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Why Arabian Sea cyclones have increased by 52% in twenty years

A new study by Indian climate scientists finds an alarming rise in the frequency, intensity and duration of cyclones in the last 20 years

A satellite image of cyclone Tauktae in the Arabian Sea from May this year.
A satellite image of cyclone Tauktae in the Arabian Sea from May this year.

One of the clearest signals of climate change is just how much the global ocean has heated up in the past fifty years. As the UN Intergovernmental Panel on Climate Change’s (IPCC) reports over the past few years have reported, since 1970, the global ocean has absorbed 90% of the excess heat generated by man-made climate change.

The effects of this are only now beginning to be seen. Marine heatwaves that cause widespread bleaching of coral reefs, more acidic ocean waters that are decimating fish stocks worldwide, and the growing number and intensity of tropical storms like hurricanes and cyclones, are just some of the effects of over-heated oceans.

Also Read: Cyclone Nivar and the shadow of climate change

A new study conducted by Indian scientists, published in the journal Climate Dynamics on 17 July, has found that there has been a 52% increase in the number of cyclones in the Arabian Sea in the past 20 years. The number of very severe cyclonic storms (VSCS), with wind speeds of 118-165 km per hour, has increased by a whopping 150% in the Arabian Sea.

The study, Changing Status Of Tropical Cyclones Over The North Indian Ocean, analyses satellite data from 1982-2019 to show that the intensity and duration of cyclonic storms have also increased quite dramatically in the north Indian Ocean, specifically in the Arabian Sea. Between 2001-2019, the duration, or the life span, of all cyclones in the Arabian Sea has increased by 80% as compared to the twenty years preceding it. What’s also alarming is that the duration of VSCS has increased by a whopping 260% in the Arabian Sea. By comparison, the duration and frequency of cyclones in the Bay of Bengal has remained relatively stable.

Also Read: The effects of climate change on cyclone Tauktae in the Arabian Sea

The study makes a clear link between this dramatic rise in the severity of cyclones in the Arabian Sea and the fact that ocean temperatures are rising. Lounge spoke to one of the co-authors of the report, climate scientist Roxy Mathew Koll of the Indian Institute of Tropical Meteorology (IITM), about the findings. He says that the Arabian Sea used to be cooler, but that it is now warming significantly. It has become what is called a ‘warm pool’ region, which the Bay of Bengal already was, where sea surface temperatures (SST) are at least 28 degree Celsius. This is a temperature threshold that is conducive to the creation of cyclonic storms. “Cyclones need high sea surface temperatures, generally above 28 degree Celsius. The Arabian Sea used to be cooler. But now because of oceanic warming, which is predominantly in the Arabian Sea region, the sea surface temperatures there have also reached that 28 degree threshold,” says Koll.

Such high SST isn’t just the perfect scenario for cyclogenesis (the creation of a cyclonic storm) but is also perfect for maintaining and intensifying the storms. This is one of the reasons why the duration and intensity of cyclones have increased. Koll gives the example of cyclone Tauktae which developed over Lakshadweep in mid-May, before moving up the Arabian Sea, in a track parallel to India’s west coast, before slamming into the Gujarat coast a few days later as a destructive VSCS. “Cyclone Tauktae absorbed so much heat and moisture from the ocean, which is why it could maintain its cyclone status even one day after making landfall. Usually once cyclones make landfall, the supply of heat and moisture from the ocean is cut off, and they dissipate quite fast. But this one maintained its status for quite a long time,” says Koll. Tauktae or the other powerful cyclones of the past two years, like Amphan, were not a part of the study, but they are indicative of the trends that Koll and his fellow scientists have observed.

Also Read: What we learnt from cyclone Tauktae

Climate change is the clear driver for such intensity. A warmer atmosphere holds more moisture; the moisture content in the air increases by 7% for every 1 degree Celsius of warming. And a combination of higher moisture content plus high SST is the perfect recipe for more powerful and destructive cyclones. Speaking specifically about the past couple of years, Koll says that scientists have been seeing record temperatures in the north Indian Ocean. “Overall, we are seeing that the SST prior to cyclones are about 1.2 degree Celsius to 1.4 degree Celsius above the average. In the last two years we’re seeing that in the pre-monsoon season, the sea surface temperatures were reaching 31-32 degrees Celsius, which is record temperatures for these basins (Arabian Sea and Bay of Bengal). 1.2-1.4 degrees Celsius is the average change, but for these two years it could be 2-3 degrees Celsius above normal.” To put this into context, when last year’s cyclone Amphan intensified into a super cyclone (with wind speeds of over 221 kmph) in about 18 hours, SST in the Bay of Bengal were as high as 32-34 degrees Celsius.

Koll says that their study corroborates earlier predictions made about the rise in extremely severe cyclones in the Arabian Sea with global heating. He also indicates that there is potential for things to get worse. “In the Arabian Sea there is potential for further increase in SST and the available moisture. It is increasing anyway and there’s a projection of further increase as well. That will mean more intense cyclones in the Arabian Sea,” he says.

Also Read: How cyclone Amphan is a grim snapshot of India's climate future

They key point to note is that the effects of climate change are already here. In fact, they’ve been here for the past 20 years. So a business-as-usual approach won’t work. Especially with regards to the west coast, apart from a few months in winter, there are rainfall events almost through the year. Whether it is cyclones in the pre-monsoon and post-monsoon months, or monsoon rains from June-September, the frequency and duration of floods, storm surges and extreme rainfall events are increasing. The preparation for these extreme events should go beyond merely evacuating people every time a disastrous storm hits. “Floods and storm surges from cyclones or heavy rains are going to increase. And we need urgent action. First we need to do risk assessment mapping on the basis of these new trends. And on the basis of that we need to take appropriate action,” says Koll.

Koll says that when it comes to infrastructure development near the coasts, whether they be expressways or any major undertaking, the risks posed by emerging climate threats have to be taken into consideration. “In fact these threats have already emerged. Cyclones have increased and intensity of rainfall has increased over the last two decades. And we have sufficient proof to show that. We have to urgently act on them.”

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