India’s climate is said to be “Tropical Monsoon”, due to its tropical location and reversal of winds seasonally. The word Monsoon is derived from the Arabic term Mausim’ which means ‘reversal of winds’ or ‘Seasons’. Monsoons are the periodic winds in which there is a reversal of wind direction periodically. On account of the variability in climatic conditions, seasonally and regionally, India is called the ‘Meteorological Unit’. The seasonal variation in the climatic condition is due to various factors like temperature, pressure, wind, rainfall etc
The monsoons are usually experienced in the tropical area roughly between 20° N and 20° S.
The climate of India is described as the ‘monsoon’ type. Most of the monsoon climate in Asia is found mainly in the south and the southeast.
There are 2 types of monsoons namely
- The southwest monsoon season – Rainfall received from the southwest monsoons, which occurs between June and September.
- The retreating monsoon season – The months of October and November are known for retreating monsoons.
Factors Influencing South-West Monsoon Formation
The differential heating and cooling of land and water create a low pressure on the landmass of India while the seas around experience comparatively high pressure.
The shift of the position of the Inter-Tropical Convergence Zone (ITCZ) in summer, is also known as the monsoon trough during the monsoon season.
The presence of the high-pressure area, east of Madagascar in the Indian Ocean. The intensity and position of this high-pressure area affect the Indian Monsoon.
The Tibetan plateau gets intensely heated during summer, which results in strong vertical air currents and the formation of low pressure over the plateau.
The movement of the westerly jet stream to the north of the Himalayas and the presence of the tropical easterly jet stream over the Indian peninsula during summer.
Southern Oscillation (SO): Normally when the tropical eastern south Pacific Ocean experiences high pressure, the tropical eastern Indian Ocean experiences low pressure. But in certain years, there is a reversal in the pressure conditions and the eastern Pacific has lower pressure in comparison to the eastern Indian Ocean. This periodic change in pressure conditions is known as the Southern Oscillations.
The Inter-Tropical Convergence Zone (ITCZ,) is the low-pressure area in equatorial latitudes. This is where the northeast and the southeast trade winds converge. This convergence zone lies more or less parallel to the equator but moves north or south with the apparent movement of the sun.
This is a name given to the periodic development of a warm ocean current along the coast of Peru as a temporary replacement for the cold Peruvian current. The presence of the El Nino leads to an increase in sea-surface temperatures and a weakening of the trade winds in the region.
The Onset of the South-West Monsoon
The location of ITCZ shifts north and south of the equator with the apparent movement of the Sun.
During June, the sun shines vertically over the Tropic of Cancer, and the ITCZ shifts northwards.
The southeast trade winds of the southern hemisphere cross the equator and start blowing in the southwest to a northeast direction under the influence of Coriolis force. These winds collect moisture as they travel over the warm Indian Ocean.
In July, the ITCZ shifts to 20°-25° N latitude and is located in the Indo-Gangetic Plain and the southwest monsoons blow from the Arabian Sea and the Bay of Bengal. The ITCZ in this position is often called the Monsoon Trough.
The shift in the position of the ITCZ is also related to the phenomenon of the withdrawal of the westerly jet stream from its position over the north Indian plain, south of the Himalayas.
The easterly Jet Stream (Somali Jet) sets in along 15°N latitude only after the western jet stream has withdrawn itself from the region. This easterly jet stream is held responsible for the burst of the monsoon in India.
As these winds approach the land, their southwesterly direction is modified by the relief and low pressure over northwest India. The monsoon approaches the Indian landmass in two branches:
The Arabian Sea branch – The monsoon winds originating over the Arabian Sea.
The Bay of Bengal branch – The Arakan Hills along the coast of Myanmar deflect a big portion of this branch towards the Indian subcontinent. The monsoon, therefore, enters West Bengal and Bangladesh from the south and southeast instead of from the south-westerly direction.
Another phenomenon associated with the monsoon is its tendency to have ‘breaks’ in rainfall. The monsoon rains take place only for a few days at a time. They are interspersed with rainless intervals. These breaks in monsoon are related to the movement of the monsoon trough.
Retreating Monsoon Season
The retreating southwest monsoon season is marked by clear skies and a temperature rise.
Owing to the conditions of high temperature and humidity, the weather becomes rather oppressive. This is commonly known as the ‘October heat’.
The weather in the retreating monsoon is dry in north India but it is associated with rain in the eastern part of the Peninsula. Here October and November are the rainiest months of the year.
The widespread rain in this season is associated with the passage of cyclonic depressions which originate over the Andaman Sea and manage to cross the eastern coast of the southern Peninsula. These tropical cyclones are very destructive.
A bulk of the rainfall of the Coromandel Coast is derived from these depressions and cyclones.
Unlike the rest of the country, which receives rain in the southwest monsoon season between June and September, the northeast monsoon is crucial for farming and water security in the southern part.
Impact of Monsoons on Life in India
More than 60% of people in India depend on agriculture for their livelihood and agriculture itself is based on the monsoon.
Regional monsoon variation in India is reflected in the vast variety of food, clothes, and house types.
Winter rainfall by temperate cyclones in north India is highly beneficial for Rabi crops.
Monsoon rain helps recharge dams and reservoirs, which are further used for the generation of hydroelectric power.
Adverse Impact of Monsoons on Life in India
Variability of rainfall brings droughts or floods every year in some parts of the country.
Sudden monsoon burst creates a problem of soil erosion over large areas in India.
In hilly areas, sudden rainfall brings landslides that damage natural and physical infrastructure subsequently disrupting human life.