The rapid intensification of Cyclone Amphan from a cyclone to a super cyclone in about 40 hours might be a sign of a warming Bay of Bengal, a consequence of anthropogenic global warming.
“The rapid intensification of Amphan into a super cyclone was because of many favourable conditions over the Bay of Bengal like high sea surface temperatures, low vertical shear winds and enough moisture in the middle layers of the atmosphere all of which, aid in the development of a cyclone,” Mrutyunjay Mohapatra, director general of the India Meteorological Department (IMD) and a well-known expert on cyclones, said.
“The development of super cyclone Amphan definitely qualifies as rapid intensification (RI) even though we may not be able to use the same definition as those developed for hurricanes and we will need to develop our own scale for the Indian Ocean cyclones,” Raghu Murtugudde, a climate scientist at the University of Maryland in the United States, said.
RI happens when there is an increase of maximum sustained winds of a cyclone by at least 55 kilometre / hour within 24 hours. This phenomenon has been recorded and studied in the case of hurricanes but not as much in the case of cyclones.
Both cyclones and hurricanes are tropical cyclones but are named differently according to the ocean in which they form. While hurricanes form in the North Atlantic and central and eastern North Pacific Oceans, cyclones form in the South Pacific and Indian Oceans.
A study published in the journal Nature Communications in February 2019 found a detectable increase in the rate of intensification of hurricanes in the Atlantic Ocean between 1982 and 2008.
It also established a link between this intensification and anthropogenic global warming but required more data and a better climate model to make exact conclusions. Though the study was global, it could not satisfactorily study the cyclone intensification rates in other parts of the world because of lack of data.
“Rapid Intensification might become more frequent in the Indian Ocean as well, especially considering the rapid and monotonic warming of the Indian Ocean. This is not good news for prediction or for impacts”, says Murtugudde.
Some RI was observed in the case of Cyclone Vayu that formed in the Arabian Sea just after the onset of the south-west monsoon season.
“There is a lack of long-term observational data for cyclone intensification rates in India. Thus, no robust statement can be made about RI over the North Indian Ocean. The precautionary principle would demand that strategies for collecting necessary data to track RI be established urgently,” advises Murtugudde.
Cyclone Amphan intensified to super cyclone category, with wind speeds in excess of 230 km / hr and gusts of up to 265 km / hr on the afternoon of May 18, 2020, according to the IMD. A super cyclone has wind speeds in excess of 220 km / hr.
This peak intensity is going to last for another 24 hours before the cyclone decreases in intensity and makes landfall somewhere along the West Bengal and Bangladesh coasts as an extremely severe cyclone with wind speeds of 165-175 km / hr, with gusts of up to 185 km / hr. IMD had earlier predicted that Amphan will make landfall as a very severe cyclone (wind speeds of 118-166 km / hr).
This is the first super cyclone to form in the Bay of Bengal after the 1999 super cyclone that hit Odisha and claimed more than 10,000 lives. It is the third super cyclone to occur in the North Indian Ocean region after 1999 which comprises of the Bay of Bengal, Arabian Sea and the northern part of the Indian Ocean.
The other two super cyclones were Cyclone Kyarr in 2019 and Cyclone Gonu in 2007.