El Nino’s Quiet Cousin UW Researchers Say Climatic Pattern Weaker But Has Decades-Long Effects

Researchers at the University of Washington say they’ve found evidence of a Pacific weather pattern that has many of the characteristics of El Nino but is spread over decades.

The phenomenon, which they call the Pacific decadal oscillation or PDO, appears to take 20 to 30 years to shift between warm and cold phases.

The climatic trend, the researchers say, may help explain why ocean water has been warmer than usual off the West Coast, why winters have been wetter than usual in the South, and why salmon have been scarce in the waters off Washington and British Columbia but plentiful in Alaska.

El Nino occurs every two to seven years, when westward-blowing trade winds weaken and a warm mass of water pushes across the tropical Pacific to South America. The warm pool now fills an area 1-1/2 times the size of the continental United States.

Forecasters say this year’s El Nino is shaping up as one of the strongest of the century, rivaling the devastating 1982-83 El Nino that brought heavy storms to much of the West.

The UW researchers say the PDO is much more subtle - a shift of only about 2 degrees in surface ocean temperatures over a period of years.

But that is enough to cause atmospheric changes that affect freezing levels, mountain snowpacks, stream flows and water supplies. It’s also enough to influence what kind of creatures thrive in Pacific coastal waters.

“This phenomenon explains much of what is happening in regional climate change,” said David Battisti, UW associate professor of atmospheric sciences.

“And if we could predict the PDO, we would have much more reliable forecasts.”

The Pacific Northwest has been in a warm phase since 1977, said John Wallace, UW professor of atmospheric sciences.

The previous cold phase of the PDO started just after World War II and lasted until 1977.

Scientists have long been tracked increases and decreases in ocean temperatures associated with El Ninos.

“But if you stand back and see the big picture, you notice there are decades when there are a lot of warm temperatures, and other times when you go for 20 to 30 years with more temperatures on the cold side,” Wallace said.

When temperatures have been on the warm side, “we have had more than our share of El Nino years, and they’ve been real strong,” he said.

In the cold phase of the PDO cycle, there appear to be fewer El Nino events.

The period between 1925 and 1945, which also was a warm cycle, was similar to the current period in terms of its effect on the climate and fishing in the Pacific Northwest, Wallace said.

Although there’s scant data from before the turn of the century, Japanese scientists have come to similar conclusions about the presence of a decades-long temperature swing based on studies of tree rings, he said.

Wallace, Battisti and UW researcher Yuan Zhang published a paper on the PDO in the May issue of the Journal of Climate, and Zhang, Wallace and UW researchers Nathan Mantua, Steven Hare and Robert Francis wrote in the June Bulletin of the American Meterological Society about the PDO’s impacts on salmon production.

Another paper on salmon effects has been submitted for publication in the journal Nature.

Mantua said the shifts in salmon production appear to be linked to the changes in ocean temperature. During the warm phase, the salmon harvest in Alaska has flourished, while salmon off the Washington and British Columbia coasts have dwindled.

The opposite is true in the cold phases.

Prior to 1977, salmon production was off in Alaska and thriving farther south, he said.

“It’s important to realize that the ocean (temperatures) play a big role in how successful salmon production will be,” he said.

Researchers are trying to determine what causes the temperature swings and how they can be used to make better forecasts to aid agriculture, fisheries and other fields.

The best scientists can do right now is to say the current warmer temperatures are likely to reverse within a decade, Mantua said.

The PDO doesn’t explain other climate changes, such as global warming, Mantua said, although the cycle’s cold years would tend to mask any increase in average global temperatures, and in warm years it would appear global warming is accelerating.

The PDO theory also doesn’t explain why Bristol Bay, a normally productive Alaska fishery, last year saw its worst salmon returns in decades.

At a session last week to discuss the Bristol Bay returns, Hare, a biologist with the International Pacific Halibut Commission at the UW, said it’s not yet known whether the PDO might be switching from a warm to a cool phase.

“It’s not nearly clear enough and coherent enough to say something has happened,” Mantua said.

“You really have to look back in time. That’s the whole difference in this thing. It plays out so slowly and one year does not a regime shift make.”