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Winter 1998

New evidence in the case for an atmosphere/solar-cycle link

by Bob Henson

This map shows correlations across the globe from 1958 to 1996 between the solar cycle and the 30 hPa (30 millibar) pressure height, the latter measured using the NCEP/NCAR reanalysis. Shaded areas denote correlations of .5 or better. These stronger correlations circle the globe in both Northern and Southern Hemispheres on either side of the equator. (Illustration courtesy of Karin Labitzke.)

How far down is the atmosphere affected by the 11-year solar cycle? Harry van Loon (NCAR) and Karin Labitzke (Free University of Berlin) maintain that the lower stratosphere rises and falls by a discernible amount with each cycle. For over a decade, they've made their case with data from three solar cycles by correlating variables such as the height of the 30-millibar pressure surface (about 24 kilometers, or 15 miles) and the solar flux at the 10.7-centimeter wavelength. Now their argument has been bolstered by correlations from a fourth solar cycle, the most recent one.

Skeptics have pointed to the fact that all kinds of phenomena--from election results to the height of hemlines--have been successfully correlated with the solar cycle, without any physical basis. However, time appears to be on van Loon and Labitzke's side. On a visit to NCAR this fall, Labitzke outlined the current state of their long-term collaborative work, a project with implications for climate modeling and other research.

The original correlations derived by Labitzke and van Loon in the late 1980s covered the period 1958-1987. Over some parts of the Northern Hemisphere, the correlation coefficient was as high as .74 for that period; correlations of .5 or better spanned half of the hemisphere. Now, with the period from 1958 to 1997 analyzed--incorporating the most recent solar cycle--the band of .5 correlation coefficients has expanded to circle the entire hemisphere, as the figure shows. "People are amazed at the statistical significance," says Labitzke. Moreover, the correlations have been found to extend to the Southern Hemisphere as well.

The solar cycle's impact on the stratosphere is enhanced during each hemisphere's summer, when it is exposed more directly to solar activity for longer periods. "What we're seeing is very likely an indirect effect of photochemical changes in the upper stratosphere," says Labitzke.

Finding solar-cycle effects below the stratosphere has been tougher. Because the sun's energy change from solar minimum to maximum occurs mainly in the x-ray and extreme ultraviolet wavelengths, much of the impact occurs in the upper stratosphere before reaching earth. Also, any effects that do make it to the lower atmosphere are probably masked by day-to-day weather, notes van Loon: "There's so much noise in the global troposphere." However, even in temperatures taken at 700 mb (about 3 km, or 2 mi), there is clear evidence of the solar cycle between the latitudes of 20 and 40 degrees N, says van Loon.

Labitzke notes an intriguing connection between the Aleutian high, a common wintertime weather feature, and the solar cycle. During solar maxima, the high seems to be more pronounced--the sun "creates a stronger Aleutian high," says Labitzke. The high is correlated with major warming episodes inside the Arctic Circle, but for a better understanding of the Arctic winter, one has to consider the quasi-biennial oscillation (QBO) of the winds in the equatorial stratosphere as well, another focus of Labitzke and van Loon.

Labitzke has been working with analyses of the Northern Hemisphere stratosphere made at the Free University of Berlin as well as with the recent global reanalyses conducted by NCAR and the National Centers for Environmental Prediction (NCEP), a data set she calls "marvelous." Both data sets yield similar results, she says. Already, the numbers from this year are showing the first signs of a positive height anomaly over Eurasia, which Labitzke and van Loon say is consistent with the encroaching solar maximum due to peak in 2000.

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Edited by Carol Rasmussen, carolr@ucar.edu
Prepared for the Web by Jacque Marshall
Last revised: Tue Apr 4 14:56:02 MDT 2000