The southwest monsoon begins in June and lasts till September. During summers, a low-pressure area is created over the northwestern part of the Indian Ocean. This attracts the southeast trade winds. These winds get deflected to their right because of the Coriolis force after crossing the equator.
- They reach the west coast as the southwest monsoon. They bring heavy rainfall accompanied by violent thunder and lightning. This violent onset of the monsoon is termed the burst of the monsoon.
- The monsoon winds get divided into the Arabian Sea branch and the Bay of Bengal branch.
Arabian Sea Branch of the Southwest Monsoon
- One branch of these winds causes heavy rainfall on the areas which lie on the windward side of the Western Ghats and the Western Coastal Plains. The areas located on the leeward side of the Western Ghats hardly receive any rainfall.
- The second branch of the Arabian Sea monsoon winds strikes the northern coast of Mumbai. It further moves along the river valleys of Narmada and Tapti and causes rainfall in central India. It then mingles with the Bay of Bengal branch after entering the Ganga plains.
- The third branch strikes the Saurashtra Peninsula and the Kachchh. It then travels to western Rajasthan and runs parallel to the Aravalli Range. Hence, western Rajasthan gets scanty rainfall.
- The Arabian Sea branch joins the Bay of Bengal branch causing rainfall in the western Himalayas.
Bay of Bengal Branch of the Southwest Monsoon
- The Bay of Bengal branch approaches Myanmar and southeast Bangladesh. Because of the presence of the Arakan Hills, they are deflected towards the Indian subcontinent. The monsoon thus enters West Bengal from the south and southeast instead of the southwesterly direction.
- This branch then divides into two—one running along the Ganga plains and the other along the Brahmaputra valley.
- These branches cause heavy rainfall in the Ganga plains, Brahmaputra valley, and Garo and Khasi Hills of Meghalaya. Mawsynram and Cherrapunji, which are located in the southern part of the Khasi Range, receive the highest average rainfall in the world.
This can be borne by the following examples:
- Mahabaleshwar located on the windward side of the Western Ghats receives heavy rainfall (250 cm). Pune, on the other hand, located on the leeward side gets less than 70 cm of rainfall annually.
- It rains heavily in the northeastern parts of the country because of the presence of the mountain ranges.
- There is a decrease in rainfall as one goes from the east to the west because the winds become dry as they shed their moisture in the course of their journey. Thus, Kolkata receives 120 cm of rainfall, Patna receives 102 cm, Allahabad receives 91 cm and Delhi gets 56 cm of rainfall.
- The coast of Tamil Nadu does not get rainfall during this season. It is because this coast is parallel to the Bay of Bengal branch of the southwest monsoon winds. It also lies in the rain shadow area of the Arabian Sea branch of the southwest monsoon.
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