A Discussion of Upper Stratospheric Ozone Asymmetry and Ozone Trend Changes

Analyses from SAGE I/II version 6.0 data exhibit upper stratospheric ozone trends which are not significantly different from those in version 5.96 data. Trend calculations show larger downward trends at mid-high latitudes in the Southern Hemisphere than in the Northern Hemisphere, particularly in 1980s. There are also indications of decreasing downward trends with time from 1979 to 1999. We have used a chemical box model and the UARS measurements of long lived gases, CH4, H2O, NO(x), and temperature to show that, with a constant Cl(sub y) trend, a hemispheric ozone trend asymmetry of 1%/decade at 45 deg. around 43 km is expected due to the hemispheric differences of temperature and CH4 during late winter/early. Also ozone trends should have been approximately 1%/decade more negative from 1979-1989 than from 1989-1999 because of the chemical feedbacks. The model results further indicate that both the reported decrease in CH4 and the increase in H2O in HALOE measurements will result in a larger downward ozone trend and a decrease in the hemispheric ozone trend asymmetry.