El Niño, La Niña, and Monsoon Systems for UPSC Prelims
1. Introduction to El Niño, La Niña, and Monsoons
El Niño and La Niña are phases of the El Niño-Southern Oscillation (ENSO), a climate phenomenon involving ocean-atmosphere interactions in the Pacific Ocean. They significantly alter global weather patterns, including monsoon systems, which are seasonal wind reversals bringing heavy rainfall to regions like South Asia. Understanding these phenomena is critical for UPSC Prelims, as they impact agriculture, water resources, and disaster management, especially in India.
Key Facts
- ENSO Phases: El Niño (warm phase), La Niña (cold phase), neutral
- Monsoon: Seasonal wind shift, critical for 70–90% of India’s rainfall
- Impact: Affects global climate, agriculture, and economies
- Significance: Drives weather variability, influences disasters
Important Facts for UPSC
- El Niño weakens India’s monsoon, causing droughts.
- La Niña strengthens India’s monsoon, leading to floods.
- Monsoons are vital for India’s agrarian economy, supporting 50% of its workforce.
2. El Niño: Mechanism and Characteristics
Overview
El Niño is the warm phase of ENSO, characterized by warmer-than-average sea surface temperatures (SSTs) in the central and eastern equatorial Pacific, weakening trade winds.
Mechanism
- Weakened Trade Winds: Reduced upwelling of cold water along South America’s coast.
- Warm Water Spread: Warm water moves eastward, altering atmospheric pressure (Southern Oscillation).
- Walker Circulation Weakens: Disrupts normal convection patterns, affecting global weather.
Characteristics
- Occurs every 2–7 years, lasts 9–12 months.
- Monitored via SST anomalies (>0.5°C above average).
- Associated with low-pressure systems in the eastern Pacific, high-pressure in the western Pacific.
Important Facts for UPSC
- El Niño is named “Christ Child” as it often peaks around December.
- The Southern Oscillation Index (SOI) measures ENSO pressure changes.
- Strong El Niño events (e.g., 1997–98) cause global climate disruptions.
3. Global and Regional Impacts of El Niño
Global Impacts
- Asia: Weak monsoons, droughts in India, Southeast Asia, Australia.
- South America: Heavy rainfall, flooding in Peru, Ecuador.
- North America: Wet winters in southern USA, mild winters in Canada.
- Africa: Droughts in southern Africa, flooding in eastern Africa.
Regional Impacts (India)
- Reduced monsoon rainfall (10–20% below average), causing droughts.
- Crop failures, water shortages, and economic losses.
- Increased temperatures, exacerbating heatwaves.
Examples
The 2015–16 El Niño caused severe droughts in India, affecting 330 million people.
Important Facts for UPSC
- El Niño reduces monsoon rainfall in India by weakening the ITCZ.
- Peru’s anchovy fisheries collapse during El Niño due to reduced upwelling.
- Global food prices rise during strong El Niño events.
4. La Niña: Mechanism and Characteristics
Overview
La Niña is the cold phase of ENSO, marked by cooler-than-average SSTs in the central and eastern Pacific, strengthening trade winds.
Mechanism
- Stronger Trade Winds: Enhanced upwelling of cold water along South America.
- Cold Water Spread: Cold water extends westward, altering pressure patterns.
- Walker Circulation Strengthens: Intensifies convection in the western Pacific.
Characteristics
- Occurs every 2–7 years, lasts 9–24 months.
- Monitored via SST anomalies (<0.5°C below average).
- High-pressure systems in the eastern Pacific, low-pressure in the western Pacific.
Important Facts for UPSC
- La Niña often follows El Niño, amplifying opposite weather patterns.
- Strong La Niña events (e.g., 2010–11) cause widespread flooding.
- La Niña enhances monsoon activity in South Asia.
5. Global and Regional Impacts of La Niña
Global Impacts
- Asia: Stronger monsoons, flooding in India, Southeast Asia, Australia.
- South America: Droughts in Peru, Chile.
- North America: Cold winters in northern USA, dry conditions in southern USA.
- Africa: Wet conditions in southern Africa, droughts in eastern Africa.
Regional Impacts (India)
- Increased monsoon rainfall (10–20% above average), causing floods.
- Improved crop yields but risks of waterlogging and infrastructure damage.
- Cooler temperatures in some regions.
Examples
The 2010–11 La Niña caused severe floods in Northeast India and Australia.
Important Facts for UPSC
- La Niña strengthens India’s monsoon by enhancing the ITCZ.
- Australia’s 2010–11 floods were among the worst due to La Niña.
- La Niña boosts Peru’s fisheries due to enhanced upwelling.
6. Monsoon Systems: Overview and Mechanisms
Overview
Monsoons are seasonal wind reversals driven by differential heating of land and sea, bringing heavy rainfall to tropical regions.
Mechanism
- Summer Monsoon: Low pressure over heated land (e.g., Northwest India) attracts moist ocean winds, causing rain.
- Winter Monsoon: High pressure over cooled land drives dry winds outward.
- ITCZ Shift: Northward movement in summer triggers monsoon onset.
Global Monsoon Regions
| Region |
Monsoon Type |
Characteristics |
| South Asia |
Southwest Monsoon |
June–September, heavy rainfall |
| East Asia |
East Asian Monsoon |
Mei-Yu rains in China, Japan |
| Australia |
Northwest Monsoon |
December–March, tropical rains |
| West Africa |
West African Monsoon |
June–September, Sahel rainfall |
Important Facts for UPSC
- Monsoons are driven by land-sea temperature contrasts and ITCZ shifts.
- South Asia’s monsoon is the strongest globally, affecting 1.5 billion people.
- Monsoon rains recharge groundwater and support agriculture.
7. Indian Monsoon and Its Variability
Overview
The Indian monsoon is a critical climatic feature, delivering 70–90% of annual rainfall between June and September.
Branches
- Arabian Sea Branch: Brings rain to Western Ghats, West India.
- Bay of Bengal Branch: Brings rain to Northeast, North India.
Stages
- Onset: June, starts in Kerala, progresses northward.
- Advance: Covers India by mid-July.
- Retreat: September–October, withdraws from Northwest India.
Variability
Monsoon rainfall varies due to ENSO, jet streams, and other factors, causing droughts or floods.
Important Facts for UPSC
- Normal monsoon rainfall in India is ~880 mm annually.
- The Western Ghats receive the highest monsoon rainfall (~2,500 mm).
- Monsoon breaks (dry spells) occur due to ITCZ shifts.
8. Influence of ENSO on Monsoons
El Niño Effects
- Weakens Walker Circulation, reducing monsoon winds.
- Shifts ITCZ southward, decreasing rainfall in India.
- Examples: 2002, 2009 droughts in India.
La Niña Effects
- Strengthens Walker Circulation, enhancing monsoon winds.
- Shifts ITCZ northward, increasing rainfall in India.
- Examples: 2010, 2020 floods in India.
Neutral Phase
Normal monsoon conditions, with variability driven by other factors like Indian Ocean Dipole (IOD).
Important Facts for UPSC
- 60% of El Niño years cause below-normal monsoon in India.
- La Niña years often lead to above-normal monsoon rainfall.
- ENSO impacts are monitored by India’s IMD for monsoon forecasts.
9. Other Factors Affecting Monsoons
Key Factors
- Indian Ocean Dipole (IOD): Positive IOD (warmer western Indian Ocean) strengthens monsoon; negative IOD weakens it.
- Jet Streams: Subtropical jet stream guides monsoon onset; Somali jet strengthens winds.
- Western Disturbances: Bring pre-monsoon rain to North India.
- Tibetan Plateau: Heating creates low pressure, attracting monsoon winds.
- Climate Change: Alters monsoon patterns, increasing extreme events.
Significance
These factors interact with ENSO, making monsoon prediction complex but critical for planning.
Important Facts for UPSC
- Positive IOD in 2019 boosted India’s monsoon despite a weak El Niño.
- Climate change increases monsoon variability, causing more floods and droughts.
- The Tibetan Plateau acts as a “heat engine” for the Indian monsoon.
10. Significance for Geography
Geographical Significance
El Niño, La Niña, and monsoons shape global and regional climate, creating diverse weather patterns and influencing ecosystems.
Environmental Significance
These phenomena drive rainfall distribution, affecting water resources, biodiversity, and disaster risks like floods and droughts.
Socioeconomic Significance
Monsoons and ENSO impact agriculture, food security, and economies, particularly in monsoon-dependent regions like India.
UPSC Relevance
This topic is critical for physical geography, climatology, and Indian geography, frequently tested in Prelims due to its socioeconomic implications.
Important Facts for UPSC
- India’s monsoon-dependent agriculture supports 15–18% of its GDP.
- ENSO forecasting aids in drought and flood preparedness.
- Monsoon variability affects India’s water management and energy sector.
UPSC Preparation Tips
- Understand El Niño and La Niña mechanisms (SSTs, trade winds, Walker Circulation).
- Memorize global and Indian impacts of ENSO phases (droughts, floods).
- Focus on Indian monsoon branches, stages, and variability factors.
- Link ENSO to monsoons and other factors (IOD, jet streams).
- Practice map-based questions on monsoon regions and ENSO-affected areas.
- Revise key terms (ITCZ, Walker Circulation, IOD, SOI) and examples.