Northeast Monsoon

The northeast monsoon is the seasonal flow of winds from the northeast that brings rain to southeastern peninsular India, mainly during October-December. Although these winds generally originate over land, their passage across the Bay of Bengal supplies moisture, while low-pressure systems help produce substantial rainfall along the southeastern coast.

Seasonal reversal of the monsoon winds

As the Sun's apparent position shifts southward after summer, the Indian landmass cools and the pressure pattern supporting the southwest monsoon weakens. The southwest monsoon consequently withdraws, and northeasterly winds become established.

• The monsoon trough shifts southward as the summer low-pressure conditions over northern India weaken.
• Relatively higher pressure over the cooling continent and lower pressure towards the warmer southern seas favour northeasterly flow.
• The withdrawal of the southwest monsoon and the establishment of the northeast monsoon are related seasonal changes, but rainfall does not result from withdrawal alone.

How the Bay of Bengal supplies rain

The crucial factor is the path taken by the winds. Northeasterlies are generally dry over much of India because they blow from land towards the sea, but winds reaching the southeastern peninsula can cross the Bay of Bengal and acquire moisture.

• After crossing the Bay, these moisture-bearing winds reach the Coromandel Coast as onshore winds, supplying water vapour for rainfall.
• Convergence and ascent associated with low-pressure systems and depressions cool the moist air, causing condensation and rain.
• Tropical cyclones over the Bay can bring intense rainfall when they approach or cross the coast; ordinary seasonal rainfall does not require a cyclone on every occasion.
• Rainfall therefore occurs in spells, with its distribution strongly influenced by the movement and strength of these weather systems.

Why southeastern peninsular India benefits

The Tamil Nadu coast faces the Bay and receives moist onshore flow during this season. This contrasts with the southwest monsoon, when the Tamil Nadu coast receives relatively little rain.

• During the southwest monsoon, much of Tamil Nadu lies in the rain shadow of the Western Ghats, while the Bay branch largely flows parallel to its coast.
• During the northeast monsoon, the changed wind direction and Bay weather systems favour rainfall over Tamil Nadu and adjoining southeastern coastal areas.
• Thus, the retreating-monsoon season is a major rainy period for the southeastern coast, even as rainfall declines across much of the rest of India.