Sep 14, 2000

La Nina's Ghost



La Niña's Ghost


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.


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September 15, 2000 -- The El Niño/La Niña one-two punch that caused unusual weather to hit much of North America over the last three years has finally come to an end, a NASA satellite image shows -- leaving behind "bruises" in the form of charred forests and an above-average hurricane season.

So the big question is: with La Niña gone, will the weather return to normal this winter? Well ... maybe.

Some scientists say weather patterns are likely to return to normal, while others point to signs of another ocean temperature pattern in the Pacific called the Pacific Decadal Oscillation (PDO), which could perpetuate La Niña-like conditions.

Above: Image from the NASA/CNES TOPEX/Poseidon satellite showing the return of normal ocean temperatures (green) to the tropical Pacific. Note also the above average temperatures (yellow, red and white) in the north, west and south Pacific, which may reveal a lingering negative phase of the Pacific Decadal Oscillation (PDO). Image courtesy of JPL.

"Anybody who makes a hard and fast forecast for this winter has definitely got a better Ouija board than me," said Dr. William Patzert, an oceanographer with NASA's Jet Propulsion Laboratory in Pasadena, CA.




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One source of ambiguity in the winter outlook is a disagreement among scientists about the existence and potential impacts of a "negative phase" of the PDO.

The PDO is a long-term cycle observed in records of sea surface temperature for the Pacific Ocean. In the "negative" half of the cycle, cooler sea surface temperatures in the east and center of the Pacific are surrounded by warmer temperatures in the north, west and south that form a "horseshoe" shape -- a pattern similar to La Niña, only much bigger.

"In many ways when the PDO is in its cool (negative) phase, it looks like a larger La Niña ," Patzert said. "El Niño and La Niña are confined to the tropics, but PDO takes up the whole Pacific. And it has longer time scales."

Each phase of the PDO lasts about 10 to 20 years, according to Patzert. The last negative phase spanned roughly 1945 to 1970, followed by a positive phase lasting roughly from 1975 to 1995. The "positive" phase of the cycle has a similar shape, but with the temperatures reversed.


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

Patzert believes that the recent image from the U.S.-French TOPEX/Poseidon satellite that shows the demise of La Niña also shows the characteristic warm horseshoe of a negative PDO.

"That (negative PDO) pattern is still quite visible in our images," Patzert said.

Potentially, those sea surface temperature patterns could divert the North American jet stream northward into Canada, producing weather effects reminiscent of La Niña.

"When you have the negative phase of the PDO ... the general tendency is to keep the jet stream a little further north, in contrast with El Niño, which dropped it so far south," Patzert said. "These tendencies look a lot like La Niña."

For the winter, that would mean colder than normal temperatures in the northwestern half of North America and warmer than normal temperatures in the Southeast. The Pacific Northwest and the Mississippi and Ohio river valleys would receive more precipitation than normal, while California and the Eastern Seaboard states south of Maryland would receive less.


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"Everything with regard to weather -- temperature, rainfall -- has to do with the path of the jet stream," Patzert said. 


Left: Jet streams are rivers of high-speed air that form along the boundaries of global air masses where there is a significant difference in atmospheric temperature. The California Regional Weather Service posts daily maps and forecasts of the jet stream over North America on the internet.

However, Patzert cautioned, the relationship between the PDO and changes in the path of the jet stream is a hot area of scientific research and still involves a certain amount of speculation.

"Actually, I'm not even making a forecast; I'm just saying there are tendencies," Patzert said. "This whole PDO thing, you have to remember, is new territory. Everybody is hedging." 

While temperature records indicate that the PDO has been going on for a long time, the scientific community only became aware of the pattern in 1996, when Steven Hare and his colleagues at the University of Washington discovered the cycle while researching fish population fluctuations.

What are the implications of the PDO for North American climate in the coming season? "People are scratching their heads, because the PDO is a relatively new phenomenon," Patzert said. "In contrast, the El Niño and the La Niña have a very strong signal, and we've been looking at them for 30 years."



PDO research is so new that scientists aren't yet sure that the cycle is indeed in its negative phase.

Dr. Vernon Kousky, a research meteorologist with the National Oceanic and Atmospheric Administration's Climate Prediction Center, uses a different index to characterize the PDO, and his conclusions differ from Patzert's.

"Our indices indicate that PDO is pretty much neutral," Kousky said. "I looked at the ocean temperature anomalies just recently for the North Pacific and there's still some residual ... negative PDO indications up there," Kousky said, "but the pattern is broken up and does not look as coherent as it did, say, about six months ago or so.


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Above: A recent daily map of sea surface temperature anomalies produced by Al Strong of NOAA.
The orange horseshoe shape in the Pacific is characteristic of a negative PDO phase.

"We expect to see a much more normal-ish (weather) pattern developing over the next couple months and into the winter," Kousky said.

Ironically, even a "normal" winter would entail greater uncertainty in the seasonal forecast for specific regions than during these last three years of El Niño and La Niña.


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Left: 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).

"Our biggest forecast tool is the El Niño/La Niña phenomenon; when that's not there, we're kind of left groping," Kousky said.

"The atmosphere can do a number of things on its own just because of its natural behavior," Kousky continued. "And when you take away the primary forcing -- which would be, say, large scale temperature patterns in the tropics with the El Niño and La Niña -- then the atmosphere's free to pretty much do what it can do, and it has a wide range of possibilities.

"Even though our forecast says we expect most areas to be leaning toward warmer than normal, it wouldn't surprise us at all to see some colder than normal temperatures somewhere in the U.S.," Kousky said.

This natural variation in the atmosphere's behavior in the absence of a strong El Niño or La Niña may help explain why this summer's weather patterns have differed from those expected for a La Niña year.

"I think we saw a gradual breakdown in the La Niña pattern as the summer progressed," Kousky said. "Since about May or so, the traditional La Niña conditions have faded away and kind of shifted into a different pattern.

"(It) probably represents just part of the natural variability of the atmosphere," Kousky said. 

In fact, the waning influence of La Niña on the summer's weather patterns raises the question of whether this summer's wildfires can be blamed solely on La Niña.


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"The pattern of drought we had (over the summer) cannot be attributed to La Niña," said Dr. James O'Brien, director of the Center for Ocean-Atmospheric Prediction Studies at Florida State University in Tallahassee, FL. 


O'Brien noted that droughts in the Montana-Idaho region and in Georgia are not typical for a La Niña summer.

"Something else is going on," O'Brien said. "Unfortunately with El Niño and La Niña, many things are blamed on it without people doing the statistics."

Above: This August 2000 image shows the latest normalized difference vegetation index (NDVI, which indicates plant vigor) anomaly data for North America. The area of stressed vegetation in the Rocky Mountains has intensified and spread compared to July 2000. It is likely that smoke from the fires that burned throughout August caused the extremely low values of NDVI anomaly shown in Idaho and Montana. [more information]

Still, La Niña probably did set the stage for the fires.

"I think (the conditions leading to the wildfires) have to be considered as probably a residual La Niña effect," Kousky said. "To have a spring or early summer warmer than normal out there (western U.S.), that is a La Niña signal. And to have it be somewhat on the dry side is also a La Niña signal."

While the firefighters battling the blazes out west might wish that La Niña was gone for good, it's only a matter of time before it returns. Scientists can't predict exactly when a La Niña or El Niño event will occur, but El Niño won't reappear until next year at the earliest, and La Niña is likely to repeat between four and seven years from now.

Web Links

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

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

To Be or Not to Be, La Niña? - An article from May discussing the first signs of the weakening of La Niña.

Los Niños May Be Gone, But Pesky Pacific Patterns Remains - The original JPL press release.

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


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