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Vinson Kurian

Thiruvananthapuram, Aug 11

An offshore trough that didn’t exist for the most part and an indifferent low-level jet (LLJ) that whizzed far too north may have combined to deny Kerala its due share of rainfall this season.

The inconsistent offshore trough is as well blamed for the dry spell along the west coast and the adjacent interior peninsula covering Maharashtra and Karnataka.

An offshore trough is an elongated trough of low pressure and is a prominent atmospheric feature that pulls in the monsoon current.

The current itself operates in the form of a cross-equatorial low level jet (LLJ) with its core at an altitude of about 1.5 km and a band of winds speeding to 80-100 km/h.

During a strong monsoon, the core of the LLJ passes through southern Kerala. During a weak spell, it bypasses India and flows to its south.

Active monsoon conditions presuppose a full-blown offshore trough and an LLJ caressing the peninsular tip during the onset phase. The LLJ gradually lifts itself and the core of southwesterlies to coastal Karnataka and further north.

Dynamics of the flows are built around the mechanism of ‘barotropic instability, explains Dr Akhilesh Gupta, Advisor to Department of Science and Technology. This means winds turn anti-clockwise (cyclonic) to the north of the LLJ and clockwise (anti-cyclonic) to its south.

Cyclonic motion aids upward motion resulting in cloud formation and rain while anti-cyclonic motion represents downward motion (subsidence) resulting in suppressed rain.

The LLJ has been anomalously directed over coastal Karnataka from very early into the onset phase. A mass of ‘subsiding air’ has come to sit over Kerala ever since.

Post-LLJ, Kerala gets rain from the offshore trough and the vortices forming as exceptionally strong flows cause the trough to loop up into a hyperactive whirl occasionally (but rarely off Kerala) and batter coastal Karnataka, Konkan and Kerala.

During this season, offshore troughs and vortices were far and few between, if nor outright absent. Hence the dry conditions, Dr Gupta said.

The burst of rains over Konkan and Goa early into the season could be attributed to the LLJ effect only.

More Stories on : Climate & Weather | Kerala

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