Climate forecasters have egg on their faces. Last May, government and private forecasters predicted that the ongoing cooling in the tropical Pacific would likely deepen this summer into a full-blown La Niña. But the cooling has done an about-face, they reported last week, forcing them to fall back to a wait-and-see forecast of neutral conditions--neither La Niña's cold nor El Niño's warmth--for the rest of the year. The reversal highlights forecasters' tenuous grip on the weather-shifting doings of the tropical Pacific, which makes such second thoughts seem almost second nature.
The May forecast for a La Niña and its attendant weather shifts--including more Atlantic hurricanes this summer and more wintertime snow for Alaska--was itself a surprise (Science, 23 May, p. 1215). A subtle cooling in March took off in April and May, prompting the La Niña forecasts. Now that cooling has nearly disappeared. "It was the most incredible turnaround," says meteorologist Anthony Barnston of Columbia University's International Research Institute for Climate Prediction in Palisades, New York, which had given La Niña a 55% chance of occurring. "The La Niña not only died, but, if anything, it's pointing the other way, toward El Niño. It's embarrassing."
The culprit was a particularly pervasive surge of atmospheric convection. With its towering thunderstorms, the stormy cloud patch marched out of the Indian Ocean and spread its contrary winds nearly across the tropical Pacific, warming the surface by cutting off the cooling influence of deeper waters. That's just one of the many factors can influence the transition from El Niño to La Niña, says meteorologist Vernon Kousky of the National Weather Service's Climate Prediction Center in Camp Springs, Maryland, which had put a 70% chance on La Niña. "Some of them are not predictable. Maybe we'll learn something from this in hindsight."