Mar 5, 2002

A Curious Pacific Wave




A massive swell of warm water is buffeting South America. Is it the first sign of a new El Niño -- or just another "Kelvin wave?"




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March 5, 2002: Somewhere on a beach in Ecuador or Peru, someone is out for stroll. A warm breeze is blowing, water laps at their feet. It's paradise.


But something is amiss. The air is a bit warmer than usual. So is the water. And, for weeks now, tides have been cresting a few inches higher. Maybe only frequent beach-walkers would notice the difference, but the changes are real.

These countries have just been hit by a gentle yet massive swell of warm water -- a so-called "Kelvin wave."

"Kelvin waves are warm bumps in the Pacific Ocean," says JPL oceanographer Bill Patzert. They form around Indonesia and travel east toward the Americas. "A typical Kelvin wave is 5 or 10 cm high, hundreds of kilometers wide, and a few degrees warmer than surrounding waters."




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Usually not much happens when a Kelvin wave arrives -- beach combers experience a bit of extra rain, perhaps, and slightly warmer surf. Nevertheless, scientists pay careful attention to them because these gentle waves occasionally herald something far more powerful: the next El Niño.

El Niños and Kelvin waves are both triggered by winds -- or a lack thereof -- in the Pacific Ocean. Patzert explains: Pacific trade winds blow from east (the Americas) to west (Indonesia). The persistent breeze pushes Sun-warmed surface waters westward and, as a result, the sea level near Indonesia is normally 45 cm higher than it is near Ecuador.

"We call that part of the ocean near Indonesia the

-- it's the biggest area of warm water on our planet," says David Adamec, a climate researcher at NASA's Goddard Space Flight Center. The pool heats the atmosphere above the western Pacific where convection cells spawn thunderstorms, lightning, and plenty of rain.


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Above: A Kelvin wave crosses the Pacific, Dec. 8, 2001 - Mar. 3, 2002. This plot shows a 500m-deep slice of the Pacific Ocean along the equator. The

is on the left, the Americas are on the right. False colors denote temperature anomalies (measured by Pacific ocean buoys) in degrees Celsius. "The warmest part of a Kelvin wave is usually 100 to 150 meters below the ocean's surface," notes Adamec. [ ]

Sometimes the trade winds falter for a few days or weeks -- researchers aren't sure why -- and warm water slides back across the Pacific toward the Americas. "That's a Kelvin wave," says Patzert. "We see at least one of them every (northern) winter."

El Niño begins when trade winds falter -- not just briefly -- but for many months. Then, says Patzert, strong Kelvin waves cross the Pacific "like a conveyor belt," depositing warm water near South America where the ocean is normally cold. This new "warm pool" alters weather all over our planet. Rains that would normally soak the western Pacific shift toward the Americas, while places like Indonesia and India become dry.




Above: El Niño conditions



Above: normal or La Niña conditions

Some people welcome the change. The last El Niño in 1997 suppressed Atlantic hurricanes, lowered winter heating bills in New England, and drew surfers to suddenly warm beaches in California. Sled dogs in the Sierra Nevada enjoyed extra sledding when it snowed in June!

Others dread it. Coastal villagers in South America fled torrential rains in 1997 that washed away their homes. Many were ready to leave anyway to escape record swarms of mosquitoes that bred in standing pools. Meanwhile, normally wet places like Indonesia were hit by terrible droughts and wildfires.

Below: Flash floods can be a side-effect of strong El Niños. Credit: NOAA.


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"El Niño can do a lot of damage," acknowledges Patzert, "but it's not all bad." For example, farmers can profit from changing weather patterns by cultivating "El Niño crops." Rice and beans, for instance, could be planted in areas normally too dry to support them. But such is only possible if growers know when El Niño is coming.

Scientists have developed computer models that do a good job forecasting El Niño's effects -- after El Niño is underway. "The hard part," says Adamec, "is predicting when El Niño will begin."

Indeed, life would be easier if El Niño came and went on a regular schedule, but the interval between El Niños varies from 3 to 7 years -- another mystery for researchers. Because the last one began in 1997, the next could begin as early as this year or as late as 2004. No one knows.

After the El Niño of 1982-83 took forecasters by surprise, the United States, Japan, and France strung 70 buoys across the equatorial Pacific. Called the "TAO array," these buoys monitor water temperature to a depth of 500 meters, as well as winds, air temperature, and relative humidity. They are designed to be an early warning system for El Niño.

Below: One of 70 buoys in the Tropical Atmospheric Ocean (TAO) array. [more]


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El Niño warnings also come from the US-French TOPEX/Poseidon satellite. It can measure the height of the ocean's surface by "pinging it" with an onboard radar. Warm water expands, so Kelvin waves appear as traveling bumps in the satellite's sea surface elevation maps.


Both TAO and TOPEX/Poseidon have tracked the latest Kelvin wave since it formed near Indonesia in Dec. 2001. "The wave crossed the Pacific in January and reached South America in February," says Patzert. Not all Kelvin waves manage to cross the vast Pacific -- but this one did. It reminds Patzert of another notable Kelvin wave: "It looks a lot like one that crossed the Pacific in early 1997 -- just before the last El Niño."

Does this mean another El Niño is near? "Not necessarily," he says. "Kelvin waves appear in the Pacific every winter; they don't all mean El Niño is coming."

Adamec agrees: "It's too soon to say. The real test will be what happens this spring." If the "conveyor belt" starts turning as it did in the (northern) spring of 1997, then we will know El Niño is back.

For now, scientists must watch ... and wait. "We're always watching the Pacific anyway," laughs Patzert. "It's the biggest thing on our planet!" And that, of course, is why Kelvin waves matter.


Web Links and more...


El Niño El No Show? -- The opposing views of experts highlight the uncertainty of an emerging science: predicting the onset of El Niño.

Recent press releases: Is El Niño Coming Back? (NOAA); Forecasters' Confidence Increases That El Niño Will Develop (NOAA); El Niño is Yawning (ESA)

"Kelvin waves are natural modes of the ocean's response to atmospheric winds," says Bill Patzert. "Kelvin waves hug the equator and are forced to travel west-to-east -- a result of Coriolis forces created by Earth's rotation." More information: Kelvin waves (US Navy); Oceanic Kelvin & Rossby Waves (JPL); Kelvin waves (University of Texas).


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Right: A map of Pacific sea-surface temperatures reveals the "warm pool" near Indonesia. Kelvin waves carry water from the warm pool toward the Americas. [more]

The El Nino/Southern Oscillation -- a tutorial from the National Oceanic and Atmospheric Administration

TOPEX/POSEIDON Sea Level Anomalies in Equatorial Pacific (1992-Present)

El Niño/La Niña: Natu46;s Vicious Cycle -- (National Geographic) It rose out of the tropical Pacific in late 1997, bearing more energy than a million Hiroshima bombs: the giant El Niño of 1997-98.

Benefits of El Nino Prediction -- (NOAA)

The Tropical Atmospheric Ocean Array -- a chain of 70 scientific buoys across the Pacific keeps watch for El Niño

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.

Eye on the Ocean -- (EarthObservtory) This article explains how El Niño is formed and influenced by a constant exchange of energy between the Pacific Ocean and our planet's atmosphere.


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