SPRINGTIME STRATOSPHERIC WATER-VAPOR IN THE SOUTHERN-HEMISPHERE AS MEASURED BY MLS

The effects of the break-up of the antarctic vortex on the water vapour distribution are studied using MLS measurements of water vapour made during September 1991 and November 1991. In early November at 22 hPa a moist area is found within the polar vortex, consistent with an observed descent of order 10 km and strong radiative cooling. As the vortex erodes (beginning of November 1991), parcels of moist air become detached from the edge of the vortex and mix rapidly (within 2-3 days) with drier mid-latitude air. When the vortex breaks up (mid-November), larger parcels of moist air from both the edge and the inner vortex migrate to mid-latitudes. These parcels have a longer lifetime than those produced by vortex erosion, probably because they are correlated with higher potential vorticity gradients. The break-up of the vortex is accompanied by a mean adiabatic equatorward transport resulting in a significant increase in midstratospheric water vapour values at mid-latitudes in late spring.