Fewer cyclones in the Bay of Bengal but frequency increased in Arabian Sea: Report

Subject: Geography

Context-

Northern Indian Ocean cyclones may have gained notoriety for causing considerable devastation, but new research has noted a decline in the Bay of Bengal.
The Arabian Sea, however, has registered an increase in the last two decades, according to researchers from the Indian Institute of Science Education and Research (IISER) in Bhopal. They attributed this trend to global warming.

IMD data on the pattern of cyclones-

The India Meteorological Data (IMD)’s data for the 130-year-long study period found an average of 50.5 tropical cyclones per decade over the region comprising the Bay of Bengal in the East and the Arabian Sea in the West.
The researchers found that 49.8 per cent of tropical cyclones occurred from October-December in the post-monsoon period, while 28.9 per cent of the cyclones occurred in the pre-monsoon season between April to June during the same 130-year period.

Increase in cyclones over the Arabian sea-

The Arabian Sea side of the north Indian Ocean, however, saw a 52 per cent increase in cyclonic storms (63-88 km per hour) from 2001 -2019.
The frequency increase in very severe cyclonic storms, extremely severe cyclonic storms and super cyclonic storms in the Arabian sea was observed during the post-monsoon months.
As an exception, 2019 witnessed five tropical storms over the Arabian Sea and three over the Bay of Bengal.
In contrast, the frequency of Bay of Bengal cyclonic storms has slightly decreased but not to a significant extent.
The researchers examined the Bay of Bengal tropical cyclones from 1982-2020 and found that El Nino Southern Oscillation (ENSO) years reported more tropical cyclonic activity over the region.

Role of vertical shear in cyclone development-

With the greenhouse gases increasing and the earth warming, not only do temperature and humidity increase but also winds change and become weaker simultaneously.
One thing in the atmosphere that inhibits the growth of cyclones is called vertical shear, which refers to how strongly the winds can change from the surface to the top of the atmosphere, for up to 10 kilometres or so.
It determines if the cyclone, which is trying to grow like a tunnel, gets chopped off or not.
If it does, then its energy gets taken away and it doesn’t grow into a strong cyclone.
In other words, strong vertical shears suppress cyclones and weak vertical shears increase cyclones.
While the global trend reflects a decrease in cyclones, there has been an increase in cyclones over certain parts of the world, including the Northern Indian Ocean (Arabian sea).