The Arabian Sea branch is one of the two principal arms of the south-west summer monsoon, the seasonal reversal of winds that delivers roughly 75–80% of India's annual rainfall between June and September. As the differential heating of the Asian landmass creates an intense low-pressure trough over north-west India and Pakistan in summer, the south-east trade winds of the southern hemisphere cross the equator near the East African coast, are deflected by the Coriolis force to become south-westerlies, and divide over the Indian peninsula into the Arabian Sea branch and the Bay of Bengal branch. The Arabian Sea branch carries the larger volume of moisture, originating over the warm waters of the western Indian Ocean and advancing onto the Indian mainland around the first week of June, with the normal date of onset over Kerala fixed at 1 June by the India Meteorological Department (IMD).
The branch operates through three distinct sub-streams. The first strikes the Western Ghats almost perpendicularly, causing heavy orographic rainfall (200–400 cm) on the windward seaward slopes — accounting for places like Mahabaleshwar — while the leeward Deccan plateau lies in a pronounced rain-shadow, receiving under 75 cm and constituting India's semi-arid interior. The second sub-stream advances up the Narmada and Tapi valleys, penetrating central India without major orographic obstruction and watering Madhya Pradesh and Chhattisgarh. The third strikes the Saurashtra–Kutch coast and the Aravalli Range; because the Aravallis run parallel to the wind direction rather than across it, they fail to force ascent, leaving Rajasthan largely dry and contributing to the aridity of the Thar Desert. This third sub-stream eventually unites over the Indo-Gangetic plain with the Bay of Bengal branch, reinforcing precipitation over north India.
The Arabian Sea branch is the key to several classic geographical anomalies tested in examinations: the dryness of the Deccan despite proximity to a moisture source, the desert conditions of Rajasthan despite the moisture-laden winds passing over it, and the contrast between the saturated Konkan–Malabar coast and the parched Telangana interior. In 2026, IMD continues to monitor its onset and progress, with seasonal forecasts increasingly linked to the El Niño–Southern Oscillation (ENSO), the Indian Ocean Dipole (IOD), and the Madden–Julian Oscillation, all of which modulate the strength of the Arabian Sea moisture flux and the position of the Somali Jet that feeds it.
For the UPSC examination, this topic falls squarely within GS Paper I (Indian and World Geography) and the Geography optional. Prelims questions typically probe the sequence of onset, the three sub-streams, and rain-shadow effects (a perennial favourite is why the Aravallis cause no rain). Mains answers should integrate the branch with monsoon mechanism theories — the classical thermal concept, the Jet Stream theory, and the Tibetan Plateau (heat-engine) hypothesis — and link rainfall distribution to agriculture, cropping patterns, and regional development disparities. Candidates must also distinguish the higher rainfall delivered by this branch on the west coast from the diminishing intensity it shows as it moves inland.
Example
In June 2023, the IMD recorded the Arabian Sea branch's onset over Kerala on 8 June, a week-long delay attributed to a developing El Niño weakening the monsoon's western arm.
Frequently asked questions
The Aravalli Range runs parallel to the wind direction and cannot force the air to rise, so no orographic rainfall occurs over Rajasthan, producing the Thar Desert. The Deccan lies in the rain-shadow of the Western Ghats, which intercept moisture on their windward seaward slopes.