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Old and tired, an El Nino hints of its end.

After upsetting global weather patterns for the last half-year, the El Nino warming in the Pacific Ocean has weakened in the past month - a trend that could signal its coming demise. Forecasters now predict that Pacific waters will soon turn abnormally cold, sparking a different weather-altering pattern that could also affect much of the globe.

While sea-surface temperatures across most of the tropical Pacific remain warmer than normal at this time, "the water right along the equator has cooled off considerably in the last four weeks," says Vernon E. Kousky of the National Weather Service's Climate Analysis Center in Camp Springs, Md. "That's usually the first sign that [a warm event] has entered its decay stage."

Warmings and coolings in the Pacific represent opposite phases of one basic pattern called the El Nino-Southern Oscillation (ENSO ENSO El Niño Southern Oscillation ) - a complex duet between ocean currents and wind streams in the tropical Pacific. During warm events, such as the one that developed late last year, the normal easterly winds (which flow east to west) weaken along the equator, allowing warm water to spread from the New Guinea New Guinea (gĭn`ē), island, c.342,000 sq mi (885,780 sq km), SW Pacific, N of Australia; the world's second largest island after Greenland.  area toward the central and eastern Pacific. Cold events, called La Ninas by some researchers, occur when the easterlies grow strong, pulling up cold, deep water from the eastern Pacific and spreading it westward across the equatorial belt. The El Nino warm events and La Nina cool events recur irregularly, about four to seven years apart.

This year's warm event in the tropical Pacific brought a devastating dev·as·tate  
tr.v. dev·as·tat·ed, dev·as·tat·ing, dev·as·tates
1. To lay waste; destroy.

2. To overwhelm; confound; stun: was devastated by the rude remark.
 drought to southern Africa
This article concerns the region in Africa. For the present-day country in this region, see South Africa; for the former country, see South African Republic.
Southern Africa
 and dry conditions to India, Indonesia and northern Australia The term northern Australia is generally considered to include the States and territories of Australia of Queensland and the Northern Territory. The part of Western Australia (WA) north of latitude 26° south — a definition widely used in law and State government policy . On the opposite side of the Pacific, severe rains hit South America's west coast, southern California Southern California, also colloquially known as SoCal, is the southern portion of the U.S. state of California. Centered on the cities of Los Angeles and San Diego, Southern California is home to nearly 24 million people and is the nation's second most populated region,  and Texas as well as other parts of the world (SN: 12/14/91, p.389; 3/7/92, p.159).

Kousky points to several recent developments that suggest the warming may soon end. Aside from the sea-surface cooling along the equator, which has reached as much as 1[degree]C below normal in some places, the equatorial easterly winds have strengthened recently.

Measurements made below the ocean surface also show significant changes in the depth of the thermocline-the border between warm surface water and colder deep water. During warm events, this boundary sinks in the central and eastern Pacific as a pool of warm water floods the region. But the thermocline ther·mo·cline  
A layer in a large body of water, such as a lake, that sharply separates regions differing in temperature, so that the temperature gradient across the layer is abrupt.
 rose rapidly during May and June in the eastern Pacific, says Kousky. He cautions, however, that these changes could prove deceptive; the present trend could stall, prolonging the warm event.

Most of the various ENSO forecast models call for a cold event to follow this year's warm one, but they differ on the timing, a factor the models have trouble predicting. A model at the Climate Analysis Center, based on the statistics of past weather patterns, predicts that Pacific Ocean temperatures will remain higher than normal for several months and then decline, with a cold event emerging by next year.

A much faster descent into cold conditions is predicted by a new model developed jointly by the Scripps Institution of Oceanography Scripps Institution of Oceanography: see California, Univ. of.  in La Jolla La Jolla (lə hoi`yə), on the Pacific Ocean, S Calif., an uninc. district within the confines of San Diego; founded 1869. The beautiful ocean beaches, in particular La Jolla shores and Black's Beach, and sea-washed caves attract visitors and , Calif., and the Max Planck Institute for Meteorology meteorology, branch of science that deals with the atmosphere of a planet, particularly that of the earth, the most important application of which is the analysis and prediction of weather.  in Hamburg, Germany. This model - a marriage between an ocean general circulation model and a statistical atmosphere model - calls for a cold event to appear this summer and reach its peak around the end of the year, says Tim P. Barnett of Scripps.

A third model, developed at Columbia University's Lamont-Doherty Geological Observatory in Palisades Palisades, cliffs along the west bank of the Hudson River, NE N.J. and SE N.Y., extending from N of Jersey City, N.J., to the vicinity of Piermont, N.Y., with a general altitude of from 350 ft to 550 ft (107–168 m). , N.Y., had previously called for cold conditions to develop next year, but more recently started showing a cooling late this year.

Like their warm counterparts, cold events can play havoc with world weather, often causing the exact opposite trends to develop over specific regions. Today's ENSO models remain far too limited to offer specific forecasts for individual areas. But past patterns during cold events would suggest dry conditions for the Gulf states, cold weather for the northern plains and above-normal precipitation in Indonesia, northern Australia and southern Africa.

With most models calling for a cold event to come, ENSO aficionados will keep close tabs on the Pacific. But cold events do not always follow warm ones, and nature may yet pull a surprise. That would make for many unhappy ENSO forecasters.
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Author:Monastersky, Richard
Publication:Science News
Date:Jul 4, 1992
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