El Niño" and "La Niña" are names for the two extremes in a huge, slow-moving, natural oscillation of the tropical Pacific Ocean.
During a La Niña phase, strong winds blowing from east to west along Earth's equator push surface waters toward Southeast Asia. Deep, cold water wells up in the eastern Pacific near South America to replace water that's blown westward. The upwelling forms a streak of cold water across the eastern half of the equatorial Pacific, a telltale signature of La Niña conditions.
When the cycle switches to an El Niño phase, the winds die down, the upwelling weakens, and the eastern Pacific becomes warmer than usual near the equator.
Above: The top graphic shows how during normal or La Niña conditions, winds push the water toward southeast Asia, forcing cold water from the deep to well up in the eastern Pacific. The bottom graphic shows conditions during El Niño, when winds over the Pacific fade, and the eastern Pacific becomes warmer than usual near the equator.
Scientists hope to figure out what the Pacific will do this year by running computer simulations.
"Some computer models ... indicate that we might get a weak (El Niño) by the end of this year; others say we won't until next year," Kousky said.
Which of these two scenarios plays out depends, in part, on another larger-scale fluctuation in the Pacific called the "Pacific Decadal Oscillation" (PDO). The PDO is a pattern of sea surface temperatures similar to El Niño/La Niña, but different in that it fluctuates over decades instead of years. It's also much larger. Some researchers liken it to a swollen version of El Niño or La Niña.