Major mood swing alters Pacific character

Afflicted with a bipolar personality, the Pacific Ocean has apparently veered from one temperature extreme to the other—a change that could alter North American weather for the next decade or two, according to climate researchers.

Map shows how ocean temperatures shift as part of a long-term pattern. Currently, a wedge of warmer-than-normal water extends east from Asia, while unusually cool water surrounds this region. When this pattern reverses, Alaskan salmon fisheries improve. David Pierce, Scripps Institute of Oceanography

“It looks like it did shift, probably a year and a half ago, maybe starting 2 years ago,” says Tim P. Barnett of the Scripps Institution of oceanography in La Jolla, Calif.

In the current pattern, a tongue of abnormally warm water extends eastward from Japan two-thirds of the way across the ocean. The rest of the North Pacific is colder than normal. If this arrangement continues in coming years, it could moisten the northwestern and southeastern United States while it dries northern Mexico, says Barnett. Scientists, however, remain unsure about exactly what causes the oceanic mood swings, so they can’t say how long the Pacific will be stuck in its latest funk.

Barnett and other researchers only recently recognized that Pacific Ocean temperatures tend to flip-flop back and forth over the decades. From 1977 through 1998, ocean temperatures along the equatorial Pacific remained generally warmer than average, while abnormally chilly waters extended far eastward from Japan. That pattern helped spur the development of record-setting El Ni±o warmings along the equator, which warped global weather in 1982-1983 and 1997-1998.

In 1998, this pattern reversed when warm waters appeared east of Japan and La Ni±a chilled the equatorial waters. That same year, scientists with the University of Washington in Seattle dubbed the bipolar swings observed in this century the Pacific Decadal Oscillation, implying that it has a regular pattern lasting 20 to 30 years.

The arrangement of temperatures first seen in 1998 has persisted until now, raising questions about whether the oscillation has officially swung opposite to its position in the 1980s and earlier 1990s. Last week, the Jet Propulsion Laboratory in Pasadena, Calif., released a satellite image suggesting that the change had taken place.

Other scientists remain cautious about the pattern. “It’s very hard to tell in real time,” says John M. Wallace, one of the Washington researchers who described the oscillation. “You need the benefit of 5, preferably 10, years hindsight to know whether one of these long-term shifts has taken place.”

He cites the period from 1958 to 1960 as an example of a false shift, when the Pacific’s temperature pattern reversed for only 2 years and then switched back.

Barnett agrees that the current shift could be short-lived, which would indicate that the Pacific Decadal Oscillation has no regular cycle. Computer climate models at this point can’t simulate ocean currents well enough to tell whether random atmospheric fluctuations or more regular ocean processes drive the oscillation. The answer will determine whether scientists can predict changes in the North Pacific.

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