The El Niño-Southern Oscillation (ENSO) is a global climate phenomenon and a recurring natural climate pattern involving the coupled ocean-atmosphere system over the tropical Pacific Ocean. It is characterized by three phases: the warm phase, El Niño; the cool phase, La Niña; and the intermediate ENSO-neutral phase. The phenomenon typically oscillates irregularly every two to seven years.
The concept has a dual origin: the term El Niño ("The Boy") was first recorded in 1892 by Peruvian sailors who noticed the warm coastal current appearing around Christmas. The atmospheric component, the Southern Oscillation, was discovered by Sir Gilbert Walker in the early twentieth century while he was studying the variability of the Indian monsoon. The combined term ENSO was established in the late 1960s when Jacob Bjerknes connected the oceanic and atmospheric changes.
The mechanism is driven by the interplay of sea surface temperatures (SST) and the overlying atmosphere, with a key process being the Bjerknes feedback. During an El Niño event, the easterly trade winds weaken, causing warm surface waters to surge eastward and SSTs in the central and eastern Pacific to warm. The strength of the atmospheric component is quantified by the Southern Oscillation Index (SOI), which measures the surface air pressure difference between Tahiti and Darwin, Australia.
ENSO is a major source of global climate variability, second only to the Earth-Sun relationship, and has strong teleconnections worldwide. For India, it is critically connected to the Indian Summer Monsoon Rainfall (ISMR). El Niño events are generally associated with reduced rainfall and droughts over the Indian subcontinent, while La Niña tends to increase monsoon rainfall. The ENSO cycle also interacts with other regional climate modes, such as the Indian Ocean Dipole.
Recently, the National Oceanic and Atmospheric Administration (NOAA) transitioned on February 1, 2026, to using the Relative Oceanic Niño Index (RONI) to monitor ENSO intensity. The RONI more accurately characterizes the atmospheric response by subtracting the tropical mean SST departures, making it less dependent on the chosen 30-year average. Furthermore, recent research suggests potential links between ENSO patterns and the long-term warming trend of human-caused climate change.