| Summary: | To investigate a descent rate in the Antarctic polar vortex, we analyzed the long-lived trace gas data derived from the Halogen Occultation Experiment on board the Upper Atmosphere Research Satellite during the 6-year period from 1992 to 1997. By comparing the Antarctic fall (February and March) and spring (September and October) methane profiles, we estimated the middle stratospheric descent for each of the six winters. Large year-to-year variations are seen (1.2–1.8 km month−1 at 0.6 ppmv), which consist of a biennial oscillation and a decreasing trend for the period analyzed. The descent rate is larger in the even years (1992, 1994, and 1996) than in the odd years (1993, 1995, and 1997). Dynamical fields for the 6 years are also analyzed using the United Kingdom Meteorological Office assimilation data. The differences between the even and odd years are clear in the midwinter. In the even years the downward and poleward movement of the westerly jet occurs earlier. The thermal wind relation infers that this event is associated with the development of a "warm pool" around the Antarctic stratopause, resulting from adiabatic heating due to the downward motion of air. Planetary wave activity over the winter season is more vigorous in the even years than in the odd years, suggesting a close relationship between the mean flow and planetary waves.
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