Don't blame sun's 18-month chill for last winter's extra bite
If you're looking for something to blame for this winter's severe weather, sorry - you can't really pin it on the inconstancy of Old Sol.
Recent news stories reported that an 18-month drop in the sun's power from February 1980 to August 1981 observed by instruments on a National Aeronautics and Space Administration (NASA) satellite was a major factor in the nation's cold and snowy winter.
But scientists who study and try to predict Earth's weather patterns - and Dr. Richard C. Willson, the Jet Propulsion Laboratory scientist who made the observations - agree that such a conclusion is premature at best.
''I applaud the effort to learn more about the sun's radiation. . . . But no one so far has demonstrated a definite link between solar activity and our weather patterns,'' says Dr. Jerome Namias of Scripps Oceanographic Institution in La Jolla, Calif. He is a leading authority on the interaction between the oceans and the atmosphere and recently conducted a workshop on last winter's weather.
''What I distrust is the correlation with the severe winter. It's generally a mistake to connect the current meteorological crisis with these things. I don't know why we keep doing it,'' comments Jack Eddy of the National Center for Atmospheric Research (NCAR).
Despite the lack of unambiguous links between solar changes and some atmospheric phenomena, for some time a number of scientists have been suspicious that gradual changes in the solar power that Earth receives may subtly affect our climate and weather patterns. These long-term variations have been ascribed to two possible causes: perturbations in Earth's orbit and slight changes in the power the sun radiates into space.
Statistical analyses of past climatic patterns and Earth's average distance from the sun have established a mathematical link between the two: During ice ages the Earth has tended to be farther from the sun than during the warm interglacial periods such as the present.
Also, various scientists have found weather phenomena that appear to follow the 22-year cycle of solar activity. The strongest example of this has been the 20-year drought cycle in North America. The height of the tropopause - the boundary between the lower and upper atmosphere - also appears to vary in step with the solar cycle. And an intricate analysis of temperatures in the northeastern US have also shown a weak correlation.
Although they consider these highly suggestive, scientists have remained cautious. They say they cannot figure out how the slight variations in solar power involved - a few tenths of a percent - could have major atmospheric effects.
An example of some of the pitfalls involved comes from the work on the drought cycles. Recently, some researchers have shown that these cycles fit as well or better to an 18.6-year variation in the moon's orbit than they do to the solar cycle.
The recent satellite measurements found that the sun's power decreased by a tenth of a percent for an 18-month period and then recovered slightly, Dr. Willson reports.
''A tenth of a percent change in the solar constant is beginning to be marginal for climate impact. It translates into roughly a one-tenth degree decrease in global temperature. When this persists for a long time, climate models start perking up their ears,'' explains Jack Eddy of the National Center for Atmospheric Research.
During its two years of operation, instruments on the Solar Maximum Mission satellite have seen a gradual decrease in the energy given off by the sun. This has been corroborated by similar but less-accurate instruments on the Nimbus satellite. If this long-term trend in solar power output varies along with the sun's sunspot cycle then ''this will be extremely interesting,'' says Dr. Eddy. It will give researchers more reason to believe that earthly weather phenomena can be driven by solar variations.
Although proof of small variations in the sun's energy output are significant , ''what they do to weather patterns is not yet known and so it doesn't help in making weather predictions,'' explains Dr. Namias.
In the few years since scientists have been trying to make seasonal forecasts , they have done well in some years and poorly in others, the veteran forecaster says. Some forecasts for last winter predicted colder-than-normal temperatures and heavier-than-usual snows. But none accurately forecast just how severe the winter weather would be, he says. Also, the winter weather seemed to come about a month late and so left late as well. As a result, if a number of forecasts were simply moved back a month they do much better, he says.
Generally, winter weather patterns are set in October and November, Dr. Namias explains. Last fall, however, there were very few clear atmospheric indicators that forecasters could find. Surface water temperatures in the North Pacific, for instance, have a major influence on winter weather in North America. But last fall there were no major cold or warm spots.
This winter demonstrates just how important conditions in the Arctic can be, Namias explains. The development of a large, high-pressure region in the Arctic pushed cold air and winter storm tracks far south of their normal position, he says. Once areas in the Midwest and Eastern US were covered with snow, this tended to maintain the abnormally cold conditions. And, because successive storm systems tend to follow snow boundaries this southerly snow cover was maintained.
''No one knows exactly how or why these Arctic highs develop,'' Dr. Namias confesses. But it's certain that he and his fellow forecasters will be keeping a much sharper eye on conditions in the Arctic than on the solar surface when they make their winter weather predictions next fall.