Annual and interannual variations of tropical upwelling in the lower stratosphere

Several idealized models of tropical upwelling are presented in order to clarify the roles of the nonlinear Hadley circulation and extratropical wave driving in determining tropical upwelling in the lower stratosphere. In particular, we show that the Hadley circulation and wave-driven circulation interact to determine the nature of tropical upwelling. We are able to explain several observed features such as maximum upwelling in the summer subtropics and the annual variation of the upwelling.; Since the net trace species exchange depends on the latitudinal and temporal correlations between the tracer concentrations and the mass flow, both spatial and temporal variations of tropical upwelling velocity are explored. Sensitivity tests are conducted to show how the morphology of tropical upwelling responds to the magnitude and structure of planetary wave drag.; The atmospheric data are analyzed to show that the magnitudes of wave drags change year to year and there exist interannual variations in the penetrations of wave drags into lower latitudes. The influence of such variations on the interannual variability of the magnitude and latitudinal distribution of tropical upwelling in the lower stratosphere are discussed, which might provide some insight into the interannual variability of stratospheric water vapor.