Time variations of descent in the Antarctic vortex during the early winter of 1997

Abstract

We analyzed long‐lived chemical constituents observed by the Improved Limb Atmospheric Spectrometer (ILAS) on board the Advanced Earth Observing Satellite (ADEOS) to study the stratospheric descent in the Southern Hemisphere polar vortex. The ILAS N2O distribution inside the polar vortex exhibited clear downward motion in February to June 1997. Average descent for the 5 months is estimated to be ∼2.1–1.7 km month−1 in the middle stratosphere. In late April to June when planetary waves are relatively active, the vertical velocity displays time variations with a period of about 10 days. These time variations also synchronize with both time variations of the temperature time change (∂/∂t) and the Eliassen‐Palm flux divergence (DF) in high latitudes. Moreover, a correlation coefficient map in the latitude‐height cross section between the vertical velocity and the temperature time change reveals an interesting four‐box pattern, suggesting warming below 10 hPa and cooling above 10 hPa in the polar region (70°–90°S) and an opposite distribution in midlatitudes (40°–70°S), when large descent is observed inside the polar vortex. It is just like the meridional circulation in response to DF induced by planetary waves, which was first illustrated by Matsuno's stratospheric sudden warming theory.