El Niño–Southern Oscillation (pron.: //, Spanish pronunciation: [el ˈniɲo], ENSO), or El Niño/La Niña–Southern Oscillation, is a band of anomalously warm ocean water temperatures that occasionally develops off the western coast of South America and can cause climatic changes across the Pacific Ocean. The 'Southern Oscillation' refers to variations in the temperature of the surface of the tropical eastern Pacific Ocean (warming and cooling known as El Niño and La Niña, respectively) and in air surface pressure in the tropical western Pacific. The two variations are coupled: the warm oceanic phase, El Niño, accompanies high air surface pressure in the western Pacific, while the cold phase, La Niña, accompanies low air surface pressure in the western Pacific. Mechanisms that cause the oscillation remain under study.
The extremes of this climate pattern's oscillations cause extreme weather (such as floods and droughts) in many regions of the world. Developing countries dependent upon agriculture and fishing, particularly those bordering the Pacific Ocean, are the most affected. In popular usage, the El Niño–Southern Oscillation is often called just "El Niño". El niño is Spanish for "the child", and refers to the Christ child, Jesus, because periodic warming in the Pacific near South America is usually noticed around Christmas.
El Niño is defined by prolonged differences in the Pacific Ocean sea surface temperatures when compared with the average value. The accepted definition is a warming or cooling of at least 0.5°C (0.9°F) averaged over the east-central tropical Pacific Ocean. Typically, this anomaly happens at irregular intervals of two to seven years, and lasts nine months to two years. The average period length is five years. When this warming or cooling occurs for only seven to nine months, it is classified as El Niño/La Niña "conditions"; when it occurs for more than that period, it is classified as El Niño/La Niña "episodes".
The first signs of an El Niño are:
El Niño's warm rush of nutrient-poor water heated by its eastward passage in the Equatorial Current, replaces the cold, nutrient-rich surface water of the Humboldt Current. When El Niño conditions last for many months, extensive ocean warming and the reduction in easterly trade winds limits upwelling of cold nutrient-rich deep water, and its economic impact to local fishing for an international market can be serious.
Although causes are still being investigated, El Niño events begin when trade winds, part of the Walker circulation, falter for many months. A series of Kelvin waves—relatively warm subsurface waves of water a few centimetres high and hundreds of kilometres wide—cross the Pacific along the equator and create a pool of warm water near South America, where ocean temperatures are normally cold due to upwelling. The weakening of the winds can also create twin cyclones, another sign of a future El Niño. The Pacific Ocean is a heat reservoir that drives global wind patterns, and the resulting change in its temperature alters weather on a global scale. Rainfall shifts from the western Pacific toward the Americas, while Indonesia and India become drier.
Jacob Bjerknes in 1969 contributed to an understanding of ENSO by suggesting that an anomalously warm spot in the eastern Pacific can weaken the east-west temperature difference, disrupting trade winds that push warm water to the west. The result is increasingly warm water toward the east. Several mechanisms have been proposed through which warmth builds up in equatorial Pacific surface waters, and is then dispersed to lower depths by an El Niño event. The resulting cooler area then has to "recharge" warmth for several years before another event can take place.
While not a direct cause of El Niño, the Madden-Julian oscillation, or MJO, propagates rainfall anomalies eastward around the global tropics in a cycle of 30–60 days, and may influence the speed of development and intensity of El Niño and La Niña in several ways. For example, westerly flows between MJO-induced areas of low pressure may cause cyclonic circulations north and south of the equator. When the circulations intensify, the westerly winds within the equatorial Pacific can further increase and shift eastward, playing a role in El Niño development. Madden-Julian activity can also produce eastward-propagating oceanic Kelvin waves, which may in turn be influenced by a developing El Niño, leading to a positive feedback loop.
The Southern Oscillation is the atmospheric component of El Niño. This component is an oscillation in surface air pressure between the tropical eastern and the western Pacific Ocean waters. The strength of the Southern Oscillation is measured by the Southern Oscillation Index (SOI). The SOI is computed from fluctuations in the surface air pressure difference between Tahiti and Darwin, Australia. El Niño episodes are associated with negative values of the SOI, meaning the pressure difference between Tahiti and Darwin is relatively small.
Low atmospheric pressure tends to occur over warm water and high pressure occurs over cold water, in part because of deep convection over the warm water. El Niño episodes are defined as sustained warming of the central and eastern tropical Pacific Ocean. This results in a decrease in the strength of the Pacific trade winds, and a reduction in rainfall over eastern and northern Australia.
During non-El Niño conditions, the Walker circulation is seen at the surface as easterly trade winds that move water and air warmed by the sun toward the west. This also creates ocean upwelling off the coasts of Peru and Ecuador and brings nutrient-rich cold water to the surface, increasing fishing stocks. The western side of the equatorial Pacific is characterized by warm, wet, low-pressure weather as the collected moisture is dumped in the form of typhoons and thunderstorms. The ocean is some 60 cm (24 in) higher in the western Pacific as the result of this motion.
Because El Niño's warm pool feeds thunderstorms above, it creates increased rainfall across the east-central and eastern Pacific Ocean, including several portions of the South American west coast. The effects of El Niño in South America are direct and stronger than in North America. An El Niño is associated with warm and very wet weather months in April–October along the coasts of northern Peru and Ecuador, causing major flooding whenever the event is strong or extreme. The effects during the months of February, March, and April may become critical. Along the west coast of South America, El Niño reduces the upwelling of cold, nutrient-rich water that sustains large fish populations, which in turn sustain abundant sea birds, whose droppings support the fertilizer industry. This leads to fish kills off the shore of Peru.
The local fishing industry along the affected coastline can suffer during long-lasting El Niño events. The world's largest fishery collapsed due to overfishing during the 1972 El Niño Peruvian anchoveta reduction. During the 1982–83 event, jack mackerel and anchoveta populations were reduced, scallops increased in warmer water, but hake followed cooler water down the continental slope, while shrimp and sardines moved southward, so some catches decreased while others increased. Horse mackerel have increased in the region during warm events. Shifting locations and types of fish due to changing conditions provide challenges for fishing industries. Peruvian sardines have moved during El Niño events to Chilean areas. Other conditions provide further complications, such as the government of Chile in 1991 creating restrictions on the fishing areas for self-employed fishermen and industrial fleets.
The ENSO variability may contribute to the great success of small, fast-growing species along the Peruvian coast, as periods of low population removes predators in the area. Similar effects benefit migratory birds that travel each spring from predator-rich tropical areas to distant winter-stressed nesting areas.
Southern Brazil and northern Argentina also experience wetter than normal conditions, but mainly during the spring and early summer. Central Chile receives a mild winter with large rainfall, and the Peruvian-Bolivian Altiplano is sometimes exposed to unusual winter snowfall events. Drier and hotter weather occurs in parts of the Amazon River Basin, Colombia, and Central America.
Winters, during the El Niño effect, are warmer and drier than average in the Northwest, northern Midwest, and northern Mideast United States, so those regions experience reduced snowfalls. Meanwhile, significantly wetter winters are present in northwest Mexico and the southwest United States, including central and southern California, while both cooler and wetter than average winters in northeast Mexico and the southeast United States (including the Tidewater region of Virginia) occur during the El Niño phase of the oscillation.
Some believed the ice storm in January 1998, which devastated parts of southern Ontario and southern Quebec, was caused or accentuated by El Niño's warming effects. El Niño warmed Vancouver for the 2010 Winter Olympics, such that the area experienced a subtropical-like winter during the games.
El Niño is credited with suppressing hurricanes, and made the 2009 hurricane season the least active in 12 years.
Most tropical cyclones form on the side of the subtropical ridge closer to the equator, then move poleward past the ridge axis before recurving into the main belt of the Westerlies. When the subtropical ridge position shifts due to El Niño, so will the preferred tropical cyclone tracks. Areas west of Japan and Korea tend to experience much fewer September–November tropical cyclone impacts during El Niño and neutral years. During El Niño years, the break in the subtropical ridge tends to lie near 130°E, which would favor the Japanese archipelago. During El Niño years, Guam's chance of a tropical cyclone impact is one-third of the long-term average. The tropical Atlantic ocean experiences depressed activity due to increased vertical wind shear across the region during El Niño years. On the flip side, however, the tropical Pacific Ocean east of the dateline has above-normal activity during El Niño years due to water temperatures well above average and decreased windshear. Most of the recorded East Pacific category 5 hurricanes occur during El Niño years in clusters.
In Africa, East Africa - including Kenya, Tanzania, and the White Nile basin - experiences, in the long rains from March to May, wetter-than-normal conditions. Conditions are also drier than normal from December to February in south-central Africa, mainly in Zambia, Zimbabwe, Mozambique, and Botswana. Direct effects of El Niño resulting in drier conditions occur in parts of Southeast Asia and Northern Australia, increasing bush fires, worsening haze, and decreasing air quality dramatically. Drier-than-normal conditions are also in general observed in Queensland, inland Victoria, inland New South Wales, and eastern Tasmania from June to August.
Many ENSO linkages exist in the high southern latitudes around Antarctica. Specifically, El Niño conditions result in high pressure anomalies over the Amundsen and Bellingshausen Seas, causing reduced sea ice and increased poleward heat fluxes in these sectors, as well as the Ross Sea. The Weddell Sea, conversely, tends to become colder with more sea ice during El Niño. The exact opposite heating and atmospheric pressure anomalies occur during La Niña. This pattern of variability is known as the Antarctic dipole mode, although the Antarctic response to ENSO forcing is not ubiquitous.
El Niño's effects on Europe are not entirely clear, but it is not nearly as affected as at least large parts of other continents. Some evidence indicates an El Niño may cause a wetter, cloudier winter in Northern Europe and a milder, drier winter in the Mediterranean Sea region. The El Niño winter of 2006/2007 was unusually mild in Europe, and the Alps recorded very little snow coverage that season.
In most recent times, Singapore experienced the driest February in 2010 since records begins in 1869, with only 6.3 mm of rain falling in the month and temperatures hitting as high as 35°C on 26 February. The years 1968 and 2005 had the next driest Februaries, when 8.4 mm of rain fell.
La Niña is the name for the cold phase of ENSO, during which the cold pool in the eastern Pacific intensifies and the trade winds strengthen. The name La Niña originates from Spanish, meaning "the girl", analogous to El Niño meaning "the boy". It has also in the past been called anti-El Niño, and El Viejo (meaning "the old man").
La Niña results in wetter-than-normal conditions in Southern Africa from December to February, and drier-than-normal conditions over equatorial East Africa over the same period.
During La Niña years, the formation of tropical cyclones, along with the subtropical ridge position, shifts westward across the western Pacific ocean, which increases the landfall threat to China. In March 2008, La Niña caused a drop in sea surface temperatures over Southeast Asia by 2°C. It also caused heavy rains over Malaysia, the Philippines, and Indonesia.
La Niña causes mostly the opposite effects of El Niño, above-average precipitation across the northern Midwest, the northern Rockies, Northern California, and the Pacific Northwest's southern and eastern regions. Meanwhile, precipitation in the southwestern and southeastern states is below average. This also allows way above average hurricanes in the Atlantic and less in the Pacific.
La Niñas occurred in 1904, 1908, 1910, 1916, 1924, 1928, 1938, 1950, 1955, 1964, 1970, 1973, 1975, 1988, 1995, 1998, 2010, and 2011. In Canada, La Niña will, in general, cause a cooler, snowier winter, such as the near-record-breaking amounts of snow recorded in the La Niña winter of 2007/2008 in Eastern Canada.  The 2010-2011 La Niña was one of the strongest ever observed. The effect on Eastern Australia was devastating.
Transitional phases at the onset or departure of El Niño or La Niña can also be important factors on global weather by affecting teleconnections. Significant episodes, known as Trans-Niño, are measured by the Trans-Niño index (TNI). Examples of affected short-time climate in North America include precipitation in the Northwest US and intense tornado activity in the contiguous US.
A strong La Niña episode occurred during 1988–1989. La Niña also formed in 1995 and from 1998–2000, and a minor one from 2000–2001. In recent times, an occurrence of El Niño started in September 2006 and lasted until early 2007. From June 2007 on, data indicated a moderate La Niña event, which strengthened in early 2008 and weakened before the start of 2009; the 2007–2008 La Niña event was the strongest since the 1988–1989 event. The strength of the La Niña made the 2008 Atlantic hurricane season one of the most active since 1944; 16 named storms had winds of at least 39 mph (63 km/h), eight of which became 74 mph (119 km/h) or greater hurricanes.
According to NOAA, El Niño conditions were in place in the equatorial Pacific Ocean starting June 2009, peaking in January–February. Positive SST anomalies (El Niño) lasted until May 2010. SST anomalies then transitioned into the negative (La Niña) and have now transitioned back to ENSO-neutral during April 2012. In early July, NOAA stated that El Niño conditions have a 50+% chance of developing during the Northern Hemisphere summer. As the 2012, Northern Hemisphere summer started to draw to a close, NOAA stated that El Niño conditions are likely to develop in August or September.
A study of climate records has shown that El Niño events in the equatorial Pacific are generally associated with a warm tropical North Atlantic in the following spring and summer. About half of El Niño events persist sufficiently into the spring months for the Western Hemisphere Warm Pool to become unusually large in summer. Occasionally, El Niño's effect on the Atlantic Walker circulation over South America strengthens the easterly trade winds in the western equatorial Atlantic region. As a result, an unusual cooling may occur in the eastern equatorial Atlantic in spring and summer following El Niño peaks in winter. Cases of El Niño-type events in both oceans simultaneously have been linked to severe famines related to the extended failure of monsoon rains.
During the last several decades, the number of El Niño events increased, and the number of La Niña events decreased, although observation of ENSO for much longer is needed to detect robust changes. The question is whether this is a random fluctuation or a normal instance of variation for that phenomenon or the result of global climate changes toward global warming.
The studies of historical data show the recent El Niño variation is most likely linked to global warming. For example, one of the most recent results, even after subtracting the positive influence of decadal variation, is shown to be possibly present in the ENSO trend, the amplitude of the ENSO variability in the observed data still increases, by as much as 60% in the last 50 years.
The exact changes happening to ENSO in the future is uncertain: Different models make different predictions. It may be that the observed phenomenon of more frequent and stronger El Niño events occurs only in the initial phase of the global warming, and then (e.g., after the lower layers of the ocean get warmer, as well), El Niño will become weaker than it was. It may also be that the stabilizing and destabilizing forces influencing the phenomenon will eventually compensate for each other. More research is needed to provide a better answer to that question. The ENSO is considered to be a potential tipping element in Earth's climate.
The traditional Niño, also called Eastern Pacific (EP) El Niño, involves temperature anomalies in the Eastern Pacific. However, in the last two decades, nontraditional El Niños were observed, in which the usual place of the temperature anomaly (Nino 1 and 2) is not affected, but an anomaly arises in the central Pacific (Nino 3.4). The phenomenon is called Central Pacific (CP) El Niño, "dateline" El Niño (because the anomaly arises near the dateline), or El Niño "Modoki" (Modoki is Japanese for "similar, but different").
The effects of the CP El Niño are different from those of the traditional EP El Niño—e.g., the new El Niño leads to more hurricanes more frequently making landfall in the Atlantic.
The recent discovery of El Niño Modoki has some scientists believing it to be linked to global warming. However, satellite data go back only to 1979. More research must be done to find the correlation and study past El Niño episodes.
There is also a scientific debate on the very existence of this "new" ENSO. Indeed, a number of studies dispute the reality of this statistical distinction or its increasing occurrence, or both, either arguing the reliable record is too short to detect such a distinction, finding no distinction or trend using other statistical approaches, or that other types should be distinguished, such as standard and extreme El Niños. Following the asymmetric nature of the warm and cold phases of ENSO, a study could not identify such distinctions for La Niña, both in observations and in the climate models.
The first recorded El Niño that originated in the central Pacific and moved toward the east was in 1986.
Extreme weather conditions related to the El Niño cycle correlate with changes in the incidence of epidemic diseases. For example, the El Niño cycle is associated with increased risks of some of the diseases transmitted by mosquitoes, such as malaria, dengue, and Rift Valley fever. Cycles of malaria in India, Venezuela, and Colombia have now been linked to El Niño. Outbreaks of another mosquito-transmitted disease, Australian encephalitis (Murray Valley encephalitis—MVE), occur in temperate south-east Australia after heavy rainfall and flooding, which are associated with La Niña events. A severe outbreak of Rift Valley fever occurred after extreme rainfall in north-eastern Kenya and southern Somalia during the 1997–98 El Niño.
ENSO may be linked to civil conflicts. Scientists at The Earth Institute of Columbia University, having analyzed data from 1950 to 2004, suggest ENSO may have had a role in 21% of all civil conflicts since 1950, with the risk of annual civil conflict doubling from 3% to 6% in countries affected by ENSO during El Niño years relative to La Niña years.
ENSO conditions have occurred at two- to seven-year intervals for at least the past 300 years, but most of them have been weak. Evidence is also strong for El Niño events during the early Holocene epoch 10,000 years ago.
El Niño affected pre-Columbian Incas  and may have led to the demise of the Moche and other pre-Columbian Peruvian cultures. A recent study suggests a strong El-Niño effect between 1789 and 1793 caused poor crop yields in Europe, which in turn helped touch off the French Revolution. The extreme weather produced by El Niño in 1876–77 gave rise to the most deadly famines of the 19th century. The 1876 famine alone in northern China killed up to 13 million people.
An early recorded mention of the term "El Niño" to refer to climate occurred in 1892, when Captain Camilo Carrillo told the geographical society congress in Lima that Peruvian sailors named the warm northerly current "El Niño" because it was most noticeable around Christmas. The phenomenon had long been of interest because of its effects on the guano industry and other enterprises that depend on biological productivity of the sea.
Charles Todd, in 1893, suggested droughts in India and Australia tended to occur at the same time; Norman Lockyer noted the same in 1904. An El Niño connection with flooding was reported in 1895 by Pezet and Eguiguren. In 1924, Gilbert Walker (for whom the Walker circulation is named) coined the term "Southern Oscillation".
The major 1982–83 El Niño led to an upsurge of interest from the scientific community. The period 1990–1994 was unusual in that El Niños have rarely occurred in such rapid succession. An especially intense El Niño event in 1998 caused an estimated 16% of the world's reef systems to die. The event temporarily warmed air temperature by 1.5°C, compared to the usual increase of 0.25°C associated with El Niño events. Since then, mass coral bleaching has become common worldwide, with all regions having suffered "severe bleaching".
Analysis of El Niño events using climate networks shows the dynamics of the climate network is very sensitive to El Niño events. Many links in the network fail during El Niño events.
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