Overview
This lecture covers the fundamentals of the Indian Monsoon system, its mechanisms, branches, influencing factors, and the distinction between southwest and retreating (northeast) monsoons, essential for both prelims and mains exams.
Fundamentals of the Indian Monsoon
- The Indian monsoon is a system of seasonal winds that reverse direction due to temperature and pressure differences between land and sea.
- In May-June, the sun is overhead in the northern hemisphere, causing intense heating and low pressure over northern India.
- Winds always flow from high-pressure to low-pressure areas, bringing moist air from the southern hemisphere to northern India.
- Coriolis force deflects winds to the right in the northern hemisphere and to the left in the southern hemisphere.
Mechanism and Branches of the Southwest Monsoon
- The southwest monsoon winds originate from the high-pressure cell in the southern Indian Ocean and move towards the low-pressure zone over northern India.
- Winds split into two branches: Arabian Sea branch (hits Kerala first, moves along Western Ghats) and Bay of Bengal branch (hits northeast India).
- The Arabian branch runs parallel to Rajasthan, causing minimal rain there; the Bay of Bengal branch provides heavy rainfall to northeast India and interior states.
Factors Influencing the Southwest Monsoon
- Intense heating of the Tibetan Plateau creates a strong low-pressure zone.
- Tropical easterly jet stream (TEJ) aids monsoon onset and influences rainfall intensity.
- A permanent high-pressure cell east of Madagascar in the south Indian Ocean influences wind movement.
- Subtropical jet streams (western disturbances) move north in summer, aiding southwest monsoon strength.
- The Intertropical Convergence Zone (ITCZ) shifts north or south with the sun, affecting monsoon onset.
Characteristics of Southwest Monsoon
- Monsoons are strong, often violent winds that bring 75% of India’s annual rainfall from June to September.
- Rainfall varies from 10 cm in western Rajasthan to over 900 cm in Meghalaya.
- Western Ghats and Bay of Bengal mountains split and intensify rainfall in their respective regions.
- The Bay of Bengal branch loses moisture as it moves west, causing less rain in Rajasthan.
Retreating (Northeast) Monsoon
- Retreating monsoon refers to the withdrawal of southwest winds, typically from September to December.
- As sun shifts to southern hemisphere, winds reverse; northeast winds pick up moisture from Bay of Bengal, causing rain in Tamil Nadu.
- Retreating monsoon winds are dry in most regions except for Tamil Nadu, which receives heavy rainfall.
- These winds are mostly upper atmospheric (jet streams), not surface winds.
Key Terms & Definitions
- Monsoon — Seasonal winds that change direction due to land-sea temperature/pressure differences.
- Coriolis Force — Deflects winds right in the northern and left in the southern hemisphere.
- Tropical Easterly Jet Stream (TEJ) — Upper atmosphere wind that helps onset and intensity of monsoon.
- ITCZ (Intertropical Convergence Zone) — Belt of low pressure near the equator where trade winds converge.
- Onshore/Offshore Winds — Onshore: sea to land (moist); Offshore: land to sea (dry).
- Retreating Monsoon — Northeast winds withdrawing from India in autumn, bringing rain mainly to Tamil Nadu.
Action Items / Next Steps
- Review and practice previous year questions (PYQs) on monsoons.
- Write a short note with a diagram explaining retreating monsoons.
- Prepare doubts and tricky questions from monsoon PYQs for discussion in the next class.