It looks like El Niño, it feels like El Niño, and if you are watching fish stocks, reservoir levels or farm production, you would say it is El Niño. But it isn't.
Researchers at the University of Washington are describing in two recent research papers what they call a decades-long climate shift in the Pacific Ocean that seems to explain many of the changing environmental patterns seen across North America, and particularly in the Pacific Northwest, since the late 1970s.
The scientists are calling this climatic phenomenon the PDO, for Pacific decadal oscillation. And, they say, its current positive cycle helps to explain why U.S. West Coast ocean temperatures have been warmer than average, why winters have been wetter than usual in the South, and why Alaska salmon harvests have been at historic highs, while there have been record declines along the West Coast.
El Niño, it appears, is only one small -- albeit exaggerated -- phase of this cycle, says David Battisti, UW atmospheric sciences associate professor, who was the first to show why El Niño recurs on an average of every four years. He describes this latest discovery as an index of sea-surface temperatures in the North Pacific, "which my guess also involves the tropics." Says Battisti: "This phenomenon explains much about what is happening in regional climate change. And if we could predict the PDO, we would have much more reliable forecasts."
However, says Nate Mantua, a UW research associate, scientists probably will not have the ability to begin making accurate forecasts for at least another five years. A PDO prediction system, he says, would allow long-term planning in such areas as fisheries, water supplies, agriculture and energy production.
"The science right now is more like our understanding of El Niño 15 to 20 years ago," says Mantua. But when a PDO forecast is developed, he says, it will become an important measure of climate across North America.
The discovery of the PDO has been something of a scientific detective story. Using high-speed computers, researchers combed the past century's meteorological records to see if they could spot any recurring patterns of climate change. In more recent decades, El Niño quickly emerged as the dominant recurring pattern of year-to-year climate variability on the planet. But when records were studied back to 1900, with the focus on the region north of Hawaii in the Pacific basin, the PDO revealed itself with positive and negative phases lasting from 10 to 30 years
With a few interruptions, researchers found that since 1977 the PDO has been in a positive phase with cool air in the Southeastern U.S., and a tendency to dry weather over the Columbia Basin and the Great Lakes. In the Northwest, winters have been largely warm and dry, water levels have been down because there have been fewer storms than normal, and snow packs have been low. In the previous negative phase of the PDO, lasting from 1947 to 1976, the Northwest's water supplies were an average of 20 percent higher than between the 1920s and the 1940s, with more precipitation and higher snow packs.
Evidence also suggests that many populations of Pacific salmon are influenced by changes in marine climate. This could explain why in the last negative phase of the PDO, when coastal ocean temperatures were cooler, coho and chinook salmon were in abundance off the coasts of Washington, Oregon and California, but Alaska's stocks were greatly depleted. Since the 1970s, warmer coastal waters have reversed these conditions. However, the UW researchers say, the present positive phase of the PDO should be expected to reverse within a decade, at which time favorable ocean conditions should return for West Coast salmon.
Many of these climate changes are felt across North America because of wave patterns -- like ripples in a stream -- in the atmosphere, which is directly affected by changes of temperature in Pacific Ocean currents. But the phenomenon is particularly evident in the Northwest because of a feature in the wind field called the Aleutian low, which directs atmospheric patterns across the region.
One of the puzzles of the PDO, says Mantua, is whether it acts as a restraint on El Niño, or whether it is a long-term response to the phenomenon. Mantua says he prefers the argument that the PDO is a slower change in the climate system of the oceans and atmosphere over the entire Pacific basin which influences how El Niño develops.
One frustrating aspect of attempting to forecast the PDO is that it develops over such a long period that a negative or a positive phase can have passed before researchers even discover it. "We can recognize the phenomenon, but we can't say what phase we're in at the time," says Battisti. "But that's only because we don't yet fully understand it. After all, it has only been in recent years that we've recognized it even exists."
Battitsti's paper, whose authors include UW professor of atmospheric sciences John Wallace and graduate student Yuan Zhang, appeared in Journal of Climate Mantua's paper, whose authors include UW quantitive biologist Steven Hare, UW professor of fisheries Robert Francis, Wallace and Zhang, appeared in the Bulletin of the American Meteorological Society