Some Closing Remarks

With the improved modeling of the troposphere-stratosphere system, model results are showing increasing consistency with the observational analyses of Labitzke, van Loon, Chanin, and others. The relatively recent work of Lesley Gray and her colleagues suggests that solar UV influences on the lower stratosphere are most sensitive to alterations in atmospheric structure in the tropical upper stratosphere, and Matthes et al. (2004) find that proper inclusion of this does indeed improve their general circulation model results. Another interesting line of research is the recent work originally due to Murry Salby that suggests a solar effect on the quasi-biennial oscillation in the stratospheric mean zonal wind in the tropics. If this is a mechanism by which solar activity affects the lower stratosphere and troposphere, there are several implications. One is that to treat this mechanism properly, one must use an atmospheric model that extends through the stratosphere and goes sufficiently high for the stratosphere to be well modeled. To account for the indirect effect of solar ultraviolet heating of the stratosphere, the ozone variations that result from the solar UV variations should also be in the model. Another point relates to comments made in the Kodera (2006) and Schmidt and Brasseur (2006) papers. This is that the solar UV ^ stratosphere ^ lower atmosphere effect on climate mainly influences regional climate whereas variations in the solar constant affect globally averaged temperatures and other climate parameters.

Finally, there is the interesting suggestion in Kodera (2006) of a completely new mechanism by which solar-induced effects in the stratosphere can affect the troposphere, and that is the effect of modulations in the Brewer-Dobson circulation on tropical convection, and its possible subsequent possible effect on the global circulation. In this regard, it should be noted that Collimore et al. (2003) have found evidence of a QBO effect on tropical convection, and they conclude that this is most likely related to QBO modulations in tropical tropopause heights.

This summary paper has discussed several of the papers that follow in this book, but it is certainly not a comprehensive presentation of all mechanisms for solar influences on climate. One such mechanism is discussed elsewhere in this volume, and that is the mechanism that involves cosmic rays on cloud amounts. The Curtius (2006) and Arnold (2006) papers address this issue.

Other topics relating to mechanisms for solar influences on the climate are also not covered here. One is the suggestion of Meehl et al. (2003) that variations in total solar insolation lead to more evaporation in the cloud-free subtropics, which in turn affects moisture transports and latent heat release. To treat the solar UV/QBO/planetary wave mechanism properly in a climate model, the model (i) should extend at least into the mesosphere; (ii) include solar modulation of ozone, (iii) ideally should account for the QBO; and (iv) include good resolution of the solar variations in the UV portion of the solar spectrum. The Meehl et al. (2003) mechanism does not require all these things. It also is not yet being researched as intensively as the solar UV/planetary waves mechanism.

It is clear that the study of solar influences on climate is now in a new phase where quantitative treatments of mechanisms are being studied. Furthermore, in the case of solar UV effects on climate, these studies contribute to the more general study of stratospheric influences on climate, where the stratospheric changes may occur from natural or anthropogenic influences. The introduction started with a rather pessimistic quote from Pittock (1978). Five years later, Pittock (1983) said the following:

"Advances have been made in the modeling of variable ultraviolet influences on middle stratosphere temperatures and ozone content, and of the modulation of reflection and absorption of tropospheric planetary wave energy by solar-induced variations in stratospheric winds and temperature profiles."

He went on to say that this solar UV/planetary wave mechanism was

"... a complex phenomenon which as yet has only been modeled in a very simplified manner."

How right he was, and how far we have come.


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