Storms Are Evidence--but Not Proof--of El Nino Connection

The fierce Pacific storms that lashed California in the past week are compelling--but so far only circumstantial--evidence that a powerful El Nino current has begun warping winter weather, several climate experts say.

Not until the winter storm season is over will climate experts be able to analyze El Nino’s effect with any certainty. Since weather is naturally so variable, the influence of El Nino can only be detected by analyzing the behavior of storm patterns across an entire season, climate experts said.

“You cannot attribute a single storm to El Nino, or even this entire regime of recent storms where you are whacked with one after another,” said climate expert Eugene Rasmussen at the University of Maryland. “It looks suspicious, but you need to see more. You have to wait until after the season is over and you can look at the whole pattern.”

For months, climate experts have watched an enormous tongue of unusually warm water protrude along the eastern equatorial Pacific, knowing that it was the signature of the largest El Nino current ever detected.

As it ebbed and swelled rhythmically, the pool of warm water--encompassing an area almost twice the size of the continental United States--released huge amounts of heat into the atmosphere, helping to make 1997 one of the warmest years overall on record.

At the same time, the evaporation from the current released more water vapor than normal into the atmosphere, which could serve as raw material for rain clouds and torrential storms, according to satellite measurements gathered by the Jet Propulsion Laboratory. The displacement of so much warm water may have shifted the path of the jet stream that directs Pacific storms.

Taken together, those elements are a prescription for storms much like those that have drenched California recently.

Indeed, the longer the current pattern of storms continues, the more likely it is that the powerful El Nino current is responsible, said Kevin Trenberth, head of the climate analysis branch at the National Center for Atmospheric Research in Boulder, Colo.

But he cautioned against attributing any of the recent storms to El Nino too quickly. “A lot of what is going on is simply winter,” he said.

Recent satellite images show that the expanse of the El Nino current has dropped almost by half since early November, suggesting that the ocean current may have peaked. Even so, its effects in the atmosphere are expected to persist for another three to five months, said experts at the University of Alabama in Huntsville.

“When you see the types of storms that California has been having, that is the type of seasonal pattern that you would expect in an El Nino event,” said Gerald Meehl at the National Center for Atmospheric Research. But “when you are right in the middle of the season it is premature to say that it is an El Nino effect.”

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Consistent, but Not Conclusive

Two Pacific storms visible in this satellite image recorded at 4 p.m. Monday are following on the heels--though somewhat to the north--of the storms that hit the West Coast in the last week. Such a series of storms is consistent with an El Nino can only be determined through analysis of storm patterns for an entire season.

(1) Today: Storm expected to pass the Los Angeles area.

(2) Thursday and Friday: Storm predicted to come onshore this week.

Source: National Oceanic and Atmospheric Administration / WeatherData Inc.