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The Model

The fully interactive chemical circulation model for the middle atmosphere MAECHAM4-CHEM (Roeckner et al, 1996) has a horizontal resolution of T30 (6^oX 6^o). Its very efficient chemical scheme (Steil et al, 1998) applies the 'family technique' and includes solid Polar Stratospheric Cloud (PSC) particles with their surface as function of temperature, water vapor and nitric acid. Settling is taken into account. Photolysis rates are computed every chemical timestep using the fast scheme of Landgraf and Crutzen (1999). The model has an improved version of the gravity wave parameterization of Manzini and McFarlane (1998) which leads to stratospheric temperatures very close to observations throughout the year. In the 3 performed 20 year-'time-slice' experiments the lower boundary conditions, including sea surface temperature, were fixed.