The monsoon is fundamentally a seasonal reversal in the direction of prevailing winds, driven by the differential heating between land and ocean. Land heats and cools faster than the sea, so in summer the heated continent draws in moist oceanic air, while in winter the flow reverses as the land cools below the ocean temperature.
The elevated Tibetan Plateau acts as a high-altitude heat source in summer, warming the middle troposphere directly rather than only at the surface. This elevated heating strengthens the meridional temperature gradient and helps establish the upper-level easterly jet that is tied to the mature summer monsoon circulation.
The arrival of the summer monsoon is not a gradual increase in rain but an abrupt transition marked by a sudden burst of rainfall and a reorganization of the large-scale circulation. This onset propagates across the subcontinent over days to weeks following a broadly predictable geographic progression.
Within a single monsoon season rainfall is not steady but alternates between active spells of heavy widespread rain and break spells of suppressed rainfall over the plains. These intraseasonal fluctuations are linked to shifts in the position of the monsoon trough and associated rain-bearing systems.
Much of the heavy rain over the subcontinent is delivered not by the broad flow itself but by discrete low-pressure systems and depressions that form over the Bay of Bengal and track inland. Viewing the season as a sequence of these travelling disturbances, rather than as continuous rain, explains the spatial and temporal clustering of heavy rainfall.
The total seasonal rainfall varies substantially from year to year, and this variability is linked to large-scale ocean and atmosphere conditions including sea-surface temperature patterns in the Pacific and Indian Oceans. Because so much of the regional economy depends on this rain, small shifts in the seasonal total carry disproportionate consequences.