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In Search of the Elusive Solar-Climate Connection
Reference
Thejll, P. Christiansen, B. and Gleisner, H.  2003.  On correlations between the North Atlantic Oscillation, geopotential heights, and geomagnetic activity.  Geophysical Research Letters 30: 10.1029/2002GL016598.

Background The authors note that "apparent relations between solar activity, or parameters closely related to solar activity, and climate data have often been reported," citing the work of Herman and Goldberg (1978), Pittock (1983), Hoyt and Schatten (1997) and van Loon and Labitzke (2000).  Noting further that a substantial portion of Northern Hemispheric climate variability is associated with the North Atlantic Oscillation (NAO), as described by Hurrell et al. (2001), they report that the activity of the NAO itself has been found to be related to various solar parameters (Bucha and Bucha, 1998; Boberg and Lundstedt, 2002; Kodera, 2002).  Hence, they probe these intriguing relationships still further.

What was done
The authors investigated various spatial and temporal relationships among a number of different parameters: the geomagnetic index (Ap), the NAO, stratospheric geopotential height and sea level pressure.

What was learned
The authors report that "significant correlations between Ap and sea-level pressures and between Ap and stratospheric geopotential heights are found for the period 1973-2000," but that "for the period 1949-1972 no significant correlations are found at the surface while significant correlations still are found in the stratosphere."  By using "Monte Carlo simulations of the[ir] statistical procedures applied to suitable surrogate data," they also conclude that these correlations are due to the existence of a "real physical link."  In the 1973-2000 period, they also note that only the winter season series are significantly correlated, which they say "is consistent with the notion that the solar-climate link works through the stratosphere."

What it means
The authors say their findings may be explained in two different ways: either the influence of the sun increased through time, reaching a strong enough level in the 1970s to finally make the correlations they studied become statistically significant, or the state of the atmosphere changed in the 1970s, so that it became more sensitive to the solar influence than it was before that time.  Their findings also strengthen the case for solar-induced perturbations being propagated downward from the stratosphere to the troposphere (Hartley et al., 1998; Carslaw et al., 2002).  Hence, although unable to identify the precise nature of the physical linkage that must exist between solar activity and climate change, the authors were able to push the pertinent science just a little bit closer to the day when that elusive goal will finally be achieved.

References
Boberg, F. and Lundstedt, H.  2002.  Solar wind variations related to fluctuations of the North Atlantic Oscillation.  Geophysical Research Letters 29: 10.1029/2002GL014903.

Bucha, V. and Bucha Jr., V.  1998.  Geomagnetic forcing of changes in climate and in the atmospheric circulation.  Journal of Atmospheric and Terrestrial Physics 60: 145-169.

Carslaw, K.S., Harrizon, R.G. and Kirkby, J.  2002.  Cosmic rays, clouds, and climate.  Science 298: 1732-1737.

Hartley, D.E., Villarin, J.T., Black, R.X. and Davis, C.A.  1998.  A new perspective on the dynamical link between the stratosphere and troposphere.  Nature 391: 471-474.

Herman, J.R. and Goldberg, R.A.  1978.  Sun, Weather, and Climate.  NASA Special Publication SP-426360.

Hoyt, D.V. and Schatten, K.H.  1997.  The Role of the Sun in Climate Change.  Oxford University Press, New York, NY.

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.

Pittock, A.B.  1983.  Solar variability, weather and climate: An update.  Quarterly Journal of the Royal Meteorological Society 109: 23-55.

van Loon, H. and Labitzke, K.  2000.  The influence of the 11-year solar cycle on the stratosphere below 30 km: A review.  Space Science Reviews 94: 259-278.


Reviewed 16 April 2003