El Nino can provide an early-warning system for worldwide weather

SOMETHING remarkable has happened to El Nino. That's the recurring warming of equatorial Pacific surface waters that affects weather around the world. Geophysicists still don't fully understand it. Yet, as they become better acquainted with the phenomenon, they find they can use it to make some valuable types of predictions.

For example, Mark Cane and Gidon Eshel at Columbia University's Lamont/Doherty Earth Observatory in Palisades, N.Y., and Roger Buckland with the Southern Africa Development Community in Harare, Zimbabwe, have found what they call an ``astonishing'' correspondence between maize (corn) crop yields in Zimbabwe and El Nino nearly half a world away. When an El Nino sets in, Zimbabwe suffers drought and maize crops dwindle.

In a research paper published last month in Nature, the scientists note that computer-based forecasts can anticipate an El Nino up to a year in advance. They suggest that such forecasts ``could provide an effective early-warning system for southern African drought-induced famines.'' Early warning can also be given to places such as Peru where El Nino brings crop-destroying rains.

El Nino is a subtle trick the ocean-atmosphere system performs every two to 10 years. It is linked to the so-called southern oscillation; hence scientists speak of an ENSO (El Nino/southern oscillation) event. In this oscillation, sea-level air pressure seesaws between the Australian-Indonesian region and the eastern subtropical Pacific.

As an ENSO develops, pressure becomes unusually high in the western subtropical Pacific and low in the East. Trade winds weaken over the central and eastern regions. The warm-water pool normally found in the western equatorial Pacific migrates eastward. The result is an expanded warm pool east of the International Date Line. This fuels strong convective activity that pumps lots of moisture and heat energy into the atmosphere. The resulting air-circulation patterns can affect weather thousands of miles away.

For reasons not well understood, the presence of an El Nino tends to suppress North Atlantic hurricane formation. Meteorologist William Gray of Colorado State University at Fort Collins uses this fact in his highly successful scheme for predicting the intensity of the Atlantic hurricane season.

An El Nino usually fades after a few years. But there is now indication its influence may linger. This month, Gregg Jacobs at the Naval Research Laboratory at Stennis Space Center, Miss., and colleagues, reported in Nature what seems to be a North Pacific reincarnation of the 1982-83 El Nino. This was the strongest ENSO of this century. The scientists suggest that the event generated a type of ocean wave that traveled across the tropical Pacific and up along the South and North American west coasts. It then was reflected westward from North America as a slow-moving so-called Rossby wave that depends on earth's rotation for its existence. After a decade, it reached the northwest Pacific where it deflected current flows to warm northern Pacific waters.

Dr. Jacobs and his colleagues have reproduced this scenario with computers. They also find evidence for it in satellite measurements of changes in sea level and sea-surface temperatures over the North Pacific. These temperature changes, in turn, could conceivably influence air-circulation patterns that affect North America. These findings need to be confirmed. Meanwhile, the scientists suggest they indicate a ``potential for decadal prediction of El-Nino-generated Rossby waves and their effects.'' That would be an early warning indeed. El Nino studies began as pure geophysical research. Now they are paying dividends with new practical forecasting techniques.

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