Planetary-Scale Disturbances in the Southern Stratosphere during Early Winter

Abstract

The early winter (mid-April to mid-July) circulation in the Southern Hemisphere stratosphere is studied. Emphasis is placed on the evolution of strong disturbances with structures dominated by the zonal wavenumber-1 component of the flow (wave 1). The approach to this investigation is based on analysis of 12 years (1979?90) of observational data and comparative analyses of control and hypothesis-testing simulations with a three-dimensional primitive equation model of the stratosphere and mesosphere.Considerable interannual variability is found in both the intensity and timing of wave-1 amplification during early winter. Though usually quasi-stationary, there are six extended periods in the dataset when wave 1 travels steadily eastward and is of large amplitude. Two of these periods (June 1980 and June 1985) are examined in detail. The evolution of the circulation in these two cases resembles that during Canadian warmings in the Northern Hemisphere in several ways. First, there is a large, eastward-moving disturbance with a nearly equivalent barotropic structure, with the largest amplitude in the lower and middle stratosphere. Second, temperature increases are smaller than those observed during final warmings in the Southern Hemisphere. Third, irreversible buckling of contours of Ertel's potential vorticity takes place in a region well away from the zero-wind line. Owing to their geographical preference for development over the South Pacific, wave-1 events in the southern stratosphere during early winter are referred to as South Pacific warmings.The hypothesis-testing simulations suggest that the development of South Pacific warmings is connected with the amplification of wave 1 at 100 mb and that the eastward propagation of the disturbances requires eastward propagation of wave 1 at 100 mb. In addition, the results suggest that development of stratospheric disturbances in the southern stratosphere during early winter depends more on the intensity of wave 1 at 100 mb than on the structure of the zonal-mean flow in the stratosphere.