What is the El Niño-Southern Oscillation?

How Does the El Niño-Southern Oscillation weather system work? How does it affect global weather? How do oceanographers distinguish El Niño from La Niña? Is the system affected by climate change? What are the economic effects? We provide all the answers.
Floods in Baton Rouge caused by strong El Niño events.
Floods in Baton Rouge, 2016 during a strong El Niño event. Photo: Dept of Agri. CC BY 2.0

The El Niño-Southern Oscillation (ENSO) is a natural weather cycle that affects winds, air pressure, temperatures and cloud formation across the tropical Pacific. Its meteorological impacts are felt worldwide, from India to Australia and from Africa to Canada.

Although not believed to be directly affected by climate change, recent studies show that unless we curb our use of fossil fuels, the effects of El Niño and La Niña are likely to intensify around the world.

The term “El Niño” is Spanish for “the boy”. It was coined originally centuries ago, when Peruvian fishermen named the weather system after the new-born Christ. “La Niña”, selected as El Niño’s opposite number, is Spanish for “the girl”. See also: How Do Oceans Influence Climate Change?

The El Niño-Southern Oscillation is one of several naturally occurring regional cycles that are responsible for much of the climate variability around the equator and in the southern hemisphere. Other cycles include the Indian Ocean Dipole (IOD), the travelling Madden-Julian Oscillation (MJO) in the Indo-Pacific and the Southern Annular Mode (SAM), or Antarctic oscillation, in the Southern Ocean around Antarctica.

Worth Knowing: What’s the Difference Between Climate & Weather?

How Does the El Niño-Southern Oscillation Work?

ENSO typically swings between three phases. 1 The most common phase, which ENSO is in for more than half the time, is the normal or neutral phase. The rest of the time it becomes either more active and enters a La Niña phase; or less active and enters an El Niño phase.

Did You know? Winds are named after their origin, not the direction in which they travel. So winds that blow from the west are called westerlies, while those blowing from the east are known as easterlies.

ENSO in Neutral Phase

During its neutral phase, steady trade winds blow across the tropical Pacific from the Americas in the east, to Southeast Asia and Australia in the west. These prevailing easterly winds blow warm surface water towards Asia piling it up in the western Pacific. (Normally sea level off eastern Indonesia is around 40-50 centimeters (15-20 inches) higher than off the coast of Ecuador

At the same time, as surface water off the Americas is pushed westwards, cold water rises up from the deep to replace it. This increases the difference in sea surface temperature (SST) between the east and west Pacific.

It’s worth noting that the tropical Pacific (the area of the Pacific Ocean between the Tropic of Cancer and the Tropic of Capricorn) receives more sunlight than almost any other region in the world, and most of it is stored in the ocean as heat. So, during ENSO’s neutral phase, the waters off SE Asia and Australia are warmer and sea levels are higher than in the eastern Pacific off the Americas.

Warm ocean water warms the air overhead, while cooler water makes the air cooler. Thus, the SST difference between east and west causes the warmer air over SE Asia/Australia to rise.

This rising air sets up a pattern of atmospheric winds across the equatorial Pacific region, with warm wet air rising at one end of the Pacific and cooler dryer air descending at the other. This huge circular air flow, occurring in the lower atmosphere or troposphere, is known as the ‘Walker Circulation’. The phrase was coined in 1969 by the meteorologist Jacob Bjerknes (1897-1975), after the pioneering research of Sir Gilbert Thomas Walker (1868-1958).

The Walker circulation model of air flow during phases of El Niño-Southern Oscillation.
The Walker circulation is a conceptual model of air flow in the lower atmosphere over the equatorial Pacific. Driven by changes in ocean temperature, currents of air follow a closed circulation in a latitudinal and vertical direction. Differing sea surface temperatures are shown in orange (warmer) and blue/green (cooler). The top image shows the Walker Circulation during ENSO-neutral conditions. Convection currents of rising air become anchored in the western Pacific, northern South America, and eastern Africa. Sinking air returns off the Californian coast. The bottom image shows the Walker Circulation during an El Niño phase. Warmer ocean waters in the central and eastern Pacific shift the column of rising air to the east of 180° longitude, while columns of sinking air are shifted so they land on the western Pacific and northern South America. Image Credit: NOAA Climate.gov /drawings by Fiona Martin.

ENSO’s normal phase is associated with ‘normal’ weather patterns in SE Asia and Australia – including normal levels of rainfall – but even a neutral phase may very occasionally cause flooding or drought.

ENSO in La Niña Phase

When the El Niño-Southern Oscillation moves into a La Niña phase, it’s like a more active or supercharged neutral phase. The trade winds become stronger, causing even more warm water to flow from the eastern Pacific to the west. This increases the east to west difference in SST, and makes the Walker circulation even stronger. In turn, a stronger Walker cell causes the trade winds to blow even harder. 2

Higher ocean temperatures in the Western Pacific means greater evaporation, therefore more clouds, therefore more rain in SE Asia and Australia. In turn, this increases the risk of widespread flooding and lower temperatures. (See also: The Water Cycle Explained.)

It also means more unsettled weather, including more tropical cyclones. This is because the warmer water adds more heat to the air, which causes the air to rise more strongly and it’s this mass of rising water vapor that creates the unsettled weather.

Sea Surface Temperatures During La Niña

Chart shows cool sea surface temperatures in the Central and Eastern Pacific during a La Niña phase.
Chart shows abnormally cool sea surface temperatures in the Central and Eastern Pacific during a La Niña phase. Image Credit: U.S. National Oceanic and Atmospheric Administration (NOAA)
Walker Circulation showing air flow during La Niña phase of the El Niño-Southern Oscillation.
Walker Circulation during a La Niña phase. Differing sea surface temperatures shown in orange (warmer) and blue/green (cooler). Warm waters in the western Pacific cause air to rise. Cooling air falls in the eastern Pacific. Rising warm air is also seen over northern South America. Image Credit: NOAA Climate.gov /drawing by Fiona Martin.

ENSO in El Niño Phase

When the El Niño-Southern Oscillation moves into an El Niño phase, the easterly trade winds falter, or even go into reverse, allowing the piled-up warmer waters to drift back towards the Americas, under pressure of gravity. These warmer waters produce more evaporation and clouds, and more unsettled weather.

The shift in ocean temperatures and atmospheric pressure causes the Walker circulation to break down, further weakening the easterly trade winds, which leads to even warmer SSTs in the east. Once this feedback loop starts, the El Niño phase is established.

As warmer ocean water (and the humid air above it) moves eastwards, it typically leaves behind a drier western Pacific. This leads to more heatwaves and drought in northern and eastern Australia, more marine heatwaves off northeastern coasts, but fewer tropical storms. The same weather patterns occur across Southeast Asia.

Also, as weaker trade winds drive less surface water in the direction of Asia, less cool water is drawn up from the depths off the Americas. This reduction in nutritious deep water typically attracts fewer fish to the usual upwelling sites along the west coast of South and Central America, which is bad news for most fisheries who depend on them for the bulk of their annual catch.

During some of the most pronounced phases of El Niño, the collapse of fish stocks has caused famine and dramatic declines in the populations of Galapagos penguins, sea lions, and seals.

Less cold water rising to the surface in the eastern Pacific also means that the overall surface temperature of the ocean is higher during an El Niño phase, than during a neutral or a La Niña phase. Hence El Niño is seen as a warming influence while La Niña is seen as cooling, although ENSO can be unpredictable. 3

Sea temperatures chart during El Niño phase of the El Niño-Southern Oscillation weather cycle.
Chart shows abnormally warm sea surface temperatures in the Eastern Pacific during an El Niño phase. © Maulucioni (CC BY-SA 4.0)

How Do Scientists Distinguish El Niño from La Niña?

Scientists use what’s called the “Oceanic Niño Index” to monitor deviations from normal sea-surface temperatures (SSTs) produced by the oscillating El Niño and La Niña phases in the east-central Pacific Ocean. La Niña events are indicated by SST decreases of more than 0.5 degrees Celsius (0.9 degrees Fahrenheit) for at least five successive three-month seasons. This shows that the waters of the east-central tropical Pacific are markedly cooler than normal. Conversely, El Niño phases are indicated by SST increases of more than 0.5 degrees Celsius, showing that the SST is markedly warmer than usual. 4

How Long Does an El Niño or La Niña Episode Last?

La Niña phases can last between one and three years, unlike El Niño, which usually lasts no more than a year. Both of these climate phenomena reach their peak during the Northern Hemisphere winter. 5

A typical ENSO phase starts in the first six months of the year and lasts until the following autumn. Sometimes an El Niño/La Niña phase can last for two or more years in a row. Although their frequency can seem quite irregular, El Niño and La Niña episodes occur on average every two to seven years.

For at least half the time, ENSO remains in its neutral or normal phase.

What Causes El Niño and La Niña?

We know that wind, sea surface temperature, and atmospheric pressure are the three key meteorological elements that determine whether ENSO enters a neutral, El Niño or La Niña phase. However, what causes these elements to shift from neutral to El Niño, or from neutral to La Niña, or from El Niño to La Niña remains a mystery. In other words, the causes of ENSO are unknown. 6

Is the El Niño-Southern Oscillation Affected by Climate Change?

ENSO is not believed to be directly affected by global warming, but scientists say that ENSO’s effects on weather events in North America and elsewhere are likely to get stronger because of rising temperatures around the world. 7

Recently, a study led by scientists in the United States and China, looked at data from 33 previous El Niño episodes dating back to 1901. The study found that our climate crisis is increasing the frequency of extreme weather events during El Niño, leading to more severe droughts, more serious flooding, and worsening hurricane patterns in the Atlantic. Researchers say if rising temperatures remain unchecked, El Niños could get more severe, with significant adverse impacts on societies around the globe. 8

Neither of these studies are conclusive. But, given the current damage being done to our climate system by the vast quantities of greenhouse gas emissions that are still being pumped into the atmosphere due to the burning of fossil fuels like coal, petroleum and natural gas, it is surely only a matter of time before global warming starts to ramp up the effects of ENSO on weather patterns around the world.

But in a sense it’s irrelevant whether or not global warming alters the actual mechanism of ENSO. What’s really relevant is the outcome of future ENSO events. So the real question is: will the effects of the El Niño-Southern Oscillation be worse in the future, owing to climate change? To which the answer must be, Yes.

Think about it. At the beginning of the last century, we had El Niño but we didn’t have much global warming. (See: When did global warming start?) Now we have El Niño and a rapidly intensifying climate crisis on our hands. Outcomes are therefore bound to be worse. Here are some examples.

• If global warming has already made forests tinder-dry and primed to ignite, then any further drying caused by El Niño-induced heatwaves is bound to have a greater impact than if global warming didn’t exist.

• As ocean warming leads to more sea level rise, the storm surges created by El Niño will do more damage than if global warming didn’t exist.

• If global warming slows down the thermohaline circulation deep-water currents that are behind the upwelling of nutrient-rich water off the Americas, and then an El Niño comes along and reduces the upwelling even further, the combined effect of both the warming and ENSO must be greater than just ENSO alone.

For this reason, global warming is certain to boost the effects of the El Niño-Southern Oscillation. (See also: Effect of Global Warming on the Oceans.)

How Does the El Niño-Southern Oscillation Affect Global Weather?

The Pacific basin covers roughly one third of the surface of the Earth. When so much heat and moisture are moved around in the Pacific during episodes of the El Niño-Southern Oscillation, it has direct knock-on effects in the atmosphere and hydrosphere around the world.

Indirect impacts of ENSO are felt around the globe in patterns that climatologists refer to as “teleconnections.”

Diagram of El Niño and La Niña Jet Streams.
Images show the typical impacts of El Niño and La Niña on the Pacific jet stream and U.S. winter climate. NOAA Climate.gov drawing by Fiona Martin. 9

Effects of El Niño on Global Weather

North America

El Niño’s impacts are most pronounced in the wintertime over North America. A warmer Pacific Ocean leads to a southward shift in the atmospheric jet stream – the strong upper-level winds that blow from west to east, about 5-7 miles high. This causes wetter-than-average conditions along the Gulf Coast from Texas to Florida, with some flooding.

Usually, warmer, drier conditions prevail over the Pacific northwest, northern US states and Canada. Usually, but not always. In 2015-16, for example, El Niño brought landslides to Northern California, and heavy rain to the Pacific Northwest, the Rocky Mountains, the Midwest, and the Ohio Valley. In general, during an El Niño, western and central Canada have a milder than average winter and spring.

SE Asia

El Niño is not caused by climate change, although evidence suggests it is influenced or destabilized by it. Even so, El Niño often results in some of the hottest years on record, simply because of the massive amount of heat that rises out of the Pacific Ocean into the atmosphere. Extreme El Niño episodes in 1982-83, 1997-98, 2015-16 and 2019-20 have resulted in some of the worst droughts, forest fires, floods and extreme weather events of the past 50 years.

Damage created by El Niño-induced droughts is often aggravated by deforestation in Southeast Asia, caused by slash-and-burn methods. When ENSO is in its neutral phase, such fires are usually extinguished by the regular rainfall of the tropics. However, during an extremely dry El Niño, the fires can often get out of control.


As we know, in the aftermath of the catastrophic Australian bushfires (2019-2020), El Niño-induced droughts also result in more intense and more widespread wildfires. These blazes are capable of emitting vast amounts of carbon dioxide into the atmosphere, ramping up the greenhouse effect and adding to global warming. Not surprisingly, the rise in atmospheric CO2 from fires is more pronounced during El Niño years.

Central and South America

During El Niño years, Central America usually enjoys warmer, drier weather, while Brazil often experiences significantly elevated temperatures, with wetland areas of the Amazon Rainforest suffering from drought-like conditions. Further south, however, Argentina, Uruguay and Paraguay typically endure unusually heavy rainfall and flooding. This brings a higher risk of mosquito-borne diseases such as Chikungunya, Dengue, and Zika.

El Niño has stronger effects in South America than in North America. An El Niño phase leads to warm and very wet weather along the coasts of northern Peru and Ecuador, causing major flooding whenever the phase is extreme. From February to April, it significantly reduces the upwelling of cold, nutrient-rich water along the west coast of South America, leading to major losses among fish and bird populations. In April 2016, for example, some 8,000 tons of sardines washed up dead along the coast of Chile, a likely casualty of El Niño related changes in ocean water.

Effects of La Niña on Global Weather

Broadly speaking, La Niña causes the opposite effects of El Niño, although there are exceptions.


La Niña typically pushes the jet stream further north, which leads to reduced rainfall with drier, sunnier weather across the southern United States, and extremely dry conditions in the North American plains. At the same time, it usually produces colder weather across the Pacific Northwest, northern California, the northern Rockies, and the northern Midwest, with increased rainfall and flooding. In Canada, La Niña typically leads to a cooler winter with higher snowfall.

La Niña is thought to have been responsible for the great Dust Bowl drought of the 1930s, and was almost certainly a major contributor to the 1988 drought in the American Midwest. 5


La Niña is associated with heavier than usual summer monsoon rains in South Asia, especially in northwest India and Bangladesh. It also tends to generate heavy rainfall over Malaysia, the Philippines, and Indonesia.


La Niña is characterized by low air pressure – therefore increased rainfall – over the western Pacific, caused by higher SSTs. The 2010 La Niña event is linked to one of the worst-ever floods in the history of Queensland, Australia. Ten thousand people were forced to leave their homes, while the financial cost of the disaster was estimated at US$2 billion.


La Niña is associated with heavier-than-usual rains in Southern Africa, and drier-than-normal conditions over equatorial East Africa over the same period.

South America

At the same time as it causes low air pressure over the western Pacific, La Niña causes higher-than-normal air pressure over the eastern Pacific. This leads to fewer clouds and therefore drier weather along the west coast of South America, including the coastal regions of Peru and Chile, and the pampas grasslands in Argentina. La Niña is associated with higher than normal rainfall in Brazil and also in the central Andes, which can sometimes cause serious flooding in Bolivia.

Economic Effects of El Niño and La Niña

The United Nations (UN) attributes 20,000 deaths and $36 billion in infrastructure damage to the El Niño of 1997-98. The UN has also stated that 60 million people across Africa, Asia, the Pacific, and Latin America required food assistance as a result of weather events caused by the 2015-16 El Niño. Meantime, the 1988-89 La Niña, one of the worst-ever La Niñas, caused an estimated $40 billion in damages in North America alone.

In addition, scientists have shown that an El Niño event, equivalent to a 1-degree Celsius deviation in sea–surface temperatures in the region of the equatorial Pacific, causes an annual growth reduction of between 1 and 2 percent. 10

But not all studies agree on the costs incurred by El Niño or La Niña.

To begin with, El Niño is considered to cause much more economic damage than La Niña. 11

Also, another report, based on the 2015 IMF Working Paper 15/89, “Fair Weather or Foul? The Macroeconomic Effects of El Niño-Southern Oscillation”, finds that El Niño has a large but highly varied economic impact across different regions, and is undecided whether or not the overall impact was negative. 12

That said, one only has to study the details of the catastrophic ecological damage caused to Australia by the 2019 El Niño – including the estimated loss of 1-3 billion animals – to see that the El Niño-Southern Oscillation can be absolutely devastating, whatever the IMF says.


  1. “What is the El Niño–Southern Oscillation (ENSO) in a nutshell?” []
  2. “The three phases of the El Niño–Southern Oscillation (ENSO).” Bureau of Meteorology. []
  3. “What are El Niño and La Niña?” []
  4. “Climate Variability: Oceanic Niño Index.” []
  5. “La Niña.” National Geographic [][]
  6. “El Niño.” NASA. []
  7. ENSO’s Changing Inﬂuence on Temperature, Precipitation, and Wildﬁre in a Warming Climate.” J. T. Fasullo et al. Geophysical Research Letters. 22 August 2018. []
  8. “Historical change of El Niño properties sheds light on future changes of extreme El Niño.” Bin Wang, et al. PNAS November 5, 2019 116 (45) 22512-22517. []
  9. “Changes in ENSO impacts in a warming world.” []
  10. “The El Niño Southern Oscillation and Economic Growth in the Developing World,” Working Papers 2017-11. Smith, Sarah C. & Ubilava, David, 2017. University of Sydney, School of Economics, revised May 2017. []
  11. “The Growth Effects of El Niño and La Niña: Local Weather Conditions Matter.” Cecile Couharde, Olivier Damette, Remi Generoso and Kamiar Mohaddes. PDF. Federal Reserve Bank of Dallas. November 10, 2019. []
  12. “El Niño Good Boy or Bad?” Finance & Development, March 2016, Vol. 53, No. 1. Paul Cashin et al. IMF. []
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