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Mechanisms by which extratropical wave forcing in the winter stratosphere induces upwelling in the summer hemisphere

Observational evidence presented here suggests that zonal body force in the middle- and high-latitude winter stratosphere can induce an extensive circulation with upwelling spreading into the summer hemisphere. Numerical model results are used to investigate the nature of the circulation anomaly and the mechanism by which it is produced. It is found that nonlinearity is important in allowing cross-equatorial flow, which connects the upwelling in the summer hemisphere to the wave forcing in the winter hemisphere. The connection between the summer and winter hemispheres in our model is through the cross-equatorial flow in the upper stratosphere in a region of low angular momentum gradient. The existence of an extensive circulation and the required cross-equatorial flow is sensitive to the location of the wave forcing in the winter extratropics. It requires that the wave forcing be present between 40 and 60 km and extends equatorward of 30° in the winter stratosphere. In the real atmosphere, planetary waves presumably break predominantly above 40 km and more equatorward in January and February. We therefore expect that such an extensive circulation exists in the atmosphere during late winter. Indeed, our model tracer, temperature, and zonal wind patterns are very similar to those observed with respect to the response of the nearly global circulation to extratropical wave forcing in winter.