Aspirant Academy

Geography

The Monsoon Mechanism

Climate of India: Monsoon, Rainfall Distribution, Climatic Regions

Paper II · Unit 3 Section 3 of 11 0 PYQs 28 min
Topic Preview Subject Inventory

Public Section Preview

The Monsoon Mechanism

2.1 What is a Monsoon?

The word "monsoon" derives from Arabic mausim (season). A monsoon is a seasonal reversal of wind direction associated with corresponding changes in precipitation. In summer, winds blow from sea to land (onshore — bringing rain); in winter, they reverse (offshore — dry).

2.2 Theories of Monsoon Formation

1. Classical Theory (Thermal Theory): Originally proposed by Halley (1686). Simple idea: in summer, the Asian landmass heats up more than the surrounding oceans → low pressure develops over land → moist oceanic air flows in from Indian Ocean to fill the low pressure → precipitation occurs.

Limitations: Does not fully explain the sudden onset, annual variability, or strength of the monsoon.

2. Modern Explanation — Role of ITCZ:
The Inter-Tropical Convergence Zone (ITCZ) — the belt where Northern and Southern Hemisphere trade winds converge — shifts northward during June–September. When ITCZ moves to about 25°N over India (attracted by the intense summer heating of the Gangetic-Thar region), it triggers the monsoon. The Somali Jet (low-level jet stream over East Africa and Arabian Sea) strengthens and drives the SW monsoon into India.

3. Role of Tibetan Plateau (PYQ 2023):
The Tibetan Plateau (average elevation 4,500 m — area 2.5 million sq km) acts as an elevated heat source. In summer:

  • The plateau surface heats up rapidly (more efficiently than surrounding atmosphere at the same altitude)
  • This creates a powerful high-level anticyclone (upper tropospheric high) over Tibet
  • This anticyclone pushes the eastward-flowing upper-level winds southward, helping maintain the divergence that sustains monsoon circulation
  • The Tibetan High is now recognized as an essential component of the monsoon machine

4. Role of El Niño/Southern Oscillation (ENSO):
This is the most important interannual control of monsoon variability (see Section 3).

2.3 Onset of SW Monsoon — Step by Step

  1. April–May: Solar heating peaks over NW India/Thar Desert; ITCZ starts migrating north; cross-equatorial flow intensifies over Arabian Sea (Somali Jet develops)
  2. Late May: Monsoon reaches Andaman & Nicobar Islands (~May 20–25); Bay of Bengal branch forms
  3. June 1 ± 7 days: Monsoon onset at Kerala coast (Thiruvananthapuram) — official declaration by IMD; southwest-to-northeast trade winds reverse to form southwesterly monsoon winds
  4. June 1–15: Monsoon covers most of South India and Bay of Bengal; Arabian Sea branch hits Western Ghats
  5. June 27–July 5: Monsoon reaches Delhi; NW India receives its share
  6. July 15: Monsoon covers entire India

2.4 The Two Branches of SW Monsoon

Arabian Sea Branch:

  • Origin: SW Arabian Sea; crosses Kerala coast first
  • Direction: Southwest to northeast over western India
  • Effect on Western Ghats: Heavy orographic rainfall on windward (west) face — Cherrapunji equivalent — 2,500–4,000 mm
  • Rain shadow: Eastern Deccan receives <700 mm; parts of interior Karnataka and Marathwada receive <500 mm
  • Also provides rainfall to Gujarat, Rajasthan (partial), and Punjab plains after crossing Aravallis

Bay of Bengal Branch:

  • Origin: Bay of Bengal; hits Meghalaya hills (Khasi, Jaintia) and NE India first
  • Mawsynram/Cherrapunji: The funnel-shaped Khasi Hills trap this moisture → 11,000+ mm/year
  • Moves northwest along Himalayas into Ganga Plain
  • Also moves west through Assam, West Bengal, and Bangladesh
type: comparison_grid
title: "SW Monsoon — Arabian Sea Branch vs Bay of Bengal Branch"
subtitle: "Comparison of the two monsoon arms"
data:
  headers: ["Feature", "Arabian Sea Branch", "Bay of Bengal Branch"]
  rows:
    - ["Origin", "SW Arabian Sea", "Bay of Bengal"]
    - ["First landfall", "Kerala coast (June 1)", "Andaman & NE India (May 20)"]
    - ["Primary rainfall area", "West coast, Western Ghats", "NE India, Ganga plains"]
    - ["Wettest station caused", "Agumbe/Mahabaleshwar (7,000+ mm)", "Mawsynram (11,871 mm)"]
    - ["Rain shadow created", "Eastern Deccan (<500 mm)", "Rain-shadow behind Meghalaya hills"]
    - ["Secondary effect", "Punjab, Gujarat plains via Aravallis", "West Bengal, Bangladesh"]
source: "IMD; NCERT"