Along the west coast of South America in the Equatorial Pacific Ocean, near the end of the calendar year, an unusually warm current of nutrient-poor tropical water replaces the cold, nutrient-rich surface water. It does not occur every year, but when it does—and because this condition frequently occurs around Christmas—local residents call it El Niño (Spanish for “child,” referring to the Christ child). Scientists now know that the El Niño is normally accompanied by a change in atmospheric circulation called the Southern Oscillation. Together, the ENSO (El Niño-Southern Oscillation) phenomenon is one of the main sources of annual variability in weather and climate around the world.

The Walker circulation (also called the Walker cell)—in which air and water flows from west to east in the tropical Pacific Ocean due to differences in high and low surface pressures—also contributes to the formation of an El Niño. When the Walker circulation is normal, it is usually warm, and there is wet weather in the western Pacific and cool and dry weather in the eastern Pacific. But when the Walker cell weakens or reverses every few years, the winds weaken, and the warmer-than-usual Pacific waters flow to the east, creating an El Niño. In contrast, during a La Niña event (see below), the Walker circulation becomes stronger, the winds increase, and cooler waters rise because of upwelling to the east.