El Niño–Southern Oscillation

• 1. Introduction

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  The strongest El Niño on record, as of early 2026, struck in 1877 and 1878. It led to a global famine that killed more than 50 million people, about 3-4 percent of everyone alive. El Niño is the warm half of the El Niño-Southern Oscillation, known as ENSO. This climate pattern grows out of shifting winds and sea surface temperatures across the tropical Pacific Ocean. Its swings reach across the tropics and subtropics, and touch distant regions through links that scientists call teleconnections. Yet the pattern keeps no fixed timetable, and its arrival is not easily predicted. Why would warm water near the equator ruin harvests on other continents? What locks the ocean and the sky into step with each other? And what finally breaks the spell?

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• 2. A Boy Named for Christmas

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  In 1892, Captain Camilo Carrillo stood before the geographical society congress in Lima and described a current. Peruvian sailors, he said, called the warm, south-flowing water El Niño, Spanish for The Boy. The name pointed to the Christ Child, because the current was most noticeable around Christmas. The longer phrase, El Niño de Navidad, is said to be centuries old and to come from Peruvian fishermen. Over time the label narrowed, and today it names only the warm phase of ENSO. Its cold counterpart is La Niña, The Girl. In the past that phase was also called an anti-El Niño, or El Viejo, meaning the old man.

  The sea itself was noticed long before anyone saw the pattern. In 1822, the cartographer Joseph Lartigue, aboard the French frigate La Clorinde under Baron Mackau, recorded a counter-current that helped ships travel south along the Peruvian coast. The phenomenon also drew interest because it disrupted the guano industry and other businesses that depend on the sea's productivity.

  Then came the realization that distant places were linked. Charles Todd suggested in 1888 that droughts in India and Australia tended to strike together, and Norman Lockyer made the same observation in 1904. Victor Eguiguren connected El Niño with flooding in 1894, and Federico Alfonso Pezet did so in 1895. In 1924, Gilbert Walker coined the term Southern Oscillation. He and others, including the Norwegian-American meteorologist Jacob Bjerknes, are generally credited with identifying the effect. The major 1982-83 event then sent a fresh wave of interest through the scientific community.

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• 3. The Tilted Pacific

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  Off the western coast of South America, the sea surface sits close to 20 degrees Celsius. Northeast of Australia, it averages 28-30 degrees. On average, then, the eastern tropical Pacific is roughly 8-10 degrees cooler than the west. Strong trade winds near the equator drive surface water away from the east and toward the west, and the Sun warms it along the way. The wind's push is balanced by a slope in the sea itself. As a result, sea level near Indonesia typically stands about 1.5 feet higher than near Peru.

  All that warm water deepens the thermocline in the west. The thermocline is the transition zone between warm surface water and the cold deep ocean. In the western Pacific it lies around 450 feet down, while in the east it is only about 90 feet below the surface. Below the thermocline, the pressure difference still drives the cold equatorial undercurrent eastward.

  Along the western coast of South America, the trade winds and the Coriolis effect push near-surface water westward. This process is called Ekman transport. Colder water rises along the continental margin to take its place, a process called upwelling. Because the thermocline is so shallow here, little separates the deep cold water from the surface. The northward-flowing Humboldt Current adds to the chill by carrying cold water from the Southern Ocean into the tropics.

  The west has no cold current, and its trade winds are usually weaker, so upwelling is limited. Its warm water supplies energy for rising air, which is why the western Pacific has more cloud and rain than the cool east. Weaken the trade winds even slightly, and every one of these numbers begins to shift.

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• 4. Feedback and Reversal

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  In 1969, Jacob Bjerknes identified the close relationship between ocean temperatures and the strength of the trade winds. He proposed that ENSO is a positive feedback system, in which a change in the ocean or the atmosphere reinforces a change in the other. Take an El Niño in its growth stage. The temperature contrast across the Pacific shrinks, the trade winds weaken, and the weaker winds feed the El Niño further.

  Bjerknes tied this idea to an east-west overturning cell of air above the Pacific. He named it the Walker Circulation after Gilbert Walker, who discovered the Southern Oscillation. In his picture, the rising branch sits over the hot western Pacific, where convection and rainfall are strong. The sinking branch lies over the cooler central and eastern waters. As the east warms, the overturning weakens, easterly surface winds ease, cold upwelling declines, and the east grows warmer still. The ocean and the air can even disagree, so the atmosphere may resemble one phase while the ocean resembles another.

  Here lies the puzzle. If Bjerknes feedback were the only process at work, ENSO would stay in one phase forever. Researchers offer two broad answers. In one view, the feedback itself triggers negative feedbacks that end and reverse the abnormal state, making ENSO a self-sustaining cycle. In the other, irregular outside events change the state, such as the Madden-Julian oscillation, tropical instability waves and westerly wind bursts. In the usual account, the warm water in the western tropical Pacific is eventually drained enough for conditions to return to normal. The exact mechanisms remain unclear and are still being studied.

  Whatever causes the reversal, the atmosphere keeps its own record of each swing. Stations in Tahiti and Darwin have kept that record since the 1800s.

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• 5. Gauges and Thresholds

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  The Southern Oscillation Index compares surface air pressure at Tahiti, out in the Pacific, with Darwin, Australia, on the Indian Ocean side. El Niño episodes produce a negative index, with lower pressure over Tahiti and higher pressure at Darwin. La Niña episodes produce a positive index, with the reverse. Low pressure tends to settle over warm water and high pressure over cold water, partly because of deep convection above the warm water.

  The record at those two stations reaches back to the 1800s, but it has a weakness. Both towns lie well south of the equator, so their pressure is less directly connected to ENSO. To fix this, researchers built the Equatorial Southern Oscillation Index. It uses two regions centered on the equator, one over Indonesia and one over the equatorial Pacific near the South American coast. Its data reach back only to 1949. Sea level responds too. El Niño raises sea surface height by several centimeters through thermal expansion, while La Niña lowers it through contraction.

  Deciding when an event has begun is left to each country, and every threshold reflects national interests. In the United States, the Climate Prediction Center watches the Niño 3.4 region. That zone runs from the 120th to the 170th meridian west, with five degrees of latitude on each side of the equator, roughly 3000 km southeast of Hawaii. When its three-month average sits more than 0.5 degrees Celsius above or below normal, an El Niño or La Niña is considered in progress. In February 2026, anomalous rises in tropical sea surface temperatures led NOAA to revise this method. The new Relative Oceanic Niño Index compares the ENSO region with the global tropics, replacing a dependence on a 30-year climate base period.

  Other agencies set their own bars. Australia's Bureau of Meteorology weighs the trade winds, the Southern Oscillation Index, weather models and temperatures in the Niño 3 and 3.4 regions. The Japan Meteorological Agency declares a start when the Niño 3 deviation exceeds 0.5 degrees for six consecutive months or longer. The Met Office in the United Kingdom also uses a several-month period. It calls a warming or cooling of seven to nine months a set of conditions, and anything longer an episode.

  Peru uses the tightest trigger of all. It declares an ENSO Costero when the deviation in the Niño 1+2 regions reaches 0.4 degrees for at least three months. That coastal version of the phenomenon can behave differently from the Pacific-wide pattern.

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• 6. Rhythms and Variants

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  Close to half of all years fall into the neutral phase, when the temperature stays within 0.5 degrees Celsius of the long-term norm. In those years, other climate patterns, such as the North Atlantic Oscillation, exert more influence than ENSO.

  El Niño comes and goes at irregular intervals of two to seven years, averaging five, and each event lasts from nine months to two years. It is thought there were at least 30 events between 1900 and 2024, with 1982-83, 1997-98 and 2014-16 among the strongest. Since 2000, El Niño has been observed in 2002-03, 2004-05, 2006-07, 2009-10, 2014-16, 2018-19 and 2023-24. Earlier major events were recorded in 1790-93, 1828-1876-78, 1891-1925-26 and 1972-73. During strong events, a second peak in sea surface temperature sometimes follows the first across the far eastern equatorial Pacific. El Niño can be the more intense phase, but La Niña may repeat and last longer. La Niña has been observed for hundreds of years, and it occurred on a regular basis in the early 17th and 19th centuries.

  The first pattern to be recognized is called Eastern Pacific ENSO. In the 1990s and 2000s, observers saw a different arrangement. The usual zone in Niño 1 and 2 stayed unaffected, while an anomaly arose in the central Pacific, in Niño 3.4. This is called Central Pacific ENSO, dateline ENSO, or ENSO Modoki, from a Japanese word meaning similar but different. The first recorded El Niño that began in the central Pacific and moved east came in 1986.

  The effects differ from the conventional type. El Niño Modoki is associated with more Atlantic hurricanes making landfall. La Niña Modoki raises rainfall over northwestern Australia and the northern Murray-Darling basin, rather than the eastern part of the country. It also increases cyclonic storms over the Bay of Bengal. Some scientists link Modoki to global warming, but comprehensive satellite data begin only in 1979. Others dispute whether this distinction is real, arguing that the reliable record is too short to detect it.

  A third form, ENSO Costero, was coined in Peru by ENFEN. Here the temperature anomalies concentrate along the South American coastline, especially Peru and Ecuador. Warm phases bring more rain to coastal Ecuador, northern Peru and the Amazon rainforest. Strong El Niño Costero events on the Coastal Niño Index include 1957-1982-83, 1997-98 and 2015-16. The index also lists cold Costera events, such as 1950-1962 and 2013, and cold phases bring drought to the Peruvian coast.

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• 7. Drought and Deluge

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  Singapore endured its driest February since records began in 1869 during 2010, when only 6.3 mm of rain fell. The next driest Februaries, in 1968 and 2005, each received 8.4 mm. In El Niño years, warm water spreads east from the western Pacific and the Indian Ocean and carries the rain with it. The western Pacific suffers extensive drought, while the normally dry eastern Pacific gets rainfall.

  Across most of Australia, El Niño and La Niña shape the climate more than any other factor. In an El Niño, the shift of rainfall away from the western Pacific can mean less rain across the continent. The Indo-Australian Monsoon arrives two to six weeks late in the tropical north. The risk of a major bushfire season in south-eastern Australia rises after an El Niño, especially when a positive Indian Ocean Dipole joins in.

  South America feels El Niño directly. From April to October, the coasts of northern Peru and Ecuador turn warm and very wet, with major flooding whenever the event is strong or extreme. A La Niña does the reverse on the coasts of Peru and Chile, where drought sets in. Heavy rain in the central Andes during La Niña also brings catastrophic flooding to the Llanos de Mojos in Bolivia's Beni Department. Such floods are documented in 1853, 1865, 1872, 1873, 1886, 1895, 1896, 1907, 1921, 1928-1929 and 1931.

  In southern Africa, summer rainfall areas get less rain during El Niño, and strong events have brought severe drought. Winter rainfall areas tend to receive more than normal. La Niña brings wetter conditions to the south from December to February, and drier ones to equatorial east Africa.

  North America shows a mixed picture. In an El Niño, the Gulf Coast between Texas and Florida turns wetter, while Hawaii, the Ohio Valley, the Pacific Northwest and the Rocky Mountains turn drier. California is a special case. Its rain depends more on the flavor of El Niño than on its presence, and only persistent events lead to consistently high rainfall. La Niña in Canada generally means a cooler, snowier winter, as in 2007-08 when eastern Canada saw near-record snow. In the spring of 2022, La Niña left Oregon with above-average precipitation and below-average temperatures. April was one of the wettest months on record there. Europe is a different matter, since El Niño is only one of several factors, and others can overwhelm its signal.

  Storms follow the same shifting pattern. During El Niño, the Atlantic usually has a quieter hurricane season, because vertical wind shear rises and the air can be drier and more stable. Activity moves toward the Pacific. In the western Pacific, cyclones form farther east without a big change in the yearly count. That makes Micronesia more likely, and China less likely, to be struck. Queensland is 50% less likely to see a cyclone landfall, while the risk rises for Niue, French Polynesia, Tonga, Tuvalu and the Cook Islands. On Guam, the chance of a tropical cyclone is more than triple the normal.

  Even a mountain wind answers to ENSO. The Tehuantepecer is a violent gap wind that funnels through the Isthmus of Tehuantepec, between the mountains of Mexico and Guatemala. It usually blows at 20-45 knots, and on rare occasions reaches 100. It is stronger in El Niño winters, because cold fronts arrive more often.

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• 8. Famine, Reefs and Warming

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  Around 1525, Francisco Pizarro landed in Peru and noted rain falling in the deserts. It was the first written record of El Niño's effects. El Niño may also have brought down the Moche around 700 AD, along with other pre-Columbian Peruvian cultures. A recent study suggests that a strong event between 1789 and 1793 ruined crops in Europe and helped touch off the French Revolution. The 1876 famine in northern China alone killed up to 13 million people.

  A modern example came in 2023. Drought cut maize production by up to 70% in parts of Zimbabwe, Zambia and Malawi, and the failed harvests forced countries to declare states of disaster and appeal for international help. East Africa, meanwhile, suffered floods that destroyed crops and infrastructure in areas still recovering from years of drought. The money effects vary by country. A 2014 Cambridge working paper found short-lived economic dips in Australia, Chile, Indonesia, India, Japan, New Zealand and South Africa. Argentina, Canada, Mexico and the United States may benefit. The IMF traced one-year changes in real GDP growth of -1.01% in Indonesia and +1.81% in Thailand.

  The sea itself pays a price. When upwelling fades, the cold nutrient-rich water that feeds large fish populations stops rising. Those fish sustain sea birds, whose droppings support the fertilizer industry, and the shortage causes fish kills off Peru. Peruvian fisheries, once the world's largest, collapsed in the 1970s after overfishing followed the 1972 loss of anchoveta. During the 1982-83 event, kelp beds died along 600 km of the Chilean coast, and two hydrocoral species in Panamá probably went extinct. Coral suffered most. The 1997-98 and 2015-16 events caused global mass bleaching, with losses of live coral of around 75-99%. The 1998 event alone is estimated to have killed 16% of the world's reef systems.

  On land, the same three events dried out tropical forests in the Amazon and Borneo, and fires followed. Insect populations fell after the 2015-16 drought and fires, and a burned forest site in Borneo temporarily lost more than 100 lowland butterfly species. In the Galápagos, a weakened Equatorial current brings warmer water and fewer nutrients. This sets off a trophic cascade that runs from primary producers up to sharks, penguins and seals. Many populations starve and die back.

  Disease follows rain. Rift Valley fever broke out severely in north-eastern Kenya and southern Somalia after extreme rainfall in the 1997-98 El Niño. Malaria cycles in India, Venezuela, Brazil and Colombia have been linked to El Niño. Scientists at Columbia University's Earth Institute analyzed data from 1950 to 2004. They suggest ENSO may have played a role in 21% of civil conflicts since 1950, with the annual risk doubling from 3% to 6% in affected countries during El Niño years compared with La Niña years.

  Climate records stretch far deeper than human history. Chemical signatures of warmer seas and heavier rain appear in coral about 13,000 years old. A 2024 study suggests that El Niño strongly shaped the hothouse climate during the Permian-Triassic extinction event. Because El Niño years push global temperatures up for about a year and La Niña years pull them down, the balance between them can tilt temperature trends over roughly ten years.

  Since about 2019, research has found that climate change is increasing the frequency of extreme El Niño events. One large ensemble experiment shows an increase of about 10% in eastern Pacific ENSO amplitude between 1901-1960 and 1961-2020. For the later period, it found twice the likelihood of strong eastern Pacific El Niño events. Warming Indian Ocean waters, fed by stronger El Niño events, have weakened the Asian Monsoon. The IPCC Sixth Assessment Report judged in 2021 that rainfall variance linked to ENSO will very likely increase in the long term. It also expressed medium confidence that ENSO strength since 1950 is higher than at any time since 1850, and possibly since 1400.

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