Ocean Changes May Signal Climate Shift
New satellite data released Wednesday show the Pacific Ocean may be undergoing a dramatic climate shift--much longer-lived than any El Nino--that could alter global weather patterns, disrupt fish stocks and perhaps lead Southern California into decades of drier than normal weather.
Scientists at the Jet Propulsion Laboratory in Pasadena, which released the data, predict a major impact, although others argue that prediction is premature.
The changing cycle, known as the Pacific Decadal Oscillation, was formally identified just three years ago. Scientists tracking its progression say it appears that the eastern Pacific is undergoing a significant ocean cooling trend.
Their interest was prompted by highly unusual temperatures throughout the Pacific Ocean. The most recent satellite images indicate that waters along the coast of the Americas are several degrees cooler than normal, while a massive horseshoe-shaped chunk of the western Pacific is abnormally warm.
“It’s bigger than El Nino,” said Bill Patzert, a satellite oceanographer at JPL. “This is a very strong signal.”
Historical records for the past century show that the Pacific Ocean climate has shifted periodically every 20 to 30 years, at times ushering in prolonged shifts in U.S. rainfall patterns, weather cold enough to freeze Florida citrus crops and severe blows to the nation’s premier salmon runs.
But whether it is shifting now remains an open question--and a topic of fierce debate. Leading climatologists this week said it was far too early to tell whether such a long-term shift was occurring. They warned that any predictions of drought were tenuous at best.
“Whether it’s going to stick around is anyone’s guess,” said Nathan Mantua, a climatologist at the University of Washington and an expert on the oscillation’s effects.
Nevertheless, Patzert theorizes that the changes signal that the Pacific is shifting to a “cool phase” that could last for decades, bringing far more rain than usual to the Pacific Northwest and less to Southern California.
Since 1977, the Pacific Ocean climate has been in a “warm phase,” marked by higher than normal rainfall in Southern California and more El Nino events. Historical records show that a cool phase persisted between 1947 and 1977 and that a warm, wetter phase persisted between 1925 and 1945.
Although the day-by-day weather effects of a shift in the oscillation are subtle, the change is important because its impacts are widespread and long-lasting. “Over time, these changes are huge,” said Aants Leetma, director of the National Atmospheric and Oceanographic Administration’s Climate Prediction Center in Washington, D.C. He added that the shift may play a role in Southern California’s current dry spell and last year’s mid-Atlantic drought.
Patzert’s new data were beamed down from the TOPEX/Poseidon satellite, jointly run by JPL and France’s space agency. The satellite uses radar to precisely measure ocean height and converts those measurements to estimates of ocean temperature. Warm water, because it expands, can be higher by several centimeters.
The images show a steady cooling of several degrees in waters throughout the eastern Pacific and abnormally warm waters in the western portion. Even small changes in ocean temperature can have massive effects on climate.
But links between the Pacific Decadal Oscillation and drought remain unproven, said Dan Cahan, director of the climate research division at the Scripps Institution of Oceanography at UC San Diego.
Cahan is among a number of climatologists who argue that there is not enough evidence to confirm any shift. Although fascinated by the recent ocean changes, many said the pattern could merely be an extended short-term event, like a La Nina, which also cools waters in the eastern Pacific.
The Pacific Decadal Oscillation “isn’t very easily separated from what’s going on with El Nino events,” said Kevin E. Trenberth, an atmospheric scientist who heads the climate analysis section at the National Center for Atmospheric Research in Boulder, Colo. “It’s very hard to get perspective until after the fact--a little like the stock market.”
The pattern could even be caused by something as random as a series of storms, said David Battisti, a professor of atmospheric science at the University of Washington, who has been monitoring the current changes for several months using records of sea surface temperature collected by ships and buoys, which he says are more accurate than satellite data.
The announcement by JPL scientists that the ocean is probably undergoing a long-term shift is “premature,” and would require several more years of temperature records to confirm, he said. “You can’t make a prediction from a snapshot.”
Effects Obvious on Fish Stocks
More clear is the relationship between the Pacific Decadal Oscillation and fish stocks. After the last shift in the mid-1970s, “salmon runs in the lower 48 [states] went to hell in a handbasket and got really good in Alaska,” said Michael M. Mullin, who directs Scripps’ Marine Life Research Group.
It was investigation of salmon decline by a University of Washington graduate student that led to the formal discovery of the Pacific Decadal Oscillation. Although a number of oceanographers had been investigating the pattern since the mid-1980s, the former student, Steven R. Hare, found salmon populations fluctuated every few decades. He christened the cycle with its current name in a 1997 paper.
The shift in 1977, he said, killed Northwestern salmon by sending nutrient-rich waters to the Alaska coast and leaving waters off the Northwest barren. “It was an oceanic desert, basically, and the smolts [young salmon]--already stressed from dams and logging--die if they have to swim far to find something to eat,” said Hare, who now works as a biologist for the International Pacific Halibut Commission in Seattle.
Although it takes a few years for fish populations to respond to oceanic changes, early indications point to a cooling trend. Columbia River Chinook runs may soon be at record levels--more evidence that a Pacific Decadal Oscillation shift may have occurred, fisheries scientists say.
Long-term ocean cycles may be largely to blame for the demise of some fish stocks, like the Monterey sardines. Analysis of fish scales left in ocean sediment finds that sardine and anchovy populations underwent cycles of boom and bust long before any commercial fishing pressures.
“Fish live long enough to get through an El Nino, but a decades-long shift affects a whole generation,” Mullin said. Fishery regulators, he said, are struggling to adjust rules “in the face of huge environmental variability we’re just groping to understand.”
Patzert, aware that his announcement would be controversial, said he waited 18 months before labeling the pattern and only did so because it was so large. “See this big, warm area?” he said, pointing to a U.S.-sized swatch of warm western Pacific water in an image on his computer screen. “It’s what I call a large disturbance in the force.”
The ocean climate picture is so complex because multiple layers of activity, from seasonal storms to century-long cycles to global warming, act in concert and interact with each other in mysterious ways.
Although the El Nino phenomenon is relatively well understood, oceanographers admit that there is no agreement on what causes the Pacific Decadal Oscillation. The field is limited, Cahan said, by having only a tiny slice of history: a mere century of sea surface temperature logs.
For the next few months, climatologists say they will be watching the behavior of the Pacific Ocean. Another year of the current temperature trend would be enough to persuade more scientists that the shift has occurred.
But if the pattern is merely the child of random winter storms churning the ocean, the seas could revert to normal in late spring.
(BEGIN TEXT OF INFOBOX / INFOGRAPHIC)
SATELLITE IMAGE TAKEN JAN. 8, 2000
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EL NIO CONDITIONS IN DECEMBER 1997
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Over the past year and a half, the eastern Pacific has cooled dramatically. In contrast, during an El Nino year, water off the West Coast is warmer. A recent climate image shows warmer water farther north and west, which may mean drier conditions in California.
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Sources: Images are from the TOPEX/Poseidon satellite, jointly sponsored by NASA and France’s space agency
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