El Niño–Southern Oscillation (ENSO)

El Niño–Southern Oscillation, or ENSO, is a natural climate "swing" in the tropical Pacific Ocean. Every few years, the sea surface in the central and eastern Pacific becomes unusually warm (this is El Niño) or unusually cool (this is La Niña). At the same time, the air pressure and winds above the ocean also change.

Because the ocean and the air change together, scientists treat them as one system and call it ENSO. It is the single biggest cause of year-to-year changes in weather around the world, including India's monsoon.

Why does it matter so much?

The tropical Pacific is the largest warm ocean on Earth. When its warmest water shifts position, the places where clouds and heavy rain form also shift. This disturbs wind patterns across the globe. So a change in the Pacific can bring drought to India and Australia, floods to Peru, and warmer winters to parts of North America. El Niño years also tend to set new global temperature records, because the ocean releases stored heat into the air.

What is "normal" in the Pacific?

To understand El Niño, first picture a normal year.

• Steady winds called the trade winds blow from east to west along the equator, from South America towards Indonesia and Australia.
• These winds push warm surface water westwards. A deep pool of very warm water (the "warm pool") collects near Indonesia.
• Near Peru and Ecuador, the surface water that was pushed away is replaced by cold water rising up from the deep. This rising is called upwelling. The cold, nutrient-rich water feeds huge numbers of fish.
• Warm water heats the air above it, so air rises over the western Pacific, forming clouds and heavy rain over Indonesia. The air then flows east high up, sinks over the cool eastern Pacific, and returns west along the surface as the trade winds. This loop of air is called the Walker Circulation.

What happens during El Niño?

1. The trade winds weaken, and sometimes even reverse.
2. The warm pool, no longer held in the west, spreads east across the Pacific.
3. Upwelling near South America weakens, so the eastern Pacific warms up.
4. Rising air and heavy rain move east, towards the central Pacific and South America.
5. Sinking, dry air now sits over Indonesia, Australia and often India. Rain there is reduced.

These steps feed each other. A warmer east weakens the winds further, and weaker winds warm the east further. This self-strengthening loop is called the Bjerknes feedback. You can think of it like a seesaw that, once tipped, keeps tipping further on its own until something stops it.

What happens during La Niña?

La Niña is roughly the opposite. The trade winds become stronger than normal. More warm water piles up in the west, upwelling near South America gets stronger, and the eastern Pacific becomes cooler than usual. Rain increases over Indonesia and Australia. For India, La Niña years usually bring a normal or good monsoon. The in-between state, when neither is present, is called ENSO-neutral.

Where did the idea come from?

• The name. Fishermen on the coast of Peru noticed that, in some years, the sea turned warm around Christmas and fish catches fell sharply. They called this warm current El Niño, Spanish for "the boy child", meaning the infant Jesus. La Niña means "the girl child".
• Gilbert Walker (1920s). Sir Gilbert Walker, head of the India Meteorological Department (IMD), was trying to predict India's monsoon after deadly famines. He found a giant see-saw in air pressure: when pressure is high over the eastern Pacific, it tends to be low over the Indian Ocean and Indonesia, and the other way round. He named this the Southern Oscillation. Today it is measured by the Southern Oscillation Index (SOI), which compares air pressure at Tahiti (central Pacific) and Darwin (northern Australia).
• Jacob Bjerknes (1969). For decades nobody linked Walker's pressure see-saw with the warm water off Peru. After the 1957–58 El Niño, scientists saw that the warm water stretched far into the middle of the Pacific. In 1969 the Norwegian-American meteorologist Jacob Bjerknes explained that the ocean warming and the pressure see-saw are two faces of one coupled system. He named the air loop the Walker Circulation in Walker's honour. From then on, scientists began studying the two together as "ENSO".

How is El Niño measured?

Scientists watch the sea surface temperature in boxes of the Pacific called the Niño regions (Niño 1+2, Niño 3, Niño 3.4 and Niño 4). The most used is Niño 3.4, which covers 5°N to 5°S and 120°W to 170°W in the central-eastern Pacific.

• An anomaly is how far the temperature is above or below its long-term normal.
• The USA's weather agency, NOAA, long used the Oceanic Niño Index (ONI): the 3-month average Niño 3.4 anomaly. An El Niño episode means the index stays at +0.5°C or more for at least five overlapping 3-month periods in a row. La Niña means −0.5°C or less for the same length.
• Strength is graded: weak (0.5 to 0.9°C), moderate (1.0 to 1.4°C), strong (1.5 to 1.9°C) and very strong (2.0°C or more).
• Since 1 February 2026, NOAA uses the Relative Oceanic Niño Index (RONI) as its official measure. RONI subtracts the average warming of the whole tropics from the Niño 3.4 value. This matters because the whole ocean is warming due to climate change, so the old index could make El Niño look stronger, or La Niña look weaker, than it really is.
• Different countries use slightly different thresholds. For example, the Australian Bureau of Meteorology uses its own rules, so one agency may declare an El Niño a few weeks before another.

How often and how long?

El Niño usually comes every 2 to 7 years and lasts about 9 to 12 months. It tends to start between March and June, reaches its peak between November and February, and fades in the following northern spring. The biggest effect on world temperature usually comes in the year after it begins. Major past El Niño events include 1982–83, 1997–98, 2015–16 and 2023–24.

How does ENSO affect India?

The link runs through the Walker Circulation. During El Niño, rising air shifts east to the central Pacific, and air sinks over the Indian region. Sinking air is dry and stops clouds from forming, so the southwest monsoon (June to September) often weakens.

• Of the 16 El Niño events since 1950, nine pulled India's monsoon rainfall below normal. So El Niño raises the risk of a weak monsoon, but it does not guarantee one.
• Drought years such as 2009 and 2015 were El Niño years. But in 1997, a very strong El Niño year, India still got normal rain. In 2023, an El Niño year, rainfall ended at 94.4% of the long-period average, close to normal. In both cases, a positive Indian Ocean Dipole helped.
• The IMD calls a season "normal" when rainfall is 96% to 104% of the long-period average (LPA), which is the average rainfall of a long base period.
• The effects reach the whole economy: lower kharif (summer crop) output, pressure on food prices and inflation, less water in reservoirs, more heatwaves, and higher demand for power.
• La Niña usually helps the monsoon, but it can also bring floods and a colder winter in north India.

Commonly confused concepts

• El Niño vs La Niña: El Niño = warm eastern Pacific, weak trade winds, often a weak Indian monsoon. La Niña = cool eastern Pacific, strong trade winds, often a good Indian monsoon.
• El Niño vs Southern Oscillation: El Niño is the ocean part (warm water). The Southern Oscillation is the air pressure part (the Tahiti–Darwin see-saw). ENSO is the combined system. A strongly negative SOI usually goes with El Niño; a positive SOI usually goes with La Niña.
• El Niño vs El Niño Modoki: In a normal (classic) El Niño, the strongest warming is in the eastern Pacific near South America. In El Niño Modoki, described by Karumuri Ashok, Toshio Yamagata and others in 2007, the warming is strongest in the central Pacific, with cooler water on both sides. "Modoki" is a Japanese word meaning "similar but different". Its effects on world weather are different from a classic El Niño.
• ENSO vs IOD: ENSO happens in the Pacific. The Indian Ocean Dipole is a separate east-west temperature swing in the Indian Ocean. The two can act together or against each other.
• Walker Circulation vs Hadley Circulation: The Walker Circulation is an east-west air loop along the equator. The Hadley Circulation is a north-south loop, where air rises near the equator and sinks near 30° latitude.

Issues, criticism and the way forward

• Prediction is hard in spring. Forecasts made before about May are less reliable, because the Pacific is in a sensitive, changing state at that time. Scientists call this the "spring predictability barrier".
• A changing link with the monsoon. A 1999 study by K. Krishna Kumar and others argued that the link between ENSO and the Indian monsoon had weakened in recent decades. Later studies suggest the link has been getting stronger again since about 2000. This shows the monsoon depends on many factors, not only ENSO.
• Climate change. Scientists are still debating whether global warming will make El Niño events more frequent or more intense. What is clear is that a strong El Niño on top of a warmer ocean pushes world temperatures to new highs. For example, the World Meteorological Organization (WMO), the UN's weather body, confirmed 2024 as the warmest year on record. It was about 1.55°C warmer than the 1850 to 1900 average. This made it likely the first calendar year above 1.5°C. Long-term warming plus the 2023–24 El Niño drove this record.
• Preparedness in India. Experts and official plans stress early action: contingency crop plans for each district, drought-tolerant seeds, water storage, timely release of food stocks, and heatwave action plans. India has improved its own forecasting through the Monsoon Mission and the IMD's dynamic forecast models, but local, district-level forecasts still need work.

Concepts to Know

• Trade winds: Steady winds that blow towards the equator from the east in the tropics. Over the Pacific they blow from South America towards Asia.
• Upwelling: Cold, deep sea water rising to the surface, usually because winds push the surface water away. It brings nutrients, so upwelling zones are rich in fish.
• Sea surface temperature (SST) anomaly: How much warmer or cooler the sea surface is compared with its long-term normal for that time of year. "+2°C" means 2°C warmer than normal.
• Thermocline: The layer in the ocean where warm surface water quickly changes to cold deep water. During El Niño this layer becomes deeper in the eastern Pacific, so cold water cannot easily rise.
• Long-period average (LPA): The average monsoon rainfall over a long base period. The IMD compares each year's rainfall with it to judge whether the monsoon was normal.
• Kharif crops: Crops sown at the start of the monsoon (June–July) and harvested around October, such as rice, cotton, soybean and pulses.