Sea surface temperature, ocean heat content, upper air divergence and weak wind shear have helped Biporjoy intensify twice until now
A map of Kutch in Gujarat showing Mandvi near the Pakistan border. Photo: Google
A combination of factors helped Cyclone Biporjoy intensify from a very severe cyclonic storm to an extremely severe cyclonic storm on June 11, 2023.
Extremely severe cyclones form when wind speeds reach 168-221 kilometres per hour (kmph). As of 11.30 am on June 12, the wind speeds were 165-175 gusting to 190 kmph, according to India Meteorological Department (IMD).
At 2.30 pm on June 10, IMD warned that the cyclone would intensify into an extremely severe cyclone.
The prediction was accurate. The cyclone developed into an extremely severe cyclone at 5.30 am on June 11 and continues to remain in the category as on June 12, 11.30 am. But this forecast was made only a few hours prior.
“The models didn’t expect it (the intensification). However, our research shows a clear trend of cyclones intensifying into extremely severe cyclones,” Roxy Mathew Koll, a climate scientist at the Indian Institute of Tropical Meteorology told Down To Earth (DTE).
His study recorded a 52 per cent increase in the number of cyclones in the Arabian Sea. And a 150 per cent increase in Very severe cyclones. This increase has been linked to rising ocean temperatures and increased availability of moisture under global warming.
Cyclone Biporjoy has intensified twice in its lifetime. It is also the fourth longest-lived cyclone in the pre-monsoon region in the Arabian Sea since 1982, according to Vineet Kumar, postdoctorate researcher at Jeju National University, South Korea.
Models were not able to predict its intensity much in advance due to certain fluctuations, which help the cyclone strengthen or weaken. These include upper air divergence, wind shear, and dry air.
The amount of wind shear is the change of the wind with height. According to the National Oceanic and Atmospheric Administration, large amounts of wind shear are generally bad news for cyclone intensification.
“The shear towards the northern Arabian Sea has been weak and favourable for cyclone intensification,” Koll explained.
Further, upper air divergence which happens when air flows (wind) away from a region, also helped the cyclone gain strength. The sea surface temperature and ocean heat content were already favouring the cyclone.
“Think of divergence as a ventilator, which helps the cyclone release air. It allows the cyclone to vent out and promotes its intensification. If this is stopped, it will weaken,” Akshay Deoras, research scientist at the National Centre for Atmospheric Science and the Department of Meteorology, University of Reading, United Kingdom.
Deoras had previously told DTE that there was an intrusion of dry air from Pakistan, which could hinder the cyclone.
On June 11, the sea surface temperature, weak shear wind, and upper air divergence overpowered the negative factor (dry air), helping Biporjoy gain strength.
As the cyclone approaches the coast, it is expected to weaken into a very severe cyclonic storm as it crosses between Mandvi (Gujarat) and Karachi (Pakistan) near Jakhau Port (Gujarat) by noon on 15th June. The maximum sustained wind speed could be 125-135 kmph gusting to 150 kmph, according to IMD.
The divergence is expected to stop and wind shear could increase. “Dry air will become prominent as it approaches the coast,” Deoras said, adding that these factors could help weaken the cyclone.
Also, these fluctuations are affecting the accuracy of model predictions. Models are not able to tell us with confidence about the intensity. “We know it will weaken but by how much is not clear,” Deoras explained.
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