Monsoon marvels: Unravelling the science behind India’s seasonal rains
Monsoon system in India — UPSC explainer
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What is a monsoon?
The term “monsoon” is derived from the Arabic word “mausim”, meaning season. In meteorology, it refers to a seasonal reversal of winds accompanied by changes in precipitation, particularly over South and Southeast Asia.
A monsoon climate is characterised by wet summers and dry winters, caused by differential heating of land and sea.
What is the mechanism of formation of the Indian monsoon?
The Indian Monsoon is a complex climatic phenomenon resulting from several interrelated factors:
Mechanism of Indian monsoon formation:
1. Differential heating of land and sea
- During summer, the Indian subcontinent heats up faster than the surrounding oceans, creating a low-pressure area over northwest India.
- The Indian Ocean remains comparatively cooler, creating a high-pressure zone.
2. Shift of Inter-Tropical Convergence Zone (ITCZ)
- The ITCZ moves northward (near the Tropic of Cancer) in summer, pulling moist air masses toward the subcontinent.
3. Trade winds and Coriolis effect
- The southeast trade winds from the southern hemisphere cross the equator, get deflected to the right due to the Coriolis force and enter India as southwest monsoon winds.
4. Presence of Tibetan plateau and Jet streams
- The Tibetan plateau acts as an elevated heat source, further intensifying the low pressure.
- The Tropical Easterly Jet helps pull monsoonal winds into the Indian subcontinent.
5. Somali Jet and Subtropical Jet Stream withdrawal
- The Somali Jet, a strong low-level jet over the Arabian Sea, feeds moisture into the monsoon system.
- The retreat of the westerly subtropical jet stream is a precursor to monsoon onset.
What is the nature of the Indian monsoon?
The Indian monsoon is seasonal, pulsating and irregular in nature. It is influenced by multiple global and regional factors.
Features:
- Bimodal: Southwest monsoon (June-September), Northeast monsoon (October-December)
- Unreliable/erratic: Onset and withdrawal dates vary; rainfall is uneven
- Orographic: Heavily influenced by topography (e.g., Western Ghats, Himalayas)
- Cyclonic: Carries moisture-laden cyclonic winds from the ocean
- Influenced by global phenomena: ENSO, IOD, Madden-Julian Oscillation (MJO)
What are the characteristics of monsoonal rainfall in India?
- Seasonal concentration: Rs 75% of annual rainfall occurs between June and September.
- Uneven distribution: Some regions receive over 1,000 cm (e.g., Mawsynram), others 200 cm)
- Western coast (Konkan, Goa, Kerala)
- Northeast India (Assam, Meghalaya, Tripura)
2. Moderate rainfall areas (100-200 cm)
- Gangetic plains, parts of eastern MP, Odisha
3. Low rainfall areas (50-100 cm)
- Punjab, western UP, Rajasthan (east)
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