El Niño is a natural climate concept that represents the warm phase of the El Niño-Southern Oscillation (ENSO), a global climate phenomenon involving variations in winds and sea surface temperatures over the tropical Pacific Ocean. The term, which means "the Christ Child" in Spanish, was first used centuries ago by fishermen in Peru and Ecuador to describe the unusually warm waters that appeared around the Christmas season, reducing their catch.
The ENSO cycle, which typically occurs every two to seven years, has three phases: El Niño (warm), La Niña (cool), and Neutral. The mechanism of El Niño involves a weakening or reversal of the easterly trade winds, which normally blow from east to west along the equator. This weakening allows a surge of warm surface water to move eastward across the Pacific, suppressing the upwelling of cold, nutrient-rich water off the coast of South America. This oceanic change is coupled with an atmospheric change known as the Southern Oscillation, where air pressure becomes higher than normal over Indonesia and the Indian Ocean and lower in the eastern Pacific. This ocean-atmosphere coupling is explained by the Bjerknes feedback mechanism, first described by Jacob Bjerknes in 1969.
El Niño is connected to its opposite phase, La Niña, which is characterized by cooler-than-average sea surface temperatures in the central and eastern tropical Pacific and stronger easterly trade winds. The ENSO phenomenon is one of the most important sources of annual global climate variability, second only to the Earth-Sun relationship. For an informed reader, it is crucial to know that El Niño events are associated with a shift in global weather patterns, often leading to drought in regions like Indonesia, Australia, and the Indian subcontinent, while causing heavy rainfall and flooding in other areas. The 2023-2024 El Niño event was one of the five strongest on record and contributed to record global temperatures. While El Niño is a natural phenomenon, its effects, such as periods of drought and heatwaves, can be intensified by long-term global warming.