ON THE MOTION OF AIR THROUGH THE STRATOSPHERIC POLAR VORTEX

Trajectory calculations using horizontal winds from the U.K. Meteorolo gical Office data assimilation system and vertical velocities from a r adiation calculation are used to simulate the three-dimensional motion of air through the stratospheric polar vortex for Northern Hemisphere (NH) and Southern Hemisphere (SH) winters since the launch of the Upp er Atmosphere Research Satellite. Throughout the winter, air from the upper stratosphere moves poleward and descends into the middle stratos phere. In the SH lower to middle stratosphere, strongest descent occur s near the edge of the polar vortex, with that edge defined by mixing characteristics. The NH shows a similar pattern in late winter, but in early winter strongest descent is near the center of the vortex, exce pt when wave activity is particularly strong. Strong barriers to latit udinal mixing exist above about 420 K throughout the winter. Below thi s, the polar night jet is weak in early winter, so air descending belo w that level mixes between polar and middle latitudes. In late winter, parcels descend less and the polar night jet moves downward, so there is less latitudinal mixing. The degree of mixing in the lower stratos phere thus depends strongly on the position and evolution of the polar night jet and on the amount of descent experienced by the air parcels ; these characteristics show considerable interannual variability in b oth hemispheres. The computed trajectories provide a three-dimensional picture of air motion during the final warming. Large tongues of air are drawn off the vortex and stretched into increasingly long and narr ow tongues extending into low latitudes. This vortex erosion process p roceeds more rapidly in the NH than in the SH. In the lower stratosphe re, the majority of air parcels remain confined within a lingering reg ion of strong potential vorticity gradients into December in the SH an d April in the NH, well after the vortex breaks up in the midstratosph ere.