The dynamically driven long-term trend in stratospheric ozone over northern middle latitudes

The cause of the observed mid-latitude decline in ozone in a 20-year integration of a stratospheric chemical transport model forced by European Centre for Medium-Range Weather Forecasts analyses for the years 1979 to 1998 is investigated. A very simple chemical scheme for ozone is used which includes no interannual variation so that any modelled interannual variability, or trend, must arise from changes in the meteorology. The integration from 1979 to 1998 does show a downward trend in mid-latitude ozone in which many of the observed features are reproduced, especially between the middle 1980s and the early 1990s. A detailed statistical trend analysis shows that the quantitative comparison between modelled and observed trend is very sensitive to the choice of period considered. However, there is good qualitative agreement in terms of the latitudinal variation of the trend. For the different periods considered, the dynamically driven model trend accounts for at least half of the observed northern mid-latitude trend averaged over December to February. The vertical variation of the modelled and observed trends agree qualitatively. The modelled total ozone correlates well with the vertical winter EP-flux at 100 hPa and the North Atlantic Oscillation index, suggesting that long-term changes in stratospheric circulation are intimately connected to the observed mid-latitude ozone trend.