Indian Monsoon Factors & Theories
- Monsoon is traditionally defined as a seasonal reversing wind accompanied by corresponding changes in precipitation.
- Seasonal Reversal of Wind is the change in wind direction during winters leading to Retreating Monsoon going out of the Indian Peninsula towards the Indian Ocean.
- Seasonal Reversal of Wind = Advancing Monsoon + Retreating Monsoon (Winters).
- Monsoon is derived from the Arabic word Mausim.
- Monsoon affects the Indian subcontinent, where it is one of the oldest and most important pattern every year from June through September.
- The unique geographical features of the Indian subcontinent, along with associated atmospheric, oceanic and geophysical factors, influence the behaviour of the monsoon.
- Monsoons all over the world are experienced in the tropical area roughly between 20° N & 20°S.
Wind forms a complete cycle of blowing from one place to other, rising and subsiding as follows:
- First, the sunlight heats up the land and causes a Low Pressure (L.P) over the land as compared to the High Pressure of the Sea (comparative H.P).
- Always remember, Winds blow from High Pressure to Low Pressure.
- Due to this L.P, wind blows from the H.P at Sea to the L.P over land. This wind is generally known as Sea Breeze.
- Sea Breeze picks up moisture from the Sea before reaching the land.
- Once this wind reaches the land, it rises up due to the heat & eventually leads to cloud formation.
- These clouds pour rain over the land.
- Now the wind completes it’s cycle by subsiding over the sea where the entire process began.
- The opposite happens for Land Breeze when the Land cools down faster than Sea and hence a comparative L.P appears over the Sea forcing the winds to blow towards the Sea.
ADVANCING vs. RETREATING MONSOONS
- Advancing Monsoon is the South-West Monsoon
- Retreating Monsoon is the North-East Monsoon.
|South-West Monsoon||North-East Monsoon|
|Moist Wind||Dry Winds|
|Two Branches: Arabian Sea + Bay of Bengal||Single Branch|
|Heavy Rainfall||Low Rainfall|
|Wind: Sea Breeze | Sea to Land||Wind: Land Breeze | Land to Sea|
REASONS FOR ONSET OF ADVANCING MONSOON
- Low Pressure over North India
- Shift of Inter-Tropical Convergence Zone (ITCZ) northwards to the Ganga Plains.
- The Himalayas acting as a barrier for the monsoon to stop it from reaching Tibet and also intensifying the Low Pressure in North India.
- Tropical Easterly Jetstreams above Tibetan Plateau helping the Monsoon maintain it’s strength.
- High Pressure at Madagascar
- Southern Oscillations (ENSO), Indian Ocean Dipole also have a role in helping or disrupting the South-West Monsoon.
- Findlater Jetstream from the coast of Somalia towards Indian Peninsula strengthen the Monsoon winds.
2 BRANCHES OF THE ADVANCING MONSOON
The South-West Monsoon on reaching the coast of the Indian Monsoon is divided into 2 branches:
- Arabian Sea Branch
- Bay of Bengal Branch
- Arabian Sea Branch: This branch hits the Western Ghats which acts as a barrier preventing the monsoon to reach the other side of the mountains. Thus, resulting in drought-like situations in various parts of Karnataka, Maharashtra & Telangana. The Monsoon winds are shifted northwards and mostly enter the Indian Peninsula through the Kathiawar Peninsula in Gujarat.
- Bay of Bengal Branch: The Bay of Bengal Branch is responsible in causing heavy rainfall in Sri Lanka, Andaman & Nicobar Islands, North-East India, & East India. These monsoon winds are deflected by the mountains in North-East India such as the Patkai Bum, Purvanchal etc and are forced to reach Central as well as North India. That is the reason why North India gets less rainfall as compared to North-East India as the intensity of the Monsoon weakens by that time.
REASONS FOR RETREATING MONSOON
- High Pressure at North India
- Westerly JetStream start flowing South of the Himalayas in winters (In summers they flow north of the himalayas towards Tibet)
- Shifting back of ITCZ to it’s original position
- While retreating, the North-East Monsoons cause catastrophic floods in Tamil Nadu (Chennai floods) as they pick moisture from the Bay of Bengal Coast of India. That is why Tamil Nadu mostly gets rainfall in winters.