Bay of Bengal branch

The Bay of Bengal branch is one of the two principal arms into which the Indian summer monsoon (the southwest monsoon) splits after the Inter-Tropical Convergence Zone (ITCZ) shifts north of the equator around June. As the southeasterly trade winds of the Southern Hemisphere cross the equator near the East African coast, they are deflected to the right under the Coriolis force and become the moisture-laden southwest monsoon. The Indian peninsula, with its triangular shape and the Western Ghats, splits this current into the Arabian Sea branch and the Bay of Bengal branch. The latter is the easterly stream that gathers moisture over the warm waters of the Bay of Bengal and advances toward the Indian subcontinent and Myanmar coast.

The Bay of Bengal branch flows northeastward and strikes the Arakan Yoma and the hills of Myanmar, which deflect a large part of it westward and northwestward over the Indo-Gangetic plain. This deflection explains why the monsoon enters India from the southeast rather than the southwest in the eastern sector. On reaching the Meghalaya hills, the branch is funnelled into the Khasi range, where the orographic uplift produces the world's heaviest rainfall at Mawsynram and Cherrapunji (Sohra), the latter recording over 1,000 cm annually. The branch normally reaches the Bengal delta and Assam around the first week of June, then bends west under the influence of the Himalayas, which act as a barrier preventing the monsoon from escaping into Central Asia and channelling it up the Ganga valley toward Punjab, where it eventually merges with the Arabian Sea branch. Rainfall consequently decreases progressively from the east (Kolkata) toward the west (Delhi, Allahabad) along the plains—a classic illustration of declining orographic and distance effects.

In a typical year, the Bay of Bengal branch governs onset over Kolkata (around 7 June), the entire Gangetic plain by mid-June, and contributes the bulk of the kharif-season rainfall to West Bengal, Bihar, Uttar Pradesh, Assam and the northeast. Its strength is modulated by the position of the monsoon trough, the formation of low-pressure systems and depressions over the head Bay, and broader drivers such as ENSO and the Indian Ocean Dipole, which the India Meteorological Department (IMD) monitors for its long-range forecasts. The withdrawal of this branch begins in September, retreating from the northwest first and from the Bay region last by October–November, when the retreating monsoon over the Bay feeds the northeast monsoon rains of the Tamil Nadu coast.

For the UPSC examination, the Bay of Bengal branch is a staple of General Studies Paper I (Indian and World Geography) and is frequently paired with the Arabian Sea branch in comparative questions. Prelims commonly tests the cause of Mawsynram/Cherrapunji's extreme rainfall, the role of the Arakan Yoma and Meghalaya hills, and the east-to-west rainfall gradient across the Gangetic plain. Mains answers should integrate the mechanism (ITCZ, Coriolis deflection, orographic uplift) with named locations and the merging of the two branches to demonstrate analytical depth.