150% increase in the frequency of very severe cyclones in Arabian Sea in past four decades: Study

Very severe cyclones in the Arabian Sea have increased by 150% and their duration has risen by 260% in the past four decades, according to a new study by scientists of the Indian Institute of Tropical Meteorology (IIT-M) under the Union Ministry of Earth Sciences. In the Bay of Bengal (BoB), however, there has been an 8% decrease in the frequency of cyclones and no significant change in duration, the research found.

The study titled, ‘Changing status of tropical cyclones over the north Indian Ocean’ analysed the frequency, duration, and intensity of cyclones over the North Indian Ocean from 1982 to 2019. It revealed that the increase in cyclone activity over Arabian Sea is tightly linked to rising ocean temperatures and increased moisture availability under global warming.

More cyclones in Arabian Sea

The frequency of cyclone storms in the Arabian Sea increased by 52% between 2001 to 2019 compared to the 19-year period between 1982 and 2002, the study noted.

The researchers divided the 38 years (1982 to 2019) into two epochs: past epoch (PE) (1982–2000) and recent epoch (RE) (2001–2019) and presented the tropical cyclones frequency. It found out that more tropical cyclones formed over the Arabian Sea in RE as compared to PE. The frequency of more intense tropical cyclones increased two-fold in the Arabian Sea whereas there was no change in BoB, it stated.

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

But there is a change from the normal behaviour, according to the study. 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.

80% increase in the total duration of cyclones in Arabian Sea

Not just the frequency, the intensity and duration of cyclones is also changing over the Arabian Sea. According to the study, there was an 80% increase in the total duration of cyclones in Arabian Sea in the recent epoch (2001-2019), while there was no significant change in the duration in the BoB.

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.

The intensity of cyclones has also increased in the Arabian Sea, by about 20% (post-monsoon) to 40% (pre-monsoon).

Due to an increase in the frequency, intensity, and duration of tropical cyclones, Accumulated Cyclone Energy (ACE) has also increased in the recent epoch (2001-2019) over the Arabian Sea, the study found. Accumulated cyclone energy is a measure of the total wind energy during a cyclone’s lifetime.

According to the study, the increase in ACE was two-fold in the pre-monsoon (April–May–June) season and four-fold in the post-monsoon (October–November–December) season for Arabian Sea tropical cyclones. However, there was no significant change in the BoB.

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