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Buoys Give Scientists A New Window On the Weather

EL Nino is back. This recurring warming of equatorial Pacific waters refused to fade out in a ''normal'' way last year. Instead, it now has returned to ''a mature stage,'' according to the National Center for Atmospheric Research in Boulder, Colo.

NCAR adds that you probably can blame California's recent floods and Australia's drought on El Nino's perverse behavior. It has shown an ability to affect weather around the world before. This time, however, scientists were ready for it. After decades of envying the ability of land-based observers to track weather, oceanographers now have a network of 70 deep-ocean anchored buoys that keeps tabs on what's happening in the El Nino area at sea. The buoys span the equatorial Pacific Ocean to form the Tropical Atmosphere-Ocean Array. They give scientists a real-time view of what's happening in those waters.

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And that sets this El Nino episode apart, notes meteorologist James O'Brien of Florida State University in Tallahassee. He explains that when the Pacific Ocean warming surged in 1992, it developed past its first phase before anyone noticed. It didn't sneak up on scientists this time.

That's good news for the rest of us. Given El Nino's global reach, the more national planners know about what's going on in the equatorial Pacific, the better they will be able to prepare for any weather changes it may bring. As Dr. O'Brien explains, computer-based forecasts can anticipate El Nino's onset several months or more in advance. But they can't predict when it will wind down. Now, he says, that doesn't matter because ''we can observe when it's going to end.''

El Nino is part of a complex interaction of air and sea that involves what meteorologists call the southern oscillation. Hence scientists' term: ENSO (El Nino/southern oscillation) event. As it develops, air pressure rises in the western subtropical Pacific and falls in the east. Trade winds slacken. A warm-water pool normally found in the western equatorial Pacific migrates east. This changes the distribution of thunderstorms and tropical storms. Vast amounts of moisture and heat are pumped into the atmosphere. Air and sea circulation patterns change to influence weather thousands of miles away.

For example, meteorologist John M. Wallace at the University of Washington in Seattle notes that there are weather changes over North America that scientists can predict with varying degrees of confidence. As an El Nino matures, he says he would be most confident in predicting warmer than normal temperatures during winter and spring over Canada and the northern tier of states from the Great Lakes westward. He also would confidently forecast cool and above-normal precipitation in the southern tier of states from Texas east.

He adds with somewhat less confidence that he would expect California weather to be ''pretty volatile'' during El Nino years. Several thousand miles west of California, jet-stream winds strengthen and develop into a second southerly jet. This can bring lashing rains to California as has happened recently. Sometimes, however, that jet turns north to bring warmth to Alaska and drought to California.

What seems unusual about El Nino now is the lack of an alternative cold event. Normally, the warming in the Pacific gives way to a substantial cooling. The warming and cooling alternate on a two-to-10-year cycle. But since 1976, there have been El Nino warmings in 1976-77, 1982-83, 1986-88, and 1991-present. But there has been only one cool episode, in 1988-89.

NCAR researcher Kevin Trenberth says ''that is unprecedented'' in the El Nino record. He speculates that this might be an early sign of man-made global warming. However, he adds that the recent lack of cool episodes could also be attributed to natural variability in the ocean-atmosphere system.

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With their new ability to track what's happening in El Nino's home region, scientists may be able to get a better concept of how that system works.