Indian Monsoon & Seasons

Master this topic with zero to advance depth.

Climate of India

India has a "Tropical Monsoon" type of climate. The word monsoon is derived from the Arabic word 'Mausim' which means the seasonal reversal of winds.

Latitude: The Tropic of Cancer divides India into the tropical zone (south) and the sub-tropical/temperature zone (north). Thus, India experiences both tropical and sub-tropical climates.
The Himalayas: Act as a massive barrier, preventing the severely cold, freezing winds of Central Asia from blowing into the subcontinent. They also trap the monsoon winds, forcing them to shed their moisture within India.
Distance from the Sea: Coastal areas experience an equable climate due to the moderating influence of the sea, while interior regions suffer from extremes of temperature (Continentality).
Relief: Affects temperature, air pressure, direction, and speed of wind, and amount and distribution of rainfall. For instance, the windward side of Western Ghats receives heavy rainfall, while the Deccan plateau is a rain shadow area.
Jet Streams: Narrow bands of strong winds in the upper atmosphere. The westerly jet stream influences winter weather, and the easterly jet stream influences the summer monsoon.

The monsoons are complex meteorological phenomena.

Proposed by Halley. It explains monsoon as huge sea and land breezes caused by the differential heating and cooling of land and water.
Summer: The landmass of India gets intensely heated, creating a low-pressure area over Northern India. The Indian Ocean is relatively cooler, maintaining high pressure. Winds blow from high pressure (sea) to low pressure (land), bringing moisture.

Shift of the ITCZ (Inter-Tropical Convergence Zone): In summer, the ITCZ shifts northwards over the Ganga plain (becoming the Monsoon Trough) due to the apparent northward movement of the sun. This shift draws the southeast trade winds of the southern hemisphere, which cross the equator, get deflected to the right by the Coriolis force, and enter India as the South-West Monsoon.
Tibetan Plateau: Intense heating of the Tibetan Plateau during summer creates strong vertical air currents and low pressure, aiding the monsoon.
Role of Jet Streams: The withdrawal of the Sub-Tropical Westerly Jet Stream from north of the Himalayas initiates the onset of the monsoon. The appearance of the Tropical Easterly Jet Stream over the peninsula steers the monsoon depressions towards India.

El Niño: A periodic warming of the ocean surface temperatures in the central and eastern equatorial Pacific Ocean. Generally associated with a weak or delayed Indian monsoon and droughts.
La Niña: The cooling phase of the Pacific water. Generally associated with a strong Indian monsoon and good rainfall.
Indian Ocean Dipole (IOD): A difference in sea surface temperatures between opposite sides of the Indian Ocean. A positive IOD usually leads to a good monsoon even during an El Niño year.

The Meteorological Department of India recognizes four distinct seasons:

Clear skies, pleasant weather, low temperature and humidity.
Western Disturbances: Shallow cyclonic depressions originating over the Mediterranean Sea travel eastwards and enter northwestern India. They cause light winter rainfall in Punjab/Haryana (highly beneficial for Rabi crops like wheat) and snowfall in the Himalayas.
Coromandel coast (Tamil Nadu) gets rainfall from the retreating Northeast trade winds picking up moisture from the Bay of Bengal.

Intense heat. The ITCZ moves northwards.
Local Storms:
- Mango Showers: Pre-monsoon showers in Kerala and Karnataka, helpful for ripening mangoes.
- Blossom Showers: Showers in Kerala beneficial for coffee flowers.
- Kal Baisakhi: Violent thunderstorms in West Bengal and Assam.
- Loo: Hot, dry, and dust-laden winds blowing over the northern plains.

Brings more than 75% of India's rainfall. Characterized by sudden onset ("Burst of Monsoon").
Bifurcates into two branches:
- Arabian Sea Branch: Strikes the Western Ghats causing heavy orographic rainfall. Its sub-branches move through Narmada-Tapi valleys and parallel to Aravallis (giving no rain to Rajasthan) before reaching the Himalayas.
- Bay of Bengal Branch: Strikes the Arakan Hills in Myanmar, gets deflected towards the Indian subcontinent, bringing heavy rain to Northeast India and West Bengal. Moves up the Ganga plain. Mawsynram (Meghalaya) receives the highest average rainfall in the world.
Break in Monsoon: Dry spells lasting a few days to weeks when monsoon trough shifts closer to foothills.

South-West monsoon winds weaken and retreat from Northern India due to the southward shift of the ITCZ.
Marked by clear skies and a rise in temperature, creating oppressive weather known as "October Heat".
Cyclonic depressions originating in the Andaman Sea hit the eastern coast, causing devastating floods in Godavari, Krishna, and Cauvery deltas.