El Niño/Southern Oscillation. A shift in the normal relationship between the atmosphere and ocean in the tropical Pacific Ocean. Normally, strong winds (called trade winds because they aided sailing ships transporting goods) blow to the west in the Pacific, moving warmer surface water away from North and South America. Simultaneously, cold water from the ocean depths rises to the surface off the west coast of South America. This upwelling brings nutrients to the surface, supporting fisheries and ecosystems in the area. In an El Niño event, these trade winds die down, causing warmer surface water to accumulate off western North and South America. This leads to increased rainfall, storm activity, and flooding in the Americas (especially the southwestern United States and Peru) and drought conditions in Australia and other areas in the western Pacific. Fisheries on the west coasts of North and South America are also seriously depleted during an El Niño year. (The movement of trade winds in the Southern Hemisphere is called the Southern Oscillation. Because El Niño events coincide with changes in these winds, El Niño is sometimes called an El Niño/Southern Oscillation event or ENSO.)

El Niño means the little one, a reference to the infant Jesus, because the event often becomes manifest in December. El Niño events occur, on average, about every four years and last for a year or more. Some climate researchers suspect that continued global warming may increase the frequency or severity of El Niño phenomena.