Every few years the central and eastern tropical Pacific Ocean warms more than usual. That warming is El Niño. The change alters atmospheric circulation across the globe, shifting rainfall patterns, storm tracks, and temperature anomalies far from the Pacific.

Trade winds weaken, allowing warm water to spread eastward. The usual upwelling of cool water along South America diminishes. Heavy rain can fall on the normally dry coasts of Peru and Ecuador, while drought may develop in Indonesia and Australia.

North America often sees milder winters in the north and wetter conditions along the southern tier during moderate to strong events. Fisheries, agriculture, and disaster planning all respond to the forecast of an El Niño year. The opposite phase, La Niña, brings cooler Pacific waters and roughly opposite impacts.

• Warming of tropical Pacific waters
• Weakens trade winds
• Alters global rainfall and temperature
• Recurs every few years