A three-dimensional model study of long-term mid-high latitude lower stratosphere ozone changes

We have used a 3D off-line chemical transport model (CTM) to study the causes of the observed changes in ozone in the mid-high latitude lower stratosphere from 1979-1998. The model was forced by European Centre for Medium Range Weather Forecasts (ECMWF) analyses and contains a detailed chemistry scheme. A series of model runs were performed at a horizontal resolution of 7.5 ◊7.5 and covered the domain from about 12 km to 30 km. The basic model performs well in reproducing the decadal evolution of the springtime depletion of ozone in the northern hemi- sphere (NH) and southern hemisphere (SH) high latitudes in the 1980s and early 1990s. After about 1994 the modelled in- terannual variability does not match the observations as well, which is probably due in part to changes in the operational ECMWF analyses - which places limits on using this dataset to diagnose dynamical trends. For mid-latitudes (35 -60 ) the basic model reproduces the observed column ozone de- creases from 1980 until the early 1990s. Model experiments show that the halogen trends appear to dominate this mod- elled decrease and of this around 30-50% is due to high- latitude processing on polar stratospheric clouds (PSCs). Dy- namically induced ozone variations in the model correlate with observations over the timescale of a few years. Large discrepancies between the modelled and observed variations in the mid 1980s and mid 1990s can be largely resolved by assuming that the 11-year solar cycle (not explicitly included in the 3D model) causes a 2% (min-max) change in mid- latitude column ozone. spite the identification of many possible contributing chem- ical and dynamical processes, and a range of modelling and observational studies, a quantitative attribution of the trend has not yet been obtained. Such quantitative attribution de- pends greatly on comprehensive model simulations which can account for all known processes. Due to the limitations of different types of model, such simulations have not yet been performed. In this paper we apply for the first time a detailed full chemistry 3D stratospheric chemical transport model to the study of decadal mid-latitude ozone changes. In contrast to the large ozone loss observed in the Antarc- tic spring, and in the Arctic during recent cold winters, ozone depletion at mid-latitudes is much smaller. At northern hemi- sphere (NH) mid-latitudes (35 N-60 N) the annual aver- aged ozone was observed to decrease by about 2% from 1979 values during the 1980s. Additional larger losses occurred during 1992-1993, following the eruption of Mt. Pinatubo in 1991. Subsequently, during the mid-late 1990s ozone levels increased and by the late 1990s the mean ozone column was around 3-4% below the 1979 values. In contrast, the mean ozone column in the southern hemisphere (SH) mid-latitudes (35 -60 S) exhibited a relatively steady decline from 1979 without a large decrease after the Mt. Pinatubo eruption. By the late 1990s mean SH mid-latitude columns were about 5% less than 1979. This hemispheric asymmetry, and the appar- ent lack of a Pinatubo signal in the SH, has not yet been ex- plained. Different processes have been proposed to explain the ob- served mid-latitude trends. Chemical processes are essen- tially related to halogen (chlorine/bromine) trends which are known to be responsible for the large seasonal depletion in the polar regions. At mid-latitudes halogen chemistry may deplete ozone by: