Now that North America is waving good-bye to three years of weird weather--brought on by back-to-back El Nio/La Nia episodes--forecasters are wondering whether things will at last return to normal this winter. The trouble is that disagreement over the dynamics of another ocean temperature pattern, the Pacific Decadal Oscillation (PDO), is making it hard to tell. "Anybody who makes a hard and fast forecast this winter has definitely got a better Ouija board then me," says William Patzert, an oceanographer with NASA's Jet Propulsion Laboratory.
Patzert believes that the PDO, a 20- to 40-year cycle affecting sea surface temperatures, is in its "negative" phase: cooler temperatures in the east and center of the Pacific are surrounded by a horseshoe of warmer waters to the north, west and south. He points to recent images from the U.S.-French TOPEX/Poseidon satellite as evidence (right). And if he is correct, this negative phase, which acts in essence like a big La Nia spread out over the entire Pacific, would make for more unusual winter weather.
But knowledge of the PDO is only four years old, and so no one, including Patzert, is making any bets. Some scientists don't even agree that the PDO is in its negative phase. "I looked at the ocean temperature anomalies just recently for the North Pacific and there's still some residual ... negative PDO indications up there," says Vernon Kousky of the National Oceanic and Atmospheric Administration's Climate Prediction Center, "but the pattern is broken up and does not look as coherent as it did, say, six months ago or so. We expect to see a much more normal-ish winter pattern." Maybe it's best to prepare for anything.