Signs of El Nino and Climate Upheaval

Article excerpt

Signs of El Nino and climate upheaval

Three years ago, climate patterns over the world were rudely altered by this century's largest El Nino -- a warming of eastern Pacific waters that occurs irregularly every three or so years and last 18 to 24 months. The last El Nino doused Ecuador with torrential rains, brought record droughts to Australia and killed much ocean life (SN: 11/5/83, p. 298).

Now a number of researchers report signs of another El Nino in the making, which, if realized, will probably by less sever than the 1982-83 episode. But more than the projected weather system itself, it is one method used in its forecast that has scientists excited. In addition to atmospheric and oceanic measurements, researchers used a new computer model that some say may represent a breakthrough in El Nino forecasting.

According to Eugene M. Rasmusson at the Climate Analysis Center of the National Weather Service in Silver Spring, Md., the present signals of a developing El Nino include warmer-than-normal waters extending from the Peruvian coast north to the equator; sea surface temperatures off Peru were lower than normal in November but during the last three months have risen faster than the usual rate, he says. Equatorial temperatures west of the date line are also higher than normal. And the southern oscillation -- an atmospheric pressure "seesaw" between the southeastern Pacific Ocean and the Australian-Indian Ocean region -- began in February to swing in a direction consistent with the development of an El Nino.

But partially because one symptom typical of an upcoming El Nino -- an increase in the tilt of sea level between the western and eastern sides of the Pacific Ocean -- has not occurred, the Climate Analysis Center has put out only an El Nino "advisory," based on current observations, rather than a forecast for the future. Rasmusson says it will probably be another two to four months before he will be able to say anything more definitive.

However, another group of scientists of Lamont-Doherty Geological Observatory in Palisades, N.Y., has made a prediction that an El Nino will strike this year and peak next winter. Mark A. Cane, Stephen E. Zebiak and Sean C. Dolan base their forecast on a simplified version of a coupled ocean-atmospher model they constructed to study the dynamics of El Ninos. Cane's group is the first to use a completely physical model to forecast El Ninos; past predictiv models hav relied on statistical comparisons of El Nino symptoms rather than on their physical causes.

In testing the model for 12 past years, three of them El Nino years, the researchers found that it was remarkably successful in forecasting whether El Ninos would occur. …