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Solar cycle linked to weather.

Solar cycle linked to weather

Atmospheric scientists have discovered a strong statistical link between the 11-year solar cycle and the weather here on earth--a finding that may eventually help explain why some winters are mild while others are unrelentingly harsh. The report was presented last week at a meeting of the American Geophysical Union in San Francisco.

Scientists have known about the solar cycle for more than a century and have long attempted to associate it with weather and a host of other phenomena. "The number of polar bears, the length of women's skirts, the stock market: Everything imaginable has been correlated with the solar cycle,' says Harry van Loon of the National Center for Atmospheric Research (NCAR) in Boulder, Colo. "The field has been in ill repute.'

The cycle is actually a minute variation in different properties of the sun. During the cycle maximum, ultraviolet and X-ray radiation increase, more sunspots appear on the surface of the sun and the total solar output is greatest.

Previous attempts to find a link between the cycle and the variations in weather have failed. When scientists look at the weather from one year to the next, temperature and air pressure and other aspects vary wildly, with no connection to the cycle. But Karin Labitzke, of the Free University in West Berlin, discovered in March that if she included only certain years, the stratospheric winter temperatures over the North Pole closely followed the solar cycle.

Labitzke grouped years according to a pattern of stratospheric winds over the tropics called the Quasi-Biennial Oscillation (QBO). During the west phase of the QBO, winter winds travel from west to east, and the opposite holds true for the east phase. On average, the wind reverses each year, but sometimes it misses a year.

During her recent visit to NCAR, Labitzke and van Loon probed deeper into this problem. By examining only the years of the western QBO they uncovered a remarkable correspondence between the solar cycle and the air pressure and temperature in certain areas, such as the eastern United States. For example, she says, "If the QBO is in the west phase, and we are in the solar minimum, the winter in Charleston [S.C.] will be normal or mild. And if we are in a solar maximum, the winter will be normal or cold.'

Over the North Pole and extending down into Canada, this correlation appears strongest, measuring as great as 0.8 on a scale of 0 to 1. This means that the link between the solar cycle and the weather accounts for 64 percent of the variability in winter temperatures and air pressure in that region. Considering all the elements the affect weather, says van Loon, this is a huge correlation.

For other areas, the connection between solar cycle and weather is weaker or nonexistent. In general, a map of the areas of correlation is a blotchy affair with no apparent pattern. (The years of the eastern QBO also show correlations, but they are weaker than during the western phase.)

Statistical tests have indicated that there is an extremely low probability that these patterns are coincidental, says Labitzke. In computer runs, the correlations emerged out of random sequences a mere 25 out of 10,000 times.

Still, the data on the QBO go back only to 1953, limiting the researchers to 3 1/2 periods of the solar cycle, and Labitzke acknowledges that the pattern could fall apart during upcoming periods.

The disreputable history of solar-cycle correlations has made scientists wary of reports of new links. And no one can yet explain the mechanism of the correlation. They wonder how a small oscillation in several solar properties can exert such a drastic influence on earthly weather.

But the statistics are beginning to speak out to scientists. "I think it's really very convincing that there's something going on,' says Brian A. Tinsley of the National Science Foundation.

Many people have wondered whether this correlation will help in making weather predictions. However, van Loon says, "This is purely statistics, and we don't understand the physical mechanism. Until we understand it, we should not use statistics to form predictions.' Labitzke believes the most important effect of her find will be to force meteorologists to consider basic questions about the role of the QBO and the solar cycle.
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Author:Monastersky, Richard
Publication:Science News
Date:Dec 19, 1987
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