A new study by a government scientist of the El Niño of 1997-98 sheds new light on "the climate event of the century" that should lead to better predictions of future El Niños and improved forecasts of their likely impact.

Fortunately, from the rapid onset of the El Niño in early 1997 to its sudden demise in May 1998, satellites and a network of buoys moored near the equator measured the surface and sub-surface conditions in the tropical Pacific that characterized the event.

Writing in the Feb. 12 issue of Science, Michael J. McPhaden of the National Oceanic and Atmospheric Administration's Pacific Marine Environmental Laboratory in Seattle, Wash., concludes that the ocean was preconditioned for the onset of an El Niño in 1995 and 1996 by a build-up of excess heat in the western Pacific. The onset of the El Niño was triggered, McPhaden reports, by powerful weather systems migrating into the western Pacific from the Indian Ocean beginning in late 1996.

"Because of the foresight of NOAA and other groups responsible for the El Niño- Southern Oscillation measurement network, scientists had the most comprehensive view ever of the physical mechanisms underlying the development of an El Niño," McPhaden said.

"Before the 1997-98 El Niño began, there was considerable spread in the model forecasts about what might actually happen in the tropical Pacific. Once the El Niño was underway, scientists were able to improve their model predictions by using these measurements of highly anomalous conditions provided by the ENSO network," he said.

"The study also suggests that ocean conditions that naturally fluctuate from one decade to another, as well as global warming trends that occur on even longer time scales, may have influenced the 1997-98 El Niño. Exactly how El Niño may interact with these longer-term climate variations is not clear, and more research is needed to answer these questions," McPhaden added.

In early 1997, a weakening and reversal of the trade winds in the western and central equatorial Pacific led to the rapid development of unusually warm sea surface waters east of the international date line. A pool of warm surface waters in the western Pacific then migrated eastward with the collapse of the easterly trade winds, while a strip, or "tongue," of cold surface waters, which normally indicates upwelling of colder subsurface waters along the equator between the coast of South America and the international date line, failed to develop in the Northern Hemisphere's summer and fall 1997.

The El Niño developed so rapidly that from June to December 1997 a new record high for sea surface temperatures in the eastern equatorial Pacific was set every month, based on measurements dating back to the middle of the last century. At the height of the El Niño in December 1997, unusually warm, 28-29 C, water filled the equatorial basin, with surface waters in the cold tongue region that were more than 4 C warmer than normal, the highest on record in the eastern equatorial Pacific.

In early 1998, sea surface temperatures in the eastern Pacific rose to over 29 C, as the anomalously warm waters added to the usual seasonal warming that occurs at this time of year. It was not until the trade winds abruptly returned to near normal strength in the eastern Pacific in mid-May 1998 that the cold subsurface waters were drawn upwards. When that happened, sea surface temperatures in the equatorial cold tongue plummeted an unprecedented 8 C during one 30-day period from May to June, marking an end to the El Niño.

For more information about El Niño, visit the following World Wide Web page:
http://www.pmel.noaa.gov/toga-tao/el-nino/nino-home-low.html