Colloquially, a sudden wet season within the tropics, but, more explicitly, a seasonal shift of air flows, cloud, and precipitation systems. Monsoons have been described in West and East Africa, northern Australia, Chile, Spain, and Texas, but the largest is the south-west monsoon. This Asian monsoon is the atmospheric response, complicated by the presence of water vapour, to the shift of the overhead sun, and therefore zone of maximum heating, from the Tropic of Capricorn in late December to the Tropic of Cancer in late June. Associated with this response are major changes in jet stream movements and a meridional shift of the rain-bringing inter-tropical convergence zone.
In winter, pressure is high over central Asia; winds blow outwards, and some depressions, guided by upper-air westerlies, move from west to east, bringing rain. During the spring, a thermal low develops over northern India. Rains, related to a trough in the upper air, enter Burma in April/May, and India in late May/early June. In early summer the direction of the upper air changes, and the tropical easterly jet stream is semi-permanent about 15° N for the rest of the summer. With this change, the monsoon ‘bursts’, giving heavy rain across the southern half of the subcontinent.
By late June there is a continuous southerly flow of warm, moist air into the monsoon trough lying across northern India. This flow is a continuation of the south-east trades, altered to south-westerlies by the Coriolis force as they move north, across the equator, bringing huge quantities of water vapour, and reaching the west coast of India as the south-west monsoon. Here, the rainfall is orographically enhanced. Monsoon depressions are formed in association with these air flows, steered by the now easterly jet. Subsiding upper air prevents rainfall in the Thal and Thar deserts of the north-west of the subcontinent. By autumn, the easterly jet stream is replaced by a narrow band of the westerly subtropical jet stream, which follows the southern Himalayas. The south-west monsoon begins to retreat in September. Thereafter, the north-east trade winds dominate, and hurricanes are common in the Bay of Bengal. See O’Hare (1997) Geo 82, 3 and 4 for an exegesis on the Indian monsoon, and Bhaskara et al. (2001) Meteor. Applications 8 on forecasting of the Indian south-west monsoon.