The El Niño Southern Oscillation (ENSO) is a major global climate concept that describes a naturally occurring, irregular fluctuation in sea surface temperatures (SSTs) and atmospheric pressure across the equatorial Pacific Ocean. It is a coupled ocean-atmosphere phenomenon that typically cycles every two to seven years, with varying intensity.
The concept's origin is twofold: the oceanic component, El Niño (Spanish for "the Christ child"), was named centuries ago by Peruvian fishermen who noticed unusually warm waters around Christmas. The atmospheric component, the Southern Oscillation (SO), was empirically identified by British climatologist Sir Gilbert Walker in the early 1920s while he was working in India to predict monsoon failures. Walker's work, which began around 1904, solved the problem of identifying a large-scale atmospheric pressure "seesaw" between the eastern Pacific (like Tahiti) and the western Pacific/Indian Ocean (like Darwin, Australia). The combined term ENSO was established in the late 1960s when Jacob Bjerknes realized the oceanic and atmospheric changes were physically connected.
ENSO works by oscillating between three phases: El Niño (warm phase), La Niña (cool phase), and Neutral. In the Neutral phase, easterly trade winds push warm surface water toward the western Pacific, causing low pressure and rainfall there, while cooler water upwells in the eastern Pacific, causing high pressure. During an El Niño event, the trade winds weaken or reverse, allowing warm water to surge eastward, which suppresses the upwelling of cold water and shifts the low-pressure zone to the central/eastern Pacific. The reverse occurs during a La Niña event, where stronger-than-normal trade winds push even more warm water west, intensifying the cooling in the eastern Pacific. This ocean-atmosphere interaction is driven by the Bjerknes feedback mechanism.
ENSO is strongly connected to the Indian Summer Monsoon Rainfall (ISMR), as it is one of the most important sources of global climate variability. El Niño events are typically associated with a weakening of the Indian monsoon, often leading to drought conditions and impacting India's agricultural output and economy. Conversely, La Niña events generally result in above-average monsoon rainfall. However, this relationship is not absolute, as other factors like the Indian Ocean Dipole (IOD) can sometimes mitigate the effect of an El Niño. Recent research suggests that the properties of ENSO may be changing, with some studies indicating a possible intensification of El Niño events.