The southwest monsoon is one of the determining factors behind the state of agricultural productivity in India and hence deciding the fate of Indian economy.
A good monsoon is a driver for the Indian economy, raising farm income, and increasing productivity. A weak monsoon on the other hand is a major factor for inflated prices and thereby harming the economy.
Monsoons are a unique climatic system which encompasses a seasonal reversal of winds. It is distinctly developed along the eastern coast of tropical lands and best developed in the South East Asia.
It occurs due to a difference in temperature between the landmass and the ocean.
Major monsoon of the world is West African and Asian-Australian monsoons.
The southwest monsoon derives its name from winds that blow from a south westerly direction in the Indian Subcontinent. These winds come from the potent Mascarene High. This is a high-pressure belt near the Mascarene Islands in the southern Indian Ocean.
This high pressure starts getting built by mid-April and is a major determiner of the intensity of Monsoon in India.
Higher the pressure, stronger will be the wind. Winds from the Mascarene basin starts blowing in a north westerly direction towards the east coast of Africa, i.e. Somalia.
From here, the winds are deflected towards the east.
On crossing the Equator, under the influence of the Coriolis force, these winds get deflected eastwards and blow from south-west to the north-east direction.
Hitting the peninsular edge of the Indian subcontinent, they split into two branches—the Arabian Sea branch and the Bay of Bengal branch.
For the Arabian sea branch of the southwest monsoon, as the wind moves northward, the precipitation falls. Also, the eastern part of the western Ghats doesn’t receive much of the rainfall.
Note: The rainfall caused due to Southwest monsoon is a type of orographic rainfall. Huge masses of air are pushed up by wind upwards along the slope of a mountain. This leads to adiabatic cooling and then condensation and then rain. The rain shadow area is the leeward side of the mountain.
It occurs when masses of air are pushed by wind upwards along the side of elevated landforms. This results in adiabatic cooling and ultimately condensation and precipitation. Along the leeward side, rain shadow is observed.
The Bay of the Bengal branch picks up moisture from the Bay of Bengal and moves towards the north eastern part of India and Bengal.
At this point, the ITCZ comes into play. It shifts North, pulling the southwest monsoon winds from the sea towards the land.
The Himalayas act as a barrier and restricts the low-pressure zone onto themselves.
The adjoining Tibetan plateau if heated more than the Himalayas will lead to an abrupt northward shift of the ITCZ. This leads to bursting of the Monsoon over the Indian subcontinent.
A reverse of this happens during the onset of winters. There is a reversal of the pressure situations on land and the sea and this cause rainfall over the eastern Indian peninsula.
The dry anticyclonic wind is no more capable of causing rain over the subcontinent once they have exhausted all their moisture. However, certain areas in the north get winter precipitation from shallow cyclonic disturbances that come from the Mediterranean Sea. These are called “Westerly Disturbances’.
This rainfall is very good for the Rabi crops for reasons like less runoff, low evaporation and the fact that moisture is confined to the root area of the crop.