Natural Disasters

Cyclone Nivar may hit harder than we think. Here’s why

IMD has upgraded the storm to a ‘very severe’ one; expect it to rapidly intensify

By Akshit Sangomla
Published: Tuesday 24 November 2020

Cyclone Nivar is rapidly intensifying in Bay of Bengal and may be much more disruptive than India Meteorological Department (IMD) earlier predicted.

IMD November 24, 2020 evening upgraded the storm from ‘severe’ to ‘very severe’: Expect winds up to 120 kilometres per hour (not 75 kmph as expected earlier) in the next 24 hours. 

That would qualify as rapid intensification, said climate scientist Raghu Murtugudde of the University of Maryland, United States. 

“A change in wind speeds of about 30 knots (50-55 kmph) in 24 hours can be generally classified as rapid intensification. The ocean and atmospheric conditions are favourable for that,” said Roxy Mathew Koll of the Indian Institute of Tropical Meteorology, Pune. 

According to IMD:

Sea-surface temperatures (SST) are 29-30 degree Celsius; there is low vertical wind shear in the region.

These can lead to rapid intensification, as was the case with Cyclone Gati that has devastated Somalia and Super Typhoon Goni. It pummelled the Philippines at an unprecedented 315 kmph.

“We saw similar conditions during Cyclone Ockhi in November 2017. SST was warmer than usual and the Madden-Julian Oscillation (MJO) was favourable,” Koll added.

The MJO is the major fluctuation in tropical weather on weekly-to-monthly timescales. It can be characterised as an eastward moving ‘pulse’ of cloud and rainfall near the equator that typically recurs every 30-60 days, according to the Bureau of Meteorology of the Australian government.

Ockhi intensified rapidly from a depression to a cyclone within nine hours and further to a very severe cyclone in 24 hours, according to research by Koll and others published in the journal Current Science in September 2020.

They found that the MJO and warm sea conditions provided favourable dynamic and thermodynamic conditions for the genesis of Cyclone Ockhi. The conditions also aided in rapid intensification of the cyclone. 

In the case of Nivar too, southern Bay of Bengal is warmer (by 0.5-1°C) than usual, according to Koll. This is over a warm pool region of 28-30°C mean SSTs.

The quantum of heat in the ocean near the east coast is also warmer than usual — that can keep a cyclone active. Also, the winds are favourable due to an active MJO, Koll said.

Murtugudde explained:

Warm SST are what drive the convective bursts in the eye wall of the cyclone and lead to rapid intensification. Low-vertical shear allows the convection to intensify because a strong shear would steal away the energy from the convection.

Eye wall is the band of the strongest winds of a cyclone, which encircle a much calmer eye of the cyclone. 

Other factors can also contribute in making Cyclone Nivar deadlier.

“Cyclones now seem to weaken relatively slower after hitting land because of the excess energy,” Murtugudde said.

This was also seen in the deep depression that brought torrents of rainfall to Andhra Pradesh and Telangana in October.

“The intense convection at the eye wall produces hot towers reaching the atmospheric ceiling and causing strong descent into the eye of the cyclone which reduces pressure and strengthens the cyclone,” Murtugudde added. 

The outer bands of Nivar are already on land but the cyclone is still intensifying. So, the destructive power of the cyclone can reach further onto land because of the warm ocean, he warned. 

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