Question.
Discuss the mechanism of monsoons.
( Chapter - 4 Climate, Cass 9 NCERT Contemporary India -I )
Answer.
The climate of India is of monsoon type as it is mainly influenced by monsoon winds.
Mechanism of monsoons refers to the way the monsoon winds act, including the onset of the monsoon, break in the monsoon, advance of the monsoon [south-west monsoon], and retreat of the monsoon [northeast winds].
The following facts are important to understand the mechanism of the monsoon:
- the difference in heating and cooling of land and water
- Change in the position of the Inter-Tropical Convergence Zone [ITCZ]
- Presence of high-pressure areas to the east of Madagascar
- The heating of the Tibetan Plateau during summer
- The speed of the westerly jet stream north of the Himalayas
- Southern Oscillation (SO)[ SO ]
- El Nino (ELNino)
- ENSO
The difference in heating and cooling of land and water:
During summer, there is low air pressure over the Indian landmass and high atmospheric pressure over the ocean, so moisture-laden winds blow from the sea towards the Indian landmass commonly known as the summer monsoon or southwest monsoon.
During winter, it is just the opposite, low atmospheric pressure builds up in the Indian Ocean and high atmospheric pressure builds up over the Indian landmass, so winds blow from the Indian landmass toward the sea as the northeast monsoon winds.
Shifting of the Inter-Tropical Convergence Zone [ITCZ] status:
The ITCZ is generally located at about 5°N to 5°S latitude. During summer, the ITCZ moves over the Gangetic plains which bring with it moist air masses from the Indian Ocean. The ITCZ is also called the Monsoon Trough.
Area of high-pressure east of Madagascar:
The presence of high-pressure areas over the Indian Ocean to the east of Madagascar at about 20°S helps in the flow of monsoon winds toward India.
The temperature of the Tibetan Plateau:
The heating of the Tibetan Plateau during summer helps in the formation of strong vertical air currents which help in instability in the atmosphere and cloud formation near the Gangetic plains.
jet streams:
Monsoon arrives in India after the western jet stream moves north of the Himalayas.
The presence of the tropical easterly jet stream helps in the formation of a depression in the Bay of Bengal and it adds moisture to the Bay of Bengal branch of the monsoon.
Southern Oscillation (SO):
Changes in atmospheric pressure conditions over the Southern Ocean also affect the monsoon rains in India.
In normal years, the tropical eastern South Pacific Ocean [west of Peru] has high pressure, and the tropical eastern Indian Ocean [north of Australia] has low atmospheric pressure.
But in some years, the position of atmospheric pressure is reversed; There is low atmospheric pressure in the Southeast Pacific Ocean than in the Southwest Indian Ocean. These changes in atmospheric conditions are called the Southern Oscillation (SO).
If the difference in pressure over the Southeast Pacific Ocean [Coast of Peru] and northern Australia is negative, it means a below-average or late monsoon over India.
El Nino (ELnino):
El Nino is a phenomenon in which warm ocean currents replace the cold Peruvian Current every 2 to 5 years. El Nino reduces monsoon rains in India.
ENSO:
Changes in the warm ocean current near the coast of Peru; and changes in low atmospheric pressure off the coast of Peru as compared to northern Australia are both known as the ENSO–El Niño Southern Oscillation.
There are two types of monsoon winds:
- advancing monsoon
- withdrawal of monsoon/retreating monsoon
Mechanism to the advancing monsoon:
The advancing monsoon is also called the southwest monsoon as it enters India in a southwesterly direction.
The South West Monsoon is not a trade wind as it is not a steady wind and it is pulsating in nature and they are influenced by different atmospheric conditions.
The southwest monsoon enters India around June 1 and lasts for about 100 to 120 days till mid-September.
At the time of the onset of the monsoon, the normal rainfall increases suddenly and persists for several days, this is called the "burst" of the monsoon.
The Arabian branch of monsoon rains first brings rain to the Malabar Coast and moves rapidly towards the northern plains.
The branch of monsoon winds from the Bay of Bengal first causes rain in Meghalaya and moves rapidly towards the northern plains.
In mid-June, the Arabian branch of the monsoon brings rain to Sauratra-Kutch and central India.
Both the Arabian and Bay of Bengal branches of the monsoon merge in the northern plains and Delhi receives rains towards the end of June.
In mid-July, monsoon rains reach Himachal Pradesh and the rest of the country.
Mechanism of retreating monsoon:
The retreating monsoon is also called the winter monsoon.
The retreating monsoon or withdrawal of monsoons is a gradual process.
Monsoon withdrawal first starts from northwest India in early September.
By mid-October, it completely withdraws from the northern half of peninsular India. The withdrawal of the monsoon is rapid over the southern part of peninsular India.
By early December, the monsoon withdraws from southern India.
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