the tropics, nonlinearity and transience each played an important role in modifying the response. In particular, the dependence of the tropical upwelling on hyperdiffusion was markedly different between the linear and nonlinear cases. Further, when a seasonal cycle in both the momentum and the radiative forcing was included, the time average response was significantly greater than the steady state response obtained with the time averaged forcings.