Offshore Trough and West Coast Monsoon

An offshore trough is an elongated zone of relatively low sea level pressure over the eastern Arabian Sea, close and roughly parallel to India’s west coast. During the southwest monsoon, it promotes convergence and ascent of moisture laden winds, thereby strengthening west coast rainfall.

How the trough intensifies rainfall

The trough modifies the lower atmospheric pressure and wind field along the coast, strengthening the following sequence:

• Moist southwesterly winds from the Arabian Sea are drawn toward the relatively lower pressure near the coast.
• The wind field develops stronger convergence, causing moist air to accumulate and rise.
• As the rising air expands and cools, water vapour condenses into clouds and produces sustained rainfall.
• A strong or persistent trough can maintain moisture supply and repeated cloud formation over the same coastal belt.

Amplification by the Western Ghats

The Western Ghats stand across the path of the moisture laden monsoon flow. Air already lifted by coastal convergence is forced to ascend further over the windward slopes, producing intense orographic rainfall along the Konkan, Goa, coastal Karnataka and Kerala sectors.

• Rainfall is generally greater on the coastal and windward sides than in the leeward rain shadow.
• Its intensity varies with the trough’s strength and position, the depth of moist winds and the speed of the Arabian Sea monsoon current.

Disaster significance

When the offshore trough remains active, prolonged heavy rainfall can saturate soil and rapidly increase surface runoff. In steep and deeply weathered hill terrain, this raises the probability of landslides, while coastal plains and short west flowing rivers face enhanced flood risk.

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