INTERANNUAL VARIABILITY OF THE STRATOSPHERIC CIRCULATION IN THE SOUTHERN-HEMISPHERE

The variability from year-to-year of the seasonal evolution in the southern hemisphere stratosphere over a period of 10 years, from 1980 to 1989, is investigated using the global geopotential height derived from the data from the Stratospheric Sounding Unit on board the TIROS-N NOAA satellites. As a measure of interannual variability, the variance of the zonal mean geostrophic wind over the 10 years was calculated for each day. Although the major warming hardly occurs in the southern hemisphere stratosphere, large variances can be observed during the winter and spring. The maximum variances appear at low latitudes in late autumn, move towards mid latitudes in midwinter and remain near 60-degrees-S in spring. The seasonal movement of the zonal mean westerly jet in the southern hemisphere stratosphere can be classified into two categories in terms of the location of the maximum westerlies at the 1 mb level in midwinter, namely HLJ (high-latitude-jet) years with maximum westerlies around 50-degrees-S, LLJ (low-latitude-jet) years with maximum westerlies around 40-degrees-S. During the late winter the core of the westerly jet moves polewards earlier in HLJ years than in LLJ years. In association with this earlier movement, the growth of the amplitudes of the planetary waves 1 and 2 during the period from winter to spring occurs earlier in HLJ years than in LLJ years. In autumn, however, the wave-I amplitude, only for LLJ years, develops vigorously in the stratosphere and even also in the troposphere, before the seasonal movement of the zonal mean westerly jet branches off into the two categories. The appearances of HLJ and LLJ years occur in groups of a few years; this is quite different from the behaviour of the equatorial quasi-biennial oscillation.