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'Tis the season for an El Nino warming.

After more than a year of perplexing signals, the Pacific Ocean has generated an El Nino warming that promises major changes in the world's weather through much of 1992, bringing floods to some regions and droughts to others. The emergence of his temporary oceanic fever signals a success for computer models that predicted when the event would occur and, perhaps more important, when it would not.

In the past month, meteorologists have observed a number of El Nino traits that did not exist earlier in the year. In particular, a large region of the central equatorial Pacific has warmed enough to generate thunderstorms, which carry ocean heat into the atmosphere, warping the jet stream's flow.

"From the observations, it looks as though we're in the middle of a warm event," Chester Ropelewski said this week at the American Geophysical Union meeting in San Francisco. "These things are important for [altering] rainfall patterns around the world, and they're also important for temperature," explains Ropelewski, a researcher at the National Meteorological Center (NMC) in Camp Springs, Md.

Generated by a complex interplay between the Pacific Ocean and the atmosphere, El Ninos recur at irregular intervals of four to seven years. They are associated with a pattern of atmospheric pressure called the Southern Oscillation. The name El Nino (Spanish for "the child") refers to the warm water's tendency to appear off South America's coast around Christmastime, ruining the region's anchovy harvest

Although these coastal waters have yet to warm, scientists say the most important traits of the El Nino have already developed. The National Weather Service has factored the new evidence into its winter forecast for the United States. For instance, it calls for above-normal precipitation in the Southeast, in part because that region tends to receive increased rainfall during El Ninos, says Gerald Bell of the NMC.

Other countries can also use the El Nino in predicting weather. In certain dry areas of Peru, Ropelewski says, farmers might plant crops that would survive the heavy rains that drench the coast during these warmings.

Scientists have theorized that El Ninos originate because the size and shape of the Pacific basin provide just the right conditions for water and air masses to slosh from west to east and back again, creating a natural engine that generates the warmings.

The last El Nino ran from late 1986 through 1987. Warm water began appearing in the central Pacific early in 1990, leading some meteorologists to expect a full-fledged El Nino later that year. While nature fooled those forecasters, some machines fared better. One computer model at the Lamont-Doherty Geological Observatory in Palisades, N.Y., has forecast for two years that an El Nino would develop now rather than in 1990, says Stephen Zebiak, who developed the model with Mark Cane, Zebiak discussed the prediction at this week's meeting.

"That's really amazing. Even that far back, it was calling for the event to be now," he told Science News. The Lamont-Doherty model resembles a simplified version of the atmospheric general-circulation models used for short-term weather forecasting. Unlike the weather models, however, this version includes an ocean that interacts with the atmosphere.

A statistical model run at the NMC also predicted an El Nino for late 1991, but that forecast did not emerge until early this summer. The NMC model, derived from one created at the Scripps Institution of Oceanography in La Jolla, Calif., uses past patterns to predict how the weather will evolve.

The modest success of these and other El Nino models offers hope that scientists can develop versions with even more predictive power. "Ten years ago, if anyone said we would have climate models that would give a handle on what will happen six months, nine months or a year ahead, people would have dismissed it," Ropelewski says. "We're really on the brink of a new era."
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Author:Monastersky, Richard
Publication:Science News
Date:Dec 14, 1991
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