El Niño, La Niña & ENSO — Climate Variability, Monsoon Impact & UPSC

Introduction

The El Niño–Southern Oscillation (ENSO) is the most powerful year-to-year climate fluctuation on Earth. It involves coupled changes in sea surface temperatures (SST) in the tropical Pacific Ocean and the atmospheric pressure pattern known as the Southern Oscillation. ENSO significantly affects rainfall, agriculture, fisheries, and disaster risk across the globe — including India's all-important southwest monsoon. For UPSC, ENSO, the Indian Ocean Dipole (IOD), and their interaction with the Indian monsoon are high-priority topics in both Prelims and Mains GS-1.

Walker Circulation — The Normal State

Under normal (non-ENSO) conditions, a large atmospheric circulation cell operates along the equatorial Pacific:

• Trade winds blow from east to west across the tropical Pacific (from South America toward Asia-Australia).
• Warm water piles up in the western Pacific (near Indonesia and Australia), creating low pressure and rising air — forming a convection cell that brings heavy rainfall.
• The eastern Pacific (near Peru and Ecuador) is cool due to upwelling of cold deep water (Humboldt Current), creating high pressure and descending air — dry conditions.
• This east-west overturning cell is called the Walker Circulation, named after British meteorologist Sir Gilbert Walker who first systematically described the Southern Oscillation in the 1920s–30s.

Key effect on India: The western Pacific warm pool drives strong convection that energizes the Hadley Cell and the Asian summer monsoon. Normal Walker Circulation = favourable conditions for Indian monsoon rainfall.

Southern Oscillation Index (SOI)

The Southern Oscillation is the atmospheric (pressure) component of ENSO. It is measured by the Southern Oscillation Index (SOI):

SOI = Standardised pressure difference between Tahiti (central Pacific) and Darwin, Australia

A sustained negative SOI (below –8) for several months typically signals an El Niño event. A sustained positive SOI signals La Niña.

El Niño — The Warm Phase

El Niño (Spanish: "The Boy/Christ Child," as it appears near Christmas off Peru) refers to the abnormal warming of sea surface temperatures in the central and eastern tropical Pacific.

Conditions During El Niño

• Trade winds weaken or even reverse direction.
• Warm water from the western Pacific sloshes eastward toward South America.
• The thermocline (boundary between warm surface water and cold deep water) deepens in the east, suppressing cold-water upwelling off Peru.
• Convection shifts eastward — from the western Pacific toward the central Pacific.
• The Walker Circulation weakens or reverses.

Atmospheric Consequences

• Indonesia, Australia, South Asia: Reduced rainfall → droughts, poor monsoon.
• Western coast of South America (Peru, Ecuador): Intense rainfall → flooding, landslides.
• East Africa, southern Africa: Drier than normal.
• Southern USA, Peru: Wetter than normal.

Impact on Indian Monsoon

El Niño is the single biggest risk factor for Indian monsoon failure. The mechanism:

1. Weakened Walker Circulation reduces moisture convergence over the Indian Ocean.
2. Convection shifts away from the Bay of Bengal region.
3. The monsoon trough weakens, leading to below-normal rainfall.

Approximately 43% of El Niño events are associated with drought in India (per research published in Geophysical Research Letters, 2019). However, the relationship is not deterministic — a positive IOD (see below) can offset El Niño's negative impact.

La Niña — The Cool Phase

La Niña ("The Girl") is the opposite of El Niño — an abnormal cooling of SSTs in the central and eastern tropical Pacific.

Conditions During La Niña

• Trade winds strengthen unusually.
• More warm water is pushed toward the western Pacific.
• Cold upwelling intensifies off Peru — eastern Pacific becomes even cooler.
• Walker Circulation intensifies.
• Convection concentrates strongly in the western Pacific.

Atmospheric Consequences

• India, Bangladesh, Pakistan, Australia: Enhanced rainfall, floods.
• South America (Peru, Chile): Drier than normal.
• Southern USA: Drier and warmer.
• East Africa: Wetter than normal.

Impact on Indian Monsoon

La Niña generally favours above-normal Indian monsoon rainfall. The intensified Walker Circulation strengthens cross-equatorial flow and moisture supply to India. Many "excess monsoon" years in India coincide with La Niña conditions.

ENSO Events and Indian Agriculture — Historical Record

Key Insight for UPSC: The 1997–98 super El Niño did not cause a major drought in India because a strong positive IOD countered its negative effect. This demonstrates that ENSO alone does not determine Indian monsoon outcomes.

Indian Ocean Dipole (IOD)

The Indian Ocean Dipole is an irregular oscillation of sea surface temperatures in the Indian Ocean. It is defined by the SST difference between the western Indian Ocean (Arabian Sea, near Somalia/East Africa) and the eastern Indian Ocean (near Sumatra/Indonesia).

Positive IOD

• Western Indian Ocean warmer than the eastern Indian Ocean.
• Greater convection and rainfall over the Arabian Sea and East Africa.
• Indonesia and Australia become drier.
• Effect on Indian monsoon: Positive IOD generally enhances Indian summer monsoon rainfall by increasing moisture supply from the Arabian Sea branch of the monsoon.

Negative IOD

• Eastern Indian Ocean warmer than western Indian Ocean.
• Convection shifts toward Indonesia/Australia.
• Western Indian Ocean and Indian subcontinent receive less moisture.
• Effect on Indian monsoon: Negative IOD tends to suppress Indian summer monsoon rainfall.

IOD–ENSO Interaction

The IOD is sometimes called the "Indian Niño" because it exerts a regional influence analogous to ENSO in the Pacific.

Madden–Julian Oscillation (MJO)

The Madden-Julian Oscillation (MJO) is a large-scale intraseasonal (30–90 day) disturbance of clouds, rainfall, and winds that travels eastward from the Indian Ocean through the Pacific. Discovered in 1971 by Roland Madden and Paul Julian of NCAR.

Relevance to India:

• MJO's active phase over the Indian Ocean can trigger monsoon onset or active spells.
• MJO's suppressed phase over India can cause monsoon breaks.
• MJO also interacts with ENSO — strong MJO events can trigger or amplify El Niño events by pushing warm water eastward.

For UPSC, MJO matters as a sub-seasonal predictor of monsoon variability, distinct from the seasonal-scale influence of ENSO and IOD.

ENSO Monitoring Systems

Climate Change and ENSO

Current scientific consensus (IPCC AR6, 2021) indicates:

• ENSO itself will continue under climate change — it will not disappear.
• El Niño events may become more intense (higher SST anomalies) even if their frequency does not change significantly.
• Extreme precipitation events associated with ENSO (floods in South America, droughts in Southeast Asia) are projected to intensify.
• The IOD may also intensify, with more frequent positive IOD events, affecting Indian Ocean rainfall patterns.

Implication for India: More extreme monsoon variability — both excess and deficit years — is projected in coming decades.

ENSO Impacts Beyond Rainfall

Key Definitions Table

Exam Strategy

For Prelims:

• Memorise the SOI definition — Tahiti minus Darwin pressure anomaly.
• Know the direction of Walker Circulation under normal conditions.
• Positive IOD → enhanced Indian monsoon; Negative IOD → suppressed.
• El Niño → weak monsoon; La Niña → strong monsoon (but not always — IOD can override).
• MJO periodicity: 30–90 days (intraseasonal).
• ENSO threshold: ±0.5°C SST anomaly in Niño 3.4 region for 5 consecutive overlapping 3-month periods.

For Mains GS-1 (Answer Framework):

• Always link ENSO → Walker Circulation → Indian monsoon chain.
• Include IOD as a modifying factor — critical for nuanced analysis.
• Use historical examples (1877, 1997–98) to illustrate.
• Mention climate change dimension for contemporary relevance.
• Connect to food security, agriculture, and disaster management for interdisciplinary answers.

Mnemonics:

• El Niño = Eastward warm water = India Dries
• La Niña = Westward warm water = India Wets
• Positive IOD = Positive monsoon
• SOI = Tahiti − Darwin (T before D alphabetically = T minus D)

Recent Developments (2024–2026)

2024 Monsoon — El Niño Overcome by Positive IOD and La Niña Transition

Despite the presence of an El Niño condition in early 2024, India's Southwest Monsoon season (June–September 2024) delivered 934.8 mm of rainfall — 108% of the Long Period Average (LPA of 868.6 mm, based on 1971–2020 data). Central India received 119% and South Peninsula 114% of LPA, while Northeast India recorded a shortfall at 86% of LPA. The excess rainfall was attributed to a transition from El Niño to La Niña conditions and a positive Indian Ocean Dipole (IOD), which together countered El Niño's typical suppressing effect on Indian rainfall. India witnessed 2,632 instances of very-heavy rainfall and 473 instances of extremely-heavy rainfall — the highest in five years — causing widespread flooding in Assam, Andhra Pradesh, and Kerala.

UPSC angle: The 2024 monsoon is an important case study of how IOD and ENSO interact — demonstrating that El Niño alone does not guarantee a deficient monsoon. Examiners frequently ask for such recent examples in Mains GS1 answers.

2025 Monsoon — Above-Normal Rainfall Under Neutral ENSO Conditions

India's 2025 Southwest Monsoon (June–September) delivered 108% of the Long Period Average (LPA) — cumulative rainfall of 937.2 mm against the LPA of 868.6 mm — marking the second consecutive year of above-normal monsoon. Regional performance: Northwest India 127% of LPA, Central India 115%, South Peninsula 110%, Northeast India 80% (below-normal). Monthly breakdown: June 109% LPA, July 105%, August 105%, September 115%. The 2025 monsoon occurred under broadly neutral ENSO conditions — neither El Niño nor La Niña — with weak La Niña-like atmospheric circulation features in the pre-monsoon period. The IMD's April 2025 Long Range Forecast (106% LPA, ±4%) was accurately verified. The Northeast's consistent below-normal performance (both 2024 at 86% and 2025 at 80% of LPA) reflects structural factors beyond ENSO — including weakening of the Bay of Bengal branch and land-use change in the Indo-Bangladesh-Myanmar corridor.

UPSC angle: Prelims — 2025 SW Monsoon: 108% LPA, 937.2 mm, neutral ENSO; Northwest India 127% LPA; Northeast 80% LPA. Mains GS1 — how monsoon variability is not solely ENSO-driven; structural factors for Northeast deficit; implications of back-to-back above-normal monsoons for agriculture and floods.

Climate Change Modifying ENSO Behaviour — 2024 Research

Peer-reviewed research published in 2024 (including work by NASA GISS) confirmed that El Niño combined with a positive IOD simultaneously reduces the production of multiple cereals across India, especially wheat and rice. Separately, climate models project that under increasing global warming, extreme El Niño events (like the 1997–98 and 2015–16 events) will become more frequent — potentially doubling in occurrence by the late 21st century. The 2024 IPCC synthesis reaffirmed that ENSO variability is expected to intensify, increasing both the severity of El Niño droughts and La Niña floods across South Asia.

UPSC angle: Climate change–ENSO interaction and its implications for India's food security and disaster frequency is a high-value GS1 analytical question and GS3 climate governance theme.

Last updated: 1 May 2026