π Topic 02 of 6 Β· Chapter 03 Β· Climate of India
Monsoon Mechanism in India
SW monsoon onset, Arabian Sea & Bay of Bengal branches, withdrawal, NE monsoon β complete notes for UPSC & PSC exams.
π§οΈ What is Monsoon?
The word “monsoon” is derived from the Arabic word “Mausim” meaning season. Monsoon refers to the seasonal reversal of winds β SW monsoon in summer (JuneβSept) and NE monsoon in winter (OctβDec).
π§οΈ South-West (SW) Monsoon β Mechanism
- In summer, the Indian landmass heats up rapidly β low pressure develops over NW India (Thar Desert)
- High pressure exists over the Indian Ocean (cooler)
- Winds blow from high pressure (ocean) to low pressure (land) β bringing moisture
- The Coriolis force deflects these winds to the right β they become SW winds
- The Tropical Easterly Jet Stream over India triggers the monsoon onset
- SW monsoon arrives at Kerala coast around June 1 (onset)
- Covers entire India by mid-July
π§οΈ Two Branches of SW Monsoon
| Feature | Arabian Sea Branch | Bay of Bengal Branch |
|---|---|---|
| Direction | Flows northward along west coast | Flows northward along east coast, then turns west |
| Rainfall areas | Western Ghats, Konkan, Goa, Kerala, Mumbai | NE India (Assam, Meghalaya), Bengal, Odisha, then UP, Bihar |
| Stronger branch | Stronger β brings more rainfall | Weaker β loses moisture before reaching NW India |
| Cherrapunji | β | Bay of Bengal branch hits Khasi Hills β Cherrapunji gets highest rainfall |
β Cherrapunji & Mawsynram:
β’ Mawsynram (Meghalaya) = highest average annual rainfall in world (~11,872 mm)
β’ Cherrapunji (Sohra) (Meghalaya) = 2nd highest; also holds record for most rain in a single month
β’ Both are on the windward side of Khasi Hills β Bay of Bengal branch hits and rises β heavy orographic rainfall
β’ Mawsynram (Meghalaya) = highest average annual rainfall in world (~11,872 mm)
β’ Cherrapunji (Sohra) (Meghalaya) = 2nd highest; also holds record for most rain in a single month
β’ Both are on the windward side of Khasi Hills β Bay of Bengal branch hits and rises β heavy orographic rainfall
π§οΈ Monsoon Withdrawal & NE Monsoon
| Event | Timing | Details |
|---|---|---|
| SW Monsoon Onset | June 1 (Kerala) | Arrives at Kerala coast; “burst of monsoon” |
| Covers all India | Mid-July | Reaches NW India (Rajasthan, Punjab) last |
| SW Monsoon Withdrawal | Sept 1 (NW India) β Nov (Kerala) | Retreats from NW India first; takes 3 months to fully withdraw |
| NE Monsoon (Retreating Monsoon) | OctβDec | Winds blow from NE (land to sea); picks up moisture over Bay of Bengal β brings rain to Tamil Nadu, Andhra Pradesh coast |
| Chennai rainfall | OctβDec | Chennai gets most of its rain from NE monsoon (not SW monsoon) |
π Key Exam Points β Monsoon:
β’ “Mausim” (Arabic) = season = origin of word “monsoon”
β’ SW monsoon onset = June 1, Kerala (normal date)
β’ Cherrapunji/Mawsynram = highest rainfall = Bay of Bengal branch + Khasi Hills
β’ Tamil Nadu gets rain from NE monsoon (OctβDec) β not SW monsoon
β’ Rajasthan = driest state = SW monsoon loses moisture before reaching it
β’ “Mausim” (Arabic) = season = origin of word “monsoon”
β’ SW monsoon onset = June 1, Kerala (normal date)
β’ Cherrapunji/Mawsynram = highest rainfall = Bay of Bengal branch + Khasi Hills
β’ Tamil Nadu gets rain from NE monsoon (OctβDec) β not SW monsoon
β’ Rajasthan = driest state = SW monsoon loses moisture before reaching it