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Once bashful El Nino now refuses to go.

It took a long time to show its face, but the El Nino warming that finally arrived last year and upset world weather has turned into a lingering guest, remaining in place far longer than expected, meteorologists with the National Weather Service reported last week.

El Ninos represent periodic blips in the global climate that develop when warm water from the western equatorial Pacific Ocean spreads eastward in concert with shifting patterns of atmospheric pressure called the Southern Oscillation. These so-called warm events alter typical weather patterns around much of the globe, bringing rains to some regions and droughts to others.

As early as 1989, some meteorologists reported seeing signs of a developing El Nino, but the real warming did not begin until late 1991, reaching full force in early 1992. The El Nino played a role in causing North America's mild winter last year and contributed to a severe drought plaguing southeast Africa, according to meteorologists at the weather service's Climate Analysis Center (CAC) in Camp Springs, Md.

By the middle of last year, a drop in sea-surface temperatures in the central Pacific and other weather changes appeared to herald the death of the El Nino. Both human forecasters and computer models called for normal or colder-than-normal conditions to develop in the equatorial Pacific by the end of 1992 (SN: 7/4/92, p. 5). But weak El Nino conditions continued throughout the year and then unexpectedly gained strength in December, says CAC's Vernon E. Kousky.

"It is fair to say that what was predicted by our model, by and large, was not correct:' says Tim P. Barnett, an oceanographer at the Scripps Institution of Oceanography in La Jolla, Calif. Barnett is one of several scientists attempting to forecast conditions in the Pacific using computer models.

Stephen Zebiak of the Lamont-Dohefty Earth Observatory in Palisades, N.Y, is less critical of the model he uses, though it also called for slightly subnormal temperatures by this time. "The model basically describes the longer term trends rather than the month-to-month changes." The current warming reported by the CAC may just be a temporary step backward in the general march toward cooler conditions in the central equatorial Pacific, Zebiak says.

Though not forecasted, the continuing warmth should make many residents of the western United States happy. The pool of warm water has spawned atmospheric convection in the central Pacific, which injects moisture into the atmosphere. That convection contributed to the storms that dropped needed rain and snow on drought-plagued California, Nevada, and Oregon during the last two months, Kousky says.

Forecasters and modelets do not yet know what to make of the lingering warmth in the central Pacific. But Barnett thinks it could reflect a long-term climate change there. Looking at temperature records of the equatorial Pacific going back to 1900, Barnett last week found indications that the average regional temperature west of the international dateline climbed considerably between the 1950s and mid-1970s.

If such a shift did actually occur, then the apparent warmth of the last few months could represent an artifact of the way meteorologists analyze temperature data. When measured against a long-term mean that includes the period prior to the climate shift, the current conditions would register as warmer than normal, leading to the appearance of a lingering El Nino. But when measured against the new average, established after the mid-century warming, the current conditions would register close to normal, Barnett says.
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Title Annotation:climate change
Author:Monastersky, Richard
Publication:Science News
Date:Jan 23, 1993
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