The search for solar activity effects on climate in atmospheric observations is made difficult by the length of the solar cycle, 11 years in the case of the Schwabe sunspot cycle, relative to the length of the observational record. Re-analyses are thought to be less reliable before about 1958 because of a lack of tropical and Southern Hemisphere radiosonde data and fewer balloons reaching high altitudes. It should also be noted that satellite data only began to be incorporated in re-analyses after 1979. A much longer and more reliable data record is thus now available for analysis than was the case when Pittock (1978) reviewed the observational evidence for solar activity effects. The research on some suggested mechanisms for solar influences on climate has also advanced a great deal since that review paper.
Any mechanism that can successfully explain why observed solar activity effects on the climate system are so much larger than expected on purely energetic grounds must somehow modulate the large energy of the natural climate system rather than just involve direct radiative forcing. More than thirty years ago, Hines (1974) suggested that a viable mechanism for solar activity effects on climate might involve direct radiatively forced changes in the upper atmosphere (the term middle atmosphere was not yet in vogue). This would modulate planetary wave propagation, thus giving rise to changes in planetary wave structure that could extend throughout the entire atmosphere. Indeed, Hines' (1974) suggestion did involve solar effects modulating the energetics of atmospheric planetary waves. Calculations relating to this mechanism followed. Geller and Alpert (1980) did experiments in which small changes were made to the zonal mean winds at various altitudes, and they found that significant changes in tropospheric planetary wave structure resulted if changes in the mean zonal winds penetrated down into the lower stratosphere. This work was followed by the work of Callis et al. (1985) in which an early two-dimensional photochemical model was used to examine the predicted changes on the middle atmosphere winds and temperatures produced directly by solar cycle modulations in solar UV radiation. These solar UV produced changes in the zonally symmetric atmospheric state were then used in the planetary wave model of Geller and Alpert (1980) to see what planetary wave changes resulted. Callis et al. (1985) found relatively modest, but noticeable, solar UV-induced changes in the middle atmosphere planetary wave structure. At about this same time, Bates (1981) also performed calculations of the changes in planetary waves that would be expected from solar UV changes.
In 1987, Karin Labitzke published her seminal paper (Labitzke, 1987) that suggested a much larger and more coherent solar influence on north polar stratospheric temperatures could be seen when the data were separated according to the phase of the quasi-biennial oscillation (QBO). At that time, these results were somewhat controversial due to the relatively short data record (about three solar cycles), the possible biases induced by segmenting the data according to the phase of the QBO, and the possibility that a natural quasi-decadal cycle in the atmosphere might by coincidence be in phase with solar activity during the examined period. Geller (1988), in commenting on the Labitzke (1987) paper, did suggest that a radiative-dynamic feedback might give a larger response to solar UV variations than expected. Quoting Geller (1988):
"It seems that radiative changes can alter the meridional (Equator-to-pole) temperature gradient, which in turn changes the polar vortex. This alters the planetary-wave propagation, which further changes the polar vortex, so that the process iterates."
The pioneering work of Haigh (1994,1996,1999) and Shindell etal. (1999) introduced an additional positive feedback, and that is the role of ozone in amplifying the direct solar UV influences on the zonal mean state of the middle atmosphere. The remainder of this introduction will discuss how the papers that follow have advanced this line of research.
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