El Nino Wikipedia

聖嬰-南方振盪現象

维基百科，自由的百科全书

（重定向自厄爾尼諾）

1997年由TOPEX/Poseidon卫星观测到的厄尔尼诺事件。南美和北美赤道区域海岸外的白色区域暗示暖水汇集。^[1]

1876-2011间的南方涛动指数的时间序列。

来自NOAA的图片显示了1950-2011期间的全球年平均温度异常，ENSO时间被用红色和蓝色标出。

聖嬰-南方振盪現象（El Niño-Southern Oscillation，簡稱ENSO），或厄尔尼诺/拉尼娜-南方振盪是发生在横跨赤道附近太平洋的一种準周期气候类型，大约每5年发生一次。南方振盪 是指东太平洋赤道区域海面温度（厄尔尼诺 事件时变暖，拉尼娜 事件时变冷）和西太平洋赤道区域的海面上气压的变动。这两种变动是相互联系的：东太平洋的暖洋阶段，即厄尔尼诺，伴随着西太平洋的高海面气压；东太平洋的变冷阶段，即拉尼娜，伴随着西太平洋的低海面气压。^[2][3]导致这种变动的机制仍在研究中。
这种气候类型变动的极端时期，即厄尔尼诺和拉尼娜事件，会在世界很多区域引起极端的天气（比如洪水和干旱）。依靠农业和渔业的发展中国家，特别是太平洋沿岸的国家，所受影响最大。厄尔尼诺-南方振盪比较流行的称呼常常省略为“厄尔尼诺”。厄尔尼诺是西班牙语，意指“小男孩”，指的是圣子耶稣，因为南美太平洋的变暖时期通常都在圣诞节附近。^[4]

名稱

太平洋平時狀態：赤道風將溫水向西吹、冷水沿南美洲海岸上湧

聖嬰現象：溫水向南美洲吹送，冷水不再上湧而使海洋變暖；注意此時太平洋東西氣壓亦隨之變動，即所謂南方振盪

早期學者以為兩現象各自獨立，但近年來的研究漸漸闡明其關係、是一體的兩面，故以聖嬰-南方振盪現象稱之，學界一般通用縮寫ENSO。

聖嬰現象

東太平洋漁民很早便發現每隔數年，該地的海水就會異常升溫，因此沿海一帶的漁民以西班牙語「El Niño」（音譯：厄尔尼诺，意為「男嬰」或「聖嬰」，因為此种气候现象通常於聖誕節前后开始发生）稱呼此異常氣候。而相反的現象稱為「La Niña」意為「女嬰」或「聖女（嬰）」，或譯作「反聖嬰」。
海水變暖的範圍主要為太平洋東部與中部的熱带海洋的海水溫度異常地持續變暖，使整个世界气候模式发生变化，造成一些地区干旱而另一些地区又降雨量过多。其出現頻率並不規則，但平均約每4年發生一次。基本上，如果現象持續期少於五個月，會稱為聖嬰情況（condition）；如果持續期是五個月或以上，便會稱為聖嬰事件（episode）。
根據海水變暖的中心區域不同，又將聖嬰分為中太平洋聖嬰Central Pacific (CP) El Niño與東太平洋聖嬰Eastern Pacific (EP) El Niño。

南方振盪

南方振盪為聖嬰現象在大氣的對應關係，不過早期學者並不了解這點。所謂的「振盪」（oscillation）是指赤道附近太平洋東西處之氣壓變化，最早由英國氣象學家沃克（Gilbert Walker）於20世紀初期發現並提出。
南方振盪的強度由所謂的「南方振盪指數」（Southern Oscillation Index，SOI）評估之，也就是選定塔希提與澳洲達爾文兩地作為東、西太平洋的代表地，指數指出了兩地之氣壓差距。^[5]當聖嬰現象時其值為負，意指大溪地（東）氣壓低於達爾文（西）。

成因

ENSO形成的原因，科学界有多种观点，比较普遍的看法是：在正常状况下，北半球赤道附近吹东北信风，南半球赤道附近吹东南信风。信风带动海水自东向西流动，分别形成北赤道洋流和南赤道暖流。从赤道东太平洋流出的海水，靠海洋底部的湧升流补充，由於底層海水溫度較低，因此使表面水温低于四周，形成东西部海温差。但是，一旦東風减弱，甚至变为西风时，赤道东太平洋地区的冷水上翻减少或停止，海水温度就升高，形成大范围的海水温度异常增暖。而突然增强的这股暖流沿着厄瓜多爾海岸南侵，使海水温度剧升，冷水鱼群因而大量死亡，海鸟因找不到食物而纷纷离去，渔场顿时失去生机，使沿岸国家遭到巨大损失。
ENSO形成的前兆包括：

• 印度洋、印尼與澳洲氣壓上升；
• 大溪地和太平洋中央、東面的海面氣壓下降；
• 南太平洋的信風減弱或往東面吹；
• 秘魯附近的暖空氣上升，令當地沙漠下雨；
• 暖空氣由太平洋西岸擴散至印度洋與太平洋東面。同時它令東面較乾燥和有乾旱的地方降雨。

影响

本条目應避免有陳列雜項、瑣碎資料的部分。 （2010年1月30日） 請協助將有關資料重新編排入適當章節或條目內。

• ENSO規模顯著的年份：1790-93、1828、1876-78、1891、1925-26、1982-83、1997-98。
• 近年、但規模較小的年份：1986-1987、1990-1994、2002-2004、2006-2007、2009-2010。
• 1982年4月至1983年7月的ENSO现象，是几个世纪来最严重的一次之一，太平洋東部至中部水面溫度比正常高出約4至5℃，造成全世界1300～1500人丧生，经济损失近百亿美元。
• 1986年至1987年的ENSO现象，使赤道中、东太平洋海水表面水温比常年平均温度偏高2℃左右；同时，热带地区的大气环流也相应地出现异常，热带及其他地区的天气出现异常变化；南美洲的秘鲁北部、中部地区暴雨成灾；哥伦比亚境内的亚马孙河河水猛涨，造成河堤多次决口；巴西东北部少雨干旱，西部地区炎热；澳大利亚东部及沿海地区雨水明显减少；中国华南地区、南亚至非洲北部大范围地区均少雨干旱。
• 1990年初又发生ENSO前兆现象。这年1月，太平洋中部海域水面温度高于往年，除赤道海域水面温度比往年高出0.5℃外，国际換日線以西的海域水面温度也比往年高出将近1℃；接近海面的28℃的暖水层比往年浅10米左右；南美洲太平洋沿岸水域的水位比平时上涨15～30厘米。
• 1997年至1998年的ENSO现象，亦是几个世纪来最严重的一次之一，太平洋東部至中部水面溫度比正常高出約3至4℃，美洲地區有持續暴雨，東南亞地區则持续干旱并發生大規模的森林大火。這次聖嬰現象緊接1990-1994年發生，頻密程度罕見。
• 同时，ENSO带动的温暖海水，影响鱼类的成群移动，破坏珊瑚礁的生长。
• 特別的是，在聖嬰現象發生當年，容易在西北太平洋和東北太平洋形成威力強大的颱風和颶風。例如：1997年太平洋颱風季當年就曾出現十個威力達到等級最高的五級颱風，在1997年太平洋颶風季也出現兩個等級最高的五級颶風分別是：颶風琳達和颶風蓋勒摩。2002年太平洋颶風季三個等級最高颶風和2009年太平洋颱風季四個等級最高的五級颱風。以及2009年太平洋颶風季僅次於1997年琳達颶風的：颶風瑞克。

ENSO和全球变暖

在过去的几十年中，厄尔尼诺事件的数目有所增加，而拉尼娜事件的数目却在减少。^[6]我们可能还需要观察ENSO更长的时间以发现稳定的变化^[7]，我们现在还不清楚这到底是随机的波动，或是ENSO的正常变化规律，还是由走向全球变暖的全球气候变化所导致。
对历史数据的研究表明，最近的厄尔尼诺的变化很可能和全球变暖有关。比如，一个最近的研究显示，即使减去十年际变化率的正影响后再看ENSO的趋势^[8]，观测数据中的ENSO变率的幅度仍然显示在增加，在过去50年里最大增加了60%。^[9]
现在还不是很确定ENSO在将来会如何变化^[10]，不同的模式得出了不同的预测结果。^[11]可能现在观测到的更强更频繁的厄尔尼诺事件只是发生在全球变暖的早期阶段，然后（比如在海洋低层也变暖后），厄尔尼诺将可能变得比现在要弱些。^[12]也有可能，影响ENSO的各种因素的最后效果被最终相互抵消。^[13]要想更好的预测ENSO的将来变化还需要更多的研究。ENSO被认为是地球气候的一个潜在的临界元素（tipping element）。^[14]

聖嬰現象(ENSO)

「聖嬰」一詞源自西班牙文，(英文翻譯為Christ Child)，意為上帝之子，是100多年前南美洲 秘魯和厄瓜多的漁民用來稱呼發生於聖誕節前後，祕魯附近海域海溫異常偏暖的現象。科學研究顯示 不僅和祕魯附近海溫的變化有關，也和熱帶東太平洋和西太平洋之間的海面氣壓的分布有關，就以 El Nino Southern Oscillation(簡稱ENSO)來表示熱帶太平洋大氣和海洋之間的變化，也就 是一般所謂的聖嬰現象。
ENSO是赤道太平洋附近大氣和海洋最顯著的現象，通常最容易發生在冬季。當赤道太平洋附近( 北緯5°至南緯5°、西經170-120°)3個月移動平均的海溫指標連續5個月比平均值高0.5℃時，就 認定為ENSO暖事件(就是一般通稱的聖嬰事件)；當3個月移動平均的海溫指標連續5個月比平均值 低0.5℃時，就認定為ENSO冷事件(就是一般通稱的反聖嬰事件)，聖嬰和反聖嬰事件發生時大氣和 海洋的狀況說明於圖1。
ENSO現象雖然主要發生在赤道太平洋附近，但卻對全球各地的氣候都會造成影響，因此，深受各國 氣象單位的重視，其中對各地較顯著之影響列於圖2。臺灣雖不在ENSO事件的主要影響區域，一般 而言，當強的聖嬰事件(即指標超過1.5℃)發生時的冬天，臺灣易出現暖冬、隔年易出現春雨偏多 的情形，而颱風生成的位置則有距離臺灣較遠的現象。當強的反聖嬰事件(即指標超過-1.5℃)發生 時的冬天，臺灣易出現冷冬、隔年易出現冷夏，颱風生成的位置會距臺灣較近。不過，研究顯示每 個ENSO事件的差異性相當大。
氣象局分析臺灣的春雨，得知除了受聖嬰影響外，也和印度洋的海溫變化有關。另外，近幾年聖嬰 現象的位置也較過去典型聖嬰現象發生時偏西，是所謂的非典型聖嬰。因此在預測臺灣的氣候時， 除了參考ENSO的預報資訊，還會參考氣候電腦模擬和統計分析的結果。
詳細聖嬰事件的分析和預報可參考本局於每月月中發布的氣候監測報告 (http://www.cwb.gov.tw/V6/statistics/watch/watch.php)，臺灣的氣候預報可參考 本局於每月月底發布的季長期天氣展望(http://www.cwb.gov.tw/V6/forecast/long/long_season.htm)。

聖嬰事件示意圖	反聖嬰事件示意圖
圖1：左圖當聖嬰事件發生時，暖海水(圖中紅與橙色部分)向東偏移、赤道地區的信風(東風)減弱出 現西風的距平，隨著暖海溫的東移，熱帶地區主要降雨的位置也會跟著往東移。右圖當反聖嬰事件 發生時，偏暖海水(圖中紅與橙色部分)位於西太平洋附近、赤道地區的東風偏強，熱帶地區主要降 雨的位置也會跟著往西移。圖片來源www.pmel.noaa.gov。

圖2：左上圖為聖嬰事件發生時夏季於印尼群島易出現乾旱、換日線附近出現多雨，中南美洲易出 現高溫少雨。左下圖為聖嬰事件發生時冬季的日本、臺灣、中南半島及加拿大西岸易出現高溫，美 國東南部易出現濕冷的天氣。右上圖為反聖嬰事件發生時夏季於臺灣及中南半島易出現偏冷的天氣、 印度及蘇門達臘群島易出現濕冷的天氣、澳洲東南部出現偏暖的天氣。右下圖為反聖嬰事件發生時 冬季於日本韓國及加拿大西岸易出現偏冷的天氣、印尼及菲律賓易出現多雨的天氣。圖片來源 www.pmel.noaa.gov。

El Niño

From Wikipedia, the free encyclopedia

The 1997 El Niño observed by TOPEX/Poseidon. The white areas off the tropical coasts of South and North America indicate the pool of warm water^[1]

Southern Oscillation Index timeseries 1876-2012.

NOAA graph of Global Annual Temperature Anomalies 1950–2012, showing ENSO

El Niño (/ɛlˈniːnjoʊ/, /-ˈnɪn-/, Spanish pronunciation: [el ˈniɲo]) is a band of anomalously warm ocean water temperatures that periodically develops off the western coast of South America and can cause climatic changes across the Pacific Ocean. There is a phase of 'El Niño–Southern Oscillation' (ENSO), which refers to variations in the temperature of the surface of the tropical eastern Pacific Ocean (El Niño and La Niña) 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.^[2][3] 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. El niño is Spanish for "the boy", and the capitalized term El Niño refers to the Christ child, Jesus, because periodic warming in the Pacific near South America is usually noticed around Christmas.^[4]

Contents

• 1 Definition
• 2 Southern Oscillation
  • 2.1 Walker circulation
• 3 Effects of ENSO warm phase (El Niño)
  • 3.1 South America
  • 3.2 North America
  • 3.3 Tropical cyclones
  • 3.4 Elsewhere
• 4 Effects of ENSO's cool phase (La Niña)
  • 4.1 Africa
  • 4.2 Asia
  • 4.3 South America
  • 4.4 North America
• 5 Transitional phases
• 6 Recent occurrences
• 7 Remote influence on tropical Atlantic Ocean
• 8 ENSO and global warming
• 9 The "Modoki" or Central-Pacific El Niño debate
• 10 Health and social impacts of El Niño
• 11 Cultural history and prehistoric information
• 12 Climate networks
• 13 See also
• 14 References
• 15 Further reading
• 16 External links

Definition

El Niño is defined by prolonged warming in the Pacific Ocean sea surface temperatures when compared with the average value. The accepted definition is a warming 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.^[5] The average period length is five years. When this warming occurs for only seven to nine months, it is classified as El Niño "conditions"; when it occurs for more than that period, it is classified as El Niño "episodes".^[6] Similarly, La Niña conditions and episodes are defined for cooling.
The first signs of an El Niño are:

1. Rise in surface pressure over the Indian Ocean, Indonesia, and Australia
2. Fall in air pressure over Tahiti and the rest of the central and eastern Pacific Ocean
3. Trade winds in the south Pacific weaken or head east
4. Warm air rises near Peru, causing rain in the northern Peruvian deserts
5. 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.

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.^[7]

Southern Oscillation

Normal Pacific pattern: Equatorial winds gather warm water pool toward the west. Cold water upwells along South American coast. (NOAA / PMEL / TAO)

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.^[8] El Niño episodes are associated with negative values of the SOI, meaning there is below normal pressure over Tahiti and above normal pressure of Darwin.
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.

Walker circulation

El Niño conditions: Warm water pool approaches the South American coast. The absence of cold upwelling increases warming.

La Niña conditions: Warm water is farther west than usual.

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.^{[9][10][11][12]}

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.^[13] 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.^[7]
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.^[14] 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

Regional impacts of warm ENSO episodes (El Niño)

See also: Effects of the El Niño–Southern Oscillation in the United States

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.^[15][16]
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.^[17] El Niño warmed Vancouver for the 2010 Winter Olympics, such that the area experienced a subtropical-like winter during the games.^[18]
El Niño is credited with suppressing hurricanes, and made the 2009 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 of 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, dryer 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]
In more 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.^[29]

Effects of ENSO's cool phase (La Niña)

Main article: La Niña

Sea surface skin temperature anomalies in November 2007 showing La Niña conditions

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").^[30]

Africa

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.^[31]

Asia

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.^[21] 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.^[32]

South America

During a time of La Niña, drought plagues the coastal regions of Peru and Chile.^[33] From December to February, northern Brazil is wetter than normal.^[33]

North America

Regional impacts of La Niña.

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.^[34] 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.^[35] 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.^{[36] [37]}

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).^[38] Examples of affected short-time climate in North America include precipitation in the Northwest US^[39] and intense tornado activity in the contiguous US.^[40]
During strong El Niño episodes, a secondary peak in sea surface temperature across the far eastern equatorial Pacific Ocean sometimes follows the initial peak.^[41]

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.

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^[42] and lasted until early 2007.^[43] 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.^[19]
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. The September 30, 2013 NOAA report indicates high probability of no El Niño or La Niña (ENSO-neutral) through Spring 2014.^[37]

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.^[44] About half of El Niño events persist sufficiently into the spring months for the Western Hemisphere Warm Pool 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]

ENSO and global warming

During the last several decades, the number of El Niño events increased, and the number of La Niña events decreased,^[48] although observation of ENSO for much longer is needed to detect robust changes.^[49] 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,^[50] the amplitude of the ENSO variability in the observed data still increases, by as much as 60% in the last 50 years.^[51]
The exact changes happening to ENSO in the future is uncertain:^[52] Different models make different predictions.^[53][54] 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.^[55] It may also be that the stabilizing and destabilizing forces influencing the phenomenon will eventually compensate for each other.^[56] 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.^[57]

The "Modoki" or Central-Pacific El Niño debate

Map showing Niño3.4 and other index regions

Map of Atlantic major hurricanes during post-"Modoki" seasons, including 1987, 1992, 1995, 2003 and 2005.

The traditional Niño, also called Eastern Pacific (EP) El Niño,^[58] 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).^[59] The phenomenon is called Central Pacific (CP) El Niño,^[58] "dateline" El Niño (because the anomaly arises near the dateline), or El Niño "Modoki" (Modoki is Japanese for "similar, but different").^{[60][61][62][63]} 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.^[64]
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.^[65]
The recent discovery of El Niño Modoki has some scientists believing it to be linked to global warming.^[66] 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.^[52]
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,^[67][68] finding no distinction or trend using other statistical approaches,^{[69][70][71][72][73]} or that other types should be distinguished, such as standard and extreme El Niños.^[74][75] 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.^[76]
The first recorded El Niño that originated in the central Pacific and moved toward the east was in 1986.^[77]

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.^[78]
ENSO conditions have also been related to Kawasaki disease incidence in Japan and the west coast of the United States,^[79] via the linkage to tropospheric winds across the north Pacific Ocean.^[80]
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.^[81][82]

Cultural history and prehistoric information

Average equatorial Pacific temperatures

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.^[83]
El Niño affected pre-Columbian Incas ^[84] and may have led to the demise of the Moche and other pre-Columbian Peruvian cultures.^[85] 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.^[86] The extreme weather produced by El Niño in 1876–77 gave rise to the most deadly famines of the 19th century.^[87] The 1876 famine alone in northern China killed up to 13 million people.^[88]
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 1990–1994 was unusual in that El Niños have rarely occurred in such rapid succession.^[89] 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.^[90] Since then, mass coral bleaching has become common worldwide, with all regions having suffered "severe bleaching".^[91]
Major ENSO events were recorded in the years 1790–93, 1828, 1876–78, 1891, 1925–26, 1972–73, 1982–83, 1997–98 and 2009–2010,^[47] with 1997-1998 being one of the strongest ever.^[92]