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The 1997–98 El Niño observed by TOPEX/Poseidon. The white areas off the Tropical Western coasts of northern South and all Central America as well as along the Central-eastern equatorial and Southeastern Pacific Ocean indicate the pool of warm water.[1]
El Niño is the warm phase of the El Niño Southern Oscillation (commonly called ENSO) and is associated with a band of warm ocean water that develops in the central and east-central equatorial Pacific (between approximately the International Date Line and 120°W), including off the Pacific coast of South America. El Niño Southern Oscillation refers to the cycle of warm and cold temperatures, as measured by sea surface temperature, SST, of the tropical central and eastern Pacific Ocean. El Niño is accompanied by high air pressure in the western Pacific and low air pressure in the eastern Pacific. The cool phase of ENSO is called "La Niña" with SST in the eastern Pacific below average and air pressures high in the eastern and low in western Pacific. The ENSO cycle, both El Niño and La Niña, causes global changes of both temperatures and rainfall.[2][3] Mechanisms that cause the oscillation remain under study.
Developing countries dependent upon agriculture and fishing, particularly those bordering the Pacific Ocean, are the most affected. In Spanish, the capitalized term "El Niño" refers to the Christ child, Jesus (literal translation "The (male) Child"). La Niña, chosen as the 'opposite' of El Niño, literally means "The (female) Child". El Niño was so named because periodic warming in the Pacific near South America is often noticed around Christmas.[4]
Definition
El Niño is defined by prolonged warming in the Pacific Ocean sea surface temperatures when compared with the average value. The U.S NOAA definition is a 3-month average warming of at least 0.5 °C (0.9 °F) in a specific area of the east-central tropical Pacific Ocean, other organizations define the term slightly differently. Typically, this anomaly happens at irregular intervals of two to seven years, and lasts nine months to two years.[5] The average period length is five years. When this warming occurs for seven to nine months, it is classified as El Niño "conditions"; when its duration is longer, it is classified as an El Niño "episode".[6]The first signs of an El Niño are a weakening of the Walker circulation and strengthening of the Hadley circulation[7] and may include[citation needed]:
- Rise in surface pressure over the Indian Ocean, Indonesia, and Australia
- Fall in air pressure over Tahiti and the rest of the central and eastern Pacific Ocean
- Trade winds in the south Pacific weaken or head east
- Warm air rises near Peru, causing rain in the northern Peruvian deserts
More generally, El Niño can affect commodity prices and the macroeconomy of different countries - and not always for the worst. It can constrain the supply of rain-driven agricultural commodities; reduce agricultural output, construction, and services activities; create food-price and generalised inflation; and may trigger social unrest in commodity-dependent poor countries that primarily rely on imported food.[9] A University of Cambridge Working Paper shows that while Australia, Chile, Indonesia, India, Japan, New Zealand and South Africa face a short-lived fall in economic activity in response to an El Niño shock, other countries may actually benefit from an El Niño weather shock (either directly or indirectly through positive spillovers from major trading partners), for instance, Argentina, Canada, Mexico and the United States. Furthermore, most countries experience short-run inflationary pressures following an El Niño shock, while global energy and non-fuel commodity prices increase.[10]
Effects of ENSO warm phase (El Niño)
South America
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.[11] 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. The reduction in upwelling leads to fish kills off the shore of Peru.[8]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.[12] 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.
North 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.[13][14]
Some believed the ice storm in January 1998, which devastated parts of New England, southern Ontario and southern Quebec, was caused or accentuated by El Niño's warming effects.[15] El Niño warmed Vancouver for the 2010 Winter Olympics, such that the area experienced a warmer than average winter during the games.[16]
The synoptic condition for the Tehuantepecer is associated with high-pressure system forming in Sierra Madre of Mexico in the wake of an advancing cold front, which causes winds to accelerate through the Isthmus of Tehuantepec. Tehuantepecers primarily occur during the cold season months for the region in the wake of cold fronts, between October and February, with a summer maximum in July caused by the westward extension of the Azores High. Wind magnitude is greater during El Niño years than during La Niña years, due to the more frequent cold frontal incursions during El Niño winters.[17] Its effects can last from a few hours to six days.[18] El Niño is credited with suppressing Atlantic hurricanes, and made the 2009 Atlantic hurricane season the least active in 12 years.[19]
Tropical cyclones
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.[20] 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.[21] During El Niño years, Guam's chance of a tropical cyclone impact is one-third higher than the long-term average.[22] The tropical Atlantic ocean experiences depressed activity due to increased vertical wind shear across the region during El Niño years.[23] 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.[24] Most of the recorded East Pacific category 5 hurricanes occur during El Niño years in clusters.Elsewhere
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.[25] 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.[26] This pattern of variability is known as the Antarctic dipole mode, although the Antarctic response to ENSO forcing is not ubiquitous.[26]
El Niño's effects on Europe appear to be strongest in winter. Recent evidence indicates that El Niño causes a colder, drier winter in Northern Europe and a milder, wetter winter in Southern Europe.[27] The El Niño winter of 2009/10 was extremely cold in Northern Europe but El Niño is not the only factor at play in European winter weather and the weak El Niño winter of 2006/2007 was unusually mild in Europe, and the Alps recorded very little snow coverage that season.[28]
As warm water spreads from the west Pacific and the Indian Ocean to the east Pacific, it takes the rain with it, causing extensive drought in the western Pacific and rainfall in the normally dry eastern Pacific. Singapore experienced the driest February in 2014 since records began 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.[29]
Transitional phases
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).[30] Examples of affected short-time climate in North America include precipitation in the Northwest US[31] and intense tornado activity in the contiguous US.[32]During strong El Niño episodes, a secondary peak in sea surface temperature across the far eastern equatorial Pacific Ocean sometimes follows the initial peak.[33]
Recent occurrences
These maps compare temperatures in a given month to the long-term average temperature of that month from 1985 through 1997. Blue shows temperatures that were cooler than average, white shows near-average temperatures, and red shows where temperatures were warmer than average. The maps are made from data collected by the Advanced Microwave Scanning Radiometer-EOS (AMSR-E) compared to historical data collected by a series of National Oceanic and Atmospheric Administration (NOAA) satellites.Some sea surface temperature anomalies are simply transient events, not part of a specific pattern or trend. Other anomalies are more meaningful. At irregular intervals (roughly every 3-6 years), the sea surface temperatures in the Pacific Ocean along the equator become warmer or cooler than normal. These anomalies are the hallmark of El Niño and La Niña climate cycles, which can influence weather patterns across the globe. This time series shows the building and subsiding of a La Niña event from early 2007 through mid-2008.
Since 2000, a number of El Niño events have been observed. El Niño events were observed in 2002–03, 2004–05, 2006–07 and 2009–10.[34] A strong El Niño has not occurred since 1997–98.[35]
In September 2014, the Climate Prediction Center issued an El Niño Watch, stating a 60-65% percent chance of El Niño development during the Northern Hemisphere fall and winter.[36]
In December 2014, the Japan Meteorological Agency had declared the onset of El Niño conditions, as warmer than normal sea surface temperatures were measured over the Pacific, albeit citing the lack of atmospheric conditions related to the event.[37] Months later both Climate Prediction Center and Australian Bureau of Meteorology had declared the arrival of El Niño conditions, in March and May 2015, respectively.[38][39]
Remote influence on tropical Atlantic Ocean
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.[40] About half of El Niño events persist sufficiently into the spring months for the Western Hemisphere Warm Pool to become unusually large in summer.[41] 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.[42] Cases of El Niño-type events in both oceans simultaneously have been linked to severe famines related to the extended failure of monsoon rains.[43]Global warming
During the last several decades the number of El Niño events increased,[44] although a much longer period of observation is needed to detect robust changes.[45] 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 as a result of 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,[46] the amplitude of the ENSO variability in the observed data still increases, by as much as 60% in the last 50 years.[47]
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.[48] It may also be that the stabilizing and destabilizing forces influencing the phenomenon will eventually compensate for each other.[49] More research is needed to provide a better answer to that question.
The "Modoki" or Central-Pacific El Niño
The traditional Niño, also called Eastern Pacific (EP) El Niño,[50] 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 (Niño 1 and 2) is not affected, but an anomaly arises in the central Pacific (Niño 3.4).[51] The phenomenon is called Central Pacific (CP) El Niño,[50] "dateline" El Niño (because the anomaly arises near the dateline), or El Niño "Modoki" (Modoki is Japanese for "similar, but different").[52][53][54][55] There are flavors of ENSO additional to EP and CP types and some scientists argue that ENSO exists as a continuum often with hybrid types.[56]
The effects of the CP El Niño are different from those of the traditional EP El Niño—e.g., the recently discovered El Niño leads to more hurricanes more frequently making landfall in the Atlantic.[57]
The recent discovery of El Niño Modoki has some scientists believing it to be linked to global warming.[58]
However, comprehensive satellite data go back only to 1979. More research must be done to find the correlation and study past El Niño episodes. More generally, there is no scientific consensus on how/if climate change may affect ENSO.[59]
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,[60][61] finding no distinction or trend using other statistical approaches,[62][63][64][65][66] or that other types should be distinguished, such as standard and extreme ENSO.[67][68]
The first recorded El Niño that originated in the central Pacific and moved toward the east was in 1986.[69] Recent Central Pacific El Niños happened in 1986–1987, 1991–1992, 1994–1995, 2002–2003, 2004–2005 and 2009–2010.[70] Furthermore, there were "Modoki" events in 1957–59,[71] 1963–64, 1965–66, 1968–70, 1977–78 and 1979–80.[72][73]
Health and social impacts of El Niño
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[citation needed]. Cycles of malaria in India, Venezuela, Brazil, 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.[74]ENSO conditions have also been related to Kawasaki disease incidence in Japan and the west coast of the United States,[75] via the linkage to tropospheric winds across the north Pacific Ocean.[76]
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.[77][78]
Cultural history and prehistoric information
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.[79]
El Niño affected pre-Columbian Incas [80] and may have led to the demise of the Moche and other pre-Columbian Peruvian cultures.[81] 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.[82] The extreme weather produced by El Niño in 1876–77 gave rise to the most deadly famines of the 19th century.[83] The 1876 famine alone in northern China killed up to 13 million people.[84]
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 north-flowing 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 1991–1995 was unusual in that El Niños have rarely occurred in such rapid succession.[85] 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.[86] Since then, mass coral bleaching has become common worldwide, with all regions having suffered "severe bleaching".[87]
Major ENSO events were recorded in the years 1790–93, 1828, 1876–78, 1891, 1925–26, 1972–73, 1982–83 and 1997–98,[43] with 1997-1998 episode being one of the strongest ever.[88]
In 1997, the comedian Chris Farley performed his last skit for Saturday Night Live in which he personifies El Niño as a 'bombastic professional wrestler',[89] saying "I am El Niño. El Niño is Spanish for… the Niño." The skit helped enter El Niño into the popular lexicon and Farley has often since been referenced in relation to El Niño.[90][91][92]
