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3. Discussion

Our preliminary analysis here presents results from the first 5 and half years of the model runs. As our initial conditions from the re-start files represent late April conditions, the first 18 months of the model simulation are considered here as a spin up period for the model calculation and are therefore omitted from the analysis.

The first topic to consider is the changes in the ozone field, as solar induced changes in the chemical module of the model alter ozone production, and with the feedback from chemistry to the GCM's dynamics various chemical, radiative and subsequent dynamical processes are involved.

Figure 1 presents the changes in the ozone field between maximum and minimum solar activity conditions with respect to the 11-year solar cycle, as annual average of the last 3 years of the model simulation at each state of the model. The top panel shows the absolute changes in ozone mixing ratio (in ppmv), and the bottom panel shows the change in percent [100*(solar max - solar min)/ solar min)]. These changes, positive over all the stratosphere, with a maximum in absolute change in the area of ozone concentration maximum, are consistent with changes between solar maximum and solar minimum conditions as calculated by the Mainz 2D chemistry model (Bruehl, 1999). These preliminary results are also in reasonable agreement with observed changes (e.g. SPARC, 1999). Negative upper tropospheric changes mainly in the tropical region indicate that dynamics play an important role in modulating the ozone field in that region, changing the shape and possibly the intensity of the Hadley cell circulation, a result reported from earlier GCM simulations of the solar activity forcing, using prescribed ozone changes.

Figure 1 (see text)


The anomalies observed in the lower stratosphere and upper troposphere region of the model suggest that the tropopause region is influenced by solar flux changes and the subsequent ozone, radiation and dynamics changes. Recent results from observation analysis (Zerefos et al., 2000) show that the tropopause temperature is varying in accordance with the solar cycle, although recent volcanic eruptions may also play an important role. The volcanic eruptions happened to have occurred at the last two solar cycle maxima, and their effect in the lower stratospheric temperature is in synergy with the solar activity for the last two solar cycles. As our model does not include volcanic aerosol effect and SST changes, a direct comparison between model and observation results in tropopause pressure is extremely complicated.



Figure 2:Average annual tropopause pressure in years with solar maximum and solar minimum activity.

However, a preliminary result from our analysis, presented in Figure 2, shows a weak response in the tropical region, and a stronger response in the vicinity of about 30N, at the area of the descending branch of the Hadley cell, indicating a stretching of the northern branch of the cell during periods of maximum solar activity. The high response seen over the high latitude and polar regions, mainly in the northern hemisphere, could be subject to changes as more model years will be added to the analysis, due to high variability in these regions of the world.