Monsoon Mechanism

Notes

The word monsoon comes from the Arabic mausim (season): a seasonal reversal of wind direction. India's climate is a monsoon climate with four seasons: cold weather (December-February), hot weather (March-May), south-west monsoon (June-September) and the retreating monsoon (October-November).

Theories

Classical (thermal) theory: the differential heating of land and sea creates a seasonal pressure reversal (like a giant land and sea breeze).
Air mass theory (Flohn): the monsoon is a seasonal shift of the Inter-Tropical Convergence Zone (ITCZ).
Dynamic and modern view: the roles of the Tibetan Plateau heating, the jet streams, the Mascarene High, the Somali jet, and links to El Nino, the Indian Ocean Dipole and snow cover.

Mechanism of the south-west monsoon

In summer the ITCZ (monsoon trough) moves north to about 25°N over the Gangetic plain, and an intense heat low forms over north-west India.
The south-east trades cross the equator, are deflected to the right by the Coriolis force, and blow as south-westerlies towards India.
Onset: about 1 June over Kerala (normal date, with a deviation of about a week), often with a burst (a sudden onset with thunderstorms).
Two branches: the Arabian Sea branch (heavy orographic rain on the Western Ghats; the rain shadow to the east, e.g., Pune) and the Bay of Bengal branch (rain in the north-east; Mawsynram and Cherrapunji in the Khasi hills receive the heaviest rainfall; then it turns west along the Himalaya into the Ganga plain).
The monsoon covers the whole country by about 8 July. Withdrawal starts from north-west India in about 17 September and ends by mid-October.
Breaks in the monsoon: dry spells during the season when the trough shifts north towards the Himalaya.
Retreating monsoon (October-November): clear skies, "October heat"; cyclones in the Bay of Bengal; the north-east monsoon brings rain to Tamil Nadu (October-December).

Factors influencing the monsoon

Factor	Effect
El Nino	Weakens the monsoon in many years
La Nina	Usually strengthens it
Positive Indian Ocean Dipole	Strengthens it
Eurasian snow cover	Heavy snow tends to weaken it
Jet streams and Tibetan heating	Control onset and withdrawal
Madden-Julian Oscillation	Active and break spells

Rainfall facts: the long-period average (LPA) of the monsoon season is about 87 cm (IMD). The IMD issues forecasts; the Mission Mausam (2024) aims to improve forecasting.

Key Points

Monsoon = seasonal wind reversal; four seasons in India.
ITCZ shifts north; SE trades become SW monsoon after crossing the equator.
Onset about 1 June in Kerala; covers India by about 8 July; withdrawal from about 17 September.
Two branches: Arabian Sea and Bay of Bengal.
Factors: El Nino, IOD, snow cover, jet streams.

MCQs

Q1. The normal date of onset of the south-west monsoon over Kerala is:

(a) 15 May
(b) 1 June
(c) 15 June
(d) 1 July
Ans: (b).

Q2. The place receiving the highest rainfall in the monsoon in the Khasi hills region is:

(a) Shillong
(b) Mawsynram
(c) Guwahati
(d) Pune
Ans: (b).

Q3. The shift of which feature to the north in summer is central to the monsoon?

(a) Polar front
(b) ITCZ
(c) Subtropical high
(d) Doldrums high
Ans: (b).