The Southwest Monsoon is a fundamental climatic concept in India, defined as a major seasonal wind system that brings substantial rainfall to the Indian subcontinent, typically from June to September. Historically, the concept's origin is linked to the seasonal reversal of winds, a phenomenon first explained by Sir Edmund Halley's Thermal Concept, which posited that the unequal heating of land and ocean creates a gigantic-scale land and sea breeze. The problem it solves is the provision of approximately 75% to 80% of India's total annual rainfall, making it the lifeline for the country's agriculture and water resources.
The mechanism works due to the intense heating of the landmass, particularly the Tibetan Plateau and the Thar Desert, during summer, which creates a strong low-pressure area over the northern plains. This low pressure attracts the relatively cooler, moisture-laden winds from the high-pressure areas over the Indian Ocean. As the southeast trade winds from the Southern Hemisphere cross the equator, the Coriolis force deflects them to blow from the southwest direction, hence the name Southwest Monsoon. A key mechanism is the northward shift of the Inter-Tropical Convergence Zone (ITCZ), which in July is located over the Indo-Gangetic Plain and is often called the Monsoon Trough. The monsoon splits into two main branches: the Arabian Sea branch and the Bay of Bengal branch. The Western Ghats and the Himalayas act as crucial orographic barriers, forcing the moisture-laden winds to rise and cause heavy rainfall.
The Southwest Monsoon connects directly to the Northeast Monsoon (or Retreating Monsoon), which occurs from October to December and affects mainly southeastern India. Related concepts include the El Niño-Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD), which significantly influence the monsoon's strength and variability. Recently, a study by the India Meteorological Department (IMD) found that the duration of the Southwest Monsoon season has extended by 1.6 days per decade between 1970 and 2020, linked to a delayed withdrawal of the monsoon winds. While the onset date over Kerala has shown no significant change, the season's contribution to annual rainfall has increased from 75% to 79%. Furthermore, there is a trend of increased short-duration, heavy rainfall events in some regions, and a decline in rainfall in others, indicating a change in the spatial distribution of the monsoon.