The El Niño-Southern Oscillation (ENSO) is a global climate concept and phenomenon defined by the interplay of oceanic and atmospheric variations over the tropical Pacific Ocean. It is a quasi-periodic cycle that typically occurs every two to seven years. The phenomenon has three phases: the warm phase, El Niño; the cool phase, La Niña; and a neutral phase.
The warm phase, El Niño (Spanish for "the child"), was first noted by Peruvian fishermen in the early 1500s due to unusually warm coastal waters around Christmas. The atmospheric component, the Southern Oscillation (SO), was described by Sir Gilbert Thomas Walker in 1923 as a "seesaw" in surface air pressure between the Pacific and Indian Oceans. Walker's work in the 1920s and 30s was driven by the problem of forecasting the strength 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 Bjerknes feedback, a positive feedback loop between the ocean and atmosphere. The atmospheric component is quantified by the Southern Oscillation Index (SOI), which measures the pressure difference between Tahiti and Darwin, Australia. During an El Niño event, the easterly trade winds weaken, allowing warm surface water to build up in the eastern Pacific. Conversely, during La Niña, the easterly winds become stronger than usual, leading to cooler-than-average sea surface temperatures in the eastern Pacific.
ENSO is one of the most important climate phenomena globally, with its effects extending to the Indian Summer Monsoon Rainfall (ISMR). Historically, El Niño events are associated with a weakened Indian monsoon and a majority of droughts in India, while La Niña events generally result in above-average rainfall. However, the correlation between ENSO and the Indian summer monsoon has decreased recently. A study led by Roxy Mathew Koll found that the relationship has considerably weakened and become non-existent over the central Indian region (core monsoon zone) in recent decades, with rainfall variability there now more dependent on the strength of the monsoon trough.