Atmospheric Chemistry and Physics

Previous studies have presented clear evidence that the Northern Hemisphere total ozone field can be sep- arated into distinct regimes (tropical, midlatitude, polar, and arctic) the boundaries of which are associated with the sub- tropical and polar upper troposphere fronts, and in the winter, the polar vortex. This paper presents a study of total ozone variability within these regimes, from 1979-2003, using data from the TOMS instruments. The change in ozone within each regime for the period January 1979-May 1991, a pe- riod of rapid total ozone change, was studied in detail. Pre- vious studies had observed a zonal linear trend of 3.15% per decade for the latitude band 25 -60 N. When the ozone field is separated by regime, linear trends of 1.4%, 2.3%, and 3.0%, per decade for the tropical, midlatitude, and polar regimes, respectively, are observed. The changes in the rel- ative areas of the regimes were also derived from the ozone data. The relative area of the polar regime decreased by about 20%; the tropical regime increased by about 10% over this period. No significant change was detected for the midlat- itude regime. From the trends in the relative area and total ozone it is deduced that 35% of the trend between 25 and 60 N, from January 1979-May 1991 is due to movement of the upper troposphere fronts. The changes in the relative areas can be associated with a change in the mean latitude of the subtropical and polar fronts within the latitude inter- val 25 to 60 N. Over the period from January 1979 to May 1991, both fronts moved northward by 1.1±0.2 degrees per decade. Over the entire period of the study, 1979-2003, the subtropical front moved northward at a rate of 1.1±0.1 de- grees per decade, while the polar front moved by 0.5±0.1 degrees per decade.