India’s densely-populated west coast is likely to witness severe cyclones in the coming months, largely because of changes occurring in the warming patterns of the subsurface ocean. And the need of the hour is reliable — and localised — weather services catering specifically to the coastal communities.
These are among the findings and recommendations of a study by top climate scientists from the India Meteorological Department (IMD) and the Cochin University of Science and Technology (CUSAT) that has been published online in the prestigious, peer-reviewed Scientific Reports – Nature journal.
With severe cyclones on the cards, the study — conducted with the help of the Union Ministry of Earth Sciences and the University of Sussex in the UK — recommends localised and reliable weather services targeting fish workers, coastal communities, and other people who frequent the coastal regions to help them deal with the coming challenge.
Need for storm-warming service
“In the Arabian Sea, tropical cyclones are more prevalent just before and at the start of the Southwestern Monsoon during March–June, and again after the season in October–December. More weather systems have developed into cyclones in recent years, pointing to a change in the environmental factors conducive to storm activity,” says the report.
“The report urges development strategies that account for the dangers posed by changing climate and weather as well as policy and technological initiatives in the areas of storm warning, impact-based local weather services, and localised reliable weather services,” pointed out Dr Mrutyunjay Mohapatra, Director General of Meteorology (DGM) at IMD, one of the authors.
“IMD’s storm-warning service is one of the best in the world,” Dr Mohapatra added.
“We favour more impact-based forecasting relevant for even better anticipatory action at local levels given the challenges of climate change, forecast users’ special needs, and our increasing scientific and technical capabilities,” he said.
Fishers demand the same
Fishers of Kerala have been demanding forecast services that give a true picture of the impact of storms in their areas.
A comprehensive IMD-CUSAT study on localised marine weather forecasts would be ready soon, said CUSAT’s Advanced Centre for Atmospheric Radar Research (ACARR) Director Prof S Abhilash, another author.
Both studies are part of a research project, “Forecasting with Fishers”, that the ACARR has led locally over the past five years.
IMD, the Indian National Centre for Ocean Information Services, and the artisanal fishing communities of Thiruvananthapuram and Kanyakumari took part in this initiative conceptualised by the University of Sussex.
“This cyclone tendency and its clustering over Eastern Arabian Sea needs attention in terms of forecasting, catastrophe risk-reduction, and climate change adaptation due to the security of coastal urban and rural habitats, livelihoods, and essential infrastructure along the coasts,” wrote the scientists who took part in the study.
A tropical cyclone is a rotating low-pressure system with high wind and heavy rain fuelled by the warm and moist air over tropical oceans. It can continue for days or weeks and travel great distances till it dissipates over land or cooler oceans.
The study pointed out that the thermodynamic structure — comprising heat, temperature and energy relationships — of the upper ocean and lower atmosphere significantly impacts the development and intensification of cyclones, technically called cyclogenesis, over the Eastern Arabian Sea.
This rise in the development and intensification of cyclones in high numbers in certain periods over the Eastern Arabian Sea is regulated by a rise in thermal instability and humidity in the middle part of the troposphere between 4-10 km above the earth’s surface.
Instability denotes the tendency for air parcels to shoot up when warmed, causing severe weather.
Reasons for cyclone formation
At the same time, changes in ocean temperature at different depths due to global warming cause processes below the ocean surface that help tropical cyclone formation.
More severe cyclones are caused by high tropical cyclone heat potential, denoting the heat content from the sea surface to the depth of 26 degrees C isotherm — an imaginary line connecting points with the same temperature— at 50–100 metres ocean depth, the study said.
“Ocean subsurface warming present in this sea region influences the development and intensification of cyclones mostly during March–June. On the other hand, mid-tropospheric relative humidity and thermal instability influence the development and intensification of an anomalously high number of cyclones within a short period (called clustering) over the Eastern Arabian Sea during the October–December season. Tropical cyclones also interact and intensify over regions of higher tropical cyclone heat potential,” the study noted.
It added: “Very severe cyclonic storm (wind speeds above 118 kmph) Ockhi formed in the Bay of Bengal when it encountered anomalously high sea surface temperature and ocean subsurface temperatures, leading to its rapid intensification over the Eastern Arabian Sea.”
Increasing trend of intense cyclones
The paper calls attention to an increasing trend of intense cyclones, especially in the post-monsoon period. The first recorded post-monsoon Extremely Severe Cyclonic Storm (ESCS, with winds above 168 kmph) of the Arabian Sea occurred in October 2014 (Nilofar), followed by two back-to-back ESCSs during the next post-monsoon season (2015 – Chapala and Megh).
In 2019, there were five cyclones, and ESCS Maha coexisted briefly with the Super Cyclonic Storm (above 222kmph) Kyarr as an unprecedented double event in the satellite era (after 1961). Meanwhile, the total duration of very severe cyclonic storms has also increased three-fold, and cyclonic storms have increased by 80 percent.
The paper’s lead author is CS Abhiram Nirmal, a doctoral researcher at ACARR), guided by director Prof S Abhilash.
The other authors are Dr Mrutyunjay Mohapatra; Dr Syam Sankar, a researcher at the National Centre for Medium-Range Weather Forecasting; Dr AK Sahai, a scientist at the Indian Institute of Tropical Meteorology; and Dr Max Martin, University of Sussex geographer affiliated with ACARR.
The research looked at weather-related hazards, their implications for artisanal fishers, and options to improve forecasts and their use. Another joint study on localised marine weather forecasts is expected to be published in a prestigious journal in the coming weeks.