VORTEX DYNAMICS AND THE EVOLUTION OF WATER-VAPOR IN THE STRATOSPHERE OF THE SOUTHERN-HEMISPHERE

The seasonal evolution of water vapour in the stratosphere of the sout hern hemisphere is studied by using water vapour measurements made by the Microwave Limb Sounder on the Upper Atmosphere Research Satellite. This evolution is interpreted with the aid of meteorological fields p roduced at the UK Meteorological Office by data assimilation. The proc esses governing the distribution of water vapour are clarified by focu sing on the physical and dynamical conditions in and around the strato spheric polar vortex. Sustained diabatic descent in the vortex causes isopleths of water vapour mixing ratio to dip down markedly in the pol ar vortex, strengthening radial gradients of water vapour in the weste rly jet, while stretching and folding of material lines in anticyclone s adjacent to the polar vortex leads to a widening zone of weak horizo ntal gradients of water vapour. The circulation is discussed in terms of the dynamics of interacting vortices. Two different flow regimes ar e identified: (a) mid and late southern winter, with a strong polar vo rtex and one or more eastward-travelling anticyclone; and (b) spring, with a relatively weak polar vortex and a quasi-stationary anticyclone . The phenomenon of merger of anticyclones, already observed in the st ratosphere of the northern hemisphere, is documented for the southern hemisphere. A detailed study of the tracer transport during merger is made by computing the isentropic advection of many thousands of partic les. A comparison is made of the seasonal evolution of water vapour in the stratosphere of the southern and northern hemispheres.