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2. Model description and experimental set-up

We present here the changes in the temperature, circulation and chemical composition in the post-Pinatubo atmosphere simulated by the UIUC 24-layer Troposphere-Stratosphere GCM with interactive chemistry. The model has been described elsewhere [Rozanov et al., 2000]. It takes into account the solar and infrared radiation perturbations by sulfate aerosol and heterogeneous reactions on/in sulfate particles. The links between the chemical and physical processes are maintained by the exchange of the mixing ratios of radiatively active species between the radiation and chemistry-transport parts of the model. This model design assures that almost all physical processes invoked by the products of the volcanic eruption are taken into account. The only exception is that the formation and development of the aerosol layer is not simulated interactively. We use the time-dependent distribution of aerosol optical properties obtained from Stenchikov et al. [1998] and Andronova et al. [1999] in the radiation code of the ST-GCM/PC to calculate the fluxes of solar and longwave radiation in the presence of the volcanic aerosol. The Pinatubo aerosol extinction at 1.02 micron from the same dataset was used to calculate the time-dependent volcanic aerosol surface area density according to the equation proposed by Thomason et al. [1997]. The data have been used for the calculation of the heterogeneous reaction constants in the chemical routine of the ST-GCM/PC.

We have carried out three 2-year-long ensemble simulations to estimate the statistical significance of the results. Each ensemble simulation consists of five separate model runs. The first ensemble run is the control run (run "C") without any changes in the external forcing. In the second run (run "F") we introduced the volcanic-aerosol perturbations into the radiation and chemical parts of the model. In the third run (run "R") we introduced the volcanic-aerosol perturbations only for the calculation of the radiation fluxes. Thus comparison of the results of runs F and R will elucidate the contribution of the chemical processes. For all the simulations we used the climatological sea surface temperature (SST). Here we present some preliminary results of the model run F. The analysis of run R will be presented elsewhere. In the figures, the regions where the changes of the different quantities are statistically significant at better than the 20% level are bounded by a red-and-black line.