The Indian Monsoon

Origin & Definition

Monsoon: Seasonal reversal of winds from High Pressure → Low Pressure that causes rainfall.

Pre-Monsoon (Mar-Apr)

ITCZ (Inter-Tropical Convergence Zone) shifts towards India. Moist local winds from BOB & Arabian Sea bring local showers.

Local names: Kaal Baisakhi (WB), Bordoichila/Nor'westers (Assam), Cherry/Coffee Showers (Karnataka), Mango Showers (Ker, TN, Karnataka)

Pre-Monsoon Winds

Theories

Theory	Scholar	Key Idea
Classical/Thermal	Edmund Halley	Temp & Pressure
Dynamic	Flohn (Germany)	ITCZ Impact
Modern	Dr. P. Koteshwar	Tibetan Plateau heating + Jet Stream

Factors for Arrival

Tibetan Plateau heats up → sub-tropical jet → anticyclonic circulation
Sub-Tropical Jet splits; southern branch converts to tropical jet
Tropical Jet forms High Pressure over Indian Ocean
ITCZ/Monsoon Trough forms Low Pressure over India; winds flow High Pressure → Low Pressure

Intensification Factors

Factor	Explanation	Effect
Somali Jet	High Pressure formation over Indian Ocean	+ve
Indian Dipole (+ve)	W. Indian Ocean → Low Pressure, E. Indian Ocean → High Pressure	+ve
Indian Dipole (-ve)	W. Indian Ocean → High Pressure, E. Indian Ocean → Low Pressure	-ve
Walker Cell	Raises warm air from Indonesia, falls on Peru coast	+ve
El Niño	Trade winds weaken, E. Pacific warms up	-ve
La Niña	Trade winds weaken, E. Pacific current becomes cold	+ve
El Niño Modoki	El Niño in Central Pacific (not Eastern)	-ve

Burst of Monsoon

Arabian Sea branch strikes Malabar Coast of Kerala on 1st June as S-W Monsoon. Torrential rainfall with thunder & lightning.

Two Branches (S-W)

Branch Details

I. Arabian Sea Branch (3 sub-branches)

1st Sub-branch

Obstructed by the Western Ghats
250-400 cm rainfall on Western Ghats
Formation of rain-shadow region on Telangana Plateau on the Eastern side of Western Ghats

2nd Sub-branch

Strikes the coast north of Mumbai
Rainfall in the Narmada & Tapi river valleys between the Vindhyas mountain range and Satpuda mountain range
15 cm rainfall in Chota Nagpur Plateau
Meets the Bay of Bengal branch on the Gangetic Plain

3rd Sub-branch

Saurashtra Peninsula & Kutch
250 cm of rain on Konsa coast, 200 cm on Konkan coast
Pune: rain-shadow, hence receives only 50-60 cm rainfall
Saurashtra-Kutch: Aravalli ranges being parallel to the monsoon winds, only 50-60 cm
90-100 cm of rain falls after hitting the Shivalik Range

II. Bay of Bengal Branch

Clashes with Arakanyoma & forms 2 branches

1st Sub-branch

Meghalaya Plateau receives over 300 cm of rainfall (Mawsynram, Cherrapunji)

2nd Sub-branch

250 cm in West Bengal, 200 cm in Bihar
130 cm in Rewa-Panna Plateau of MP
Meets Arabian Sea branch at Shivaliks

Monsoon Break

No rain for 2 weeks during monsoon season.

Decreasing tropical cyclones
ITCZ at Shivalik foothills
Winds parallel to west coast
Temperature inversion in Rajasthan

Withdrawal (Oct-Nov)

Sun's Dakshinayana (southward shift). High Pressure over India, Low Pressure over Indian Ocean. Direction: NE → SW. Called North-East Monsoon. Tamil Nadu coast gets ~100cm. Dry evergreen forests.

Mawatha (Western Disturbances)

In December, sub-tropical jet blows over Ganga plains (Punjab, HR). Absorbs moisture from Mediterranean & Black Sea. Warm in nature.

These winds get struck by the Himalayas & cause rainfall in N-India along with other parts (sometimes).

Months: Oct-Nov
Direction: West → East (Westerlies)
J&K 20cm, Punjab 15cm, Allahabad 10cm, Patna 15cm
Importance: Essential for Rabi crops. Ensures availability of water throughout the year as it prevents dry spells during the dry winter months.

The Monsoon

Origin & Definition

Pre-Monsoon (Mar-Apr)

Pre-Monsoon Winds

Theories

Factors for Arrival

Intensification Factors

Burst of Monsoon

Two Branches (S-W)

Branch Details

I. Arabian Sea Branch (3 sub-branches)

1st Sub-branch

2nd Sub-branch

3rd Sub-branch

II. Bay of Bengal Branch

1st Sub-branch

2nd Sub-branch

Monsoon Break

Withdrawal (Oct-Nov)

Mawatha (Western Disturbances)