DESCENT OF LONG-LIVED TRACE GASES IN THE WINTER POLAR VORTEX

Recent observations of CH4 and HF from the UARS Halogen Limb Occultati on Experiment (HALOE) suggest that vigorous descent occurs within the polar winter vortex with ''mesospheric'' values of CH4 evident down to 30 mbar. This study shows that a highly accurate two-dimensional mode l advection scheme coupled with a modern radiation scheme, parameteriz ed planetary and gravity wave drag algorithms can produce tracer distr ibutions consistent with HALOE observations. The modeled tracer distri bution within the polar vortex is found to be principally dependent on the strength of dynamical drag in the middle atmosphere and the stren gth of the planetary wave forcing. However, the strong downward transp ort of tracers at the poles during winter can be disrupted in midwinte r by planetary wave mixing. Thus the weaker planetary wave forcing in the southern hemisphere winter allows for a more coherent descent of l ong-lived tracers from the mesosphere than during the northern hemisph ere winter. Multiple-year integrations of the model reveal a general c irculation of the stratosphere which lofts tracers to mesospheric alti tudes. Material removed from the mesosphere returns to the stratospher e principally within the polar regions. Upward vertical transport of m aterial is found to be enhanced by horizontal planetary wave mixing.