El Niño–Southern Oscillation

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El Niño–Southern Oscillation

(ENSO) (ĕl nēn`yō), large-scale climatic fluctuation of the tropical Pacific Ocean and the overlying atmosphere. The El Niño [Span.,=the child] is the South American term for the warm surface current that usually appears around Christmas in the Pacific off Ecuador and Peru and disappears by the end of March, but every two to seven years it persists for up to 18 months or more as part of the ENSO. The term El Niño is now used more broadly as a synonym for the warm phase of the ENSO in the central and E central Pacific. The ENSO results from the dynamic and thermodynamic interactions among the atmosphere, oceans, and land surfaces, but exactly what initiates the oscillation in pressure changes and wind currents that causes an El Niño is unclear.

In a typical El Niño, the strong easterly winds of the equatorial Pacific weaken due to a reversal of the normal atmospheric pressure gradient above the S Pacific between Australia and Tahiti. This allows warm eastward-flowing subsurface waters to rise, increasing surface temperatures 1–2°C; (2–3.5°F;), and sometimes as much as 4–6°C; (7–11°F;), in the central and E central Pacific. Along the west coast of South America, El Niño's warm waters persist and deepen, and cold, upwelling, nutrient-rich waters fail to reach surface waters; the resulting warm, nutrient-poor waters devastate coastal fisheries. Heavy rain falls along the South American coast, and heavy rainfall also moves from the western to central Pacific, causing drier than normal conditions in Indonesia and nearby areas. An El Niño also affects the climate of the northern latitudes, particularly North America, which experiences warmer temperatures along the N Pacific coast, increased precipitation in the Southwest and Gulf states and N Mexico, and weaker Atlantic hurricanes.

Severe El Niño events can be economically disruptive worldwide; those of 1982–83, 1997–98, and 2015–16 are regarded as the three strongest on record. The most devastating events have caused droughts in such areas as Africa, India, Southeast Asia, Australia, and the W Pacific, flooding along the Pacific coast of South America, in California, and the SE United states, and devastating coastal storms in California. The 1982–83 El Niño was blamed for 1,300–2,000 deaths and more than $13 billion in damage to property and livelihoods. The 1997–98 event, generally regarded as the strongest El Niño, also was blammed for a severe ice storm in the NE United States and neighboring parts of Canada; estimates of the losses due to it worldwide range from $32 billion to three times that. El Niños can also have positive economic effects; warmer weather in the N United States, for example, can lead to lower energy expenses. It has been estimated that in the United States the 1997–98 event in fact had a positive economic impact overall.

The effects of an El Niño were documented in Peru as early as the Spanish conquest in 1525. By the end of the 19th cent. the phenomenon was being studied by Peruvian oceanographers, although the effects were thought to be limited to the coast of W South America. It was not until the systematic studies of the International Geophysical YearInternational Geophysical Year
(IGY), 18-month period from July, 1957, through Dec., 1958, during a period of maximum sunspot activity, designated for cooperative study of the solar-terrestrial environment by the scientists of 67 nations.
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 of 1957–58 that the extent of the meteorological impact of El Niño was recognized.

La Niña, a related climatic fluctuation that can be thought of as the cold phase of the ENSO, involves the abnormal cooling of the waters off Ecuador and Peru. Among its effects are colder weather than normal in North American along the N Pacific coast and into the N central United States and wetter than normal conditions in the Philippines, Malaysia, and N Australia. A La Niña in 2011–12 led to a prolonged dry spell in the South Pacific that created serious water shortages in several island nations; it also contributed to increased rainfall and significant flooding in E Australia.


See M. H. Glantz, Currents of Change: El Niño's Impact on Climate and Society (1996); B. Fagan, Floods, Famines, and Emperors: El Niño and the Fate of Civilizations (1999).