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

Northern Indian Ocean cyclones may have gained notoriety for causing considerable devastations, but a 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. 

Raghu Murtugudde, visiting professor at the Indian Institute of Technology Bombay’s department of Climate Studies, explained this phenomenon to Down To Earth (DTE): 

With the greenhouse gases increasing and the earth warming, not only does 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 the 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,” Murtugudde said.

In other words, strong vertical shears suppress cyclones, the expert added. “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).” 

Read more: Cyclone Sitrang surprise: Landfall already complete in Bangladesh

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. 

Overall, 80 per cent of the tropical cyclones occurred in the pre- and post-monsoon months, said the IISER researchers.

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, according to another study published this April by IISER.

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.

Pankaj Kumar, associate professor at IISER and lead researcher of both studies, told DTE that Arabian sea cyclones are increasing significantly. He added: 

In comparison to the Bay of Bengal, the proportion of Arabian sea cyclones was initially 1:4, but in our study, we found that it has become 2:4 from 2001-2020.

“We studied the characteristics of the Arabian Sea cyclones using three wind-energy drive metrics, namely power dissipation index (PDI), accumulated cyclone energy (ACE) and accumulated cyclone intensity (ACI), which demonstrated to us that the length, energy and intensity of these cyclones increased over the last 20 years,” Kumar said.

In contrast, the frequency of Bay of Bengal cyclonic storms has slightly decreased but not to a significant extent. The reason is that there is a warming threshold for sea surface temperature (SST) over any ocean, the expert added. “It is generally between 26 degrees Celsius and 30 degrees Celsius, which has already been achieved in the Bay of Bengal.”

However, sea surface temperature is still gradually increasing over the Arabian sea, Kumar said. “Some studies have reported a 0.5-0.7 degree Celsius increase of SST over the Indian Ocean, but more studies have to be done for greater accuracy.”

Read more: Why cyclones like Sitrang stoke fear? Loss & damage worth billions due to Indian Ocean storms

The researchers examined the Bay of Bengal tropical cyclones from 1982-2o20 and found that El Nino Southern Oscillation (ENSO) years reported more tropical cyclonic activity over the region.

They reported a shift in tropical cyclogenesis during the time, which refers to the various stages of its formation, in terms of sea surface temperature, humidity, force, disturbance and atmospheric instability.

“As we are in a La Nina year, the Cyclone Sitrang headed eastwards towards Southeast Asia, skipping the Indian sub-continent. Correspondingly, during El-Nino years, the cyclones head westwards towards our country and beyond,” Kumar said.

Murtugudde is cautious regarding the patterns observed. The trouble with ‘trends’ is that only six out of 10 years may have more cyclones, he noted. “And the Indian Ocean region has a strong decadal variability and so, we have to be careful about the so-called ‘trends’ unless we have long-term reliable data.” 

Despite a La Niña, we had a relatively quiet cyclone season since last year and so, we always have to be humble about our claims, he added. 

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