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Defying predictions, El Nino still simmers.

The current El Nino warming in the Pacific Ocean has surprised most human forecasters and computer models by hanging on far longer than predicted, promising continued disruptions in the typical weather patterns for many parts of the planet, according to researchers from the National Meteorological Center (NMC) in Camp Springs, Md.

Having lasted almost two years so far, this El Nino is the longest in the last 50 years. "It is an unusual event that we're seeing now;' says Vernon E. Kousky, an NMC meteorologist who last week presented an update on the warming.

El Ninos, which recur irregularly two to three times a decade, start when winds along the equator slacken, allowing a pool of warm water from the western Pacific to spread eastward. As sea surface temperatures climb in the central and eastern Pacific near the equator, thunderstorms develop over this part of the ocean, which normally lacks significant rainfall. This alters the storm patterns over Asia, the Americas, and even Africa.

The present warming began in mid1991 and reached its mature phase by early 1992, bringing excessive rain to central South America, northern Mexico, and the western coast of the Gulf of Mexico. At the same time, the abnormal weather caused droughts in southeastern Africa, the Philippines, and northern Australia, according to the NMC.

By mid-1992, the El Nino had lost strength as sea surface temperatures dropped in the equatorial Pacific, leading Kousky and others to announce that the warming was nearing its end. At the same time, several experimental computer models predicted that this part of the Pacific should revert to normal temperatures or even reach cool conditions by the end of the year.

Contrary to expectations, water temperatures in the central Pacific began climbing once again in late 1992, and the warmth spread toward the South Ameri* can coastline, reestablishing the El Nino, says Kousky (SN: 1/23/93, p. 53). During April, observations of sea surface temperatures continued to show a large patch of warm water, both along the equator and spreading to the tropics of the northern and southern hemispheres.

The warm conditions in the central Pacific probably will persist through the middle of 1993, but the equator could cool off quickly because the pool of warm water there is thin, covering only the top surface of the ocean, says Kousky

Moisture injected into the atmosphere by the warm water in the central Pacific played a role in the intense rains that washed California this year, Kousky says. The lingering El Nino also continued the drought in southeast Africa and will likely weaken the monsoon rains in India and Indonesia this year, he adds.

While most computer models missed the call this year, a new one under development at the NMC did make the right prediction. During the summer of 1992, the NMC ran the first real forecast on the model, which correctly called for a winter warming.

This experimental version is more complex and has better resolution than other models currently in use, but its developers say they need to test it further before they can judge its accuracy. The first prediction "was a success:' says NMC meteorologist Ming Ji. "It could be luck, but it was encouraging."
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Title Annotation:El Nino Pacific Ocean warming
Author:Monastersky, Richard
Publication:Science News
Date:May 8, 1993
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