Vortex dynamics and the evolution of water vapour in the stratosphere of the southern hemisphere

The seasonal evolution of water vapour in the stratosphere of the southern 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 produced at the UK Meteorological Office by data assimilation. The processes governing the distribution of water vapour are clarified by focusing on the physical and dynamical conditions in and around the stratospheric polar vortex. Sustained diabatic descent in the vortex causes isopleths of water vapour mixing ratio to dip down markedly in the polar vortex, strengthening radial gradients of water vapour in the westerly jet, while stretching and folding of material lines in anticyclones adjacent to the polar vortex leads to a widening zone of weak horizontal gradients of water vapour. The circulation is discussed in terms of the dynamics of interacting vortices. Two different flow regimes are identified: (a) mid and late southern winter, with a strong polar vortex 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 stratosphere 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 particles. A comparison is made of the seasonal evolution of water vapour in the stratosphere of the southern and northern hemispheres.