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El Niño and La Niña – UPSC Geography Notes
Overview: El Niño and La Niña are major ocean-atmosphere phenomena occurring in the Pacific Ocean that influence global climate, monsoons, rainfall, droughts, cyclones, and ocean currents. Understanding these events is crucial for UPSC Geography, Environment, and Disaster Management.
Table of Contents
- 1. Introduction
- 2. Normal Conditions in the Pacific Ocean
- 3. What is El Niño?
- 4. Effects of El Niño
- 5. What is La Niña?
- 6. Effects of La Niña
- 7. ENSO (El Niño–Southern Oscillation)
- 8. Impact on Indian Monsoon
- 9. Global Effects
- 10. UPSC Quick Revision Notes
- 11. Previous Year Question
1. Introduction
El Niño and La Niña represent two opposite phases of the climate pattern known as the El Niño–Southern Oscillation (ENSO).
- El Niño: Abnormal warming of the central & eastern Pacific Ocean.
- La Niña: Abnormal cooling of the same region.
These changes in sea surface temperature disturb global wind patterns and rainfall distribution.
2. Normal Conditions in the Pacific Ocean
Under normal conditions:
- Trade winds blow from east to west (South America → Australia).
- Warm water piles up near Australia and Indonesia.
- Cold, nutrient-rich water rises near South America → Peru Current upwelling.
- Rainfall is high over Western Pacific and low over Eastern Pacific.
3. What is El Niño?
El Niño is the periodic warming of sea surface temperatures in the central and eastern Pacific Ocean.
- Trade winds weaken or even reverse.
- Warm water shifts from Indonesia towards South America.
- Upwelling near Peru stops → fisheries collapse.
Threshold: Sea Surface Temperature rises by **≥ 0.5°C** above normal for at least 3 months.
Causes of El Niño
- Weakening of Trade Winds
- Shift in atmospheric pressure (Southern Oscillation)
- Changes in Ocean Current patterns
4. Effects of El Niño
(A) Global Effects
- Drought in India, Australia, Indonesia, parts of Africa.
- Floods in South America (Peru, Ecuador).
- Warmer winters in North America.
- Weak monsoon circulation.
- Coral bleaching due to warmer oceans.
(B) Effects on Marine Life
- Upwelling slows → nutrient supply decreases.
- Fish population collapses (especially anchovies in Peru).
5. What is La Niña?
La Niña is the cooling phase of ENSO, opposite of El Niño.
- Stronger-than-normal Trade Winds.
- Warm water pushed further west (Indonesia/Australia).
- Stronger upwelling near South America → rich fisheries.
Threshold: Sea Surface Temperature **≥ 0.5°C below normal** for at least 3 months.
6. Effects of La Niña
(A) Global Effects
- Heavy rains and floods in Australia, Indonesia.
- More cyclones in the Western Pacific.
- Colder winters in North America.
- Drought in South America.
(B) Marine Effects
- Stronger upwelling → high fish productivity.
- Cooler water supports rich marine ecosystems.
7. ENSO (El Niño–Southern Oscillation)
ENSO is the combined ocean-atmosphere cycle involving interaction between:
- El Niño (warming)
- La Niña (cooling)
- Southern Oscillation (pressure changes)
The Southern Oscillation Index (SOI) measures pressure differences between:
- Tahiti (High pressure)
- Darwin, Australia (Low pressure)
Negative SOI → El Niño Positive SOI → La Niña
8. Impact on Indian Monsoon
(A) El Niño → Weak Monsoon
- Reduced rainfall.
- Higher chances of drought in India.
- Major drought years (e.g., 2002, 2009) linked to El Niño.
(B) La Niña → Strong Monsoon
- Higher-than-normal rainfall.
- Stronger monsoon winds.
- Flooding in some regions.
9. Global Effects (Summary)
| Phenomenon | Pacific Ocean Condition | Global Impact |
|---|---|---|
| El Niño | Warmer eastern Pacific | Droughts in India/Australia, floods in South America, fisheries collapse |
| La Niña | Colder eastern Pacific | Stronger monsoon, floods in SE Asia, drought in Peru/Chile |
10. UPSC Quick Revision Notes
La Niña → cooling of eastern Pacific.
ENSO = El Niño + La Niña + Southern Oscillation.
El Niño → weak monsoon, drought in India.
La Niña → strong monsoon, floods in SE Asia.
Normal → strong trade winds, high upwelling near Peru.
El Niño → weak trade winds; La Niña → strong trade winds.
SOI: Negative = El Niño, Positive = La Niña.
11. Previous Year Question
Q. El Niño is associated with:
(a) Increase in sea surface temperature in the eastern Pacific
(b) Strengthening of Trade Winds
(c) Heavy rainfall in India
(d) Strong upwelling near Peru
Answer: (a) Increase in sea surface temperature in the eastern Pacific
Related Keywords
El Niño UPSC Notes, La Niña, ENSO, Indian Monsoon, Pacific Ocean Currents, Climate Change UPSC, Oceanography Notes.
Sources: NCERT Class XI – Fundamentals of Physical Geography, IMD & NOAA ENSO Reports (simplified for UPSC), Goh Cheng Leong.
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