The concept of El Niño is the warm phase of the El Niño-Southern Oscillation (ENSO), a global climate phenomenon involving coupled variations in sea surface temperatures (SST) and atmospheric pressure over the tropical Pacific Ocean. The term, which means "the little boy" in Spanish, was originally used by Peruvian fishermen centuries ago to describe the appearance of unusually warm ocean water near the coast of Peru and Ecuador around Christmas time. The modern scientific concept of El Niño refers to sustained warming of the central and eastern tropical Pacific Ocean, which typically occurs irregularly every two to seven years.
The mechanism of El Niño involves a positive feedback loop known as the Bjerknes feedback, which links oceanic and atmospheric changes. Normally, strong easterly trade winds push warm surface water toward the western Pacific, allowing cold, nutrient-rich water to upwell in the east. During an El Niño event, the easterly trade winds weaken or reverse, causing a surge of warm surface waters to move eastward and suppressing the cold water upwelling. This results in warmer SSTs, a weaker Walker circulation (an east-west atmospheric overturning circulation), and even weaker trade winds.
El Niño is intrinsically connected to the atmospheric component, the Southern Oscillation, which is an oscillation in surface air pressure between the tropical eastern and western Pacific. The strength of the oceanic component is officially tracked by the Oceanic Niño Index (ONI), which is the running three-month average SST anomaly in the Niño 3.4 region (5°N–5°S, 120°W–170°W). To be classified as a full-fledged El Niño episode, the ONI must be at or above +0.5°C for at least five consecutive overlapping three-month periods.
A recent change occurred in February 2026, when the U.S. National Oceanic and Atmospheric Administration (NOAA) introduced the Relative Oceanic Niño Index (RONI) to supplement the ONI. This new index was developed because global warming has increased overall ocean temperatures, making the fixed 30-year baseline used by the traditional ONI less precise. The RONI addresses this by comparing the temperature anomaly in the Niño 3.4 region to the average temperature anomaly of the entire global tropics, rather than just a fixed historical baseline.