Active and Break Cycles of the Indian Monsoon

The Indian summer monsoon alternates between active phases, when rainfall is widespread and vigorous over the monsoon core zone, and break phases, when rainfall there becomes markedly subdued for several days. These are intraseasonal fluctuations produced mainly by changes in convection, moisture-bearing winds, low-pressure systems and the position of the monsoon trough.

Movement of the monsoon trough

The monsoon trough is an elongated low-pressure zone extending across northern India towards the Bay of Bengal. Its shifting position strongly controls where moist air converges and rises.

• During an active phase, the trough remains near or south of its normal position, promoting strong convection and widespread rainfall over central and adjoining India.
• During a break, the trough shifts northward towards the Himalayan foothills; rainfall decreases over much of the monsoon core zone but increases along the foothills and in the Brahmaputra valley.

Convection, circulation and weather systems

Active and break conditions reflect changes in the coupling between atmospheric convection and the monsoon circulation.

• Active conditions feature stronger low-level westerlies, abundant moisture transport from the Arabian Sea and Bay of Bengal, and enhanced moisture convergence over the subcontinent.
• Low-pressure areas and depressions forming over the Bay of Bengal often travel west-northwestward along the trough and reinforce active rainfall.
• Break conditions involve suppressed convection, weaker moisture convergence and greater descending motion over central India; fewer rain-bearing disturbances traverse the monsoon trough.

Role of intraseasonal oscillations

Large-scale convective anomalies periodically move northward from the equatorial Indian Ocean into the monsoon region. This northward propagation strengthens or suppresses rainfall and helps shift the trough between active and break configurations.

• The Boreal Summer Intraseasonal Oscillation and the Madden-Julian Oscillation can modulate convection, low-level winds and the formation of monsoon depressions.
• These oscillations interact with regional land-ocean heating and synoptic systems, so active-break transitions are not perfectly periodic or controlled by a single mechanism.