An observational study of the circulation in the Southern Hemisphere stratosphere

The dynamical processes that shape the three-dimensional evolution of the circulation in the Southern Hemisphere stratosphere during winter and spring are studied using observational data. Limited use is also made of a three-dimensional primitive-equation model of the circulation in the middle atmosphere. The warming during spring 1982 is analyzed in detail and other events from an eight-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. Repeating life cycles of growth, eastward-movement and decay of anticyclones in the stratosphere are described and related to the behavior of quasi-stationary wave 1 and eastward-traveling wave 2. Evidence that the orography exerts a strong influence on the evolution of the final warming is presented. During the winter, the stratospheric jet stream shifts downward in response to seasonal variations in thermal forcing. The poleward and downward shift of the jet axis in an individual year is usually abrupt and occurs in association with an enhancement of planetary wave activity. The wave driving of the mean flow in the stratosphere generally has a dipolar structure with easterly accelerations near 40 S and westerly accelerations in polar latitudes. In the three examples (1980, 1984, and 1985) of large early winter stratospheric disturbances with a predominately zonal wavenumber one pattern in the dataset, the high and low centers developed in the same geographical region. In all three cases, observations suggest, that the growth of the stratospheric disturbances is due to the amplification of the zonal wavenumber 1 component of the flow at 100 mb.