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 vapou r 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 obse rved descent of order 10 km and strong radiative cooling. As the vorte x erodes (beginning of November 1991), parcels of moist air become det ached from the edge of the vortex and mix rapidly (within 2-3 days) wi th drier mid-latitude air. When the vortex breaks up (mid-November), l arger parcels of moist air from both the edge and the inner vortex mig rate 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 a ccompanied by a mean adiabatic equatorward transport resulting in a si gnificant increase in midstratospheric water vapour values at mid-lati tudes in late spring.