Cousin of El Nino Haunts Indian Ocean

Science News, Sept 25, 1999 by R. Monastersky

For decades, climate researchers have regarded the Indian Ocean as a boring basin, lacking the commotion that El Nino and La Nina stir up in the Pacific. Now, two teams of scientists have independently identified the climatic kin of El Nino in the Indian Ocean, where it caused widespread weather problems in 1997.

The finding raises the hope that forecasters will soon learn to predict abnormal torrents and droughts in East Africa, India, and Indonesia.

"It's like we've found a raw gem. Now, we have to polish it and see how good it is," says Peter J. Webster of the University of Colorado at Boulder, the leader of one team. His group published its results in the Sept. 23 NATURE, as did a trio of Japanese and Indian researchers.

The two sets of scientists noticed that water temperatures along the equatorial Indian Ocean have a habit of flip-flopping. Usually, warm water accumulates in the eastern part of the ocean near Indonesia, while the western section near Africa remains relatively cool. This pattern can occasionally reverse, however, when winds and ocean currents conspire to chill the eastern Indian Ocean, contend the scientists.

Historical data show this kind of extreme turnabout has happened six times in the past 40 years, reports Toshio Yamagata at the University of Tokyo and his colleagues. When the western Indian Ocean warms dramatically, it can bring devastating rains to Kenya and neighboring countries, which in turn spawn epidemics there (SN: 7/17/99, p. 36).

Scientists had previously detected hints of this temperature reversal while studying the climate of 1961, a catastrophically wet year in East Africa. They had, however, thought that the 1961 episode was unique, says Webster. The pattern sprang to life again in 1997 but got overshadowed by the century's biggest El Nino.

"All of the commentaries in 1997 and 1998 were referring to the great El Nino, and here we had this enormous temperature variation in the Indian Ocean, and nobody had noticed it," says Webster.

The Colorado scientists used measurements of sea level, thunderstorm activity, and ocean temperatures to reconstruct the events in the Indian Ocean. The unusual ocean pattern started to develop in early 1997, when a broad swelling in the sea surface rippled westward, piling up warm water near the African coast. In June, winds along the coast of Sumatra in Indonesia pulled up cool ocean water.

As East Africa warmed and the Sumatran region cooled, the winds that normally blow eastward along the equator reversed direction. The shift enhanced the temperature disparity, feeding the growth of storms over East Africa and fostering record drought and fires in Indonesia. In mid-1998, monsoonal winds sweeping across the Indian Ocean wiped out the unusual pattern, says Webster.

The Indian Ocean reversal is an independent beast that has no clear link to El Nino, contend the two teams. Although the two events coincided in 1997, the Indian pattern occurred on its own in 1961 and at other times in recent decades.

If researchers can discern how the temperature shift in the Indian Ocean affects weather around the basin, forecasters might better predict conditions months in advance, in much the same way that El Nino gives a forewarning of unusual climatic events.

"That is the hope, but it remains to be demonstrated," says David Anderson of the European Centre for Medium-Range Weather Forecasts in Reading, England.

At present, most computer climate-forecasting models ignore the Indian Ocean, but the two reports and other new studies will likely redirect researchers' attention. For example, Lisa Goddard and Nicholas E. Graham of the Scripps Institution of Oceanography in La Jolla, Calif., used a model to show that Indian Ocean temperature plays a much bigger role than Pacific temperatures in influencing African rains, they report in an upcoming issue of the JOURNAL OF GEOPHYSICAL RESEARCH.

"It's important," says Graham. "You can't leave out the Indian Ocean, as some people do."