A quick start. Unusually cool waters (blues) on the equator that mark La Niña barely showed in March (top) but took off in April (bottom) and continue to cool in May.

A Surprise La Niña

Climate experts announced today that El Niño's opposite number, La Niña, seems to be emerging in the tropical Pacific Ocean. La Niña's unusually cool waters signal less troublesome weather extremes around the world than El Niño's warmth, but the development highlights troubles for forecasters: Neither they nor their computers saw La Niña coming.

Until recent weeks, the tropical Pacific seemed to be behaving well enough. Human forecasters, if not their computers, had anticipated the onset of last winter's El Niño by half a year (Science, 26 July 2002, p. 497). They had even pegged its magnitude. And into April, it had been fading away pretty much by the book. In March, meteorologist Vernon Kousky and his colleagues at the National Weather Service's Climate Prediction Center (CPC) in Camp Springs, Maryland, did note that some ongoing cooling in the eastern tropical Pacific "supports the possibility of the development of La Niña later this year."

Despite that early hint, in mid-April researchers at Columbia University's International Research Institute for Climate Prediction (IRI) predicted near-normal conditions through September, as did CPC. But rapid developments in the tropical Pacific have now driven forecasters to think again. The cooling glimpsed in March took a dive, chilling the upper few hundred meters of the eastern and central Pacific. On Friday, researchers at IRI upped the odds on a full-blown La Niña this fall from 25%--essentially a random guess--to 55%. And Kousky today said that "oceanic and atmospheric conditions indicate that a transition to La Niña is already under way." The CPC forecast now stands at a 70% chance.

If the tropical Pacific follows through as predicted, it will mean a mixed bag of weather. La Niña would spur hurricanes in the tropical Atlantic this summer and fall, meteorologist Gerry Bell of CPC said today. Other parts of the world could benefit. India would be less likely to suffer a failure of its crucial summer monsoon, Barnston notes. The African Sahel, which suffered decades-long drought late in the 20th century, would be wetter than average. And Alaska's unusually mild winters of late could get back to normal, says Kousky. Last winter, during the El Niño, the renowned Iditarod dogsled race there had to be relocated northward for lack of snow.

Related sites
CPC's El Niño/La Niña Home
CPC's El Niño/Southern Oscillation (ENSO) Diagnostic Discussion