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El Nino's Atlantic counterpart.

El Nino's Atlantic Counterpart

Call it son of El Nino -- or, perhaps, brother or cousin. Scientists have found in the Atlantic Ocean a phenomenon that is the image of El Nino -- the periodic change of water temperature and winds that occurs in the Pacific Ocean, triggering ecological upsets worldwide.

Scientists report in the July 17 NATURE that they measured El Nino-like conditions in the Atlantic in 1984. Near the equator, the surface water was unusually warm and winds were very light. The band of clouds that spans the ocean from South America to Africa hung over the equator longer than usual that spring -- causing heavy rains in eastern Brazil and western Angola and intensifying drought to the north, near the Amazon and in western sub-Saharan Africa.

This Atlantic phenomenon occurs periodically, though not as often as El Nino, according to George H. Philander, senior oceanographer at the Geophysical Fluid Dynamics Laboratory at Princeton (N.J.) University, who wrote the first in a series of six articles in NATURE describing the 1984 event. Whereas El Nino occurs every three to five years, the last Atlantic event prior to this one appears -- from weather and ocean temperature records -- to have been in 1963, Philander reports.

As El Nino in 1982-83 decimated the anchovy, sardine and mackerel populations off southwest Peru, the warm Atlantic in 1984 killed the same kinds of fish near Angola, according to biological oceanographer Richard T. Barber of Duke University's marine lab in Beaufort, N.C.

And as El Nino altered weather around the world (SN:2/26/83,p.135), the Atlantic event affected rainfall and temperatures in western Europe and Africa. However, these effects were relatively mild, Philander says, because the Atlantic is three times smaller than the Pacific.

Although the cause of oceanic warming spells is not well understood, scientists believe it has to do with the interaction of sea-surface temperatures and winds. Normally, in both the Pacific and Atlantic, the water is warmer on the west side of the basin than on the east, and the surface winds blow westward.

During 1984 in the Atlantic -- as during El Ninos -- the eastern basin warmed to match the west, and winds were unusually weak. Near Angola, where in June 1983 the water averaged 21[deg.]C, the average temperature in June 1984 was 26[deg.]C -- the same as the water near Brazil.

The question is, which comes first, warm water or weak winds? A meteorologist would say it's the water, according to Philander, because winds are drawn to the warmest areas. But an oceanographer would say the winds change first, because without them, the warm water does not move west. "It's a chicken-and-egg situation," Philander says.

The weather changes because the warmer ocean holds the east-west band of clouds near the equator. The band usually migrates about 15[deg.] northward in March, as the sun warms the water to the north. In 1984, the Atlantic clouds did not move north until late June.

However, a second major weather change caused by El Ninos did not occur in the Atlantic. During El Nino, clouds over the western islands are drawn eastward to the warmer ocean, causing drought in Australia, Indonesia and the Philippines. In 1984, the clouds over Brazil did not similarly move east -- probably because Brazil's large, warm land mass was enough to hold them, Philander says.

"It's as though, in the Atlantic, it was half an El Nino," he says.

Philander hopes the new data will help scientists understand El Ninos. "By comparing the Atlantic and ... Pacific events," he says, "we should be able to understand both of them better."
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Author:Murray, Mary
Publication:Science News
Date:Jul 19, 1986
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