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NASA Science News for March 14, 2003

A Quirky El Nino
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The 2002-03 El Nino has resisted stereotypes with its unpredictable
behavior.

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

"Every El Nino 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 Nino 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 Nino weather, too. 

El Nino 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. 

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 Nino has
been far milder than the mammoth El Nino 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. 

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

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

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

Patzert speculates that the 2002 El Nino 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." 

Somehow--no one knows the details--the PDO influences El Nino: If El
Nino begins during a cool phase of the PDO, El Nino tends to be milder 
and less predictable. El Ninos 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 Nino.
Since then the PDO has cooled. "Tropical ocean surface temperatures
surrounding El Nino'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 Ninos 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 Nino stereotype that some 
forecasters expect. 

Patzert, however, thinks this El Nino 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 Nino is giving
way to a cool band of water that might herald "La Nina." 

"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 Nina. She has weather patterns all
her own. In this case La Nina would probably mean continuing drought
across much of the western U.S." he says. 

La Nina often, but not always, follows El Nino. 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 Nino of 2002
may have more surprises in store. Anything else would be out of
character. 

Credits & Contacts
Authors: Dr. Tony Phillips, Patrick L. Barry 
Responsible NASA official: Ron Koczor 
Production Editor: Dr. Tony Phillips 
Curator: Bryan Walls 
Media Relations: Steve Roy 

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