Mar 5, 2001

After Three Strikes, Is La Nina Out?


After Three Strikes, Is La Niña Out?


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.


Marshall Space Flight Center


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March 6, 2001 -- Last autumn scientists thought La Niña had faded, but recent NASA satellite images reveal La Niña-like conditions lurking in the Pacific for the third year in a row. Will they linger for a fourth? Some computer climate models predict La Niña will vanish in 2001 and that a weak El Niño could take its place.

A shift from La Niña to El Niño conditions would likely trigger more rainfall in California, where swelling rivers will increase the output of hydroelectric dams, providing the state with some much-needed electricity.

Above: Shasta Dam on the Sacramento River in central northern California. Hydroelectric power from dams like these account for roughly 20 percent of the state's electricity. Image courtesy of the U.S. Bureau of Reclamation. 

Over the next few months, scientists plan to monitor the situation using data from several satellites -- including the NASA-French TOPEX/Poseidon spacecraft -- to see if the La Niña pattern of Pacific sea surface temperatures remains or departs. Northern spring is a crucial time for such monitoring.




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"It's almost like it's a new ball game every year between March and May," said John Wallace, professor of atmospheric science at the University of Washington. "That's when (the Pacific) sort of forgets what it's been doing and decides what it's going to do next year,"

If La Niña wanes, an El Niño could begin in the fall. But if it does, it won't be like the Super-El Niño of 1997 and 1998. The next El Niño is likely to be mild. 

"Because we've just had a big El Niño, it's very unlikely that we'd have another big one this year. The big El Niños tend to be separated by weaker ones," said Vernon Kousky, a research meteorologist with the National Oceanic and Atmospheric Administration's Climate Prediction Center.

The return of El Niño would mean more rain in California and in the Southeast U.S., cooler temperatures in the Southwest U.S. and generally warmer temperatures along the west coast of Canada and Alaska.

El Niños recur every 4 years on average -- and this fall marks 4 years after the onset of the last one -- but the time between El Niños varies widely.

"The period ranges anywhere from as short as 2 years to as long as 7 years," Kousky said.



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Above: normal or La Niña conditions
Below:El Niño conditions
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"El Niño" and "La Niña" are names for the two extremes in a huge, slow-moving, natural oscillation of the tropical Pacific Ocean.

During a La Niña phase, strong winds blowing from east to west along Earth's equator push surface waters toward Southeast Asia. Deep, cold water wells up in the eastern Pacific near South America to replace water that's blown westward. The upwelling forms a streak of cold water across the eastern half of the equatorial Pacific, a telltale signature of La Niña conditions.

When the cycle switches to an El Niño phase, the winds die down, the upwelling weakens, and the eastern Pacific becomes warmer than usual near the equator. 

Above: The top graphic shows how during normal or La Niña conditions, winds push the water toward southeast Asia, forcing cold water from the deep to well up in the eastern Pacific. The bottom graphic shows conditions during El Niño, when winds over the Pacific fade, and the eastern Pacific becomes warmer than usual near the equator.

Scientists hope to figure out what the Pacific will do this year by running computer simulations.

"Some computer models ... indicate that we might get a weak (El Niño) by the end of this year; others say we won't until next year," Kousky said.

Which of these two scenarios plays out depends, in part, on another larger-scale fluctuation in the Pacific called the "Pacific Decadal Oscillation" (PDO). The PDO is a pattern of sea surface temperatures similar to El Niño/La Niña, but different in that it fluctuates over decades instead of years. It's also much larger. Some researchers liken it to a swollen version of El Niño or La Niña.


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Above: Typical ocean temperature patterns for the phases of the Pacific Decadal Oscillation (PDO). Image courtesy of JPL.

The PDO has a positive phase and a negative phase. The positive phase resembles El Niño -- a warm "wedge" of water around the equator surrounded by a cool "horseshoe" to the north, west, and south. In the negative phase, which resembles La Niña, the temperatures are reversed.

The negative phase of the PDO is so much like La Niña that researchers aren't entirely sure which we're experiencing now. It matters little, however, since the short term climate effects are similar.

"The present 'cool' or 'negative' phase of the PDO looks a lot like and tends to produce impacts similar to the La Niña of the past two winters and springs," says William Patzert of NASA's Jet Propulsion Laboratory in a recent press release. "The big debate among climate scientists is whether we are entering a long-lasting negative PDO episode."

Below: The warm (red) horseshoe and cool (blue/green) wedge pattern in this recent map of Pacific ocean temperatures is characteristic of a negative PDO and of a weak La Niña. [more information] Credit: the U.S.-French TOPEX/Poseidon mission.


During the negative phase of the PDO, El Niños tend to be weaker and less frequent. Since 1976, the PDO has been in a positive phase, which tends to favor strong El Niños. But now the PDO may be switching back to the negative part of its cycle. If so, the upcoming El Niño is likely to be weak.

Climate scientists caution that it's too early to tell if the PDO has really switched.

"I think [we won't] know except with the benefit of hindsight. ... It's kind of like people commenting on the stock market after it's already happened," Wallace said.

Only time will tell if this year's La Niña-like ocean temperature pattern will vanish. Meanwhile, western power consumers might well cross their fingers and wish it gone. After all, a bit of El Niño-like weather might prove to be a welcome change for California's beleaguered utilities.


Web Links

NASA's Jet Propulsion Laboratory -- Visit the JPL home page.

Daily sea surface temperature anomaly chart -- Produced by Al Strong of NOAA.

NOAA Climate Prediction Center -- The latest outlook for hurricanes, droughts, El Niño/La Niña, heat waves, etc. for the U.S.

La Niña's Ghost -- Science@NASA article, 9/15/00 -- 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.

To Be or Not to Be, La Niña? -- Science@NASA article, 5/18/00 -- Just last month, scientists were predicting that current La Niña conditions would persist, but now data from Earth-orbiting satellites show that it may be on the decline.


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