Indian monsoon

The Indian monsoon (Arabic mausim, "season") is the large-scale seasonal reversal of surface winds over South Asia, driven primarily by the differential heating of land and the Indian Ocean and the consequent migration of the Inter-Tropical Convergence Zone (ITCZ). The classical thermal theory of Sir H.F. Blanford and the later dynamic explanation associated with the work of M.T. Yin and others hold that intense summer heating of the Tibetan Plateau and the Thar Desert creates a deep low-pressure trough over northwest India, drawing in moisture-laden maritime air from the high-pressure cell over the southern Indian Ocean. The Tibetan Plateau's role as an elevated heat source, the formation of the Tropical Easterly Jet, and the seasonal shift of the Subtropical Westerly Jet north of the Himalayas are central to the modern dynamic understanding articulated by P. Koteswaram and refined by later monsoon research.

The southwest summer monsoon normally sets in over Kerala around 1 June and advances in two branches — the Arabian Sea branch and the Bay of Bengal branch — covering the whole country by mid-July, before withdrawal begins from northwest India in September. The Arabian Sea branch yields the heaviest orographic rainfall on the windward Western Ghats, while the Bay of Bengal branch, deflected by the Himalayas, gives Mawsynram and Cherrapunji their record totals and travels up the Ganga plain. The retreating or northeast monsoon (October–December) brings rain to Tamil Nadu and coastal Andhra Pradesh as winds pick up moisture crossing the Bay of Bengal. Monsoon rainfall is characterised by "break" periods, El Niño–Southern Oscillation (ENSO) teleconnections, the Indian Ocean Dipole, and the role of the Madden–Julian Oscillation in intraseasonal variability.

The India Meteorological Department (IMD), established 1875 and headquartered at New Delhi, issues operational Long Range Forecasts; it defines a "normal" monsoon as seasonal rainfall within 96–104% of the Long Period Average (about 87 cm). El Niño years such as 2009, 2015 and 2023 produced deficient rainfall, whereas La Niña conditions generally favour abundant monsoons. As of 2026 monsoon prediction increasingly relies on dynamical coupled ocean-atmosphere models alongside the older statistical Sixteen-Parameter and successor models. The monsoon's reliability underpins Indian agriculture — roughly half of net sown area remains rain-fed — making it decisive for food-grain output, reservoir storage, hydropower and rural demand.

For the UPSC examination the Indian monsoon is a core theme of Geography (both the Civil Services Mains Geography optional and General Studies Paper I, which covers physical geography of India and key natural resources). Typical question angles include the mechanism and theories of monsoon origin (thermal versus dynamic), the role of jet streams and the Tibetan Plateau, the distinction between southwest and northeast monsoons, the influence of ENSO and the Indian Ocean Dipole on rainfall variability, and the socio-economic implications of monsoon failure. Prelims items frequently test the onset date over Kerala, the two branches, IMD's "normal" definition, and pairings of rainfall regimes with regions.