El Nino-Southern Oscillation

A recurring, irregular climate pattern centered on the tropical Pacific Ocean, in which sea surface temperatures and the trade winds that normally push warm surface water westward swing between two opposite phases every two to seven years. In its warm phase, commonly called El Nino, the trade winds weaken and warm water that is normally piled up in the western Pacific spreads east, suppressing the upwelling of cold, nutrient-rich deep water off the coast of South America and disrupting fisheries there, while shifting rainfall patterns across much of the globe. Its cool phase, La Nina, is close to a mirror image, with stronger trade winds and stronger-than-usual upwelling. Because the ocean and the atmosphere are so tightly coupled across a third of the planet's surface, a strong swing in either phase measurably shifts temperature and rainfall patterns far beyond the tropical Pacific itself, which is why national weather agencies track and forecast it as one of the single largest sources of year-to-year climate variability on Earth.