ENSO (El Niño–Southern Oscillation)

What is ENSO?

ENSO stands for the El Niño–Southern Oscillation — a recurring climate pattern in the tropical Pacific Ocean that couples ocean temperatures with atmospheric pressure and wind patterns. ENSO has three phases:

1. El Niño — Warm phase: central and eastern tropical Pacific is warmer than average.
2. La Niña — Cool phase: central and eastern tropical Pacific is cooler than average.
3. Neutral — Neither warm nor cool; conditions are close to long-term average.

ENSO is the single largest source of year-to-year climate variability on Earth, ranking second only to the seasonal cycle itself in influence on weather. A single ENSO cycle from El Niño through neutral to La Niña typically lasts 3–7 years, though there is huge variability — some phases last under a year, others persist for 2–3.

For South Asia, ENSO is the most consequential climate signal after the monsoon itself. ENSO state strongly influences:

• Indian monsoon rainfall
• Bay of Bengal cyclone frequency and intensity
• Pre-monsoon heatwaves
• Winter rainfall (western disturbances)
• Pakistan and Bangladesh flooding risk

How ENSO works

ENSO operates as a coupled ocean-atmosphere system. Two principal components:

Ocean (the “El Niño” part):

• Warm surface waters in the tropical Pacific shift east-west on multi-year timescales.
• During El Niño, warm water pools in the central and eastern Pacific.
• During La Niña, warm water pools in the western Pacific (Indonesia, Philippines).

Atmosphere (the “Southern Oscillation” part):

• The Walker Circulation — a giant convective loop with rising air over Indonesia and sinking air over the eastern Pacific — varies in strength.
• During El Niño, the Walker Circulation weakens or reverses.
• During La Niña, it strengthens.

The coupling: ocean SST patterns drive atmospheric circulation, which drives ocean currents, which drive SST patterns. The feedback loop allows the system to persist in each phase for months to years.

How ENSO is measured

Several indices track ENSO state:

Oceanic Niño Index (ONI) — the most widely used. A 3-month running mean of SST anomalies in the Niño 3.4 region (5°N–5°S, 120°W–170°W).

• ONI ≥ +0.5°C = El Niño conditions
• ONI ≤ −0.5°C = La Niña conditions
• −0.5°C < ONI < +0.5°C = Neutral

NOAA officially declares El Niño or La Niña when ONI exceeds the threshold for 5+ consecutive overlapping 3-month periods.

Southern Oscillation Index (SOI) — based on sea-level pressure difference between Tahiti and Darwin (Australia). Historical record-keeping; less commonly used in modern forecasts.

Multivariate ENSO Index (MEI) — combines SST, surface pressure, wind, cloudiness, sea-level pressure. Captures the full coupled system.

ENSO and the Indian monsoon

The statistical relationship between ENSO and Indian monsoon is well established:

El Niño years:

• About 60% of El Niño years see deficient monsoon (10%+ below normal).
• Examples: 2002 (19% deficit), 2009 (23% deficit), 2015 (14% deficit), 2023 (6% deficit).
• Counter-example: 1997, one of the strongest El Niños on record, had a NEAR-NORMAL Indian monsoon — proving the relationship is not deterministic.

La Niña years:

• Often see a surplus monsoon, but not always.
• Examples: 1988 (19% surplus), 2010 (2% surplus), 2020 (9% surplus), 2022 (6% surplus).

Neutral years:

• Roughly equal mix of normal, above-normal and below-normal monsoons.

Why isn’t it deterministic? Other climate drivers — the Indian Ocean Dipole (IOD), Madden-Julian Oscillation (MJO), Eurasian winter snow cover — can offset or amplify ENSO’s effect. A strong positive IOD in 2019 rescued the Indian monsoon despite a weak El Niño.

ENSO 2023-26 cycle

The Pacific entered El Niño in mid-2023 and the event peaked in late 2023 / early 2024 as one of the strongest on record (ONI peak ≈ +2°C). It ended in mid-2024.

A La Niña then emerged in December 2024 and gave way to ENSO-neutral in March 2025; the Pacific returned to weak La Niña in September 2025, and it lasted into early 2026. NOAA’s Climate Prediction Center (CPC) reported ENSO-neutral conditions by April 2026 and El Niño conditions by June 2026; in October 2026 the El Niño was still strengthening.

For the 2026 Indian monsoon, IMD’s April forecast was below normal, 92% of the long period average (LPA) ± 5%, and the season ended at 87% of LPA.

ENSO and tropical cyclones

ENSO affects cyclone basins differently:

Bay of Bengal & Arabian Sea (North Indian Ocean):

• La Niña: more cyclone activity, higher intensification.
• El Niño: less activity overall, but counter-examples exist.

Northwest Pacific (typhoons):

• La Niña: fewer typhoons but more landfalls along Philippines/China.
• El Niño: more typhoons in central Pacific, fewer landfalls.

Atlantic (hurricanes):

• La Niña: more active hurricane season.
• El Niño: suppressed hurricane activity (more wind shear).

How to track ENSO

For South Asians planning farm operations, water management, energy procurement or infrastructure projects, ENSO forecasts matter. Key resources:

• NOAA Climate Prediction Center — weekly ENSO discussion.
• Australian Bureau of Meteorology (BoM) — ENSO outlook with model ensemble.
• Japan Meteorological Agency (JMA) — ENSO monitoring.
• India Meteorological Department (IMD) — incorporates ENSO into monsoon forecasts.
• IRI Columbia — ENSO probabilistic forecast plumes.

Modern ENSO forecasts at 3-6 month lead times now have considerable skill, though forecasting through the “spring predictability barrier” (March-May) remains difficult.

Frequently asked questions

What is ENSO in simple terms? ENSO is a natural climate cycle in the Pacific Ocean that swings between warm (El Niño), cool (La Niña), and neutral phases. The phase affects weather patterns worldwide, including the Indian monsoon, Bay of Bengal cyclones and heatwaves in South Asia.

How often does El Niño or La Niña happen? Each phase recurs every 2–7 years on average. El Niño is slightly more frequent than La Niña. A typical event lasts 9–12 months, but some persist 18–24 months. Three-year “triple dip” La Niña events (like 2020-23) are rare.

Is ENSO becoming stronger due to climate change? The IPCC AR6 finds no climate model consensus on a change in ENSO-related sea surface temperatures over the coming century, but judges it very likely that rainfall variability linked to El Niño and La Niña over the east-central tropical Pacific will increase under intermediate to very high emissions. Importantly, ENSO impacts now occur on top of underlying warming — making heatwaves and rainfall events more extreme.

Why is ENSO important for India and Pakistan? ENSO state is the single most important predictor of seasonal rainfall used by IMD and PMD. El Niño years carry drought risk; La Niña years carry flood risk. Both affect agriculture, water security, hydropower and food prices.

Where can I see how ENSO is affecting my city? Mausam Online shows live weather and AQI on every city page. For seasonal context, see our Monsoon India 2026 guide (Hindi). Live forecasts: Mumbai, Delhi, Kolkata, Chennai, Dhaka, Karachi.

Related terms

• El Niño
• La Niña
• Monsoon
• Indian Ocean Dipole
• Drought