Multiannual simulations with a three-dimensional chemical transport model

An off-line three-dimensional stratospheric chemical transport model (CTM) has been developed and integrated in a series of 6-year experiments coverin g 1991 to 1997. The model has a detailed chemistry scheme and is forced by the meteorological analyses for the temperatures and horizontal winds, whil e the vertical motion is diagnosed using a radiation scheme. By using the m eteorological analyses, the model captures the interannual variability in c hlorine activation in the Arctic winter lower stratosphere, as well as the much more regular, strong activation in the Antarctic. The model fields sho w generally good agreement with long time series of ground-based observatio ns. In particular, the model simulations of column O-3 are excellent in ter ms of capturing the magnitude and day-to-day, seasonal, and interannual var iations at all latitudes. However, a large model/observation discrepancy fo r O-3 occurs at high latitudes during the summer, when the model overestima tes the O-3 profile throughout the lower and mid-stratosphere. The model al so reproduces many features in ground-based observations of HCl, ClONO2, HN O3, and NO2, such as short-term variability, the seasonal cycles, and winte r/spring enhancements of ClONO2 at northern midlatitudes. By running the mo del without cold, chlorine activating heterogeneous reactions the effect of polar land subpolar) processing on midlatitudes has been estimated. Polar processing results in 2-3% less O-3 at 50 degrees N throughout the year, an d around 5% less O-3 at 50 degrees S. Direct chlorine activation on enhance d midlatitude aerosol contributes 1% to column ozone depletion at mid and l ow latitudes in early 1992. The enhanced aerosol also caused column ozone r eductions of around 3% at 45 degrees N throughout 1992 and 1993. However, t he CTM runs confirm the suggestion interannual dynamical variability (which may be partly driven by the radiative effects of the aerosols) contributed to the large negative ozone anomaly in northern midlatitudes in 1992/1993.