Interannual Changes of the Stratospheric Circulation: Influence on the Tropics and Southern Hemisphere

Abstract

Interannual changes of dynamical structure and ozone are investigated in the Tropics and Southern Hemisphere over the 1980s and 1990s. Changes of dynamical structure over the winter hemisphere are accompanied by coherent changes over the summer hemisphere, but of opposite sign. They are most noticeable during northern winter, when amplified planetary waves of the Northern Hemisphere drive strong downwelling in the Arctic stratosphere that penetrates well into the troposphere. Changes over the summer hemisphere operate coherently and in phase with weaker changes over the Tropics. Coherent changes appear even inside the tropical troposphere, where they coincide with regions of deep convection. Changes in the summer hemisphere and Tropics both operate coherently but out of phase with changes over the Arctic, which in turn operate coherently with anomalous forcing of the residual mean circulation. Anomalous summertime structure modulates the polar low in the upper troposphere and lowermost stratosphere. It modifies the wintertime spinup of westerlies and the storm track of the Southern Hemisphere. Very similar changes are found in total ozone. Like dynamical structure, anomalous ozone over the summer hemisphere operates coherently with anomalous ozone in the Tropics. Both are out of phase with anomalous ozone over the Arctic, which in turn operates coherently with anomalous forcing of the residual circulation. Anomalous ozone has the same basic structure as anomalous temperature. The two are consistent with anomalous upwelling over the Tropics and Southern Hemisphere that compensates anomalous downwelling over the Arctic. Compensation is also evident in systematic changes of ozone during the 1980s and 1990s. Interannual changes over the Southern Hemisphere during southern winter are weaker than changes over the Northern Hemisphere during northern winter. However, they have the same character. They operate coherently with anomalous forcing of the residual circulation, resembling the Southern Hemisphere counterpart of the Arctic Oscillation. Accompanying changes of ozone, which are as large as 50?100 DU, cover a wide area of the Southern Hemisphere. When mixed with chemically depleted polar air that is released during the spring breakdown of the vortex, they can make a significant perturbation to the net hemispheric overburden of ozone.