El Nino, Wetter Winter in Forecast

Forecasters say El Nino is on the way again, bringing the promise of another wetter-than-normal winter in Southern California.

But it probably won’t be as wet as the drenching El Nino winter of 1997-98, and there’s no guarantee that the crucial snowpack in the High Sierra will be deep enough to provide a big boost in hydroelectric generation of power for energy-strapped California.

“With El Nino, the snowpack could be deeper--the odds favor those things,” said Tim Barnett, a climatologist with the Scripps Institution of Oceanography in La Jolla.

“But we don’t want to offer false hopes,” he said. “It’s not a gimme. With the weather, there are no gimmes.”

Barnett said computer models show that this El Nino--an oceanic and meteorological phenomenon named for the Christ child because its effects were first noted during the Christmas season--should start building later this summer, reaching its maximum strength in late December or early January.

“It doesn’t look like a real big one at this point,” he said.

Meteorologists say El Nino--and its often drier-than-normal counterpart, La Nina--are generated by fluctuating ocean surface temperatures in the equatorial Pacific Ocean. They said the Pacific is essentially a vast basin, “with the water sort of sloshing back and forth.”

During normal conditions, low-level equatorial trade winds blow from east to west, shoving water ahead of them that tends to pile up near Indonesia, raising the sea level there a foot higher than it is off the coast of Peru. During El Nino, these trade winds slacken or even reverse, and the water sloshes back toward South America.

The sloshing surface water presses down on cooler water beneath it that normally wells to the surface off the coast of Peru. As a result, the surface water there stays warmer than usual, and starts ebbing back westward across the Pacific.

This vast pool of warmer-than-usual surface water, often depicted on television as a reddish smear spreading west from Peru, interacts with the atmosphere above it to disrupt weather patterns.

During most El Nino winters, high-altitude Pacific jet stream winds are intensified and extended all the way across the ocean to California and Baja California. A large low-pressure center over the central Pacific spins off an unusual number of storms, and the amplified jet stream storm track tends to funnel more precipitation than usual into Southern California.

That’s what happened in the fall, winter and spring of 1997-98, when 31.03 inches of rain fell on downtown Los Angeles, more than twice the normal amount and the third-highest total on record.

But that was one of the strongest El Ninos ever measured, and Barnett said the one expected this winter probably won’t be as strong.

And while the effects of El Nino on Southern California are pretty consistent, with above-average rainfall during eight of the last nine years, the effects on Central California are less certain.

“That’s sort of a never-never land up there,” Barnett said.

Central California’s Sierra snowpack, which melts during the spring and early summer to provide the bulk of the water used to generate hydroelectric power, was well below normal last winter, and Barnett said that even with uncertainties, El Nino should help.

“Last winter was a bummer,” he said. “There should be some improvement.”

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Defining El Nino

El Nino --Spanish for “the child” --refers to an ocean current in the South Pacific, named for the Christ child because it appears off Peru around Christmas.

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Normal Year

Trade winds blow to the west along the equator

Winds drag warm surface water west

Cold, deep water rises to replace warm water, forming a strip of cooler water along the equator

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El Nino Year

Trade winds weaken or retreat to the east

Warm surface water surges back along the equator and the cold strip disappears

Cold water upwelling decreases, sea level rises in the east

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Source: Scripps Institution of Oceanography