Background
Many people believe that the changes in total solar irradiance that are observed at the top of the atmosphere over the 11-year solar cycle are simply too small to have a significant impact on earth's climate.  Nevertheless, numerous studies have identified an 11-year cycle in various types of proxy climate data throughout the world [see Solar Effects (Decadal-Scale Cycles) in our Subject Index for some examples].  Logic thus suggests that serious consideration be given to the hypothesis that the 11-year climate cycles are caused by the 11-year solar cycle, which is precisely what the authors of this model study do.

What was done
The authors begin by noting that several prior studies (Labitzke and Van Loon, 1995; Haigh , 1996, 1999; Shindell et al., 1999) have indicated "the possibility of an indirect dynamical response of the lower atmosphere to the [weak, solar-induced] radiative forcing of the upper atmosphere," but that "a complete mechanism for the influence of solar irradiance changes on atmospheric circulation is still missing."  To forge ahead on this front, they thus conduct a model study in which they "use realistic variations in solar UV and total solar irradiance as external forcing to a fully interactive 3-D coupled chemistry-general circulation model ? to study the nature and extent of the tropospheric response."

What was learned
The authors say their "radiative forcing results show that the 11-year solar cycle effect on global mean temperature is negligible, but simulated responses of sea level pressure do suggest that regional effects are probably significant."  Indeed, they conclude that "the solar effect is most probably real and its magnitude is sufficiently large to exceed the natural variability over 20 years."  They also note that their results are "in line with recent work by Kodera (2002) on the NAO [North Atlantic Oscillation] modulation by the solar cycle."  Tourpali et al. thus conclude that "realistic changes of the solar UV radiation influence the stratosphere-troposphere system in a significant way," producing circulation changes that "give rise to regional changes of weather and climate" and "add to the credibility of the recent claim by Baldwin and Dunkerton (2001) that stratospheric processes may act as a precursor of anomalous weather regimes."

What it means
This study, as well as the others it cites, clearly demonstrates that the 11-year solar cycle may indeed influence regional climate in a possibly profound way.  Being regional, however, and switching back and fourth as rapidly as it does, the 11-year cycle's impact on the long-term global temperature is not apparent.  However, if there are longer-term solar cycles -- which we know there are [see, for example, Solar Effects (Centennial-Scale Cycles and Millennial-Scale Cycles) in our Subject Index] -- the study of Tourpali et al. suggests, in our view, that the climatic consequences of these much longer cycles may well be much greater than what is implied by the nigh-unto-negligible global mean temperature impact of the 11-year solar cycle.  Hence, more work of the nature of that described in this study would appear to be very important; and Tourpali et al. note that "a fruitful approach is perhaps by analyzing vertical wave propagation, as suggested in a number of other studies (Perlwitz and Graf, 2001; Kodera, 1995)."

References
Baldwin, M.P. and Dunkerton, T.J.  2001.  Stratospheric harbingers of anomalous weather regimes.  Science 294: 581-584.

Haigh, J.D.  1996.  The impact of solar variability on climate.  Science 272: 981-984.

Haigh, J.D.  1999.  A GCM study of climate change in response to the 11-year solar cycle.  Quarterly Journal of the Royal Meteorological Society 125: 871-892.

Kodera, K.  1995.  On the origin and nature of the interannual variability of the winter stratospheric circulation in the Northern Hemisphere.  Journal of Geophysical Research 100: 14,077-14,087.

Kodera, K.  2002.  Solar cycle modulation of the North Atlantic Oscillation: Implication in the spatial structure of the NAO.  Geophysical Research Letters 29: 10.1029/2001GL014557.

Labitzke, K. and Van Loon, H.  1995.  Connection between the troposphere and stratosphere on a decadal scale.  Tellus Series A 47: 275-285.

Perlwitz, J. and Graf, H.-F.  2001.  Troposphere-stratosphere dynamic coupling under strong and weak polar vortex conditions.  Geophysical Research Letters 28: 271-274.

Shindell, D., Rind, D., Balachandran, N., Lean, J. and Lonergan, P.  1999.  Solar cycle variability, ozone and climate.  Science 284: 305-308.