A thick knot of interlocking forces give birth to a cyclone like Vardah that smashed into the Chennai coast and Nellore and Chittoor districts of Andhra Pradesh on December 12 last year, with wind speeds reaching 100-110 km per hour. Among its salient features, Vardah is the fourth consecutive cyclone with a recurving track, after cyclones Roanu, Kyant and Nada. It was also the most intense.
“Its track was not straightforward, but it was a cyclone changing its tracks, what we call a recurving cyclone,” said Mrutyunjay Mohapatra, head of the Cyclone Warning Division at India Meteorological Department. A recurving cyclone has the potential to cause widespread damage. The forces that propel these cyclones include upper ocean heat content, sea surface temperatures, freshwater impact, global warming and sea level rise.
The intensity of cyclones has increased in recent years in the Bay of Bengal, which has been a topic of study by scientists around the world. Although they have been reluctant to link any single cyclone to global warming and climate change, much of the research point to increasing heat content in the upper ocean being connected to the more violent cyclonic storms.
Global warming
Since the industrial revolution in 1850s, average global temperature has increased by 1 degree Celsius. Scientists have predicted that increasing sea-level rise due to climate change will increase storms and coastal flooding, as stated in a paper published in 2013 in the journal Nature by Jonathan Woodruff and colleagues.
There are three things during a storm that cause most of the damage — howling winds that smash and impale things; storm surges that swamp low lying areas, and inland flooding due to torrential downpours.
“Sea-level rise makes coastal regions surrounding the Bay of Bengal more vulnerable to the devastating impacts of cyclone storm surge under climate change,” said ocean scientist Karthik Balaguru at Pacific Northwest National Laboratory in Seattle, Washington, who followed Cyclone Vardah closely.
Fuel for cyclones
Cyclones or storms live and die by the availability of water vapour from the warm sea or ocean surface, or the lack of it. As long as it’s available, they rage, whipping up winds and pouring rains. There was plenty of water vapour available to Cyclone Vardah. “(It was) part chance but no doubt enhanced by high ocean temperatures that have a global warming component,” said Kevin Trenberth, senior climate scientist at the National Centre for Atmospheric Research in Boulder, Colorado.
Although the time for cyclones is after the monsoon season gets over in India, where they go is largely a matter of chance and it depends on the weather situation, the high and low pressure systems etc. The fuel for such storms is from moisture in the atmosphere that condenses in heavy rains and the latent heat released intensifies the storm, explained Trenberth. “The moisture is strongly linked to how warm the ocean is.”
Surface temperatures
A lot of heat was pumped into Vardah, fuelling its intensity. That’s due to upper ocean heat content — the amount of heat held in the ocean from the surface down to a fixed depth, typically 100m, or from the surface down to the depth where the temperature of the ocean reaches 26 degrees Celsius. “It is an indicator of not only how warm the water is, which is related to the sea surface temperature, but also how deep the layer of warm water extends below the surface,” said Balaguru.
A paper published in 2014 in Geophysical Research Letters by Balaguru and colleagues shows that post-monsoon (October-November) cyclones have been intensifying over the Bay of Bengal due to increasing heat content in the upper-ocean. The researchers analysed major tropical cyclones — with wind speeds above 49 metres per second, or about 176 km per hour during 1981-2010 — and found that their intensity has increased.
The paper states that changes in environmental ingredients are driving their intensity. “Increases in sea surface temperatures and upper ocean heat content made the ocean conducive to tropical cyclone intensification, while enhanced convective instability made the atmosphere more favourable for the growth of tropical cyclones,” the paper said.
When sea surface temperature increases, the temperature of air in contact with the ocean surface also increases. Since warm air is less dense, it makes the air column unstable and promotes convection (transfer of heat) in the atmosphere. This is known as convective instability, explained Balaguru.
“In our study, we found that sea surface warming occurred in the eastern Bay of Bengal, which enhanced the convective instability for the atmosphere in that region. These conditions are favourable for the intensification of cyclones,” he said.
The sea surface temperatures were above normal and this contributed to the formation of a category-2 tropical cyclone, reckons T. N. Krishnamurti, a meteorologist and professor emeritus at the Florida State University. “The warmer than normal sea surface temperatures over South China sea especially and the southwestern Bay of Bengal were major factors.”
Thermocline
In the ocean, solar heat is received at the top and hence the surface is relatively warm. As we go deeper, the temperature starts decreasing but slowly. Later, there comes a region where the temperature decreases rapidly. This region is known as the thermocline.
Typically, when a storm buzzes above the ocean, the strong winds throw the water in turmoil, exhume cooler water from the thermocline, and help cool the surface ocean, thus depleting the fuel for the storm and weakening it. This mechanism tones down the intensity of the storm.
However, in situations where the ocean heat content is large, the availability and depth of that cold-water region is so far down below the surface that mixing induced by storms doesn’t cool the surface temperatures by much, Balaguru explains.
Which means this process maintains the warm temperatures at the surface, which means its wrecking capacity keeps firing on all cylinders. As winds blow and rains deluge, the rivers swell, and empty into the Bay of Bengal.
Freshwater Impact
During the monsoon months of June-September, the major rivers in the Bay of Bengal basin —Ganga, Brahmaputra and Irrawaddy — receive copious amounts of freshwater, and eventually flow into the Bay of Bengal. The freshwater loads up on salty seawater because of its low salinity during the post-monsoon months of November-December. It creates what Balaguru calls “a lens of freshwater, which acts as a barrier layer and prevents cooler deeper water from getting into the surface warm layer, and reduces storm-induced mixing.”
In a recent paper, published in November 2016 in Nature Communications, Balaguru and colleagues analysed western North Pacific typhoons — so called because they form in the Pacific — for the period between 1961 and 2008. They state that “freshening of the upper ocean, caused by greater rainfall in places where typhoons form, tends to intensify STYs (super typhoons) by reducing their ability to cool the upper ocean.”
Furthermore, a 2012 paper in the Journal of Geophysical Research Oceans, published by National Institute of Oceanography, Goa, researchers S. Neetu and colleagues state, “Surface cooling induced by tropical cyclones (TCs) is about three times larger during pre-monsoon than during post-monsoon season in the Bay of Bengal.”
Yet another paper, published in 2007 in Atmospheric Science Letters by researchers at the Centre for Atmospheric and Oceanic Science at Indian Institute of Science, Bengaluru, says: “Even the strongest post-monsoon cyclones do not cool the north Bay of Bengal.” In the region, the paper states, a shallow layer of freshwater coming off the runoff, covers warm layer, and so the storm-induced mixing is not deep, favouring intense cyclones.
This means the ocean surface is still yet warm, which turns storms steroidal. As if this is not enough, Vardah was also receiving the La Niña effect from afar. La Niña, according to NOAA, is characterised by unusually cold ocean temperatures in equatorial Pacific, as opposed to El Niño, characterised by unusually warm ocean temperatures there.
La Niña Phase
A paper, published in 2015 in the journal Theoretical and Applied Climatology by researchers at Indian National Centre for Ocean Information Services, Hyderabad, shows that the tropical cyclones intensify during La Niña phase of climate.
“This is the year after the El Niño, the Pacific trades are stronger than normal, that contributed to strong disturbances towards the Bay of Bengal from the southern part of the south China sea,” says Krishnamurti.
All of these changes are not happening in isolation. They are all threaded by global warming. Scientists, however, have been hesitant to relate single events to climate change in the past, according to an article published in The Conversation, a news website.