Figure 41 shows results from a similar model experiment for the tropospheric mean meridional circulation. The upper panel shows the mean climatological values: positive values (warm colours) indicate clockwise circulation and negative values (cold colours) anti-clockwise. The lower panel shows the anomaly associated with solar cycle variability. At solar maximum the winter Hadley cell (the major circulation in each season) is clearly weaker and broader than it is at solar minimum.
The modelled signals for zonal wind (Figure 40) and mean meridional circulation (Figure 41) are broadly similar to those deduced from observational (NCEP reanalysis) data (Figure 16 and Figure 17), although somewhat weaker in magnitude. The models used in these studies had fixed sea surface temperatures which essentially restricted them from responses involving feedbacks between SSTs, clouds and circulation and the results suggest that at least part of the solar signal in the troposphere comes from a response to changes in the atmosphere above.
Recently an atmosphere-ocean GCM with fully coupled stratospheric chemistry has been run (despite huge computational demands) to simulate the effects of changes in solar irradiance between the Maunder Minimum and the present (Shindell et al., 2006). As in the previous studies the results show a weakened Hadley circulation when the Sun is more active, and they also suggest an impact on the hydrological cycle with greater tropical precipitation. Furthermore, they provide additional evidence that coupling with stratospheric chemistry enhances the solar signal near the surface.
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