6.4 Model studies of the influence of varying solar UV on the troposphere

The first GCM studies of the impact of solar UV-induced variations in stratospheric temperature and ozone on the dynamical structure of the troposphere were presented by Haigh (1996Jump To The Next Citation Point). In that paper a number of experiments were carried out using a variety of assumptions concerning ozone changes. All the experiments showed the same pattern of response in tropospheric temperatures and winds but with different magnitudes depending on the specifications of the UV and ozone changes. An example of the impact on zonal wind is shown in Figure 40View Image from a run in which ozone was specified based on results of a 2D model calculation. The anomaly shows that at higher solar activity the mid-latitude jets are weaker and move slightly poleward.

Figure 41View Image 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.

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Figure 40: Above: January field of zonal mean zonal wind (ms–1). Below: difference between fields at solar maximum and solar minimum of zonal wind calculated in a GCM. From Haigh (19961999). UV changes were prescribed according to Lean (1989), ozone changes from the results of the 2D model experiments of Haigh (1994).
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Figure 41: Mean meridional circulation. Above: mean fields for January and July. Below: difference between values at solar maximum and minimum. From Larkin (2000); similar figures in Larkin et al. (2000).

The modelled signals for zonal wind (Figure 40View Image) and mean meridional circulation (Figure 41View Image) are broadly similar to those deduced from observational (NCEP reanalysis) data (Figure 16View Image and Figure 17View Image), 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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