El Niño is the warm phase of the El Niño–Southern Oscillation (ENSO), a coupled ocean-atmosphere climate pattern in the tropical Pacific. During El Niño, the normally strong trade winds weaken or reverse, allowing the pool of warm surface water (normally piled up in the western Pacific) to spread eastward towards South America. Sea surface temperatures in the central and eastern Pacific rise 1–3 °C above normal, with major consequences for global weather.
El Niño events occur irregularly every 2–7 years and typically last 9–12 months, peaking in December (hence the name, "The Christ Child", given by Peruvian fishermen who noticed warm waters appearing around Christmas). Effects include: torrential rains and flooding in Peru and Ecuador, drought in Indonesia and Australia, reduced Atlantic hurricane activity (due to increased wind shear), warmer winters in northern North America, and disrupted monsoon patterns in India and Southeast Asia.
For Europe and Spain, El Niño's influence is more subtle and indirect. Studies suggest it favours wetter and stormier winters in the western Mediterranean (more Atlantic depressions reaching the Iberian Peninsula) and can influence the strength and position of the jet stream over Europe. The strongest El Niño events on record — 1972–73, 1982–83, 1997–98, and 2015–16 — caused global temperature spikes and billions of dollars in weather-related damage worldwide. ENSO monitoring uses the Niño 3.4 index (SST anomaly in the central Pacific) and the Southern Oscillation Index (pressure difference between Tahiti and Darwin).