El Nino-Southern Oscillation

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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, but the term El Niño is often used more broadly as a synonym for ENSO. An ENSO results from the dynamic and thermodynamic interactions among the atmosphere, oceans, and land surfaces, but 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. A recent study suggests that some of these effects depend on whether the warming in the Pacific is stronger in its eastern or central waters.

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. A La Niña in 2011 led to a prolonged dry spell in the South Pacific that created serious water shortages in several island nations.

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).

El Niño is a phenomenon that occurs every three to seven years in the Pacific Ocean. During a normal year sea temperatures warm up briefly during the Christmas season off the coasts of Peru and Ecuador (El Niño is Spanish for the infant Jesus). However, during an El Niño these warm temperatures persist for many months, coinciding with aberrant changes in air pressure and wind patterns throughout the southern Pacific, known as El Niño Southern Oscillations (ENSOs). Periodically, the Intertropical Convergence Zone (ITCZ) in the atmosphere, which normally sits over the Indonesian region, migrates eastwards over the central Pacific. The ITCZ is a zone of high atmospheric pressure where the rising air triggers heavy rainfall and high air temperatures. This warms the ocean surface and triggers a planetary wave that travels eastwards. This wave is slow moving and takes about a year to cross the Pacific along the equator. Its effect is to deepen the thermocline so that the upwelling that occurs along the equator no longer brings cool nutrient-rich water from below the thermocline up to the surface. Normally the cool waters that occur off the coasts of Peru and Ecuador are not only highly productive, but they also keep the coastal climate arid.

During an El Niño event the major fishery for anchovetta (Centegraulis mysticetus), normally one of the biggest in the world, collapses and there are mass mortalities of seabirds, such as pelicans and guanay cormorants, whose droppings were the main source of nitrates used to manufacture the gunpowder used in 19th-century European wars. The warm seas also trigger heavy rainfall over the normally arid coastal regions, resulting in disastrous flooding. When the planetary wave encounters the continental margin of America it is diverted both north and south, resulting in similarly anomalous warm seas and heavy rainfall along the normally arid coasts of North and South America. The repercussions are felt in other regions such as South Africa, India, and South-East Asia, where seasonal rains fail resulting in disastrous droughts and human misery.

The El Niño events of 1982 and 1997 were the most extreme on record. What determines the irregular occurrences of ENSO events is not understood, but many scientists believe that the recent increase in their frequency and severity is one expression of global climate change. If this is indeed so then these events may become the norm with severe implications for human societies.

See also environmental issues.

www.elnino.noaa.gov/

M. V. Angel

El Niño (Sp. (Christ) ‘child’) Warm surface current that flows in the equatorial Pacific Ocean towards the South American coast. It occurs around Christmas time. An easing or reversing of the trade winds over the s Pacific Ocean, causes warm surface waters that have ‘piled-up’ in the w Pacific to flow back and warm the coastal waters of South America by 2 to 3°C. It has a dramatic effect on climate patterns in Australia and Southeast Asia, and may affect rainfall as far away as Africa. In normal years, trade winds blow e to w along the Equator, dragging warm surface waters into a pool off n Australia and monsoon rains to Indonesia. In the w Pacific, the Humboldt Current pushes the surface waters away from the coast of Peru, bringing cold water to the surface. This upwelled, nutrient-rich water stimulates phytoplankton production and swells the population of anchovies, a mainstay of the Peruvian fishing industry. In an El Niño year, the upwelling ceases and the biological productivity of the area collapses. In 1982–83, the anchovy catch fell by 600%. In addition, mean sea-level along the coast of Latin America may increase by as much as 50cm (20in), causing widespread flooding. Some scientists believe that the frequency (presently every 2–10 years) and effects of El Niño may be increasing.

http://www.elnino.noaa.gov

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.

El Niño A warm-water current which periodically flows southwards along the coast of Ecuador. It is associated with the Southern Oscillation (these effects are collectively known as an El Niño—Southern Oscillation, or ENSO, event) and with climatic effects throughout the Pacific region. A similar phenomenon may also occur in the Atlantic. Approximately once every seven years, during the Christmas season (the name refers to the Christ child), prevailing trade winds weaken and the Equatorial countercurrent strengthens. Warm surface waters, normally driven westward by the wind to form a deep layer off Indonesia, flow eastwards to overlie the cold waters of the Peru current. In exceptional years, e.g. 1891, 1925, 1953, 1972–3, 1982–3, 1986–7, 1994–5, and 1997–8 the extent to which the upwelling of the nutrient-rich cold waters is inhibited causes the death of a large proportion of the plankton population and a decline in the numbers of surface fish.

El Niño A weakening of the Equatorial Current, allowing warm water to accumulate off the S. American Pacific coast; it is associated with the southern oscillation (these two effects are known collectively as an El Niño–Southern Oscillation or ENSO event) and with climatic effects throughout the Pacific region. A similar phenomenon may also occur in the Atlantic. Approximately once every seven years, during the Christmas season (the name refers to the Christ child), prevailing trade winds weaken and the Equatorial Countercurrent strengthens. Warm surface waters, normally driven westward by the wind to form a deep layer off Indonesia, flow eastwards to overlie the cold waters of the Peru Current. In exceptional years (e.g. 1953, 1972–3, 1982–3, and 1997–8) the extent to which the upwelling of the nutrient-rich cold waters is inhibited causes the death of a large proportion of the plankton population and a consequent decline in the numbers of surface fish.

Ni·ño, El • see El Niño.

El Niño •bagnio •dal segno, jalapeño •cursillo, Trujillo •caudillo • El Niño • yo-yo