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Updated: Nov 09, 2019 05:37 IST

: Bulbul , a very severe cyclonic storm in the Bay of Bengal, which is likely to intensify further and cross the Sunderbans delta in West Bengal and Bangladesh, is the seventh cyclonic storm this year, of which four have unusually been in the Arabian sea.

The high overall number -- between 2011 and 2018, the number of cyclones was less than five a year, according to India Meteorological Department (IMD) data -- as well as the activity in the Arabian sea is being seen by scientists as yet another impact of climate change, which causes surface sea temperatures to rise.

Four cyclones—Vayu, Hikka, Kyarr and Maha -- developed over the Arabian Sea this year and three in the Bay of Bengal—Pabuk, Fani and Bulbul.

Among these, Kyarr, which developed last month, is the first super cyclonic storm to develop in the Arabian Sea in the post monsoon season, according to analysis by Roxy Mathew Koll, scientist at the Climate Research Lab at the Indian Institute of Tropical Meteorology.

“We need to analyse long-term data to see if the frequency of storms has increased. But we have noticed that the Arabian Sea in recent years is much more fertile with respect to cyclones in the post monsoon season. This is mainly because of rapid warming of the Indian Ocean. Cyclones draw most of their energy from the ocean. Now, the Arabian Sea is more conducive for cyclones to form and this is projected to increase. The potential for more potent and frequent cyclones will go up because of warming,” Koll said.

Nilofar ( in 2014) was the first extremely severe cyclonic storms to form over Arabian Sea in the post monsoon season.

“Normally there are three to four cyclones annually. This year there are more cyclones and the Arabian Sea is so active is because of a positive Indian Ocean Dipole (IOD). Sea surface is warmer than usual,” said Mahesh Palawat, vice president, climate change and meteorology, Skymet Weather. Changes in the difference between sea surface temperatures of the tropical western and eastern Indian Ocean are known as IOD.

IMD has said there have been more than one or two cyclonic formations in the Arabian Sea seen in recent years. “When a deep depression forms into a cyclonic storm and has a speed of 34 nautical miles and above, they are named. So, all these storms have intensity. Climatologically, the ratio of storms in Bay of Bengal and Arabian Sea is 4:1 but this year more storms have formed in the Arabian Sea,” said Sunita Devi, senior scientist and head of cyclones at IMD.

“There are three main reasons why the Arabian Sea is active—higher sea surface temperatures, more moisture in the middle level of atmosphere and intertropical convergence zone (narrow zone near the equator where northern and southern air masses converge) is active,” added Sunita Devi.

This year’s South West monsoon started withdrawing almost a month later than usual, only on October 10.

The Intergovernmental Panel on Climate Change’s (IPCC) “The Ocean and Cryosphere in a Changing Climate” report released in September says anthropogenic (caused by humans) climate change has increased observed precipitation, winds , and extreme sea level events associated with some tropical cyclones. This, it adds, has increased intensity of multiple extreme events and associated cascading impacts. It also says there is emerging evidence for an increase in annual global proportion of Category 4 or 5 tropical cyclones in recent decades.

IMD in its Friday bulletin said Bulbul will cross as a Severe Cyclonic Storm with maximum sustained wind speed of 110-120 kmph gusting to 135 kmph. Several districts in coastal Odisha and Gangetic West Bengal are likely to receive very heavy rainfall and strong winds under its influence.