El Niño/La Niña-Southern Oscillation, or ENSO, is a quasiperiodic climate pattern that occurs across the tropical Pacific Ocean roughly every five years. It is characterized by variations in the temperature of the surface of the tropical eastern Pacific Ocean—warming or cooling known as El Niño and La Niña respectively—and air surface pressure in the tropical western Pacific—the Southern Oscillation. 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.[2][3] Mechanisms that cause the oscillation remain under study.
ENSO causes 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 little boy" and refers to the Christ child, because periodic warming in the Pacific near South America is usually noticed around Christmas.[4]
El Niño is defined by prolonged differences in Pacific Ocean 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 2–7 years and lasts nine months to two years.[5] The average period length is 5 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".[6]
The first signs of an El Niño are:
El Niño's warm rush of nutrient-poor tropical 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.[7]
Although its 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.[8] 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.[9] Rainfall shifts from the western Pacific toward the Americas, while Indonesia and India become drier.[10]
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.[11] 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.[12] The resulting cooler area then has to "recharge" warmth for several years before another event can take place.[13]
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.[14] 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.[15] 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.[16]
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.[17] El Niño episodes are associated with negative values of the SOI, meaning that the pressure difference between Tahiti and Darwin is relatively small.
High atmospheric pressure tends to occur over warm water and Low pressure occurs over cold water, in part because deep convection over the warm water acts to transport air. 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 centimetres (24 in) higher in the western Pacific as the result of this motion.[18][19][20][21]
Winters, during the El Niño effect, are warmer and drier than average in the Northwest, Northmidwest, and Northmideast United States, and therefore 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.[22][23]
In Canada, both warmer and drier winters (due to forcing of the Polar Jet further north) over much of the country occur, although less variation from normal is seen in the Maritime Provinces. The following summer is warmer and sometimes drier creating a more active than average forest fire season over Central/Eastern Canada. Some believed that 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.[24] El Niño warmed Vancouver for the 2010 Winter Olympics, such that the area experienced a subtropical-like winter during the games.[25]
Summers, during the El Niño effect, are wetter than average in the Northwest, Northmidwest, Northmideast, and mountain regions of the United States.
El Niño is credited with suppressing hurricanes and made the 2009 hurricane season the least active in twelve years.[26] El Niño is also associated with increased wave-caused coastal erosion along the United States Pacific Coast.
There is some evidence that El Niño activity is correlated with incidence of red tides off the Pacific coast of California.
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.[27] 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.[28] During El Niño years, Guam's chance of a tropical cyclone impact is one-third of the long term average.[29] The tropical Atlantic ocean experiences depressed activity due to increased vertical wind shear across the region during El Niño years.[30]
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. There are also drier than normal conditions 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 [31]. 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 [32].
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. There is some evidence that 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.[33]
In most recent times, Singapore experienced the driest February in 2010 since records begins in 1869. With only 6.3 millimetres of rain fell in the month and temperatures hitting as high as 35 degrees Celsius on 26 February. The years 1968 and 2005 had the next driest Februaries, when 8.4 mm of rain fell.[34]
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").[35]
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.[36]
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.[28] In March 2008, La Niña caused a drop in sea surface temperatures over Southeast Asia by an amount of 2 °C. It also caused heavy rains over Malaysia, Philippines, and Indonesia.[37]
During a time of La Niña, drought plagues the coastal regions of Peru and Chile.[38] From December to February, northern Brazil is wetter than normal.[38]
La Niña causes mostly the opposite effects of El Niño. La Niña causes above-average precipitation across the North Midwest, the Northern Rockies, Northern California, and the Pacific Northwest's southern and eastern regions. Meanwhile there is below-average precipitation in the southwestern and southeastern states.[39]
La Niñas occurred in 1904, 1908, 1910, 1916, 1924, 1928, 1938, 1950, 1955, 1964, 1970, 1973, 1975, 1988, 1995.[40] 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.[41]. [42] The 2010-2011 La Niña was one of the strongest ever observed. The effect on Eastern Australia was devastating.[43]
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.[44] About half of El Niño events persist sufficiently into the spring months for the Western Hemisphere Warm Pool (WHWP) to become unusually large in summer.[45] 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.[46] Cases of El Niño-type events in both oceans simultaneously have been linked to severe famines related to the extended failure of monsoon rains.[47]
During the last several decades the number of El Niño events increased, and the number of La Niña events decreased.[48] 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 that the recent El Niño variation is most likely linked to global warming. For example, one of the most recent results is that, even after subtracting the positive influence of decadal variation, shown to be possibly present in the ENSO trend,[49] the amplitude of the ENSO variability in the observed data still increases, by as much as 60% in the last 50 years.[50]
It is not certain what exact changes will happen to ENSO in the future: Different models make different predictions (cf.[51]) 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.[52] It may also be that the stabilizing and destabilizing forces influencing the phenomenon will eventually compensate for each other.[53] More research is needed to provide a better answer to that question, but the current results do not completely exclude the possibility of dramatic changes. The ENSO is considered to be a potential tipping element in Earth's climate.[54]
The traditional Niño, also called Eastern Pacific (EP) El Niño,[55] involves temperature anomalies in the Eastern Pacific. However, in the last two decades non-traditional 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).[56] The phenomenon is called Central Pacific (CP) El Niño,[55] "dateline" El Niño (because the anomaly arises near the dateline), or El Niño "Modoki" (Modoki is Japanese for "similar, but different").[57]
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.[58]
The recent discovery of El Niño Modoki has some scientists believing it to be linked to global warming.[59] However, Satellite data goes back only to 1979. More research must be done to find the correlation and study past El Niño episodes.
The first recorded El Niño that originated in the central Pacific and moved toward the east was in 1986.[60]
A joint study by the National Aeronautics and Space Administration and the National Oceanic and Atmospheric Administration concluded that climate change may contribute to stronger El Niños. El Niño "Modoki" events occurred in 1991-92, 1994–95, 2002–03, 2004–05, and 2009-10.[61] The strongest such Central Pacific El Niño event known occurred in 2009-2010.[62]
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.[63]
ENSO may be linked to civil conflicts. Scientists at the Earth Institute of Columbia University have analyzed data from 1950 to 2004 and suggest that 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.[64][65]
ENSO conditions have occurred at two- to seven year intervals for at least the past 300 years, but most of them have been weak. There is also evidence for strong El Niño events during the early Holocene epoch 10,000 years ago.[66]
El Niño affected pre-Columbian Incas [67] and may have led to the demise of the Moche and other pre-Columbian Peruvian cultures.[68] A recent study suggests that a strong El-Niño effect between 1789–93 caused poor crop yields in Europe, which in turn helped touch off the French Revolution.[69] The extreme weather produced by El Niño in 1876–77 gave rise to the most deadly famines of the 19th century.[70]
An early recorded mention of the term "El Niño" to refer to climate occurs 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 that 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 from 1990–1994 was unusual in that El Niños have rarely occurred in such rapid succession.[71] 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.[72] Since then, mass coral bleaching has become common worldwide, with all regions having suffered "severe bleaching".[73]
Major ENSO events were recorded in the years 1790–93, 1828, 1876–78, 1891, 1925–26, 1972–73, 1982–83, and 1997–98, with 2010–2011 being one of the strongest ever.[47]
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