Image: U.S.-French TOPEX/Poseidon Mission

After three years of weather dominated by La Nia, El Nio may be trying for a timid comeback, and power-starved California could profit from its return. El Nio and La Niaeither extreme of a cycle of water movement in the Pacific Oceangained notoriety when a "super-El Nio" devastated portions of the Western United States in 1997 and 1998.

During La Nia, strong winds blow from east to west. This push actually raises the sea level on the coast of Australia by more than 150 centimeters (almost five feet) at times, much like sand piled up against a wall. Eventually the wind pressure subsides and the water starts to flow back, creating a similar push in the opposite direction. Along the west coast of the Americas (indicated by the blue and green in the image here), there is an updraft of cold water that fills the gap left by the surface water pushed westward. As La Nia ends, the surface water flows back and the coast is hit with unusually warm water, which results in more rainfall. It is this rainfall that could feed the rivers and fill the lakes that provide California with hydroelectric power.

The next El Nio is unlikely to have the devastating effects its predecessor did. "Because we've just had a big El Nio, it's very unlikely that we'd have another big one this year," explains Vernon Kousky, a research meteorologist with the National Oceanic and Atmospheric Administration's Climate Prediction Center. "The big El Nios tend to be separated by weaker ones." Experience shows that El Nio usually takes over from La Nia after about four years, so scientists are getting ready to take a closer look this spring. "It's almost like it's a new ball game every year between March and May," says John Wallace, professor of atmospheric science at the University of Washington. "That's when [the Pacific] sort of forgets what it's been doing and decides what it's going to do next year."