Every few years, the waters of the tropical Pacific Ocean warm anomalously and trigger a cascade of effects that alter the climate in regions thousands of kilometres away. This warming — and the atmospheric pattern that accompanies it — is called El Niño, one of the two extreme phases of the cycle known as ENSO (El Niño-Southern Oscillation). The opposite phase, with cooler-than-normal waters, is La Niña.
Understanding El Niño is far from academic: its consequences affect agriculture, fisheries, water management and the frequency of extreme weather events across all five continents. In this guide we explain how it works, what effects it has and why each episode matters even more in the context of global warming.
What is El Niño and how does the ENSO cycle work?
Under normal conditions (neutral phase), the trade winds blow from east to west along the equatorial Pacific. These winds push warm surface water westward (towards Indonesia and Australia), while cold, nutrient-rich water wells up from the deep off the South American coast (upwelling). The result is a sea-surface temperature difference of up to 8 °C between the two sides of the Pacific.
El Niño phase (warm)
Periodically, the trade winds weaken or even reverse. Without that push, the warm water mass accumulated in the western Pacific migrates toward the central and eastern ocean. Sea-surface temperatures (SST) in the Niño 3.4 region rise at least 0.5 °C above the long-term average for several consecutive months.
This warming is not just oceanic: the atmosphere responds immediately. Deep convection (large storm formation) shifts toward the central Pacific, which alters the position of the jet stream and, with it, rainfall and temperature patterns across half the globe.
La Niña phase (cold)
In La Niña the opposite occurs: trade winds strengthen, upwelling off South America intensifies, and SSTs drop below average. Convection concentrates in the western Pacific, and the global climate effects are broadly reversed compared with El Niño (though not always symmetrically).
Neutral phase
Between episodes, the Pacific may spend months or years in an intermediate state with no significant anomalies. The full El Niño → neutral → La Niña → neutral cycle typically lasts 2 to 7 years, although duration and intensity vary enormously.
How is El Niño measured?
- ONI (Oceanic Niño Index): 3-month running mean of SST anomaly in the Niño 3.4 region. An El Niño episode is declared when the ONI exceeds +0.5 °C for at least 5 consecutive overlapping seasons.
- SOI (Southern Oscillation Index): sea-level pressure difference between Tahiti and Darwin (Australia). Persistently negative values indicate El Niño.
- TAO/TRITON buoy array: a network of moored buoys across the equatorial Pacific measuring temperature, currents and winds in real time.
- Altimetric satellites: measure sea level, which is higher where the water is warmer (thermal expansion).
Global effects of El Niño by region
South America
- West coast (Peru, Ecuador): torrential rain, flooding, collapse of the anchovy fishery as cold upwelling disappears.
- Southern Brazil, Uruguay, northern Argentina: increased rainfall, especially in austral autumn-winter.
- Colombia, Venezuela: drought, reduced river flows, water stress.
North America
- Southern US (California, Texas, Florida): wetter winters with frequent storms.
- Northern US and Canada: warmer and drier winters than usual.
- Mexico: drought in the central-south, more rain in the north-west.
Europe and Mediterranean
- The impact on Europe is more subtle, but strong El Niño episodes tend to be associated with:
- Colder, drier winters in northern Europe.
- Rainier autumns-winters in the western Mediterranean (including southern and eastern Spain).
- Greater frequency of anticyclonic blocking patterns over the North Atlantic.
Asia and Oceania
- Australia: severe drought, increased bushfire risk, reduced wheat harvests.
- Indonesia, Philippines, Malaysia: drought, forest fires (especially in Borneo and Sumatra peatlands), poor air quality.
- India: weakened summer monsoon, reduced rainfall and agricultural stress.
Africa
- East Africa (Kenya, Tanzania, Somalia): increased rainfall, sometimes with severe flooding.
- Southern Africa (South Africa, Zimbabwe, Mozambique): drought, reduced maize harvests.
Notable historic episodes
| Episode | Intensity | Peak ONI | Key effects |
|---|---|---|---|
| 1982-83 | Very strong | +2.2 °C | Catastrophic flooding in Peru and Ecuador; drought in Australia and Indonesia |
| 1997-98 | Very strong | +2.4 °C | The most studied: 23,000 deaths, $35 bn losses; fires in Indonesia, flooding in California |
| 2015-16 | Very strong | +2.6 °C | Strongest by ONI; 2016 global temperature record; mass coral bleaching |
| 2023-24 | Strong | +2.0 °C | Contributed to 2024 global temperature record; heavy rain in California and southern Brazil |
El Niño and climate change
- El Niño episodes temporarily amplify global warming. Years with a strong El Niño tend to be the warmest on record (2016, 2024).
- Possible increase in extreme episodes: several climate models suggest that with more CO₂, very strong El Niño events could become more frequent.
- Impacts are compounded: El Niño effects (droughts, heatwaves, floods) are superimposed on an already warmer baseline climate, amplifying damage.
Prediction: can El Niño be anticipated?
Yes, with limitations. Current models can predict the onset of an episode 6-9 months in advance. The main reference centres are the CPC/IRI (NOAA, USA), BoM (Australia), ECMWF (Europe) and JMA (Japan).
Frequently asked questions
How long does an El Niño episode last?
Typically 9 to 12 months, although some very strong episodes can last up to 18 months.
Does El Niño cause more hurricanes?
In the Atlantic, El Niño tends to reduce hurricane activity (due to increased vertical wind shear). In the eastern Pacific, the opposite is true: more tropical cyclones.
Sources: NOAA Climate Prediction Center, World Meteorological Organization (WMO), Bureau of Meteorology (Australia), IPCC AR6.