What is El Nino?

Under normal conditions, trade winds push warm surface water westward across the tropical Pacific, allowing cold water to well up along South America. During El Nino those winds weaken or reverse, warm water sloshes back eastward, and the upwelling of cold nutrient-rich water shuts down. The atmospheric circulation shifts with it, relocating the regions of rising air and heavy rainfall, which is why the effects propagate far beyond the Pacific. The full system is called ENSO, the El Nino Southern Oscillation, and its cool counterpart is La Nina, in which the normal pattern intensifies.

Events occur irregularly every two to seven years and typically peak in the Northern Hemisphere winter. Typical El Nino effects include wetter conditions across the southern United States and drier conditions in the Pacific Northwest and Ohio Valley, drought in Australia, Indonesia, and parts of Southeast Asia, disrupted monsoon patterns in India, heavy rainfall along the coast of Peru and Ecuador, and a suppressed Atlantic hurricane season alongside a more active eastern Pacific one. These are tendencies rather than guarantees, and individual events vary considerably in strength and character.

Two broader points. El Nino years tend to be warmer globally because heat stored in the ocean is released to the atmosphere, which is why several of the warmest years on record have coincided with strong events, and why analysts separate the ENSO signal from the longer-term warming trend when interpreting annual temperature records. And the economic consequences are large and specific: fisheries off South America collapse when the nutrient upwelling stops, agricultural yields shift across continents, and food prices respond. Forecasting has improved substantially, with useful skill several months ahead, which allows agricultural and disaster planning that was impossible before the mechanism was understood in the 1960s and 1970s.