El Niño–Southern Oscillation (ENSO) is a global climate pattern that happens because of changes in wind patterns and sea surface temperatures in the tropical Pacific Ocean. These changes do not follow a regular schedule but sometimes repeat in cycles. Scientists cannot predict when ENSO will occur. It influences weather in many tropical and subtropical regions and is connected to weather patterns in areas farther from the equator. When sea surface temperatures are warmer than usual, it is called "El Niño." When they are cooler, it is called "La Niña." The Southern Oscillation refers to changes in air pressure that happen at the same time as changes in ocean temperatures.

During El Niño, air pressure is higher than normal over Indonesia, Australia, and parts of the Indian and Atlantic Oceans. During La Niña, air pressure is higher over the central and eastern Pacific and lower in most other tropical and subtropical areas. These events usually last about a year and happen every two to seven years, with periods of weaker ENSO activity in between. El Niño events can be stronger, but La Niña events may last longer and happen more often.

A key part of ENSO is the Bjerknes feedback, named after Jacob Bjerknes in 1969. This process happens when changes in the atmosphere affect ocean temperatures, which then change wind patterns, creating a cycle that strengthens itself. Weaker trade winds (winds that blow from east to west) cause warm surface water to move eastward and reduce the upward movement of cold water near the equator. This leads to warmer ocean temperatures (El Niño), weaker atmospheric circulation patterns, and even weaker trade winds. Eventually, the warm water in the western Pacific is used up, and conditions return to normal. Scientists are still studying the exact reasons for these changes.

Each country that tracks ENSO uses different standards to define El Niño or La Niña events, based on their specific needs. These events change global weather patterns, causing heavy rain in some areas and droughts in others. El Niño events briefly raise global average temperatures, while La Niña events briefly lower them. Over time, the number of El Niño events compared to La Niña events can influence global temperature trends. Countries most affected by ENSO are often developing nations near the Pacific Ocean that rely on farming and fishing.

In climate science, ENSO is considered an example of natural climate changes that happen without outside causes. How ENSO might change in the future due to climate change is not certain, but climate change is expected to make droughts and floods worse. According to the IPCC Sixth Assessment Report from 2021, scientists believe that changes in ENSO will likely increase rainfall differences in the future. They also think that changes in ENSO’s effects on weather patterns will cause major changes in specific regions.

Definition and terminology

The El Niño–Southern Oscillation (ENSO) is a climate pattern that changes over time between three phases: Neutral, El Niño, and La Niña. El Niño and La Niña are opposite phases that happen when certain ocean and air conditions are met.

The term "El Niño" ("The Boy" in Spanish) was first used in 1892 by Captain Camilo Carrillo during a meeting in Lima, Peru. He explained that Peruvian sailors called a warm ocean current flowing south along the coasts of Peru and Ecuador "El Niño" because it was most noticeable around Christmas.

The term "El Niño" is connected to the birth of Jesus, as the warm ocean current near South America often appears around Christmas. Originally, the term described a weak, seasonal current that flowed south along the coasts of Peru and Ecuador near Christmas. Over time, the term came to describe the warm phase of ENSO. The phrase "El Niño de Navidad" (El Niño of Christmas) was used centuries ago by Peruvian fishermen to name the weather pattern after the newborn Christ.

La Niña ("The Girl" in Spanish) is the cooler phase of ENSO. It is also called an anti-El Niño or El Viejo ("the old man").

During El Niño, air pressure over Indonesia and the western Pacific is higher than usual, while pressure over the eastern Pacific is lower than usual. During La Niña, the opposite happens: pressure over Indonesia is lower than usual, and pressure over the eastern Pacific is higher than usual.

Fundamentals

The average temperature of the ocean surface in the tropical East Pacific is about 8–10 °C (14–18 °F) cooler than in the tropical West Pacific. The sea surface temperature (SST) in the West Pacific, northeast of Australia, averages around 28–30 °C (82–86 °F). In the East Pacific, near the western coast of South America, SSTs are closer to 20 °C (68 °F).

Strong trade winds near the equator push water away from the East Pacific and into the West Pacific. As this water moves west along the equator, the Sun warms it. The wind stress on the ocean surface creates a sea surface slope, which causes sea levels near Indonesia to be about 0.5 m (1.5 ft) higher than near Peru.

Warm surface water accumulates in the western Pacific, making the thermocline—where the warm surface water meets the cooler deep water—much deeper there. In the western Pacific, the thermocline averages about 140 m (450 ft) deep, while in the East Pacific, it averages about 30 m (90 ft) deep. Below the thermocline, the pressure difference between the East and West Pacific still drives the eastward flow of cold equatorial undercurrents.

Cooler deep ocean water replaces the outgoing surface water in the East Pacific through a process called upwelling. Along the western coast of South America, trade winds and the Coriolis effect push surface water westward, a process known as Ekman transport. This causes colder water from deeper in the ocean to rise along the continental margin to replace the surface water.

Upwelling cools the East Pacific because the thermocline is closer to the ocean surface, reducing the distance between the deep cold water and the surface. The Humboldt Current carries colder water from the Southern Ocean into the tropics of the East Pacific. Together, the Humboldt Current and upwelling keep the waters near Peru cooler.

The West Pacific lacks a cold ocean current and has less upwelling because trade winds are usually weaker than in the East Pacific. This allows the West Pacific to reach warmer temperatures. These warm waters provide energy for air to rise, leading to more cloudiness and rainfall in the West Pacific compared to the East Pacific.

ENSO describes repeating changes in ocean and atmospheric conditions over the tropical Pacific. These changes affect weather patterns worldwide. The tropical Pacific can be in one of three ENSO states: neutral, El Niño, or La Niña. The neutral phase occurs when conditions are close to average.

If trade winds are weaker than average, upwelling in the East Pacific weakens, and warm surface waters move less toward the West Pacific. This results in a cooler West Pacific and a warmer East Pacific, shifting cloudiness and rainfall toward the East Pacific. This situation is called El Niño. If trade winds are stronger than average, the opposite occurs: the West Pacific warms, and the East Pacific cools, leading to La Niña, which increases cloudiness and rainfall in the West Pacific.

Jacob Bjerknes first identified the relationship between ocean temperatures and trade wind strength in 1969. He proposed that ENSO is a feedback system where changes in the ocean or atmosphere reinforce each other. This process is called Bjerknes feedback. For example, during El Niño, reduced temperature differences across the Pacific weaken trade winds, further strengthening the El Niño state.

Although changes in the ocean and atmosphere often occur together, the atmosphere and ocean may appear to be in different ENSO phases. ENSO variations arise from changes in both the ocean and atmosphere, not just one. Models explaining ENSO generally support the Bjerknes feedback hypothesis. However, ENSO would remain in one phase if Bjerknes feedback were the only process.

Several theories explain how ENSO shifts between states despite positive feedback. Some suggest that negative feedbacks naturally end and reverse abnormal conditions, making ENSO self-sustaining. Others propose that external factors, such as the Madden–Julian oscillation, tropical instability waves, and westerly wind bursts, trigger changes.

Bjerknes described ENSO as an east-west atmospheric circulation pattern over the Pacific, called the Walker Circulation, named after Gilbert Walker. The strength of this circulation depends on the east-west temperature gradient along the equator. Warm water in the West Pacific causes rising air, convection, and rainfall, while cooler water in the East Pacific causes sinking air.

During El Niño, increased sea surface temperatures in the East Pacific reduce the temperature gradient, weakening the Walker Circulation. This weakens trade winds, reduces upwelling, and leads to even warmer sea surface temperatures in the East Pacific.

The Southern Oscillation is the atmospheric part of ENSO. It involves changes in surface air pressure between the tropical eastern and western Pacific. The Southern Oscillation Index (SOI) measures this pressure difference between Tahiti and Darwin, Australia.

El Niño episodes have negative SOI, meaning lower pressure over Tahiti and higher pressure over Darwin. La Niña episodes have positive SOI, meaning higher pressure over Tahiti and lower pressure over Darwin.

Low atmospheric pressure occurs over warm water, and high pressure occurs over cold water, partly due to deep convection over warm areas. El Niño is defined by sustained warming in the central and eastern Pacific, weakening trade winds and reducing rainfall in eastern and northern Australia. La Niña is defined by sustained cooling in the same region, strengthening trade winds and increasing rainfall in Australia compared to El Niño.

Although the SOI has data dating back to the 1800s, its reliability is limited because Tahiti and Darwin are far from the equator, affecting the accuracy of pressure measurements.

Three phases of sea surface temperature

The El Niño–Southern Oscillation (ENSO) is a climate pattern that changes between three phases: Neutral, El Niño, and La Niña. El Niño and La Niña are opposite phases that occur when specific changes in the ocean and atmosphere happen.

During the cool phase of ENSO, called La Niña, sea surface temperatures (SST) in the eastern Pacific are lower than average. At the same time, air pressure is higher in the eastern Pacific and lower in the western Pacific. The ENSO cycle, which includes both El Niño and La Niña, affects global weather patterns, including temperature and rainfall.

If the temperature difference from the average is less than 0.5°C (0.9°F), ENSO conditions are considered neutral. Neutral conditions occur between the warm (El Niño) and cool (La Niña) phases. During this time, ocean temperatures, rainfall, and wind patterns are close to normal. About half of all years fall into neutral conditions. When ENSO is neutral, other climate patterns, such as the North Atlantic Oscillation or Pacific–North American teleconnection, have a greater influence on weather.

El Niño happens when the Walker circulation weakens or reverses, and the Hadley circulation strengthens. This leads to warm ocean water forming in the central and eastern Pacific, near the west coast of South America, because cold water rising from the deep ocean is reduced or stops. This warming changes atmospheric circulation, causing higher air pressure in the western Pacific and lower pressure in the eastern Pacific. Rainfall decreases in Indonesia, India, and northern Australia, while rainfall and tropical storms increase in the tropical Pacific. Trade winds, which normally blow from east to west along the equator, weaken or reverse direction.

El Niño events occur irregularly every two to seven years and last between nine months and two years, with an average duration of five years. If warming lasts seven to nine months, it is called El Niño "conditions." If it lasts longer, it is called an El Niño "episode." Between 1900 and 2024, at least 30 El Niño events have been recorded, with the strongest ones occurring in 1982–83, 1997–98, and 2014–16. Since 2000, El Niño events have been observed in 2002–03, 2004–05, 2006–07, 2009–10, 2014–16, 2018–19, and 2023–24. Major ENSO events have also been recorded in 1790–93, 1828, 1876–78, 1891, 1925–26, 1972–73, 1982–83, 1997–98, 2014–16, and 2023–24. During strong El Niño events, a second rise in ocean temperatures sometimes occurs in the eastern Pacific.

La Niña is the cold phase of ENSO, where sea surface temperatures in the eastern Pacific are 3–5°C (5.4–9°F) cooler than average. Strong winds push warm surface water from South America toward Indonesia, allowing cold water from the deep ocean to rise near South America. This movement of heat across the Pacific affects weather worldwide. During neutral or La Niña conditions, colder water areas, called tropical instability waves, are often visible on ocean temperature maps.

La Niña is a climate pattern that happens every few years and can last longer than five months. El Niño and La Niña can signal changes in weather globally. Hurricanes in the Atlantic and Pacific may differ in strength and frequency due to changes in wind patterns and ocean temperatures. La Niña events have occurred for hundreds of years, with regular appearances in the 17th and 19th centuries. Since 1900, La Niña events have been observed in many years, including 1900–01, 1903–04, 1907–08, 1910–11, 1914–15, 1918–19, 1924–25, 1928–29, 1931–32, 1938–39, 1940–41, 1945–46, 1949–50, 1953–54, 1956–57, 1963–64, 1965–66, 1973–74, 1975–76, 1988–89, 1998–99, 2007–08, 2010–11, 2017–18, 2020–21, and 2023–24.

Transitional phases between El Niño or La Niña can also influence global weather through teleconnections. These phases may affect short-term climate patterns, such as rainfall in the Pacific Northwest of the United States and tornado activity in the central United States.

Variations

The first ENSO pattern identified is called Eastern Pacific (EP) ENSO. It is named to help distinguish it from other types and involves unusual temperature changes in the eastern Pacific. In the 1990s and 2000s, scientists noticed changes in ENSO conditions. These changes showed that the usual area of temperature changes (Niño 1 and 2) was not affected, but unusual temperatures also appeared in the central Pacific (Niño 3.4). This pattern is called Central Pacific (CP) ENSO, "dateline" ENSO (because the unusual temperatures occur near the dateline), or ENSO "Modoki" (Modoki means "similar, but different" in Japanese). Scientists have also found other ENSO variations beyond EP and CP. Some researchers believe ENSO exists as a continuous range, with hybrid types that mix EP and CP features.

The effects of CP ENSO differ from EP ENSO. El Niño Modoki is linked to more hurricanes making landfall in the Atlantic. La Niña Modoki increases rainfall in northwestern Australia and the northern Murray–Darling basin, unlike the usual EP La Niña, which affects the eastern part of the country. La Niña Modoki also increases cyclonic storms in the Bay of Bengal but reduces severe storms in the Indian Ocean overall.

The first recorded El Niño that began in the central Pacific and moved east occurred in 1986. Other Central Pacific El Niños happened between 1986–87, 1991–92, 1994–95, 2002–03, 2004–05, and 2009–10. Modoki events occurred between 1957–59, 1963–64, 1965–66, 1968–70, 1977–78, and 1979–80. Some sources say the El Niños of 2006–07 and 2014–16 were also Central Pacific El Niños. Recent La Niña Modoki events happened between 1973–1974, 1975–1976, 1983–1984, 1988–1989, 1998–1999, 2000–2001, 2008–2009, 2010–2011, and 2016–2017.

The discovery of ENSO Modoki has led some scientists to think it might be connected to global warming. However, satellite data only go back to 1979, so more research is needed to study past El Niño events and find any links to climate change. Scientists do not yet agree on how climate change might affect ENSO.

There is also debate about whether ENSO Modoki is a real phenomenon. Some studies question if the differences between EP and CP ENSO are meaningful or if the data are not reliable enough to confirm this. Other research suggests that other ENSO types, such as standard and extreme events, should be considered separately.

Some studies have also found that La Niña events may not always show the same patterns as El Niño. For example, La Niña Modoki can involve cooler waters in the central Pacific and average or warmer temperatures in both the eastern and western Pacific. In these cases, ocean currents in the eastern Pacific may flow in the opposite direction compared to traditional La Niña events.

ENSO Costero, also called ENSO Oriental, refers to a type of ENSO where unusual sea-surface temperatures are mainly near the South American coast, especially in Peru and Ecuador. This event can occur alongside or separately from EP ENSO. Sometimes, ENSO Costero shows opposite conditions to Modoki events in other regions.

ENSO Costero events usually have more localized effects. During warm phases, they increase rainfall along the coasts of Ecuador, northern Peru, and the Amazon rainforest and raise temperatures along northern Chile. During cold phases, they cause droughts on the Peruvian coast and increase rainfall while lowering temperatures in mountainous and jungle areas of Peru.

Because ENSO Costero events affect the global climate less than other ENSO types, they are less strongly connected to other ENSO features. They are not always caused by Kelvin waves or linked to the Southern Oscillation. Strong El Niño Costero events include 1957, 1982–83, 1997–98, and 2015–16. Strong La Niña Costera events include 1950, 1954–56, 1962, 1964, 1966, 1967–68, 1970–71, 1975–76, and 2013.

Monitoring and declaration of conditions

Currently, each country uses different standards to determine when an El Niño event occurs, based on their specific needs. For example:

• In the United States, an El Niño is declared when the Climate Prediction Center predicts that the sea surface temperature in the Niño 3.4 region will be 0.5 °C (0.90 °F) or more above average for the next several seasons. The Niño 3.4 region spans from the 120th to 170th meridians west longitude, crossing the equator within five degrees of latitude on either side. It is about 3,000 kilometers (1,900 miles) southeast of Hawaii. The most recent three-month average temperature for the area is calculated. If the temperature is more than 0.5 °C (0.90 °F) above or below normal for that period, an El Niño or La Niña is considered to be happening. In February 2026, NOAA changed the method used to distinguish between La Niña and El Niño events. The new method uses the Relative Oceanic Niño Index (RONI), which compares the ENSO region to the global tropics instead of relying on a 30-year climate base period.

• The Australian Bureau of Meteorology considers trade winds, the Southern Oscillation Index, weather models, and sea surface temperatures in the Niño 3 and 3.4 regions before declaring an ENSO event.

• The Japan Meteorological Agency declares an ENSO event when the average five-month sea surface temperature deviation in the Niño 3 region is 0.5 °C (0.90 °F) or more for six consecutive months or longer.

• The Peruvian government declares an ENSO Costero event if the sea surface temperature deviation in the Niño 1+2 regions reaches or exceeds 0.4 °C (0.72 °F) for at least three months.

• The United Kingdom’s Met Office uses a several-month period to determine the ENSO state. If warming or cooling lasts seven to nine months, it is classified as El Niño/La Niña "conditions." If it lasts longer than that, it is classified as El Niño/La Niña "episodes."

Effects of ENSO on global climate

In climate change science, ENSO stands for El Niño-Southern Oscillation. It is one of the natural climate patterns that cause changes in weather over time. The other two main patterns are the Pacific decadal oscillation and the Atlantic multidecadal oscillation.

El Niño and La Niña are parts of ENSO. El Niño can cause short-term increases in global average temperatures, while La Niña can cause short-term cooling. These events change weather patterns worldwide, leading to heavy storms in some areas and droughts in others. Over time, the number of El Niño events compared to La Niña events can influence long-term temperature trends.

The effects of ENSO on weather, such as extreme heat, heavy rain, or droughts, are becoming more common because of climate change. Recent studies since 2019 show that climate change may be making extreme El Niño events happen more often. Earlier research had different opinions about whether climate change would make El Niño events stronger or weaker, longer or shorter.

Although more observations are needed to confirm changes in ENSO, experiments using climate models show that the strength of ENSO events in the eastern Pacific increased by about 10% between the periods 1901–1960 and 1961–2020. Compared to climate models with pre-industrial greenhouse gas levels, models from 1961–2020 show that strong eastern Pacific El Niño events are twice as likely, and strong central Pacific La Niña events are nine times as likely.

The IPCC Sixth Assessment Report (2021) summarized research on ENSO as follows:
– Scientists believe that changes in rainfall linked to ENSO will likely increase in the long term.
– Changes in how ENSO affects weather patterns may lead to major changes in specific regions.
– There is some confidence that ENSO events have been stronger and more frequent since 1950 than they were between 1850 and possibly as far back as 1400.

ENSO is considered a possible tipping element in Earth’s climate. Global warming may make ENSO-related weather patterns more extreme. For example, more frequent and intense El Niño events have caused warmer-than-usual temperatures in the Indian Ocean, which affects the Asian Monsoon.

In the past, scientists studied whether ENSO could act as a tipping element. Normally, strong winds blow from South America toward Australia across the Pacific Ocean. Every 2 to 7 years, these winds weaken, and the middle of the Pacific warms, changing weather patterns globally. This is called El Niño and often causes droughts in India, Indonesia, and Brazil, and flooding in Peru. In 2015–2016, this caused food shortages for over 60 million people. El Niño-related droughts may increase the risk of wildfires in the Amazon. Scientists estimated that ENSO could become a permanent El Niño state if global warming reaches 3.5°C to 7°C. This has happened before in Earth’s history, such as during the Pliocene era, but ocean conditions were different then. So far, no clear evidence shows ENSO has changed in behavior. The IPCC Sixth Assessment Report concluded that ENSO will likely remain the main cause of yearly climate changes in a warmer world. Because of this, the 2022 report no longer lists ENSO as a likely tipping element.

Effects of ENSO on weather patterns

El Niño changes global weather patterns, causing strong storms in some areas and dry conditions in others.

Most tropical cyclones form near the equator on the side of the subtropical ridge. These storms often move toward the poles, cross the ridge, and then turn toward the main area of the Westerlies. Regions west of Japan and Korea usually see fewer tropical cyclones during El Niño and neutral years. During El Niño, the break in the subtropical ridge often occurs near 130°E, which increases the chances of cyclones forming near Japan.

Based on data from models and observations, El Niño years typically lead to fewer hurricanes in the Atlantic Ocean but more tropical cyclones in the Pacific Ocean. In contrast, La Niña years usually bring more hurricanes in the Atlantic and fewer in the Pacific.

In the Atlantic Ocean, stronger wind shear increases, which makes it harder for tropical cyclones to form and grow. The air over the Atlantic may also be drier and more stable during El Niño, further limiting cyclone development. In the Eastern Pacific, El Niño reduces wind shear, leading to more hurricanes than usual. However, the effects of El Niño in this region depend on other climate patterns. In the Western Pacific, El Niño causes tropical cyclones to form farther east, but the total number of cyclones remains similar. This shift makes Micronesia more likely to be affected by cyclones and China less likely. In the Southern Pacific, between 135°E and 120°W, cyclones are more likely to form in the Southern Pacific than in Australia. As a result, cyclones are 50% less likely to hit Queensland, while island nations like Niue, French Polynesia, Tonga, Tuvalu, and the Cook Islands face a higher risk.

Studies show that El Niño events in the equatorial Pacific are often followed by warmer temperatures in the tropical North Atlantic during the next spring and summer. About half of El Niño events last long enough for the Western Hemisphere Warm Pool to become unusually large in summer. Sometimes, El Niño strengthens easterly trade winds in the western equatorial Atlantic, causing cooler temperatures in the eastern equatorial Atlantic during spring and summer after El Niño peaks in winter. Events where El Niño occurs in both the Pacific and Atlantic Oceans at the same time have been linked to severe famines caused by long-term failures in monsoon rains.

Impacts on humans and ecosystems

When El Niño conditions last for many months, large amounts of ocean warming and weaker easterly trade winds reduce the movement of cold, nutrient-rich deep water to the surface. This can seriously affect local fishing industries that supply international markets. Countries that rely heavily on agriculture and fishing, especially those near the Pacific Ocean, are often most impacted by El Niño. During this phase of the climate cycle, the warm water pool in the Pacific near South America is usually warmest in late December. The 2023 El Niño disrupted weather patterns worldwide, causing droughts that reduced maize production by up to 70% in parts of Zimbabwe, Zambia, and Malawi. These reduced harvests led some countries to declare states of disaster and request help from other nations to avoid widespread hunger. At the same time, El Niño brought heavy rainfall to East Africa, causing floods that damaged crops and infrastructure in areas still recovering from years of drought.

El Niño can also influence the prices of goods and the overall economies of different countries. It may reduce the supply of crops that depend on rain, lower agricultural production, construction, and services, raise food prices, and increase the risk of social unrest in poor countries that rely on imported food. A study from the University of Cambridge found that countries like Australia, Chile, Indonesia, India, Japan, New Zealand, and South Africa often see a short-term drop in economic activity during El Niño events. However, other countries, such as Argentina, Canada, Mexico, and the United States, may benefit from El Niño through direct or indirect effects, such as trade with other nations. Most countries also experience short-term increases in inflation and higher prices for global energy and non-fuel resources after El Niño events.

The International Monetary Fund (IMF) has linked El Niño events to economic changes. In one year, El Niño was associated with a -1.01% drop in Indonesia’s real GDP growth, a -0.72% drop in South Africa’s growth, a +0.5% increase in the United States, a +1.57% increase in Mexico, and a +1.81% increase in Thailand.

Extreme weather linked to El Niño is connected to changes in disease outbreaks. For example, El Niño increases the risk of mosquito-borne diseases like malaria, dengue fever, and Rift Valley fever. Malaria cycles in countries such as India, Venezuela, Brazil, and Colombia have been linked to El Niño. Outbreaks of another mosquito-borne disease, Murray Valley encephalitis (MVE), occur in southeast Australia after heavy rainfall and flooding, which are often linked to La Niña events. A major Rift Valley fever outbreak happened in northeastern Kenya and southern Somalia during the 1997–98 El Niño.

El Niño conditions have also been connected to increases in Kawasaki disease cases in Japan and the west coast of the United States, due to changes in wind patterns over the Pacific Ocean.

El Niño may also be linked to civil conflicts. Research from The Earth Institute at Columbia University found that El Niño events may have contributed to 21% of all civil conflicts since 1950. The risk of civil conflict doubles from 3% to 6% in countries affected by El Niño compared to years influenced by La Niña.

During the 1982–83, 1997–98, and 2015–16 El Niño events, large areas of tropical forests experienced long dry periods that caused widespread fires and changed forest structures and tree species in the Amazon and Borneo regions. These effects are not limited to plants, as insect populations declined after extreme drought and fires during the 2015–16 El Niño. In burned Amazonian forests, declines were observed in bird species that depend on specific habitats and large mammals that eat fruits. In Borneo, more than 100 lowland butterfly species temporarily disappeared from a burned forest area.

In seasonally dry tropical forests, which are more resistant to drought, researchers found that El Niño-induced drought increased seedling deaths. A 2022 study in Thailand’s Chiang Mai National Park observed that El Niño increased seedling mortality even in these forests, potentially affecting entire ecosystems over time.

The Pacific Marine Environmental Laboratory linked the first large-scale coral bleaching event in 1997–1998 to the warm ocean temperatures caused by the El Niño event, possibly combined with human-caused climate change.

Major global coral bleaching events occurred in 1997–98 and 2015–16, with 75–99% of live coral lost worldwide. Attention was also drawn to the collapse of anchovy populations in Peru and Chile, which caused severe fishery crises after El Niño events in 1972–73, 1982–83, 1997–98, and 2015–16. In 1982–83, rising ocean temperatures likely caused the extinction of two hydrocoral species in Panama and the death of kelp beds along 600 km of Chile’s coastline. Kelp and related biodiversity slowly recovered in some areas even after 20 years. These findings show that El Niño events are a powerful force driving ecological changes globally, especially in tropical forests and coral reef ecosystems.

Impacts by region

Since 1950, scientists have observed that the effects of El Niño and La Niña events depend on the time of year. While some changes are expected during these events, it is not certain whether they will occur. Most El Niño events bring below-average rainfall to Indonesia and northern South America, and above-average rainfall to southeastern South America, eastern equatorial Africa, and the southern United States. La Niña events typically cause wetter-than-normal conditions in southern Africa from December to February and drier-than-normal conditions in equatorial east Africa during the same period.

El Niño affects rainfall in southern Africa differently in summer and winter rainfall areas. Winter rainfall areas usually receive more rain than normal, while summer rainfall areas receive less. The impact on summer rainfall areas is stronger and has led to severe droughts during strong El Niño events.

Sea surface temperatures near the west and south coasts of South Africa are influenced by El Niño and La Niña through changes in wind strength. During El Niño, south-easterly winds that bring cold water to the surface are weaker, leading to warmer coastal waters. During La Niña, these winds are stronger, causing colder coastal waters. These wind changes affect large-scale weather patterns, including the tropical Atlantic and the South Atlantic High-pressure system, as well as westerly winds further south. Other factors not related to El Niño and La Niña also influence these changes, and some El Niño or La Niña events do not produce the expected effects.

In the high southern latitudes near Antarctica, El Niño causes high-pressure areas over the Amundsen and Bellingshausen Seas, leading to reduced sea ice and increased heat transfer in these regions and the Ross Sea. The Weddell Sea tends to become colder and have more sea ice during El Niño. The opposite occurs during La Niña. This pattern is called the Antarctic dipole mode, though not all areas of Antarctica respond the same way to El Niño and La Niña.

In Western Asia, during the November–April rainy season, El Niño increases precipitation, while La Niña decreases it. During El Niño years, warm water moves eastward from the western Pacific and Indian Ocean, bringing rain to the eastern Pacific and causing drought in the western Pacific. For example, Singapore had its driest February in 2010 since records began, with only 6.3 mm of rain. The years 1968 and 2005 had the next driest February months, with 8.4 mm of rain.

During La Niña years, tropical cyclones and weather patterns shift westward across the Pacific, increasing the risk of storms hitting China. In March 2008, La Niña caused sea surface temperatures in Southeast Asia to drop by 2°C. It also brought heavy rains to the Philippines, Indonesia, and Malaysia.

Across most of the world, El Niño and La Niña have a greater impact on climate than any other factor. Stronger La Niña events are linked to greater changes in rainfall, with larger differences in sea surface temperatures and atmospheric pressure leading to more significant rainfall changes.

In Australia, El Niño can reduce rainfall, especially in the south, where warmer temperatures and more mobile weather systems lead to drier conditions. The start of the Indo-Australian Monsoon in northern Australia is often delayed, reducing rainfall in the tropics. El Niño increases the risk of severe bushfires in southeastern Australia, especially when combined with a positive Indian Ocean Dipole event. La Niña brings above-average rainfall to Australia’s eastern coast, often causing floods due to stronger winds from the Pacific. El Niño weakens these winds, reducing moisture and increasing fire risk.

El Niño and La Niña events in Australia are major climate drivers. Since 1900, there have been 28 El Niño and 19 La Niña events, including the current 2023 El Niño, declared in September 2023. These events typically last 9 to 12 months but can last up to two years. The ENSO cycle usually spans one to eight years.

In the United States, El Niño impacts are most noticeable between October and March. During this time, the Gulf Coast from Texas to Florida often sees wetter-than-average conditions, while Hawaii, the Ohio Valley, Pacific Northwest, and Rocky Mountains experience drier conditions. La Niña brings opposite effects, with above-average rainfall to the northern Midwest, northern Rockies, Northern California, and parts of the Pacific Northwest, and below-average rainfall to the southwestern and southeastern United States. La Niña also increases hurricane activity in the Atlantic and reduces it in the Pacific.

In Europe, the effects of El Niño are complex and hard to predict, as other weather factors can override its influence. In Canada, La Niña typically brings colder, snowier winters, such as the record-breaking snowfall in eastern Canada during the 2007–2008 La Niña winter. In the spring of 2022, La Niña caused above-average rainfall and below-average temperatures in Oregon, with April being one of the wettest months on record.

In Puerto Rico, El Niño increases snowfall in the southern Rockies and Sierra Nevada, while reducing it in the Upper Midwest and Great Lakes. La Niña increases snowfall in the Pacific Northwest and western Great Lakes.

Studies of recent weather in California and the southwestern United States show that the relationship between El Niño and rainfall is not always consistent. The strength of the El Niño event and other factors play a major role in determining rainfall outcomes. While El Niño has historically been linked to heavy rainfall in California, its effects depend on these additional conditions.

History

Strong evidence shows El Niño events happened around 10,000 years ago during the early Holocene epoch. Ancient climate records show different types of El Niño-like events, which were influenced by the Earth's geological, atmospheric, and ocean conditions at the time. These records help scientists understand how to protect the environment today.

Scientists found chemical signs in coral from about 13,000 years ago that show warmer ocean temperatures and more rainfall caused by El Niño.

A 2024 study suggests El Niño events strongly affected Earth’s climate during the Permian-Triassic extinction event. Stronger and longer El Niño events were linked to volcanic activity, which caused plants to die, increased carbon dioxide in the air, and disrupted weather patterns.

El Niño events have occurred every 2 to 7 years for at least 300 years, but most were weak.

El Niño may have caused the decline of the Moche culture and other ancient Peruvian societies around 700 AD. In 1525, Francisco Pizarro noted unusual rainfall in the Peruvian desert, the first written record of El Niño’s effects. A study suggests a strong El Niño between 1789 and 1793 led to poor harvests in Europe, which contributed to the French Revolution. Extreme weather from El Niño in 1876–77 caused deadly famines, with up to 13 million people dying in northern China alone.

People have long been interested in El Niño because of its impact on industries like guano production, which rely on ocean life. In 1822, a French mapmaker named Joseph Lartigue noted a "counter-current" along the Peruvian coast that helped ships travel south.

In 1888, Charles Todd observed that droughts in India and Australia often happened at the same time. This pattern was also noted by Norman Lockyer in 1904. Victor Eguiguren and Federico Alfonso Pezet reported El Niño-related flooding in 1894 and 1895, respectively. In 1924, Gilbert Walker introduced the term "Southern Oscillation," and he and others, like Jacob Bjerknes, are credited with identifying the El Niño effect.

The major El Niño event of 1982–83 increased scientific interest in the phenomenon. From 1991 to 1994, El Niño events occurred unusually quickly. A very strong El Niño in 1998 caused about 16% of the world’s coral reefs to die. This event temporarily raised air temperatures by 1.5°C, much higher than the usual 0.25°C increase linked to El Niño. Since then, widespread coral bleaching has become common globally, with all regions experiencing severe bleaching.

Related patterns

The Madden–Julian oscillation (MJO) is the biggest cause of changes in the tropical atmosphere that happen every 30 to 90 days. It was discovered in 1971 by Roland Madden and Paul Julian of the American National Center for Atmospheric Research (NCAR). It connects large-scale air movement with deep rain clouds in the tropics. Unlike the El Niño–Southern Oscillation (ENSO), which is a fixed pattern, the MJO moves eastward through the atmosphere above the warm parts of the Indian and Pacific oceans at about 4 to 8 meters per second (14 to 29 kilometers per hour; 9 to 18 miles per hour). This movement often causes unusual rainfall.

The MJO’s activity changes a lot from year to year. Sometimes it is very active, and other times it is weak or not present. These changes are partly connected to ENSO. In the Pacific, strong MJO activity often happens 6 to 12 months before an El Niño event begins, but it is rare during the strongest parts of some El Niño events. MJO activity is usually stronger during La Niña events. Strong MJO events over several months in the western Pacific can speed up the development of El Niño or La Niña but usually do not start these events. For example, the 1982–1983 El Niño began quickly in July 1982 because of a wave caused by an MJO event in late May. The MJO’s structure changes with the seasons and ENSO, which can increase its influence on ENSO. For instance, strong winds linked to active MJO rain are more common when El Niño is forming, and strong opposite winds are more common when La Niña is forming. Globally, the MJO’s yearly changes are mostly caused by air movement patterns, not by conditions on Earth’s surface.

The Pacific decadal oscillation (PDO) is a strong and repeating pattern of ocean and air changes over the mid-latitude Pacific Ocean. It is seen as warm or cool surface water in the Pacific Ocean north of 20°N. Over the past 100 years, this pattern has changed in strength over time periods from a few years to several decades. Evidence shows that the pattern reversed direction around 1925, 1947, and 1977. The last two reversals were linked to major changes in salmon populations in the North Pacific Ocean. This pattern also affects coastal ocean temperatures and air temperatures from Alaska to California.

ENSO can affect weather patterns thousands of kilometers away from the equatorial Pacific through the "atmospheric bridge." During El Niño events, rain and heat transfer to the atmosphere increase over unusually warm ocean areas. This tropical change creates Rossby waves that move toward the poles and east, then return toward the tropics. These large waves form in the North and South Pacific, and their effects appear within 2 to 6 weeks. ENSO patterns change surface temperatures, humidity, wind, and cloud distribution in the North Pacific, which alters heat, wind, and water movement, leading to changes in ocean surface temperatures, salt levels, and the depth of the ocean’s mixed layer.

The PMM is not the same as ENSO, but evidence shows PMM events can start ENSO events, especially Central Pacific El Niño events. The PMM can also affect hurricane activity in the East Pacific and typhoon activity in the West Pacific, as well as rainfall on land near the Pacific Ocean. The South Pacific has a similar pattern called the "South Pacific Meridional Mode" (SPMM), which also influences ENSO.

In the early 2000s, the strength of the 2014–16 El Niño and the active 2018 hurricane and typhoon seasons were linked to strong PMM events. With human-caused global warming, PMM activity is likely to increase. Some scientists suggest that loss of sea ice in Antarctica and the Arctic may cause future strong PMM events.