CORE Southwest Monsoon Onset and Branches
Southwest monsoon onset over Kerala is the operational opening of India's rainy season, not a fixed historical event. India Meteorological Department states that the southwest monsoon normally sets in over Kerala around 1 June and advances northwards in surges, usually covering the country around mid-July. The onset is declared through rainfall over 14 stations in Kerala and neighbouring areas, wind field and outgoing longwave radiation over the southeast Arabian Sea. This turns a familiar calendar date into a measured meteorological threshold. After onset, the Arabian Sea branch strikes the Western Ghats and gives heavy windward rainfall, while another stream enters through the Bay of Bengal, moves toward the northeast and then bends westward along the Ganga plain. Rajasthan's connection is indirect but important: the Arabian Sea stream crossing Saurashtra and Kutch weakens before reaching western Rajasthan, and the Bay branch loses moisture as rainfall decreases from east to west across the northern plains. The onset date, branch paths, wind reversal and moisture supply together define the monsoon type of Indian climate. The two branches also explain spatial sequencing. Kerala, coastal Karnataka and Konkan receive early heavy rain from the Arabian Sea side; Assam and Meghalaya receive early intense rain from the Bay side; Delhi and Rajasthan wait for inland advance. A delayed or weak branch affects sowing dates, reservoir inflow, power demand and heat persistence differently in each region. The onset signal also affects migration of labour, fodder planning and urban water demand in drought-prone districts. The same signal guides state advisories on nursery sowing and contingency cropping.