As Nisarga makes landfall, experts explain why Arabian Sea is witnessing more cyclones

Severe cyclone Nisarga, which made the landfall on Wednesday afternoon near Alibag in Maharashtra’s Raigad district, indicates an increasing frequency of severe cyclones developing in Arabian Sea in the past decade.

Studies have linked this trend to climate change.

In the past two years, the Arabian Sea has witnessed seven cyclones. However, according to the India Meteorological Department (IMD), the ratio of cyclones in the Arabian Sea and the Bay of Bengal - which just witnessed the destructive cyclone Amphan on May 21 - is still 1:4.

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The Intergovernmental Panel on Climate Change (IPCC) last year came out with a special report, Ocean and Cryosphere in a Changing Climate, in which it said that extreme rainfall and extreme sea level events associated with some tropical cyclones are being seen to have a cascading impact on coastal areas.

“There is emerging evidence for an increase in annual global proportion of category 4 or 5 tropical cyclones in recent decades,” the IPCC report had said.

A category 4 cyclone has a wind speed of 209-251 kmph, and is referred to as an extremely severe cyclonic storm, while a category 5 cyclone has a wind speed of more than 252 kmph, and is referred to as a super cyclone. Cyclone Amphan, which hit Sundarbans on May 21, was categorised as a super cyclone, but made landfall as an extremely severe cyclonic storm.

The IPCC report refers to a 2017 study led by Atmospheric and Oceanic Sciences Programme, of the United States’ Princeton University, which found that in 2014 and 2015, post-monsoon extremely severe cyclonic storms (ESCS) were first observed over the Arabian Sea causing widespread damage to the West coast.

The study had concluded that climate change had led to an increase in the occurrence of ESCSs in the Arabian Sea. Hiroyuki Murakami is the lead author of the study who is also among few scientists globally who specialise in cyclones in the Arabian Sea.

“Rising mean sea levels will contribute to higher extreme sea levels associated with tropical cyclones (very high confidence). Coastal hazards will be exacerbated by an increase in the average intensity, magnitude of storm surge and precipitation rates of tropical cyclones,” the IPCC report further stated.

“The IPCC reports indicate that pre- and post-monsoon storms may increase over Indian Ocean in the future in a warming world,” said Mathew Roxy Koll, climate scientist at Indian Institute of Tropical Meteorology and one of its authors.

“This year, both Arabian Sea and Bay of Bengal were about 1 degree warmer than normal (in early May) and hence the conditions were conducive to increasing the strength of the cyclones. A warmer ocean does not automatically mean there will be more cyclones but if the cyclones are born they will become stronger on account of a warmer sea. The number of cyclones in a given ocean depends on the existence of circulation patterns that are not related to climate change,” said J Srinivasan, Distinguished Scientist, Divecha Centre for Climate Change, Indian Institute of Science.

“Since 1990, we are seeing an increasing trend of intense cyclonic storms over Arabian Sea. Some studies have attributed this increase to climate change but with low confidence,” Mrutyunjay Mohapatra, director general, IMD had said on Monday.

“During Amphan, warmer oceans did allow the system to reach a super cyclone status but not in this case so far,” said Mohapatra. Warmer ocean temperatures are not the only parametre for rapid intensification of cyclones, he said. “We need to consider other parameters such as relative humidity in middle troposphere, and instability in the atmosphere allowing moist air to form aiding cloud formation through the help of convection. However, the frequency of cyclones has been abnormally high in Arabian Sea since last year and it is a fact that this zone is witnessing an increase in cyclones,” he said.