Unusual Ocean Temperatures May Have Played Part in 1988 Drought
Some of this year's drought in the Midwest may have been caused by ocean temperature abnormalities near the equator in the Pacific Ocean, according to a new computer study reported Thursday. Such droughts could be anticipated if the temperature abnormalities turn out to be predictable, one of the authors said in the report appearing in Friday's issue of Science magazine. The authors are Kevin E. Trenberth and Grant W. Branstator of the National Center for Atmospheric Research in Boulder, Colo., and Phillip A. Arkin of the Climate Analysis Center of the National Oceanic and Atmospheric Administration in Camp Springs, Md. They noted that when asked what caused the drought that hit much of North America in 1988, meteorologists often reply ``the jet stream was displaced northward of its usual position so that storms, which tend to track along the path of the jet stream, were similarly displaced northward.'' ``Such an answer is, however, just a brief description of the weather patterns associated with the drought but does not get at the cause. A more satisfying response would address why the jet stream was displaced northward,'' the team wrote. Their proposed answer focuses on the development in April, May and June of drought in the Midwest, where several states recorded less rain than at any time since 1895. By July the weather pattern they studied was breaking up, and continuing dryness in the study area and elsewhere probably had other causes, Arkin said. But during the April-June period there were alternating high and low pressure centers across much of the northern half of the Western Hemisphere: A high-pressure center north of Hawaii, a low in the Gulf of Alaska, a high in central Canada extending down into the northern Great Plains states and a low on the East Coast. In this period, Pacific Ocean temperatures ranged up to 5.4 degrees Fahrenheit below normal in a narrow band extending about 4,000 miles along the equator westward from the coast of South America, with the coolest spot midway along the band. At the same time, a bit to the north of this band, surface temperatures ranged up to 0.9 degrees above normal. When this temperature pattern was fed into the computer, the pattern of stationary alternating high and low pressure systems was reproduced. The below-normal equatorial temperature by itself did not give such a result. ``We haven't proved anything; all we've done is shown that it's a possibility,'' Arkin said. The global atmosphere is so complicated that repeated running of a more detailed computer model would be needed to show that these abnormal temperatures are likely to be associated with drought, he said. If the model holds up, the work will be the first demonstration of tropical sea surface temperatures affecting weather outside the tropics in the summer, Arkin said. The authors took note of computer models predicting increased frequency of drought with the buildup of the ``greenhouse effect'' global warming caused by accumulation of gases like carbon dioxide in the atmosphere. ``The greenhouse effect may tilt the balance such that conditions for droughts and heat waves are more likely, but it cannot be blamed for an individual drought,'' they said.
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