Ozone trends in the stratosphere viewed as a system of meteorological regimes

Analysis of the ozone field in the Northern and Southern Hemispheres, using both TOMS and Dobson data, has shown that the total ozone field can be separated into three distinct meteorological regimes. These are defined as (1) the tropical regime - between the equator and the upper troposphere subtropical front (2) the midlatitude regime - between the subtropical and polar fronts, (3) the polar regime - between the polar front and the polar vortex Within each regime, the daily mean total ozone is relatively constant, with a clearly separate value for each regime. The trends in total ozone in the Northern Hemisphere for the latitude zone between 25 and 60N for the period 1979-92 within each regime are between -1.6% and -1.9% per decade, much less than the trend obtained for the whole zone of -3.2 % per decade. This difference can only be resolved if, during the period 1979 to 1992, there was a northward movement of the subtropical and polar fronts. The zonal average now includes a larger area of the tropical regime with lower ozone, and a smaller area of the polar regime with higher ozone, leading to a larger negative trend. We have examined total ozone data over the period 1965 to 2000, and find the same correlation between ozone variability and frontal movement. This is direct evidence that the observed mid-latitude total ozone trends are the result of both chemistry and dynamics. This paper will present the results of the analysis that leads to this conclusion, and also examines the implications on the trends within the ozone profile.