A seasonal reversal of prevailing winds affecting the Indian Ocean and western Pacific, bringing heavy rainfall on its wet phase.
In practice
For a master planning a voyage through the Indian Ocean or Arabian Sea, the monsoon season determines route selection, speed, port call sequence, and cargo securing standards. Vessels bound for Indian west coast ports during the south-west monsoon face significant swells and restricted port access; some ports are effectively closed or operate at reduced capacity during the peak monsoon. Voyage planning under SOLAS Regulation V/34 must account for the monsoon forecast, and weather routing services provide specific guidance.
Regulatory detail & full definition
The monsoon is a large-scale seasonal reversal of prevailing winds affecting the Indian Ocean, the Arabian Sea, the Bay of Bengal, and adjacent seas, caused by differential heating between the ocean and the Asian land mass. The south-west monsoon runs roughly from June to September, bringing strong south-westerly winds, heavy rain, and a heavy swell to the Arabian Sea and the west coast of India. The north-east monsoon, from November to February, delivers drier, more moderate conditions with north-easterly winds. Both monsoons profoundly affect vessel routing, port operations, and cargo handling in the region.
For a master planning a voyage through the Indian Ocean or Arabian Sea, the monsoon season determines route selection, speed, port call sequence, and cargo securing standards. Vessels bound for Indian west coast ports during the south-west monsoon face significant swells and restricted port access; some ports are effectively closed or operate at reduced capacity during the peak monsoon. Voyage planning under SOLAS Regulation V/34 must account for the monsoon forecast, and weather routing services provide specific guidance.
Cargo insurance and P&I Club warranties may specify restrictions on trading during monsoon months. Officers must note in the deck log the observed monsoon conditions at regular intervals and update weather reports accordingly. Crew welfare is also affected, as sustained heavy weather during the monsoon can cause fatigue and increase the risk of accidents on deck.