he central tropical Pacific is in hot water. That means El Niño is knocking.

Five years after the onset of the most intense El Niño on record, forecasters at the National Oceanic and Atmospheric Administration's Climate Prediction Center (CPC) are once again tracking conditions that herald another event.

Yet for all the improvements in detecting its signals, the phenomenon remains a forecasting challenge, researchers say.

"The El Niño-Southern Oscillation is probably the most predictable large-scale climate fluctuation on the planet, but our crystal ball is still blurry," acknowledges Michael McPhaden, a research meteorologist at NOAA's Pacific Marine Environmental Laboratory in Seattle.

Forecasters monitoring the tropical Pacific first noticed El Niño's feeble signals about the middle of last year, according to Vernon Kousky, a research meteorologist at the CPC.

"Things crept along until the end of last year, but now we're seeing more rapid development," he says. Unusually warm sea-surface temperatures are being recorded in the central tropical Pacific as a vast pool of warm water heads east.

This week, the CPC issued an El Niño update indicating that during the next few weeks, waters off Peru and Ecuador should begin to warm, with El Niño reaching full strength sometime within the next three months - although no one has any idea yet how strong it will be.

Formally known as the El Niño-Southern Oscillation, the pattern repeats every four to five years. (See chart for explanation of how the phenomenon occurs.) An El Niño can last for 12 to 18 months.

While El Niño has its most pronounced effect on the tropical Pacific and nearby regions, its reach is worldwide. By one estimate, the United States experienced an economic gain during the 1997-98 El Niño of $16 billion and 650 fewer fatalities than might have otherwise occurred, because El Niño brought milder than normal winters and suppressed the formation of Atlantic hurricanes.

Globally, however, researchers estimate that the event triggered $36 billion in damage and killed 22,000 people.

Thus, issues of strength, timing, and regional impact weigh heavily on researchers. The hope is that improved forecasts, properly used, can help reduce casualties and damage.

"Compared with the previous two or three events, 1997 proved very challenging," says Stephen Zebiak, director of modeling and prediction at the International Research Institute for Climate Prediction (IRI) at Columbia University's Lamont-Doherty Earth Observatory in Palisades, N.Y. "The magnitude was unprecedented and took forecasters by surprise. And our inability to anticipate changes more than a few months in advance was a problem."

Research that could improve the forecasts is focusing on the long-term and short-term patterns that affect air and sea circulation in the tropical Pacific.

One puzzle has been the rise of more frequent, more intense, and longer-lasting El Niños since the mid-1970s.

Analyzing wind and water-current data collected between 1950 and 1999, Dr. McPhaden and colleague Dongxiao Zhang document a slowdown in Pacific Ocean circulation patterns that drive warm water from the tropics to higher latitudes, where it cools, sinks, and returns to the equator. There, upwelling drives the water back to the surface to be heated and to repeat the cycle.

This slowdown, reported in today's issue of the journal Nature, began in the 1970s, the two calculate. As a result, the reduced upwelling has allowed water temperatures at the surface along the equator to rise by about 0.8 degrees C.

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