Global ozone trends in potential vorticity coordinates using TOMS and GOME intercompared against the

Global total column ozone trends were calculated by applying a least squares regression model to five homogenized satellite data sets from November 1978 to December 1998. Drifts and offsets in the satellite data were removed through comparisons of coincident satellite overpass and ground-based Dobson spectrophotometer measurements and through intersatellite comparisons. The satellite data were remapped into a potential vorticity coordinate (equivalent latitude) before zonal means were calculated. This remapping preserves steep meridional gradients in ozone and ozone trends across the winter polar vortex boundary and thereby reveals statistically significant negative trends just poleward of the Antarctic vortex boundary (∼70°S equivalent latitude) during May (−0.51±0.42% yr−1 (2σ)), June (−0.65±0.44% yr−1) and July (−0.85±0.44% yr−1). This feature is indistinct in trends derived as a function of geographic latitude most likely as a result of the smoothing by zonal averaging across steep ozone gradients. Comparison with similar earlier analysis (November 1978 to May 1991) indicates that Antarctic ozone trends have weakened (from −3.71±1.80% yr−1 to −2.95±0.40% yr−1), most likely as a result of saturation, while Arctic ozone trends are now much larger (from −1.05±0.96%yr−1 to −1.93±0.40% yr−1) following severe wintertime Arctic ozone depletion in recent years. Midlatitude negative trends calculated over the longer time period are also slightly reduced.

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