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Science File

Weather Forecast: Wait

Scientists say we are due for the global effect known as El Nino, but no one can say when one is coming or how strong it will be.

April 29, 2002|USHA LEE McFARLING | TIMES STAFF WRITER

Will it be an El Nino? Or an El No Show?

The country's top weather predictors--scientists at the National Oceanic and Atmospheric Administration--have said since February that an El Nino is on the way. But they can't say when it will arrive or how strong it will be.

Longer-range experimental forecasts that attempt to predict El Ninos up to one year in advance disagree on what's ahead. And some of those who closely monitor ocean temperatures say there's not much to report from the tropical Pacific waters that spawn the global weather phenomenon.

"It's not happening yet," said Bill Patzert, an oceanographer at JPL. "It could be an El No Show."

El Nino is a temporary disruption in the waters of the tropical Pacific and the atmosphere above it that can alter weather patterns around the world. The massive El Nino of 1997-1998 brought torrential rains to California and drought to Indonesia and Australia.

Because of these widespread effects, scientists and government officials are anxious to predict the phenomenon well in advance. Many groups--from energy traders to roofers--await any El Nino news.

Phenomenon Is Linked to Ocean Temperatures

It is about time for an El Nino. They generally occur every three to seven years. But forecasting long-term weather and climate patterns is never an easy game. This year is turning out to be more challenging than usual.

El Ninos are closely linked to ocean temperatures and tropical trade winds.

Warm water generally pools in the western Pacific because trade winds blow from east to west. Every few years, a brief period of weakening trade winds or warming ocean temperatures in the eastern Pacific leads to a yearlong period of warm temperatures in the eastern part of the ocean and weak tropical trade winds.

These interactions, a natural positive feedback loop, crank up to produce an El Nino or its cold, windy cousin, La Nina. Both patterns influence the atmosphere and can disrupt weather around the globe.

The El Nino of 1997-1998 was much easier to get a handle on. Huge pools of warm water quickly spread across the ocean and then warmed the atmosphere. "You saw it coming, then it hit," said Wayne Higgins, a lead El Nino forecaster at NOAA's Climate Prediction Center. "This year, the evolution has been really slow. We're being very cautious."

Higgins said there was about a 70% chance an El Nino would develop. He was unable to say how weak or strong it might be, but said that it would be unusual to have another strong event after the El Nino of 1997-1998. Major El Nino events occur every two to three decades.

"When you look in the historical record, you don't find back-to-back El Ninos," he said.

Nathan Mantua, an atmospheric scientist at the University of Washington who monitors conditions in the tropical Pacific, said he sees what looks like weak El Nino conditions developing.

"I'd be surprised if this cranked up into a major event," he said. "It might be among the weakest El Ninos we can classify."

Surges of warm water similar to those seen in 1997-1998 have been reported by South American fishermen, and they have coincided with major changes in coastal fisheries and regional flooding in Peru and Ecuador. But these "coastal El Ninos," Mantua said, are not always linked to the larger-scale atmospheric changes that yield global El Nino events.

While talk of El Nino often prompts fears of massive floods and houses tumbling down rain-soaked cliffs, the net effect of the wet and warm El Nino generally is considered positive for the United States. Because of lowered winter heating bills in the northern parts of the country and fewer Atlantic hurricanes, the weather pattern results in savings estimated to be billions of dollars and hundreds of lives.

Weak El Nino Could Be Bad News for California

In California, the lack of a strong El Nino may actually be bad news. The state has been drier than average for the last four years, and a weak El Nino is unlikely to change that. "People shouldn't dis El Nino," Patzert said. (The current drought on the East Coast is not thought to be linked to El Nino or to the drier La Nina pattern.)

Several groups are trying to predict El Nino events six months to a year in advance, using models of how the oceans and atmosphere behave.

A group at the Scripps Institution of Oceanography is predicting a mild El Nino for the coming winter and next spring, said David Pierce, an analyst at the Climate Research division at Scripps.

But the forecasts of various groups using different ocean and atmosphere models do not agree as well as they did in 1997-1998. "It's been a little uncertain this year," Pierce said. "This time around, I guess we'll find out which models are better."

Forecasting El Nino remains a challenge. Some aspects of the system--like the slow-moving waves that carry heat across the ocean--can be predicted. But other aspects, like small-scale weather patterns, remain random. "We don't know how much will ultimately be predictable," Pierce said.

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