52% rise in frequency of cyclones in Arabian Sea over two decades: Study

The frequency and intensity of cyclones developing over the Arabian Sea has increased in the last two decades, while fewer such storms have been seen over the Bay of Bengal, researchers have found in a paper that underlines the increasing risk of disasters hitting the west coast of India if the trend continues to hold over the years.

A 52% increase was noticed in the frequency of cyclones over the Arabian Sea between 2001 and 2019 , and an 8% decrease over the Bay of Bengal, compared to the 19-year period between 1982 and 2002, according to a new study by scientists of the Indian Institute of Tropical Meteorology (IITM) under the Union ministry of earth sciences.

Normally four to five cyclones form over the north Indian Ocean region (includes both Bay of Bengal and Arabian Sea) in a year, with the majority -- about three-four of them -- developing over the Bay of Bengal.

But this is fast changing. For instance, 2019 witnessed eight cyclones in the north Indian Ocean, out of which five formed in the Arabian Sea. The total figure was seven for 2018, including three over the Arabian Sea.

Not just the frequency, the intensity and duration of the cyclones over the Arabian Sea too are changing, according to the paper published in Springer’s Climate Dynamics journal on Saturday. Apart from a higher than usual number of cyclones over Arabian Sea in recent years, back-to-back “extremely severe cyclonic storms” (with wind speeds between 167-221kmph) occurred within a single month over the Arabian Sea in 2015, according to the paper.

This trend means India’s west coast may have to prepare for more frequent and stronger storms, researchers said.

“Until now the west coast was least prepared for very severe cyclones. Our study shows that the number of very severe cyclones in the Arabian Sea has gone up by 150% during the last two decades. This means that we need to be prepared, not only in terms of forecasts but also have a risk assessment. This risk assessment should be based on the overlapping impacts of storm surges, heavy rains and a rising sea level that can act together, resulting in prolonged floods in the west coast. More cyclones in the Arabian Sea means the chances of (storms) some getting closer to the west coast are more, like we saw in the last four years,” said Roxy Mathew Koll, climate scientist and co-author from IITM.

The study, titled Changing status of tropical cyclones over the north Indian Ocean, said cyclones over the Arabian Sea have become extremely intense in recent decades.

The research also noted that the duration for which a cyclonic storm lasts over the

Arabian Sea is on a rise. The total time span of cyclonic storms over the Arabian Sea rose 80% between 2001 and 2019 when compared to the 1982 to 2002 period. Similarly, there was a three-fold increase in the duration of “very severe cyclonic storms” (which see wind speeds in the range of 118-166 kmph) for the same period.

The findings consolidate what has been theorised for a few years: abnormal warming of the Arabian Sea is creating a recipe for deadlier cyclones compared to the Bay of Bengal, which was earlier considered to host ocean-atmospheric conditions that are more conducive to the development of cyclones.

When asked what could be causing this switch in cyclone patterns, Sunitha Devi, in-charge of cyclones at IMD agreed that there may be connections to the climate crisis, but said more studies are needed. “This is happening because of an increase in the ocean thermal energy, enhanced mid tropospheric moisture content as well as a reduced vertical wind shear compared to the past over the Arabian Sea,” she said.