Indian Monsoon

Notes

The Indian monsoon is the seasonal reversal of winds that brings about 75% of India's annual rainfall during the south-west monsoon (June-September). It determines agriculture (the kharif season), water supply, hydropower and the economy; the Union Budget is often described as a "gamble in the monsoon".

Characteristics

Onset over Kerala about 1 June; it covers the country by about 8 July; withdrawal from the north-west from about 17 September.
Two branches: the Arabian Sea branch (heavy rain on the Western Ghats; its rain-shadow on the Deccan interior) and the Bay of Bengal branch (heavy rain in the north-east, then along the Gangetic plain). The monsoon trough (the ITCZ over the plains) shifts north and south and controls active and break spells.
Uneven and uncertain: large variation in time and space; breaks, floods and droughts.
Rain-shadow effect in Pune, interior Karnataka and Tamil Nadu (they get the NE monsoon).
The north-east (retreating) monsoon in October-December brings rain to Tamil Nadu, coastal Andhra Pradesh and Kerala.

Rainfall distribution

Above 250 cm: the Western Ghats windward side, Meghalaya (Mawsynram and Cherrapunji), north-eastern hills.
100-200 cm: south Odisha, east Madhya Pradesh, Chhattisgarh, the Himalayan foothills.
50-100 cm: western Uttar Pradesh, Haryana, Punjab, east Rajasthan, Gujarat, interior Deccan.
Below 50 cm: west Rajasthan, Kutch, parts of Ladakh (Jaisalmer is among the driest).

Classification of monsoon rainfall by IMD (percentage of the Long Period Average)

Deficient: below 90%; Below normal: 90-95%; Normal: 96-104%; Above normal: 105-110%; Excess: above 110%. The long-period average of the season is about 87 cm.

Drivers of variation: ITCZ and monsoon trough, El Nino/La Nina, the Indian Ocean Dipole, Eurasian snow cover, the jet streams (the subtropical westerly jet shifts north of the Himalaya and the Tropical Easterly Jet forms), the Mascarene High, the Somali (Findlater) jet, and the Madden-Julian Oscillation.

Impacts: agriculture and food security, reservoir storage and power, floods (Assam, Bihar, Kerala), droughts (Marathwada, Rajasthan), landslides in the hills, and urban flooding.

Key Points

The south-west monsoon gives about 75% of the annual rainfall.
Two branches: Arabian Sea and Bay of Bengal; onset about 1 June in Kerala.
Mawsynram and Cherrapunji get the highest rain; Jaisalmer is among the driest.
IMD: normal = 96-104% of LPA (about 87 cm).
Tamil Nadu gets rain from the north-east monsoon.

MCQs

Q1. The south-west monsoon contributes about what share of India's annual rainfall?
(a) 25%
(b) 50%
(c) 75%
(d) 90% Ans:
(c).

Q2. Rainfall of 105-110% of the Long Period Average is classified by IMD as:
(a) Normal
(b) Above normal
(c) Excess
(d) Deficient Ans:
(b).

Q3. Tamil Nadu receives most of its rainfall from the:
(a) South-west monsoon
(b) North-east monsoon
(c) Western disturbances
(d) Pre-monsoon showers Ans:
(b).