How climate change made Arabian Sea conducive for severe cyclones

According to a paper in Elsevier’s Earth Science Reviews published last year, the sea surface temperatures over the Arabian Sea increased by 1.2°C to 1.4°C in recent decades

Arabian Sea was always relatively cooler in comparison to the Bay of Bengal in the north Indian Ocean until it started undergoing a major change of character around four decades ago.

The duration of very severe cyclones has increased by 260%. (ANI image)

According to a paper in Elsevier’s Earth Science Reviews published last year, the sea surface temperatures over the Arabian Sea increased by 1.2°C to 1.4°C in recent decades compared to four decades ago.

In 2021, a research paper in Nature journal said there is a significant increasing trend in the intensity, frequency, and duration of cyclonic storms (CS) and very severe CS (VSCS) is observed over the Arabian Sea during the study period of 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 during the last two decades.

The duration of very severe cyclones has increased by 260%.

The change in 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 6% of the global tropical cyclones annually and 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 has pointed.

“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,” the paper by Vineet Kumar Singh, researcher from Typhoon Research Centre, Jeju National University and Mathew Roxy Koll, climate scientist at Indian Institute of Tropical Meteorology stated.

On June 6, when Biparjoy was forming, Koll had posted on his official Twitter account 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 Madden-Julian Oscillation (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.

“At least in the last ten years, the Arabian Sea has been very warm. One reason for Biparjoy’s intensification is the warm ocean but also various other atmospheric factors. There is also 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 two anti-cyclones on both sides. India Meteorological Department (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.

“Parts of central Bay of Bengal and Arabian Sea both are extremely warm at around 31°C during this season. For a cyclone to form you need only 26°C SST, so ocean heat and wind direction is helping. The upper air divergence and interaction with monsoon flow is also aiding the system,” added a senior IMD official.

There was no model consensus on the track and landfall of Biparjoy until Saturday.

“We wanted to be absolutely sure that it is moving towards Gujarat before providing warning,” the official said.

“The oceans have become warmer already on account of climate change. In fact, recent study shows that the Arabian Sea has warmed up by almost 1.2°C since March, thus conditions are very much favourable for the rapid intensification of the system so it has potential to sustain the strength for a longer period,” said Raghu Murtugudde, Professor, department of atmospheric and oceanic science, University of Maryland and IIT Bombay said in a statement on June 7.