INTERANNUAL VARIABILITY OF THE STRATOSPHERIC CIRCULATION IN THE SOUTHERN-HEMISPHERE

The variability from year-to-year of the seasonal evolution in the sou thern hemisphere stratosphere over a period of 10 years, from 1980 to 1989, is investigated using the global geopotential height derived fro m the data from the Stratospheric Sounding Unit on board the TIROS-N N OAA satellites. As a measure of interannual variability, the variance of the zonal mean geostrophic wind over the 10 years was calculated fo r each day. Although the major warming hardly occurs in the southern h emisphere stratosphere, large variances can be observed during the win ter and spring. The maximum variances appear at low latitudes in late autumn, move towards mid latitudes in midwinter and remain near 60-deg rees-S in spring. The seasonal movement of the zonal mean westerly jet in the southern hemisphere stratosphere can be classified into two ca tegories in terms of the location of the maximum westerlies at the 1 m b level in midwinter, namely HLJ (high-latitude-jet) years with maximu m westerlies around 50-degrees-S, LLJ (low-latitude-jet) years with ma ximum westerlies around 40-degrees-S. During the late winter the core of the westerly jet moves polewards earlier in HLJ years than in LLJ y ears. In association with this earlier movement, the growth of the amp litudes of the planetary waves 1 and 2 during the period from winter t o spring occurs earlier in HLJ years than in LLJ years. In autumn, how ever, the wave-I amplitude, only for LLJ years, develops vigorously in the stratosphere and even also in the troposphere, before the seasona l movement of the zonal mean westerly jet branches off into the two ca tegories. The appearances of HLJ and LLJ years occur in groups of a fe w years; this is quite different from the behaviour of the equatorial quasi-biennial oscillation.