E-Paper
Why cyclones such as Biparjoy have become severe and frequent, explained
Sea surface temperatures over the Arabian Sea have increased by 1.2 to 1.4 degrees C in recent decades compared to four decades ago
Behind extremely severe cyclone Biparjoy’s evolution is a gradual but undesirable change in the nature of the Arabian Sea: it was always relatively cooler compared to the Bay of Bengal in the north Indian Ocean.
Then, four decades ago, it started undergoing a major change of character .
Also read: 'Biparjoy' on course to become cyclone with longest lifespan in Arabian Sea: IMD
Sea surface temperatures over the Arabian Sea have increased by 1.2 to 1.4 degrees C in recent decades compared to four decades ago according to a paper in Elsevier’s Earth Science Reviews published last year. The result? More frequent cyclones that were stronger and often lasted longer.
According to a 2021 paper published in Nature, there is a significant increasing trend in the intensity, frequency, and duration of cyclonic storms (CS) and very severe cyclonic storms (VSCS) observed over the Arabian Sea during the study period, 1982 to 2019. There is a 52% increase in the frequency of CS during the recent epoch (2001–2019) in the Arabian Sea, while there is a decrease of 8% in the Bay of Bengal, the paper highlighted. Further, there has been an 80% increase in the total duration of cyclones in the Arabian Sea over two decades to 2021. The duration of very severe cyclones increased by 260% in the same period, the paper added.
The change in the Arabian Sea’s character has also led to more severe cyclones forming and sustaining over it which also means India’s west coast is now more vulnerable.
The north Indian Ocean accounts for only 6% of the global tropical cyclones annually , yet some of the most devastating cyclones have formed in this basin, causing extensive damage to the life and property in the north Indian Ocean rim countries, the Earth Science paper said. “Rapid warming in the north Indian Ocean, associated with global warming, tends to enhance the heat flux from the ocean to the atmosphere and favour rapid intensification of cyclones. Monitoring and forecasting rapid intensification is a challenge, particularly due to gaps in in-situ ocean observations. Changes in ocean-cyclone interactions are emerging in recent decades in response to Indian Ocean warming, and are to be closely monitored with improved observations since future climate projections demonstrate continued warming of the Indian Ocean at a rapid pace along with an increase in the intensity of cyclones in this basin,” said the paper by Vineet Kumar Singh, researcher from Typhoon Research Centre, Jeju National University, South Korea and Mathew Roxy Koll, climate scientist at Indian Institute of Tropical Meteorology, Pune.
On June 6, when Biparjoy was forming, Koll tweeted about the link between an exceptionally warm Arabian Sea and the strength of the cyclone. “IMD forecasts indicate a very severe cyclone in the Arabian Sea, driving the moisture away, instead of the expected monsoon onset. An exceptionally warm Arabian Sea, a weak monsoon onset, and favourable MJO conditions are favouring this cyclone…SSTs are 31-32°C, 2-4°C above climatological mean. This is clearly the climate change link, as Arabian Sea warming is favouring more intense cyclones. Increasing cyclone activity and global warming The increase in cyclone activity in the Arabian Sea is tightly linked to the rising ocean temperatures and increased availability of moisture under global warming. Arabian Sea used to be cool, but now it is a warm pool,” he said in a series of tweets.
There may be other factors behind Biparjoy too, according to another expert
Photos: Cyclone Biparjoy brings heavy showers, high tides, gusty winds to Mumbai
“At least in the last 10 years, the Arabian Sea has been very warm. One reason for Biparjoy’s intensification is the warm ocean but (there are) also various other atmospheric factors. There is a 30-year cycle for frequency of Arabian Sea cyclones which is well known. That may have an impact too. The cyclone had been moving extremely slowly because of the anti-cyclones on both sides. IMD expects the intensity of the cyclone to reduce before landfall which will reduce the impact but it’s important to know that Biparjoy can still cause widespread damage during landfall,” explained M Rajeevan, former secretary, ministry of earth sciences.
Anti-cyclone is a high pressure around which winds move in clockwise direction. Rajeevan referred to a 30 year cycle when cyclones are more severe over Arabian Sea followed by not so severe ones.
