Cyclone Nisarga: Rise in sea temperatures could lead to more pre-rain cyclonic storms, say experts
Owing to rising sea surface temperatures, rapid intensification of these weather systems are being observed.Updated: Jun 02, 2020 05:52 IST
With the formation of Cyclone Amphan in Bay of Bengal, followed by the likely formation of Cyclone Nisarga in Arabian Sea within two weeks, scientists have warned about the possibility of more pre-monsoon cyclonic storms in coming years due to warmer ocean temperatures.
Dr Roxy Mathew Koll, senior scientist, Indian Institute of Tropical Meteorology (IITM), said, “We are aware that post-monsoon cyclones have increased, but climate models and projections are indicating the likelihood of more cyclones before the onset of the southwest monsoon in the Arabian Sea.”
Sea surface temperatures in the Arabian Sea were plotted on a map, showing the difference between long-term average temperatures and temperatures on May 30, even as the weather system that is yet to intensify into Cyclone Nisarga developed.
“While the temperatures in the Bay of Bengal, were between 30-33 degrees Celsius prior to Amphan, surface temperatures over the Arabian Sea recorded 30-32 degrees Celsius prior to the depression which is now evolving as cyclone Nisarga. Such high temperatures aid rapid intensification of these cyclonic systems, which many weather models fail to capture,” Koll said.
According to IITM, sea surface temperatures between 24 and 26 degrees Celsius are normal.
The India Meteorological Department (IMD), however, said that higher ocean temperatures were not a factor for the intensification of Cyclone Nisarga.
“During Amphan, warmer oceans allowed the system to reach a super cyclone status but not in this case so far,” said Mrutyunjay Mohapatra, director general, IMD.
Warmer ocean temperatures are not the only parameter for rapid intensification of cyclones, said Mohapatra. “We need to consider other parameters, such as relative humidity in middle troposphere and instability in the atmosphere, allowing moist air to form aiding cloud formation through the help of convection. There are also other dynamic parameters to be considered. However, the frequency of cyclones has been abnormally high in the Arabian Sea from the past one year and it is a fact that this zone is witnessing an increase in cyclones,” said Mohapatra.
Owing to rising sea surface temperatures, rapid intensification of these weather systems are being observed, added Dr Koll.
“What is more surprising is that the models are unable to pick up the rapid intensification of these systems. In case of Amphan and Nisarga, we saw that atmospheric conditions were not that favourable and may not develop into a cyclone, but warm ocean conditions are helping convection and allowing these systems to intensify faster into full-blown cyclones even during pre-monsoon period,” he said, adding that these were not conclusive results but indications were clear.
Last year, the Arabian Sea had recorded five cyclones, equalling a 117-year-old record, according to data from IMD. Cyclone Vayu, a very severe cyclonic storm, was witnessed from June 10-17 last year, while Cyclone Hikka, a very severe cyclonic storm, was seen in September 22-25. In October and November, India had witnessed twin cyclones Kyarr and Maha, which was a rare event according to IMD, while Cyclone Pawan was formed in December.
Flooding inevitable in low-lying areas: Ex-IMD DG
Former IMD director general (DGG) Dr KJ Ramesh said the predicted landfall dates of June 3 by IMD is in close proximity to the full moon date of June 6. “Three to four days before the full moon, the gravitational effect of moon and earth will be high, which means the wave action will be at the high- tide level. The storm surge will be over, resulting in large-scale flooding across several low-lying areas, up to the high-tide line. Wind speed of 80 kilometre per hour (kmph) and above will be more than enough to cause massive damage. This will be applicable from south Gujarat up to Raigad, and efforts to safeguard infrastructure have to be considered,” he said.
Mumbai at highest risk as cyclones become the new normal: Climate scientist
One of the authors of the Intergovernmental Panel on Climate Change’s (IPCC’s) special report on Ocean and Cryosphere pointed out that warming of oceans is the main factor for increasing the intensity and occurrence of tropical cyclones, especially in the Arabian Sea. “The most-affected city is Mumbai,” said Dr Anjal Prakash, research director and adjunct associate professor, Bharti Institute of Public Policy, Indian School of Business, Hyderabad Campus. He highlighted concerns raised by the Madhav Chitale committee after the July 2005 deluge. “The committee reiterated the reasons of the flooding which was known earlier – inadequate drainage system, rapid developments and loss of ponds that used to hold water, encroachment by the slums on and over the existing drainage systems, and reduction in the coastal mangrove areas. These are the adaptation measures which Mumbai must focus on,” he said, adding that the combination of the pandemic and extreme weather events could be dangerous, especially for slum residents living in low-lying areas of the city.