The Hindu Business Line : Monsoon onset window open as ‘storm’ blows over

Thiruvananthapuram, May 26 Threat of a cyclone in the Arabian Sea seems to have blown over, and the causative ‘low’ will now act as an important cog in the wheel for the incoming monsoon caravan.

Easterly vertical wind shear and lack of ‘ventilation’ towards the top combined to nix the would-be cyclone, according to the US Navy’s Joint Typhoon Warning Centre (JTWC).

This, by default, will help consolidate the flows being directed across the Arabian Sea. India Meteorological Department (IMD) said in an update on Monday that conditions were becoming favourable for the onset of monsoon over Kerala during the next four days.

Fairly widespread rainfall is likely over Lakshadweep, Kerala and coastal Karnataka over the next few days, the IMD said. On Monday, the monsoon advanced into some more parts of south and central Bay and parts of northeast Bay of Bengal.

An offshore trough extending from the Karnataka coast to the Kerala coast has appropriated the ‘rogue low’ situated in the southeast and adjoining east-central Arabian Sea, as an embedded feature.

FLOWS TO DEEPEN

Meanwhile, numerical models suggested the strengthening and deepening of the cross-equatorial wind flow over south Arabian Sea during next four days.

Another important development is the ‘east-west shear zone of turbulence’ getting established over the south peninsula.

Shear zone is where the winds change speed and/or direction rapidly over a given distance. The shear zone is set up by low-level monsoon westerlies and upper-level easterlies criss-crossing the peninsula. The core of the easterlies (housing the Easterly Jet) was seen converging around the 10 deg N latitude on Monday.

The shear zone peaks into peak activity when a circulation gets embedded into it. The turbulence gets driven manifold, often resulting in storms in the nearby seas and heavy rains along the coast and progressively into interior peninsula.

A European Centre for Medium-Range Weather Forecasts (ECMWF) update predicted that the shear zone could feature circulations on either side of the peninsula by June 1, which could represent an active phase of the monsoon.

FRESH WESTERLY

What could unsettle the build-up temporarily is the expected arrival of a fresh western disturbance within the next two days, leading to a let-up in peninsular rainfall. But the westerly is expected to fade out by May 31, allowing elbowroom for a ‘low’ to pop up over the northeast Bay of Bengal, off Myanmar.

The ‘low’ will survive the aggressive forays by a follow-up westerly dipping in around June 2.

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