On the night of November 25, at 11:30 p.m., Cyclone Nivar began to make landfall in Puducherry, on the coast of Tamil Nadu. A severe cyclonic storm (SCS) with wind speeds of 120-30 km / h, according to the Indian Meteorological Department (IMD), Nivar finished making landfall at 2.30am on November 26. It subsequently weakened to a cyclonic storm (CS), but not before dumping large amounts of rain on the coastal areas around Chennai and Puducherry. By then, Puducherry had logged 237mm of rain in 18 hours. In that same time, Cuddalore had recorded 246mm and Chennai, 83mm.
The good news is that IMD had pinpointed the path of the cyclone on November 23, and this helped with proper warnings and evacuation from shore. As of the 25th, thousands of families were reportedly evacuated in Chennai and the coastal districts of the state. “Up to 1,21,152 people have been evacuated across the state, including Cuddalore, Villupuram, Nagapattinam, Ariyalur, Perambalur,” said Phanindra Reddy, chief secretary and commissioner of the Tamil Nadu disaster mitigation and management department. Hindustan Times. Although this helped keep deaths low, the true extent of the damage caused by the cyclone has yet to be determined.
A NASA satellite image of Cyclone Nivar. (Photo: AP / PTI)
(AP)
A year of storms
2020 has been a year of tropical storms of unusual ferocity; storms overloaded over the open ocean by abnormally high sea surface temperatures. It has been the case of one of the worst scenarios of impacts of climate change in real time. And India has felt it more than most.
The fact that a pandemic has swept the world may have distracted people from how alarming it is. Between the afternoon of May 16 and the morning of May 18, Cyclone Amphan, contrary to all predictions, had become a monstrous supercyclone, the equivalent of a Category 5 hurricane. The reason for this was a heat wave. that swept across the Bay of Bengal, raising sea surface temperatures to unprecedented levels. Heat is energy, says climate scientist Roxy Mathew Koll of the Indian Institute of Tropical Meteorology (IITM) in Pune, and cyclones rapidly intensify by converting potential energy stored in the ocean into kinetic energy.
Something similar happened with cyclone Nivar. It rapidly intensified between November 23 and 24, from a deep depression (DD) to a severe cyclonic storm (SCS), and by November 25, to a very severe cyclonic storm (VSCS). Koll says there are two reasons for this. The first and foremost reason is the warmer seas. “The Bay of Bengal is already a ‘warm pool region’, where surface temperatures are permanently above 28 degrees Celsius. In November, the sea surface temperatures here are usually 28-29 degrees Celsius, sometimes 30 degrees Celsius, ”he says. Observers were expecting a cyclone, especially since cooler La Niña conditions (a weather pattern over the Pacific Ocean) this year would have created favorable environmental conditions for cyclogenesis in the Bay of Bengal. But this is where global warming distorted the natural process.
The trail of Cyclone Nivar, showing how the cyclone intensified over the Bay of Bengal. (Photo: Indian Meteorological Department)
According to the 2019 Intergovernmental Panel on Climate Change (IPCC) Special report on the ocean and cryosphere in a changing climate (Srocc), the global ocean has absorbed 90% of the excess heat generated by greenhouse gas (GHG) emissions since 1970. This has caused anomalous ocean warming, which in turn causes cyclones to intensify quickly. Before the formation of Nivar in the Bay of Bengal, climate change had warmed the Bay of Bengal. “This time, the sea surface temperatures were 0.5 to 1 degree Celsius warmer, in some regions it is almost 1.2 degrees Celsius above normal,” says Koll. He adds that every 0.1 degree warmer seas gives a cyclone additional energy to feed itself.
The other, minor factor for Nivar intensification is an annual fluctuation in the tropical climate called the Madden-Julian Oscillation (MJO). “It’s a band of clouds and winds that starts in the western Indian Ocean, then moves to the central Indian Ocean and then to the eastern Indian Ocean and then to the Pacific Ocean. So depending on where it is, it can affect local weather conditions, ”says Koll. Given its current position south of Bay of Bengal, the OMJ would have provided favorable winds for a cyclone to develop.
Koll finds similarities between Nivar and Cyclone Okchi of November 2017. Then as of now Okchi started in the south of the Bay of Bengal, rapidly intensified due to warmer seas, and with the help of the MJO, it followed a long road of destruction through Sri Lanka and crossed into the Arabian Sea. More than 800 people died in Sri Lanka. Koll says more analysis is needed, but his observations suggest that the rapid intensification of cyclones is more dependent on higher sea surface temperatures. “The role of a warmer ocean is greater. If we look at the cyclones that developed during the pre-monsoon or monsoon start time, Amphan and Nisarga, there were no favorable winds and yet the cyclones still intensified, ”he says.
The Indian Ocean hot spot
The Indian Ocean (which includes both the Arabian Sea and the Bay of Bengal) is a hot spot for climate change. According to a comprehensive study from 2017, Improved estimates of ocean heat content from 1960 to 2015, published in Scientific advances, the Indian Ocean has contributed to 25% of the excess ocean heat absorption induced by global climate change. This despite the fact that the Indian Ocean represents only 13% of the world’s ocean surface. And this is important for the global climate. “MJO, for example, sometimes travels as far east as the Atlantic Ocean, affecting the climate in the tropics, in the United States and also in Europe. Changes in the Indian Ocean also affect global atmospheric circulation. It also affects global ocean circulation, ”says Koll.
Koll says more and better observing systems are needed in the Indian Ocean. In 2006, a multinational network of integrated observing systems was established for the Indian Ocean, called the Indian Ocean Observing System (IndOOS). Koll is one of more than 60 scientists from around the world who provide recommendations and reports based on IndOOS. In a three-year evaluation report co-authored by Koll, published in early November, scientists have called for better and deeper observing systems under a proposal for IndOOS-2. “In the years since it was established, the (observation) priorities have changed. Temperatures are increasing rapidly in the Indian Ocean, the sea level is changing rapidly and these changes are affecting monsoon patterns, affecting cyclones, marine ecosystems, ”he says.
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