Jul 2, 2025
10
mins read
Background: El Niño and La Niña are opposite phases of the El Niño-Southern Oscillation(ENSO), a natural climate cycle in the tropical Pacific. Under normal conditions, steady easterly trade winds blow warm surface water westward across the Pacific, building a warm “pool” near Indonesia and causing heavy rainfall there. Cold, nutrient-rich water upwells along the South American coast, keeping the eastern Pacific cool. This east–west temperature and pressure contrast drives the Walker circulation, with rising air and low pressure in the west and high pressure in the east.
ENSO or El Niño-Southern Oscillation, is a recurring climate pattern involving fluctuating ocean temperatures in the tropical Pacific and changes in the overlying atmosphere. It's characterized by three phases: El Niño (a warming phase), La Niña (a cooling phase), and a neutral phase. These fluctuations significantly impact weather patterns worldwide and recur irregularly every 2–7 years.
El Nino was first recognized by Peruvian fishermen off the coast of Peru as the appearance of unusually warm water.
The Spanish immigrants called it El Nino, meaning “the little boy” in Spanish.
Walker Circulation: Under neutral ENSO, warm western Pacific waters (red in diagram below) create low pressure and convection, with cool upwelled waters in the east (blue) and descending air over South America. This loop (Walker cell) weakens or reverses in ENSO events.
El Nino (Warm Phase)
El Niño years are marked by weak trade winds and eastward drift of the warm Pacific “pool.” This flattens the thermocline and suppresses upwelling off South America. SSTs in the central/eastern Pacific become anomalously warm (>0.5°C above normal), which shifts convection eastward .
Easterly trade winds weaken, allowing warm water to move eastward.
The central/eastern Pacific warms, lowering air pressure there. Convection shifts east, and the Walker cell weakens or collapses.
El Niño often peaks around Dec–Jan (hence “Navidad”).
Monsoon impact: Historically, strong El Niños tend to suppress the Indian summer monsoon, causing below-normal rains and drought risk. For example, since 1950 India has seen 16 El Niño years, about half of which produced monsoon deficits. (Other factors like the Indian Ocean Dipole can modulate this effect.)
La Niña (Cool Phase)
La Niña is the opposite ENSO phase. During La Niña: trade winds are stronger, pushing the warm pool farther west toward Indonesia, and enhancing upwelling of cold water along the eastern Pacific. SST anomalies are negative in the central/eastern Pacific. Key points:
Stronger easterly winds intensify the Walker cell. Warm water piles up in the far western Pacific while the central Pacific cools.
The eastern Pacific experiences increased upwelling and higher pressure.
Indian monsoon: La Niña years generally produce normal or above-normal monsoon rainfall across India, which boosts agriculture. (Exception: some regions in NW/NE India can still get deficits.)
Oceanic Niño Index (ONI):
The Oceanic Niño Index (ONI), is a measure of the departure from normal sea surface temperature in the east-central Pacific Ocean, is the standard means by which each El Nino episode is determined, gauged, and forecast.
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Impact of El Nino
Background: Normal Pacific Conditions
Under non-El Niño conditions in the Pacific Ocean, strong trade winds blow westward, pushing warm surface water from South America toward Asia—driving the Walker circulation loop: rising air over the western Pacific and sinking over the east.
This normal state supports upwelling of cold, nutrient-rich waters off the coasts of Ecuador, Peru, and Chile, sustaining rich fisheries.
Oceanic Impacts
When El Niño develops, the trade winds weaken and warm water shifts eastward, disrupting normal ocean circulation.
The Oceanic Niño Index (ONI) registers this warming, which:
Alters sea-surface temperatures.
Disrupts ocean currents and nutrient cycles.
Devastates coastal fisheries by reducing upwelling.
Increased Rainfall & Flooding
Warm surface waters boost convection, leading to drastically increased precipitation in South America—often causing coastal flooding and erosion.
For example, the 1997–98 El Nino-Southern Oscillation UPSC event triggered severe floods in Peru alongside intense droughts in Asia and Australia.
Health Impacts: Floods & Droughts
Floods and droughts from ENSO events create fertile ground for cholera, dengue, malaria, and wildfires that worsen respiratory health.
Positive Effects
Interestingly, El Niño tends to reduce Atlantic hurricane frequency, due to altered wind shear over the western Atlantic.
Regional Contrasts
South America: wetter conditions and floods.
Indonesia & Australia: suffer from drought, threatening reservoirs, agriculture, and water security.
In the eastern Pacific, upwelling diminishes; the thermocline deepens, impacting fisheries and reducing dietary protein sources.
Impact of La Lina
Europe
Northern Europe (e.g., UK): La Niña often leads to milder winters due to a weakened polar vortex and a more zonal jet stream.
Southern/Western Europe: Experiences colder and snowier winters, with increased precipitation in regions like the Mediterranean.
North America
United States: La Niña conditions enhance vertical wind shear, suppressing Atlantic hurricane activity but increasing the frequency and intensity of hurricanes in the Caribbean and central Atlantic.
Tornadoes: The phenomenon contributes to a higher incidence of tornadoes across various states.
South America
Peru and Ecuador: La Niña leads to cooler sea surface temperatures, reducing upwelling and negatively impacting the fishing industry by dispersing anchovy populations.
Fishing Industry: While artisanal fisheries may adapt, industrial fishing faces challenges due to the displacement of key species.
Western Pacific & Asia
Australia: La Niña brings increased rainfall and cooler daytime temperatures, leading to more tropical cyclones and potential flooding.
Asia & China: Enhanced monsoon activity and increased cyclone frequency are observed, impacting agriculture and infrastructure.
Oceans & Climate
Sea Surface Temperatures: La Niña is characterized by cooler-than-normal sea surface temperatures in the central and eastern Pacific, influencing global weather patterns.
Indian Ocean & Somalia: Elevated temperatures in these regions can exacerbate drought conditions and affect local climates.
Aspect | Normal | El Niño Phase | La Niña Phase |
Walker Circulation | Strong east–west atmospheric loop intact | Weakened significantly; warm-Pacific ascent weakens trade winds | Strengthened, reinforcing upwelling |
Trade Winds | Strong easterlies (Pacific‑to‑Asia winds) | Weak or reversed (westerlies may appear) | Strengthened easterlies |
Ocean Upwelling | Cold, nutrient-rich water rises at surface | Suppressed or halted, limiting nutrient renewal | Enhanced upwelling, cooler SST |
Sea Surface Temps & Currents | Balanced Pacific distribution | Warm pools shift east; currents reorganize eastwards | Cold anomalies in the east; stronger thermocline gradient |
Regional Weather | Balanced global precipitation | Drier Asia/Australia; wetter Americas | Opposite pattern; wetter Asia, drier Americas |
Fisheries & Disease | Stable marine ecosystems, low disease | Fisheries decline; cholera, dengue outbreaks rise | Strong marine productivity |
Atlantic Hurricanes | Normal season frequency | Reduced frequency due to higher wind shear | More frequent/intense hurricane seasons |
El Nino Events
El Nino events of 1982-83 and 1997-98 were the most intense of the 20th century.
During the 1982-83 event, sea surface temperatures in the eastern tropical Pacific were 9-18° F above normal.
The El Nino event of 1997-98 was the first El Nino event to be scientifically monitored from beginning to end.
The 1997-98 event produced drought conditions in Indonesia, Malaysia, and the Philippines. Peru and California experienced very heavy rains and severe flooding.
The Midwest experienced record-breaking warm temperatures during a period known as “the year without a winter.
La Lina Events
The 2010 La Nina event correlates with one of the worst floods in the history of Queensland, Australia.
More than 10,000 people were forced to evacuate, and damage from the disaster was estimated at more than $2 billion.
Static and Current: UPSC aspirants must know ENSO or El Niño-Southern Oscillation fundamentals (definitions of El Niño, La Niña, ONI, Walker circulation) for Prelims and Geography papers.
Analytical (Mains): Understanding ENSO or El Niño-Southern Oscillation’s mechanism and its modulation of the Indian monsoon (e.g. why El Niño often causes drought) is vital for GS3 (Environment & Ecology) and GS1 (Geography). Examples (1982–83, 1997–98 El Niños) illustrate impacts on agriculture, economy and policy responses.
Current Affairs: Recent ENSO developments (IMD forecasts, 2024–25 neutral/La Niña conditions) often appear in Current Affairs. Linking ENSO to Indian initiatives (like agricultural advisories during monsoon) is important.
ENSO Phase | Pacific SST Pattern | Walker Circulation | Indian Monsoon (SW) Impact |
Normal | Warmest in West, cool East | Strong zonal Walker cell (west Pacific low pressure) | Normal rainfall distribution (baseline climatology) |
El Niño | Warm anomalies in central/east Pacific | Weakened/shifted Walker cell (warm pool drifts east) | Monsoon tends to weaken (often below-normal rains) |
La Niña | Cool anomalies in central/east Pacific | Strengthened Walker cell (warm pool stays west) | Monsoon tends to strengthen (often above-normal rains) |
ENSO (El Niño and La Niña) is a cornerstone topic for UPSC geography and environment. Aspiring candidates should focus on cause–effect linkages: how Pacific temperature shifts alter wind patterns (Walker circulation) and in turn modulate the Indian monsoon. Mastery of ENSO dynamics and indices (e.g. ONI) with UPSC-relevant examples ensures readiness for both Prelims and Mains.
Q. What are El Niño and La Niña UPSC phenomena?
A: These are opposite phases of the El Niño-Southern Oscillation, a recurring climate cycle in the tropical Pacific. El Niño represents the warm phase with weakened trade winds and eastward-moving warm water, while La Niña is the cool phase with enhanced trade winds and stronger upwelling.
Q. How do they affect the Walker circulation?
A: In El Niño, the Walker circulation weakens or collapses, reducing convection in the western Pacific. In contrast, La Niña strengthens the Walker cell by intensifying trade winds and boosting convection in the west
Q. What is the Oceanic Niño Index (ONI) used in ENSO tracking?
A: The ONI, central to El Niño-Southern Oscillation monitoring, measures 3-month average sea surface temperature anomalies in the eastern-central Pacific. Positive values indicate El Niño; negative ones indicated La Niña .
Q.What is the impact of El Niño on Indian monsoon UPSC?
A: El Niño weakens monsoon circulation, causing below-normal rainfall in India, droughts, crop losses, rising food inflation, and slower GDP growth .
Q. How does La Niña UPSC influence India?
A: During La Niña, enhanced trade winds and strengthened Walker circulation bring above-normal monsoon rains, boosting agricultural yields. It also induces colder winters through southward intrusion of cold air via a north–south trough
El Niño and La Niña, as the opposing phases of the ENSO or El Niño-Southern Oscillation cycle, play pivotal roles in global climate. El Niño is characterized by unusually warm Pacific waters and tends to suppress monsoons in India, while La Niña brings cooler Pacific waters and usually enhances monsoon rains. These ENSO phases can dramatically alter rainfall, temperatures, and storm activity around the world. For UPSC aspirants and policymakers alike, a clear understanding of El Niño and La Niña dynamics is essential. Accurate prediction of these events allows better preparation for their consequences – from agricultural planning to economic policy – and helps mitigate their adverse impacts on the environment and society
Internal Linking Suggestions
How to Begin Your UPSC Preparation : The Ultimate Guide For Beginners
UPSC Previous Year Question Papers with Answers PDF - Prelims & Mains (2014-2024)
Upsc optional subject list and syllabus-for-cse-exam-2025-complete-guide
How to Prepare Current Affairs for UPSC Exam: A Comprehensive Guide
51st G7 Summit 2025 – Countries, Key Issues, India’s Role & UPSC
External Linking Suggestions
UPSC Official Website – Syllabus & Notification: https://upsc.gov.in/
Press Information Bureau – Government Announcements: https://pib.gov.in/
NCERT Official Website – Standard Books for UPSC: https://ncert.nic.in/
