Arabian Sea emerging as a cyclone hotspot
Faster warming in recent years due to increased human activity may be making India’s west coast more vulnerable to devastating storms, scientists say.
doi:10.1038/nindia.2021.86 Published online 18 June 2021
Severe cyclones in the Bay of Bengal on India’s east coast are common but till recently were rare in the Arabian Sea on the west coast. In the last few years, a spate of severe cyclones in the Indian Ocean have hit the country’s west. These are likely due to increased surface temperatures of the Arabian Sea, making it a breeding ground for more frequent extreme weather events, scientists say.
India’s coastline was ravaged in May this year by back-to-back high intensity cyclones – Tautkae churned the Arabian Sea making landfall on 17 May and peaking at a wind speed of 220 km/hour, followed by Yaas in the Bay of Bengal on 26 May ascending up to 120 km/hour.
Cyclones have become an annual affair on India’s west coast providing evidence of a shift in pattern. Nilofar (2014), Chapala and Megh (2015), Vayu (2019), Nisarga (2020) and Tautkae this year show the rapid rise in cyclone count in the Arabian Sea puzzling weather scientists.
Deviation from the norm
The Bay of Bengal is warmer than the Arabian Sea and thus the waters form a natural hotspot for cyclones. “High intensity cyclones were relatively less frequent in the Arabian Sea than in the Bay of Bengal because of lower sea surface temperature (SST) and unfavourable wind shear,” says Preeti Tewari, an associate professor of geography at the University of Delhi.
Different dynamics operate in the Arabian Sea keeping it cooler than Bay of Bengal, according to Alejandra Sanchez-Franks, an oceanographer at UK’s National Oceanography Centre at Southampton.
But the Arabian Sea has been warming faster, partly due to global warming, says Madhavan Nair Rajeevan, secretary in India’s ministry of Earth Sciences. “Warming is seen not only in the top layers, but also in the deeper ocean layers — we call this the ocean heat content.” Due to increases in the SST and ocean heat content, Arabian sea cyclones are intensifying, he says.
S Prasanna Kumar, a CSIR emeritus scientist at the National Institute of Oceanography (NIO) in Goa, and his team, are studying cyclones in the Arabian Sea since 2007. They found1 “a strong link between the accelerated warming of the Arabian Sea and the frequency and intensity of cyclones in the sea.” While the Bay of Bengal experiences 2 to 4 cyclones per year, annual cyclones in the Arabian Sea of category 2 and 3 have increased only after 1995, their analysis shows. Category 4 cyclones started appearing in the Arabian Sea only after 1995.
Out of five cyclones in 2020, two were in the Arabian Sea. Five of the eight cyclones in 2019 were in the Arabian Sea, a marked shift from the eastern bay being the hub of cyclones.
The other recent unusual phenomenon is the simultaneous occurrence of cyclones in the Arabian Sea and the Bay of Bengal — like Luban (Arabian Sea) and Titli (Bay of Bengal) in 2018. Similarly, in 2020, Nisarga (Arabian Sea) was followed by Amphan (Bay of Bengal).
Genesis of frequent intense cyclones
The faster rate of warming of the Indian Ocean – concentrated in the Arabian Sea – than the rest of the world’s oceans has created just the right milieu for cyclone genesis.
Prasanna Kumar’s study shows the Arabian Sea has warmed at an overall rate of 10.1 millidegrees per year (0.0101°C per year) from 1960 to 2011. The rate of warming increased since the mid-1990s (from 7.7 millidegrees per year from pre-mid-1990s to 13.4 millidegrees per year post mid-1990s).
Hiroyuki Murakami, a scientist at the Geophysical Fluid Dynamics Laboratory in Princeton, New Jersey, corroborates the increasing temperature of the Arabian Sea. The Indian Ocean, specifically the Arabian Sea, is where SST has been rising markedly since 1980 than the other open oceans all over the world, he says. Tropical cyclone activity correlates with SST— higher SST is favourable for tropical cyclones, he adds. The increase in severe cyclones in the Arabian Sea is consistent with the increasing trend in SST.
How Arabian Sea was shielded
The Arabian Sea experiences cooling twice a year. Once during June to September (summer monsoon season) due to a ‘upwelling’ which brings colder, deeper subsurface waters to the surface. The mixing cools the surface. The second time is between November and February when the northern Arabian Sea cools due to 'winter convection' when the cold atmosphere and air cools the ocean surface. This increases the density of the surface water which sinks and is replaced by colder waters that rise up.
The heat accumulated over a year — due to solar heating and heating by carbon dioxide — is removed by upwelling and winter convection. Until the mid-1990s, these two processes were able to offset the Arabian Sea’s excess heat.
But since then upwelling and winter convection have proved inadequate to remove the excess heat input by ever-increasing carbon dioxide levels. The warming of the Arabian Sea has been accelerated, and with it the conditions necessary for cyclones.
Indian Ocean dipole, a phenomenon in which the eastern Indian Ocean becomes colder than normal and the western Indian Ocean warmer than normal, is another offender. In years when the Indian Ocean dipole is active, the Arabian Sea becomes warmer than normal increasing the chances of cyclones.
It is this difference in temperatures that accounts for the differing propensity towards cyclone genesis in the western and eastern Indian Ocean, says Sanchez-Franks. More than any single factor, an interplay of factors accounts for the increase in frequency and intensity of cyclones in the Arabian Sea.
Role anthropogenic aerosols play
Recent increases in anthropogenic aerosols have also been reported to cause an increase in storm intensity over the Arabian Sea2.
Sanchez-Franks says, “Oceans are absorbing more of the excess anthropogenic heat – about 90% –than any other component of the climate system. This heat is absorbed, stored and then redistributed by the oceans.”
The Indian Ocean, warming much faster than any other ocean basin, roughly accounts for about a quarter of the excess global heat uptake. Sanchez-Franks says that the rapid rise in temperature of the Indian Ocean has been linked to extreme weather events in India.
Murakami has a word of caution. Cyclones form in two seasons: pre-monsoon (April to June) and post-monsoon (October to December). Their study3 suggests that the increase in severe storms in the Arabian Sea is more apparent in the post-monsoon season rather than in the pre-monsoon season. This is inconsistent with the two cyclones that have already occurred in the 2021 pre-monsoon season.
“Cyclone genesis is complex. It is challenging to attribute any individual storm to anthropogenic climate change,” Murakami says. While there may be a potential influence of anthropogenic climate change on recent extreme cyclones in Arabian Sea, it is uncertain and difficult to estimate the extent of its contribution to the two cyclones, he says.
1. Kumar, S. P. et al. Response of the Arabian Sea to global warming and associated regional climate shift. Marine Environmental Res. 68, 217-2222 (2009) Doi: 10.1016/j.marenvres.2009.06.010
2. Evan, A.T. et al. Arabian Sea tropical cyclones intensified by emissions of black carbon and other aerosols. Nature 479, 94–97 (2011) Doi: 10.1038/nature10552
3. Murakami, H. et al. Increasing frequency of extremely severe cyclonic storms over the Arabian Sea. Nat. Clim. Change 7, 885-889 (2017) Doi: 10.1038/s41558-017-0008-6