Monsoon Winds of the Arabian Sea

The monsoon winds that originate over the Arabian Sea do not travel as one single current. Instead, they split into three distinct branches, each taking a different path across the Indian landmass and producing very different rainfall outcomes. The behaviour of each branch is controlled by the physical barriers it meets — chiefly the Western Ghats, the river valleys of central India, and the arid tracts of Rajasthan.

The first branch and the Western Ghats. One branch of the Arabian Sea monsoon is obstructed by the Western Ghats. These winds are forced to climb the slopes of the Ghats from a height of about 900 to 1200 metres. As they rise, they cool rapidly, and this cooling causes heavy condensation. The result is very heavy rainfall on the windward side — that is, the western side of the Sahyadris and the Western Coastal Plain — receiving between 250 cm and 400 cm of rain. Once the winds cross the crest of the Ghats and descend on the eastern side, they warm up again. This heating reduces the humidity in the air, so the winds carry far less moisture by the time they reach the land east of the Ghats. Consequently, this eastern region receives very little rainfall and is known as the rain-shadow area. To appreciate this contrast, one can compare the rainfall at Kozhikode and Mangalore (on the windward coast) with that at Pune and Bengaluru (in the rain shadow) — the difference is striking.

The second branch and central India. Another branch of the Arabian Sea monsoon strikes the coast north of Mumbai. From there it moves inland along the valleys of the Narmada and Tapi rivers, bringing rainfall to extensive areas of central India. The Chotanagpur plateau receives about 15 cm of rainfall from this part of the branch. After this, the winds enter the Ganga plains, where they merge with the Bay of Bengal branch of the monsoon. …