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‘There’s a long way before we can hope to tame tropical cyclones’

Vinson Kurian Thiruvananthapuram | Updated on June 01, 2021

A policeman directs people to leave as waves approach the shore ahead of Cyclone Tauktae in Veraval in the western state of Gujarat, India, May 17, 2021. REUTERS/Amit Dave

India’s long coast is a sitting duck to nature’s fury

India has a total coastline of 7,000 km or so with the states sharing nearly 5,500 km of mainland coastline while the islands of Lakshadweep and Andaman & Nicobar Islands too have hundreds of km of coastline that they need to protect. The entire coastline, therefore, is vulnerable to storms.

Even Union Territories, excluding of Jammu & Kashmir, Ladakh, Chandigarh and Delhi, too are unsafe since they open out into either the Arabian Sea or the Bay of Bengal. For instance, Puducherry, Andaman & Nicobar, Lakshadweep, Daman-Diu and Dadra Nagar-Haveli.

Cyclone seasons for India

“We have a long coastline under direct threat, with a full-flooded cyclone season divided into two phases, pre-monsoon and post-monsoon, to contend with. And the threat is usually more in the post-monsoon, I would say,” GP Sharma, President, Meteorology and Climate Change, Skymet Weather, points out.

“In recent times, we’re able to see this balance is changing and even the pre-monsoon season is getting as active and threatening as the post-monsoon,” Sharma told BusinessLine. Still, a pre-monsoon cyclone each crossing the coast on either side of the peninsula in close succession as happened this year is not that usual.

“I went through the records. The last time this happened was in 2010 when severe cyclone Laila that was active in the Bay of Bengal from May 17-21. But it had weakened into conventional cyclone when it made a landfall over Bapatla in Andhra Pradesh in what is a known trend with land-falling cyclones.”

Cyclones Laila, Bandu

Almost simultaneously on the other side of the peninsula, cyclone Bandu had reared in the Arabian Sea and was active during May 19-23. But it pulled away from our West Coast towards the Somalia coast, which was again in line with a pattern associated with pre-monsoon systems in the Arabian Sea.

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Last year (2020) also, two powerful cyclones brewed in the oceans though not in close succession, compared to ‘Tauktae’ or ‘Yaas’ this year. “But the gap wasn’t very much either in 2020”, Sharma said in reference to super cyclone ‘Amphan’ (Bay of Bengal, May 16-20) and very severe cyclone ‘Nisarga’ (Arabian Sea, June 1-4).

There is a precedent in the Indian seas wherein two storms have reared close to each other with almost 75 to 80 per cent of these forming in the Arabian Sea storms tending to head towards the Somalia-Yemen-Oman direction. But landfalls on the Indian coast make for an emerging exclusive phase.

Threat is more in the Bay

“Only a very few travels in close proximity to India’s West Coast. So, the danger for us from the Arabian Sea is very infrequent, I would say. But in the Bay storms threaten the entre coast from Tamil Nadu to West Bengal at any given time during the season. Only the target area changes from month to month,” Sharma said.

During the post-monsoon (October-November-December), the October cyclones will go to Odisha and West Bengal; the November ones aim Andhra Pradesh and adjoining Odisha; and the December ones target Tamil Nadu. Only the track changes, but the entire coast is vulnerable throughout the season.

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Cyclones have a tendency to weaken while approaching coast, due to various reasons. Among these are the cooler waters or the high vertical wind shear near the coast. “These two are very detrimental for the health of the storm systems and can kill them along their track over a period of time,” Sharma said.

Cyclone weakening factors

Storms pack enormous amounts of moisture and sustain over the smooth and warm waters. They are also huge in size, and their wing spans graze land features much earlier than the core engine does and suffers weakening due to frictional impact. This is a major factor weakening cyclone strength near the coast.

They also have to contend with dry air entrainment in which prevailing dry or hot air from land intrudes into the storm system compromising the prevailing moisture level, the fuel that sustains the storms. Vertical wind shear refers to the change in wind direction and strength with height, which too kills a storm.

“This was on show demonstrably with very severe cyclone Yaas this year. But the exact time and the drop-level are very difficult to tell in advance. Amphan was a super cyclone over the Bay waters, but while striking the coast it wound down to a grade lower. As a rule, all storms weaken as they brace for landfall,” Sharma said.

Published on June 01, 2021

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