Monsoon Winds of the Bay of Bengal

The Bay of Bengal branch of the southwest monsoon is not a single, uniform current of air. Its path is shaped by the land it meets, and the story of this branch explains both the heaviest rainfall on Earth and the surprising dryness of one coastal region.

The branch first strikes the coast of Myanmar and part of southeast Bangladesh. Here, the Arakan Hills, running along the Myanmar coast, act as a physical barrier. They deflect a large portion of this branch towards the Indian subcontinent. Because of this deflection, the monsoon enters West Bengal and Bangladesh not from the usual south-westerly direction, but from the south and southeast.

Once the branch enters this region, it splits into two under the combined influence of the Himalayas to the north and the thermal low over northwest India. One branch moves westward along the Ganga plains, travelling as far as the Punjab plains. The other branch moves up the Brahmaputra valley towards the north and northeast, causing widespread rains across that region.

From this second branch, a sub-branch strikes the Garo and Khasi hills of Meghalaya. It is here that Mawsynram, located on the crest of the Khasi hills, receives the highest average annual rainfall in the world. The orographic effect — moist air forced upward over the hills — is at its most extreme at this location.

The section then raises a natural question: why does the Tamil Nadu coast remain dry during this season? Two factors are responsible.