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El Nino Repellent?

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El Niño Repellent?

New satellite images of the Pacific Ocean hint that El Niño will not return this winter. Instead, La Niña-like weather patterns will persist thanks to a "Pacific Decadal Oscillation" that might also repel strong El Niños.

NASA
Marshall Space Flight Center

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June 28, 2001 -- During the winter of 1997 and '98, I learned to really dislike El Niño. At my mountain home in California it wouldn't stop snowing. I spent several hours a day shoveling the stuff, mainly out of my dog yard, the home of a dozen Siberian Huskies. Whenever the snow level reached about five feet they could jump the fence en masse and wreak havoc around the neighborhood. So rather than dog sledding, which is what we usually did when it snowed, I shoveled.

Most of the snow was precipitation that usually falls somewhere over the western Pacific, perhaps in Indonesia or Australia. But in 1997, for reasons scientists still don't understand, Pacific trade winds slackened --a telltale signature of El Niño-- and the weather came our way instead. My problems were small compared to South Americans who lost entire villages to El Niño-driven floods, but I was still passionate when I hoped El Niño would leave and not return.

Above: The question on every dog's lips is "When is the snow coming back?" Recently, their sledding season has been cut short thanks to the influence of La Niña.

Be careful what you wish for, they say, it might come true.

Indeed, El Niño didn't return after 1998. But what came next, La Niña, was in its own way just as bad -- only longer-lasting.

"La Nina conditions developed soon after the '97-98 El Niño," says Bill Patzert, an oceanographer at NASA's Jet Propulsion Laboratory. "That's normal. Big El Niños are always followed by a few years of La Niña -- an extreme anti-El Niño weather pattern."

In 1997, El Niño pulled North America's jet stream farther south than usual, delivering strong rains to much of the US. Then, beginning in late 1998, La Niña pushed the jet stream back north again, reinforcing dry weather trends. Wildfires and droughts became common in the very same places where torrential rains and mudslides had vexed Americans during El Niño.

Above: Dams like this one on the Sacramento River in northern California generate hydroelectric power and account for roughly 20 percent of the state's electricity. During dry years there's less hydro-power to go around. Image courtesy of the U.S. Bureau of Reclamation.

"Droughts aren't caused entirely by La Niña," cautions Patzert. "Other factors, like the human impact on farmland, are important too. But La Niña does contribute." Even California's energy crisis is due, in part, to La Niña conditions. Hotter-than-average weather means more air conditioners running full blast, while dry conditions reduce stores of hydroelectric power.

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When the Pacific Speaks, the World Listens

Most years, trade winds in the tropical Pacific blow from east to west -- that is, from South America towards Indonesia -- pushing warm surface waters toward Asia. The wind pushes so much water westward that the sea level near Indonesia is usually 18 inches higher than it is near Mexico!


Above: El Niño conditions


Above: normal or La Niña conditions

El Niño happens when the trade winds subside, and that great mass of warm water -- bigger than Canada -- sloshes back toward South America. Rains that would normally soak the western Pacific shift toward the Americas, while places like Australia, Indonesia, and India experience drought. Satellite images reveal El Niño as a band of warm water accumulating near South and Central America.

Below: The upper false-color globe shows El Niño (white represents the warmest water), while the lower globe shows evidence of La Niña (purple represents cooler water).

El Niño normally lasts for one year. Then, resurgent trade winds shove the warm surface waters back across the Pacific with a vengeance. Cold water rises from the ocean depths to replace the warm, forming the La Niña signature: a band of chilly water near western American coasts. Whatever extremes El Niño did, La Niña does the opposite. Floods become droughts. Cold weather grows warm.

The back and forth rhythm of La Niña and El Niño is a normal part the Pacific Ocean's "lifecycle." There are similar oscillations in other oceans -- like the North Atlantic Oscillation, which affects weather in New England and northern Europe. However, El Niño and La Niña are best known because of their globe-straddling impact on climate. "The Pacific Ocean is the biggest thing on our planet," says JPL's Bill Patzert "And when the Pacific speaks, the world listens."

Although few of us long for the stormy perils of 1997, there are some Californians who feel that a nice wet El Niño --perhaps just a modest one-- wouldn't be such a bad thing.

Alas, the latest sea surface temperature (SST) data from the US-French TOPEX/Poseidon satellite show that, while La Niña has subsided, there are no signs of a resurgent El Niño. Instead, "it looks like we're heading for more of the same hot, dry weather," Patzert says. "I wouldn't be surprised if we have a record-breaking fire season in the US this year." Already in Florida unusual wildfires have scorched the Everglades. Firefighters are meanwhile bracing for the worst in western states, which are tinderbox dry.

Right: Dry conditions lead to wildfires. Image credit: John McColgan (Bureau of Land Management, Alaska Fire Service) and the Alaskan Type I Incident Management Team.

What's holding El Niño at bay? "It could be the Pacific Decadal Oscillation," Patzert says.

The Pacific Decadal Oscillation, or "PDO" for short, is a widespread pattern of sea-surface temperatures in the Pacific Ocean that cycles back and forth between two opposite phases every 10 to 20 years (for comparison, El Niño-La Niña appear every 2 to 7 years). In the "negative" phase of the PDO cycle cooler surface waters in the eastern part of the Pacific are surrounded by warmer waters in the north, west and south. (The positive phase is just the opposite.) Negative PDOs appear in sea surface temperature maps as a "horseshoe" of warm water surrounding a cooler "wedge."

Recent SST maps drawn from satellite and ocean buoy measurements by NASA and NOAA reveal just such a pattern in the Pacific. "When the PDO is in its cool (negative) phase, Pacific sea surface temperatures resemble those of La Niña," says Patzert, "and it has many of the same effects on seasonal weather." That's why dry conditions are likely to persist in many parts of North America even though La Niña has faded. The negative PDO is picking up where La Niña left off!

Left: This June 7, 2001, sea surface elevation map from the U.S.-French TOPEX/Poseidon satellite reveals a wedge of cool (blue/purple) water in the Eastern Pacific is surrounded by a vast horseshoe of warmer water to the north, west, and south. [more]

It seems that "negative PDOs can also act as El Niño repellents," says Patzert. In the 1950's, 60's, and 70's, decades characterized by a negative PDO, El Niños were less frequent and weaker. In the 1980's and 90's, when the PDO was positive, strong El Niños happened more often. If history is a reliable guide --and this is still controversial, cautions Patzert-- the ongoing negative PDO could increase our wait for a new El Niño and weaken it when it does arrive.

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"People are still scratching their heads about all this, because PDOs are new to us," Patzert says. While temperature records indicate that the ocean-girdling oscillation has been going on for a long time, the scientific community only became aware of the subtle pattern in 1996, when Steven Hare and his colleagues at the University of Washington discovered the PDO while researching fish population fluctuations. "In contrast, El Niño and La Niña have a stronger signal, and we've been looking at them for 30 years," adds Patzert.

The coming years and decades will be an exciting time for researchers as they monitor the PDO and its shorter-lived cousins El Niño and La Niña. "We're just beginning to learn all the rhythms of the Pacific," says Patzert, "and how they mold weather across the globe. Satellite measurements of these patterns are critical to improve our climate models and forecasts."

Meanwhile, there's a yard full of sled dogs in California who couldn't care less about climate models, but they sure would enjoy some snow and they're willing to live through another El Niño to get it. It seems they'll have to wait at least another year -- bad news for sledding, perhaps, but a welcome respite from snow shoveling!

Editor's note: It's tempting to think of El Niño and La Niña -- both extreme weather patterns -- as bad. After all, they can trigger floods and fires, wash away villages, and even contribute to the energy shortage! But that's not the whole story. "For example," notes Patzert, "the El Niño winter of '97-98 was so mild in the northern United States that the price of oil dropped because of lower demand for heating oil. El Niños also suppress destructive Atlantic and Gulf hurricanes. And west coast surfers love El Niño because it brings waves and warm water." Good or bad? It all depends on your perspective.

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Web Links

La Niña's Ghost -- Science@NASA story La Niña has faded away, but will weather patterns change? Some scientists expect the Pacific Decadal Oscillation to pick up where La Niña left off.

After Three Strikes, is La Niña Out? -- Science@NASA story La Niña-like conditions that have persisted in the Pacific Ocean for three years might finally subside this Fall. The change could pave the way for a weak El Niño -- and a surge of hydroelectricity for power-starved California.

Dust Begets Dust -- Science@NASA story La Niña's not the only factor that reinforces drought. Land management practices have an effect, too.

USA Drought and Summer Outlook -- from NOAA

Daily Sea Surface Temperature maps -- from NOAA

La Niña Earth -- See what a La Niña sea surface temperature pattern looks like.

El Niño Earth -- See what an El Niño sea surface temperature pattern looks like.

The Pacific Decadal Oscillation -- learn more about the PDO at JPL's TOPEX/Poseidon ite.

TOPEX/Poseidon -- A partnership between the U.S. and France to monitor global ocean circulation, discover the tie between the oceans and atmosphere, and improve global climate predictions. Every 10 days, the TOPEX/Poseidon satellite measures global sea level with unparalleled accuracy.

Pacific Remains Locked in Three-Year-Old Pattern -- June 21, 2001, JPL press release


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