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

Recent observations of CH4 and HF from the UARS Halogen Limb Occultation 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 model advection scheme coupled with a modern radiation scheme, parameterized planetary and gravity wave drag algorithms can produce tracer distributions consistent with HALOE observations. The modeled tracer distribution within the polar vortex is found to be principally dependent on the strength of dynamical drag in the middle atmosphere and the strength of the planetary wave forcing. However, the strong downward transport of tracers at the poles during winter can be disrupted in midwinter by planetary wave mixing. Thus the weaker planetary wave forcing in the southern hemisphere winter allows for a more coherent descent of long-lived tracers from the mesosphere than during the northern hemisphere winter. Multiple-year integrations of the model reveal a general circulation of the stratosphere which lofts tracers to mesospheric altitudes. Material removed from the mesosphere returns to the stratosphere principally within the polar regions. Upward vertical transport of material is found to be enhanced by horizontal planetary wave mixing.