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Solar Activity, Clouds and Climate: Data and Concepts Evolve
Reference
Kristjansson, J.E., Staple, A. and Kristiansen, J.  2002.  A new look at possible connections between solar activity, clouds and climate.  Geophysical Research Letters 29: 10.1029/2002GL015646.

Background
In the words of the authors, solar irradiance "varies by about 0.1% over the 11-year solar cycle, which would appear to be too small to have an impact on climate."  Nevertheless, they report that "persistent claims have been made of 11-year signals in various meteorological time series, e.g., sea surface temperature (White et al., 1997) and cloudiness over North America (Udelhofen and Cess, 2001)."  Hence, Kristjansson et al. say the purpose of their paper "is to re-evaluate the statistical relationship between low cloud cover and solar activity adding 6 years of ISCCP [International Satellite Cloud Climatology Project] data that were recently released."

What was done
For the period 1983-1999, the authors compared temporal trends of solar irradiance at the top of the atmosphere with low cloud cover derived from different sets of satellite-borne instruments that provided two measures of the latter parameter: full temporal coverage and daytime only coverage.

What was learned
The authors report that "solar irradiance correlates well with low cloud cover," with the significance level of the correlation being 98% for the case of full temporal coverage and 90% for the case of daytime only coverage.  As would be expected if the variations in cloud cover were driven by variations in solar irradiance, they also report that lagged correlations between the two parameters reveal a maximum correlation between solar irradiance and low cloud cover when the former leads the latter by one month for the full temporal coverage case and by four months for the daytime only situation.

The authors' observation that "low clouds appear to be significantly inversely correlated with solar irradiance" compelled them to suggest a possible physical mechanism that could explain this phenomenon.  Very briefly, this mechanism, in their words, "acts through UV [ultraviolet radiation] in the stratosphere affecting tropospheric planetary waves and hence the subtropical highs, modulated by an interaction between sea surface temperature [SST] and lower tropospheric static stability," which "relies on a positive feedback between changes in SST and low cloud cover changes of opposite sign, in the subtropics."  Based on experimentally determined values of factors that enter into this scenario, they obtain a value for the amplitude of the variation in low cloud cover over a solar cycle that "is very close to the observed amplitude."

What it means
Things appear to be looking up for the postulated solar-climate connection with the promise of even brighter days ahead.

References
Udelhofen, P.M. and Cess, R.D.  2001.  Cloud cover variations over the United States: An influence of cosmic rays or solar variability?  Geophysical Research Letters 28: 2617-2620.

White, W.B., Lean, J., Cayan, D.R. and Dettinger, M.D.  1997.  Response of global upper ocean temperature to changing solar irradiance.  Journal of Geophysical Research 102: 3255-3266.


Reviewed 9 July 2003