The Indian Monsoon is a complex climatic phenomenon, a seasonal reversal of wind direction derived from the Arabic word mausim, meaning "season". It is a double system of seasonal winds that flows from sea to land in summer and from land to sea in winter. The concept's first scientific study was conducted by Arab traders who relied on the seasonal wind patterns for trade with India. The classical theory of its origin, the 'thermal concept,' was explained by Sir Edmund Halley in 1686, who proposed it as a large-scale land and sea breeze caused by the differential heating of land and sea.
The system operates through two main phases: the Southwest Monsoon (June to September) and the Northeast Monsoon (October to December). The primary rainy season, the Southwest Monsoon, is driven by the intense heating of the Thar Desert and Tibetan Plateau, which creates a low-pressure zone over the land. This low pressure draws moisture-laden winds from the high-pressure Indian Ocean. A key mechanism is the northward shift of the Inter-Tropical Convergence Zone (ITCZ), a low-pressure belt, which pulls the moist winds into the Indian interior. The Northeast Monsoon is the retreating monsoon, where winds blow from the cooler land to the warmer sea, bringing rainfall mainly to the southeastern coast, such as Tamil Nadu.
The monsoon's mechanism is also connected to global teleconnections like the El Niño-Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD), which influence its strength and variability. Modern climatologists have moved beyond Halley's thermal theory, emphasizing dynamic factors like the shifting of the ITCZ (Dynamic Concept by H. Flohn in 1951) and the role of Jet Streams. Recently, a 2019 study proposed a new definition for the Northeast Monsoon season, identifying the average onset and demise dates as November 6 and March 13, based on surface temperature analysis. Furthermore, a trend of delayed withdrawal of the southwest monsoon has been observed, leading to increased rainfall in October in nearly 48% of tehsils.