The Brewer-Dobson circulation: Dynamics of the tropical upwelling

Recent advances in our understanding of the dynamics of the stratospheric c irculation have led to the concepts of "downward control" and the "extratro pical pump." However, under the assumptions on which these concepts are bas ed, midlatitude wave driving cannot explain the fact that mean stratospheri c upwelling is located in the Tropics. Nevertheless, using a nonlinear two- dimensional model it is shown here that a steady and (in the lower stratosp here) linear circulation with a qualitatively reasonable upwelling can be p roduced, provided the wave drag extends to within about 20 degrees of the e quator. In a linear analysis of the problem, it is shown that the effects o f weak model viscosity (some 50 times weaker than thermal relaxation) are c rucial in permitting flow across angular momentum contours within a tropica l boundary layer whose width is of order LRP1/4, where L-R is the equatoria l Rossby radius and P a Prandtl number (the ratio of radiative to viscous r elaxation times). Provided the wave drag extends into this boundary layer, upwelling is distributed across the Tropics. These considerations put limit s on the generality of the concepts of the extratropical pump and downward control and, inter alia, open the possibility that diabatic heating alone c an drive a meridional circulation within the Tropics. On the basis of simpl e representations of wave drag and diabatic healing in a nonlinear, zonally symmetric model, it is found that, although driving by wave drag is the do minant mechanism, stratospheric (and perhaps tropospheric) heating may make a significant contribution to the net upwelling and may help explain its s tructure. Just what, in reality, might play a role analogous to that of vis cosity in the model is an open question.