Natural Disasters

Cyclone Sitrang: ‘Unusual’ quickening of pace, say weatherpeople

T-20 in the last few overs of cyclonic system? Speed doubled within span of four hours between 2 IMD bulletins

By Jayanta Basu
Published: Wednesday 26 October 2022
The cyclonic storm sitrang made an early landfall in Bangladesh. Photo: @ArnobK_ / Twitter

Note: An earlier version of this article incorrectly stated the westerly influence on cyclone.

Cyclone Sitrang rapidly doubling its pace and making an early landfall in Bangladesh has been termed as ‘unusual’ by meteorologists. Westerly winds might have played a role in the quickening of the cyclone system, which ultimately led to less destruction in West Bengal, India. 

Sitrang was projected to make landfall in early October 25, 2022 by India Meteorological Department’s (IMD) national bulletin number 17, issued at 10.15 pm October 24. However, the cyclone pulled a Viral Kohli in Melbourne by increasing its speed rapidly and made landfall about three hours earlier. 

This has been termed unusual by senior IMD officials. However, all other activities of Sitrang happened as per IMD predictions, said Mrutyunjay Mohapatra, director general of Meteorology for the department. 

Read more: Why cyclones like Sitrang stoke fear? Loss & damage worth billions due to Indian Ocean storms

“The cyclone mostly followed the projected track and there was a shift of hardly 20 kilometres in landfall point. Even the wind speed was close to projections,” Mohapatra told this reporter on October 26, 2022. 

The wind speed was reduced by about 10 kilometres per hour during landfall compared to earlier IMD predictions of 110 kmph. But IMD had already spelt out the revised wind speed in its last bulletin before landfall happened, claimed Mohapatra. 

Bangladesh sources, however, claimed that wind speed was even less. 

Rapid change in four hours 

Bulletins 17 and 18 were issued with a difference of four hours — underscoring the rapidity of the turn of events. 

IMD bulletin number 18 stated: 

The cyclonic storm Sitrang…moved rapidly north–northeastwards with a speed of 56 kmph during the past six hours. It crossed the Bangladesh coast between Tinkona and Sandwip close to Barisal between 2130 hrs IST and 2330 hrs IST on October 24 with a maximum sustained wind speed of 80-90 kmph, gusting to 100 kmph

The bulletin was issued at 11.30 pm on the night of October 24, just after the cyclone made landfall. 

The earlier IMD note, on the other hand, had stated, “the cyclonic storm…moved north-northeastwards with a speed of 28 kmph during past six hours (and) It is very likely to continue to move north-northeastwards and cross Bangladesh coast between Tinkona Island and Sandwip close to Barisal during early hours of October 25, 2022”.

Clearly, the cyclone speed doubled, just like the last few overs of a T-20 cricket match. “The speed increased rapidly in the last few hours and there was an early landfall compared to earlier projection,” said G K Das, head of IMD Kolkata, agreeing to a T-20 analogy.

Westerly influence

The westerlies are prevailing winds from the west toward the east in the middle latitudes. The westerly influence played a key role in the cyclone shifting gear rapidly, claimed experts. Though such quickening of speed is not a common phenomenon, the speed of the cyclonic systems often gets enhanced by the westerly influence. 

“Westerly influence is the main cause of quickening of the speed of the Sitrang cyclone system in last few hours. The speed was earlier hovering around 18 kmph but then shot up to about 30 and then to 56 kmph rapidly,“ explained Sanjib Banerjee, a deputy director general of IMD in Kolkata.

Under the influence, the upper part of the cloud got disconnected and moved ahead compared to the rest of the system, the expert explained.

“The impact, including rainfall, was less than initially predicted despite the projected speed remaining the same, because the system moved very quickly in the last few hours,“ said Banerjee.

The high speed of the system also ensured that it picked up less moisture from the atmosphere, said another expert.

Read more: Cyclone Sitrang: 7 Odisha districts on alert, evacuation efforts intensified

The cyclone structure got unorganised to an extent and hence, it took a sharp 90-degree turn instead of a bend and went for a quick landfall.

“The cyclone got into the westerly trough and, hence, the turn was a sharp 90 degrees and there was no bending,” said Das. Both Banerjee and Das could not recollect another recent cyclone that showed such sharp speed variation within such a short span.

“Under the westerly influence, an increase of the speed is common but almost doubling up the speed is unusual,” added Pulak Guha Thakurta, a cyclone expert in IMD.

The dry and cold wind coming from the north-westerly direction has started to dominate the system once the southern-westerly monsoon wind is cleared off, pointed out experts from Indian Institute of Tropical Meteorology. 

The extremely severe cyclonic storm Sidr also similarly enhanced speed in the last few hours before landfall. Sidr had resulted in one of the worst natural disasters in Bangladesh in 2007. 

Early landfall reduced damage

Though Bangladesh experienced a significant impact due to Sitrang, the damage was much less in West Bengal, India. Experts point out that the damage in both Bengal, particularly in the Sunderbans area and around, could have been much more if the cyclone had followed its predicted timeline.

“The sharp bend that took away the cyclone further away from West Bengal coastline, high speed of the system and no overlap with the high tide contributed to West Bengal not being much impacted,” observed an expert.

Read more: Just how many cyclones does the world have now

About 35 people died in 15 districts of Bangladesh due to Sitrang impact, apart from large-scale infrastructural destruction and damage to the livelihoods and shelters of the people.

About seven lakh people had to be evacuated to safe places, confirmed a source in the Bangladesh disaster management ministry to this reporter.

“We did not have any significant damages apart from breaching embankments at about 13 points in South and North 24 Parganas; but those are not of serious concern,” stated a senior official of West Bengal‘s disaster management department.

Kumirmari island, closest to the landfall point on the Indian side, and the Sunderbans tiger reserve forest adjacent to the international border also were not much affected, as per local inputs.

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