La Niña is a naturally occurring climate pattern, which is the cold phase of the El Niño-Southern Oscillation (ENSO), a major form of climate variability. The concept is a quasi-periodic abnormal cooling of sea surface temperatures (SST) in the central and eastern equatorial Pacific Ocean. The name, meaning "the little girl" in Spanish, was coined by scientists as the opposite counterpart to El Niño ("the Christ Child"), which was a term used by Peruvian fishermen as early as the 1600s to describe an unusually warm current that appeared around Christmas. The phenomenon was recognized as part of a larger global climate cycle, the ENSO, in the 20th century.
La Niña works through a coupled ocean-atmosphere mechanism: it is characterized by unusually cool SST anomalies of 0.5°C or greater in the Niño 3.4 region of the Pacific (between 170°W and 120°W longitude and 5°N and 5°S latitude). This cooling is caused by stronger-than-normal easterly trade winds, which push warm surface water westward toward Indonesia and enhance the upwelling of cooler, deeper water along the South American coast. This process intensifies the Walker Circulation, a major atmospheric circulation pattern, and shifts the area of concentrated heavy rainfall to the western Pacific. The ENSO cycle, which includes La Niña, El Niño, and a neutral state, typically oscillates every two to seven years.
La Niña connects directly to its warm counterpart, El Niño, and the overarching El Niño-Southern Oscillation (ENSO), which is tracked using the Oceanic Niño Index (ONI). It also connects to the Southern Oscillation Index (SOI), where a high SOI indicates the stronger trade winds characteristic of La Niña. Recently, the understanding of ENSO dynamics was challenged by a rare "triple-dip" La Niña that persisted for three successive years from late 2020 to early 2023. Furthermore, a newer metric called the Relative Oceanic Niño Index (RONI) has gained ground to better assess whether warming events are ENSO-driven or due to global ocean warming, which is a change in the measurement approach.