Mar 14, 2003

A Quirky El Nino

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A Quirky El Niño


The 2002-03 El Niño has resisted stereotypes with its unpredictable behavior.




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March 14, 2003: Sometimes Earth scientist Bill Patzert wishes he had a degree in psychology. It might help him understand El Niño.


"Every El Niño has a personality all its own, and the latest one has been very quirky," says Patzert, who works at NASA's Jet Propulsion Laboratory. "Here in southern California we expect El Niño to bring heavy rains. But the weather this winter has had a split-personality, alternating between warm and dry months to very cold and wet months."

Strange. But it's not just southern California. Other parts of the world have had quirky El Niño weather, too.

Right: Rains and flooding? Not this time. Southern California has been warm and dry for much of the El Niño winter of 2002. Copyright Michael Pole, all rights reserved.

El Niño is a global weather disturbance that comes along every 4 to 7 years when trade winds blowing across the Pacific Ocean weaken or even reverse. (Why they do this, no one knows.) Normally these winds blow from the Americas toward Australia, pushing sun-warmed surface waters from east to west. "Warm water accumulates near Australia in a region we call 'the warm pool,'" says Patzert.




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What happens when the trade winds falter? "That warm water sloshes eastward back across the Pacific Ocean. We can see it in NOAA and NASA satellite maps of sea surface temperature and sea surface height: a band of warmer, higher water stretching along the equator from the mid-Pacific to the coasts of Peru and Ecuador."

This warm strip has multiple influences on global patterns of winds and humidity. For instance, it diverts the course of the jet stream--a "conveyor belt" for storms--which in turn affects weather over much of the globe, especially the North American continent.

In terms of Pacific sea surface temperatures, the 2002-03 El Niño has been far milder than the mammoth El Niño of 1997-98. Recent images from the NASA/CNES Jason-1 satellite show that the mid-equatorial Pacific was only 2°C warmer than average in 2002, compared to the huge, long-lasting tongue of water that was as much as 5°C warmer than average and piled up along the coasts of the Americas in 1997.


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Above: A comparison of El Nino's warm strip in Dec. 1997 and Dec. 2002. Sea surface temperature anomalies in these maps were computed from measurements of sea surface temperature collected by the AVHRR sensor on the NOAA polar orbiting satellites. [more]

But don't call this El Niño "weak." It's more complicated than that. In some places its effects have been unusually severe. "Take Australia, for example," says Patzert. "El Niño typically causes dry weather and droughts there--and this year is no exception. What's curious is that the drought of 2002 (when El Niño was mild) is worse than the drought of 1997 (when El Niño was strong)."

Meanwhile in Ecuador and Peru, countries where El Niño usually brings torrential rains and flooding, the 2002 El Niño has had little effect. New England has also experienced contrary weather: The El Niño winter of 1997 was unusually warm. This winter it has been bitterly cold.

"This El Niño definitely has a different personality than the one in 1997-98," Patzert says.

Patzert speculates that the 2002 El Niño is both milder and quirkier than the last one because of something called "the Pacific Decadal Oscillation" (PDO). The PDO is a subtle pattern of sea surface temperatures in the Pacific Ocean. "It looks like a horseshoe nearly as large as the Pacific Ocean itself, slowly shifting between warm and cool phases every 20-30 years."


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Above: The colors in these maps represent temperature anomalies--differences from the average sea surface temperature during the cool and warm phases of the PDO. [more]

Somehow--no one knows the details--the PDO influences El Niño: If El Niño begins during a cool phase of the PDO, El Niño tends to be milder and less predictable. El Niños that come during a warm PDO are stronger and "better behaved, in the sense that we can predict their consequences," he says.

In 1997-98 the PDO was in a warm phase--hence the strong El Niño. Since then the PDO has cooled. "Tropical ocean surface temperatures surrounding El Niño's warm patch now are less like they were in '97-98 and more like they were during the '50s, '60s and early '70s--the last time the PDO was in a cool phase and El Niños were relatively mild," says Patzert.

What's next?

The jury is still out on what the coming months hold. Southern California might yet get a dose of wet weather and the Northeast could still have a warm spell, more like the El Niño stereotype that some forecasters expect.


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Patzert, however, thinks this El Niño is nearing its end. "The latest sea surface temperature and sea level maps from space show a cooling trend in the equatorial Pacific." The warm strip of El Niño is giving way to a cool band of water that might herald "La Niña."


"When the trade winds return, they often return with a vengeance," he explains. Not only do they push the warm surface waters back toward Australia, but also they drag cooler-than-usual waters up from the depths. "This is what we call La Niña. She has weather patterns all her own. In this case La Niña would probably mean continuing drought across much of the western U.S." he says.

Right: This Feb. 2003 sea surface temperature map shows cooler waters emerging near the coast of South America. A telltale sign of La Niña? Bill Patzert thinks so.

La Niña often, but not always, follows El Niño. In 1998 the transition happened in the month of May. This time it seems to be happening in March--again, quirky.

"We'll have to wait and see," cautions Patzert. The El Niño of 2002 may have more surprises in store. Anything else would be out of character.




Web Links


A Curious Pacific Wave -- (Science@NASA) A massive swell of warm water buffeting South America was first sign of the 2002 El Niño.

El Niño Repellent -- (Science@NASA) La Niña-like weather patterns persist thanks to a "Pacific Decadal Oscillation" that may repel strong El Niños.




Above: El Niño conditions



Above: normal or La Niña conditions

more: The NOAA El Niño Page; Comparison of Different El Niño and La Niña Events (NOAA); Tracking El Niño (PBS); When the Pacific Ocean Speaks, Earth Listens (JPL)


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