El Nino-Southern Oscillation

El Niño-Southern Oscillation (ENSO) , large-scale climatic fluctuation of the tropical Pacific Ocean. The El Niño [Span.,=the child] itself is a 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 an ENSO. While the ENSO results from the dynamic and thermodynamic interactions among the atmosphere, oceans, and land surfaces, exactly what initiates an ENSO is unclear. It seems certain that pressure changes and wind currents play a vital role. Some researchers have implicated the greenhouse effect (see global warming ), while others have attributed it to activity occurring on the ocean floor, such as underwater earthquakes.

In a typical ENSO, the strong easterly winds of the equatorial Pacific weaken, which 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 Pacific. Along the W 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 ENSO also affects the climate of the northern latitudes, particularly North America, which experiences warmer temperatures along the Pacific coast, increased rainfall in the Gulf states, and weaker Atlantic hurricanes.

Severe ENSO events can be economically disruptive worldwide. Of the 29 ENSOs that occurred between 1700 and 1999, the 1982-83 El Niño was the strongest and most devastating. It caused droughts in Africa, Australia, India, Indonesia, and the Philippines, flooding in Peru and Ecuador, and devastating coastal storms in California. The ENSO was blamed for 1,300-2,000 deaths and more than $13 billion in damage to property and livelihoods.

The effects of 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 W coast of South America. It was not until the systematic studies of the International Geophysical Year of 1957-58 that the extent of the meteorological impact of El Niño was recognized.

La Niña, a similar climatic fluctuation, involves the abnormal cooling of the waters off Ecuador and Peru. Penetrating westward, the cold current is believed to affect weather in areas in the middle latitudes in the western Pacific Ocean and to cause extremely hot summers in Japan.

Bibliography: 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).

southern oscillation A fluctuation of the intertropical atmospheric circulation, in particular in the Indian and Pacific Oceans, in which air moves between the SE Pacific subtropical high and the Indonesian equatorial low, driven by the temperature difference between the two areas. The general effect is that when pressure is high over the Pacific Ocean it tends to be low in the Indian Ocean, and vice versa. The phenomenon is strongly linked to El Niño.

El Niño A weakening of the Equatorial Current, allowing warm water to accumulate off the S. American Pacific coast; it is associated with a change in the atmospheric circulation known as a southern oscillation, the two together comprising an El Niño—Southern Oscillation (ENSO) event. Its climatic effects are felt throughout the Pacific region. (A similar phenomenon may occur in the Atlantic.) About once every 7 years, during the Christmas season (midsummer in the southern hemisphere), prevailing trade winds weaken, the Equatorial counter-current strengthens, and warm surface waters that are normally driven westwards by the wind to form a deep layer off Indonesia flow eastwards to overlie the cold waters of the northward-flowing Peru current. In exceptional years (e.g. 1953, 1972–3, 1982–3, and 1997–8) the severity with which the upwelling of nutrient-rich cold water is inhibited causes the death of a large proportion of the plankton population and a consequent decline in the numbers of fish.