EVOLUTION OF MICROWAVE LIMB SOUNDER OZONE AND THE POLAR VORTEX DURINGWINTER

The evolution of polar ozone observed by the Upper Atmosphere Research Satellite Microwave Limb Sounder is described for the northern hemisp here (NH) winters of 1991/1992, 1992/1993, and 1993/1994 and the south ern hemisphere (SH) winters of 1992 and 1993. Interannual and interhem ispheric variability in polar ozone evolution are closely related to d ifferences in the polar vortex and to the frequency, duration and stre ngth of stratospheric sudden warmings. Ozone in the midstratospheric v ortices increases during the winter, with largest increases associated with stratospheric warmings and a much larger increase in the NH than in the SH. A smaller NH increase was observed in 1993/1994, when the middle stratospheric vortex was stronger. During strong stratospheric warmings in the NH, the upper stratospheric vortex may be so much erod ed that it presents little barrier to poleward transport; in contrast, the SH vortex remains strong throughout the stratosphere during winte rtime warmings, and ozone increases only below the mixing ratio peak, due to enhanced diabatic descent. Ozone mixing ratios decrease rapidly in the lower stratosphere in both SH late winters, as expected from c hemical destruction due to enhanced reactive chlorine. The interplay b etween dynamics and chemistry is more complex in the NH lower stratosp here and interannual variability is greater. Evidence has previously b een shown for chemical ozone destruction in the 1991/1992 and 1992/199 3 winters. We show here evidence suggesting some chemical destruction in late February and early March 1994. In the NH late winter lower str atosphere the pattern of high-ozone values (typical of the vortex) see n in mid-latitudes is related to the strength of the lower-stratospher ic vortex, with the largest areal extent of high ozone outside the vor tex in 1994, when the lower stratospheric vortex is relatively weak, a nd the least extent in 1993 when the lower stratospheric vortex is str ongest.