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Abstract

The three-dimensional evolution of the final warming that takes place in the stratosphere of the southern hemisphere during spring is studied using data from a satellite. The event of spring 1982 is discussed in detail, and other events from an 8-year set are briefly surveyed. The zonal-mean westerly jet moves poleward and downward in spring as strong, planetary-scale disturbances develop which contribute to the weakening of the stratospheric westerly vortex. The processes governing this weakening are discussed by reference to isentropic maps of Ertel's potential vorticity and associated area diagnostics. The vortex breaks down first in the upper stratosphere and then later (and more slowly) in the middle stratosphere. This behaviour is broadly reproduced year after year. Repeating life-cycles of growth, eastward movement and decay of anticyclones in the stratosphere are described and related to the behaviour of quasi-stationary wave 1 and eastward-travelling wave 2. Evidence that the topography of the southern hemisphere exerts a strong influence on the evolution of the final warming is presented. An association is found between the location of anticylones in the upper stratosphere, warm pools of air in the lower stratosphere and a climatological split of the westerly jet stream in the upper troposphere.