In the last 50 years, the coastal region of Andhra Pradesh — from Srikakulam to Nellore — would have been hit by at least 70 cyclones, a few of them categorised as Category 3 to Category 5 levels of cyclonic storm.
India basically has two seasons prone to tropical cyclones — one from April to May, which is just prior to the onset of southwest monsoon, and the other from October to November, a period when the southwest monsoon ends and the northeast monsoon sets in.
But among the two seasons, the period from October-November, is said to produce more devastating cyclonic storms, primarily due to the cooling of the ocean temperatures, which creates instability in the upper air column.
But of late, it is seen that the season is extending to December, which is a clear indication of climate change, due to global warming, says Prof. S.S.V.S. Ramakrishna of the Department of Meteorology and Oceanography, Andhra University.
More intensity
Climate researchers point out that it is not only the frequency that has increased in the last 30 years, but also the intensity of the tropical storms to a great extent.
According to Prof. Ramakrishna, the first recorded category-4 cyclone or super cyclone to hit India was the November 1996 Konaseema Cyclone. According to the Joint Typhoon Warning Centre (JTWC), this cyclone struck with a wind speed touching close to 215 kmph and destroyed 250 villages along a 60-km stretch within a minutes and left over 1,077 dead.
Though this was much smaller than the November 1977 Andhra Pradesh Cyclone that made its landfall near Chirala and left over 14,000 dead, the intensity of Konaseema cyclone was much higher. This clearly indicates that with climate change, the tropical storms or cyclones are getting more intensified, says Prof. Ramakrishna.
Thereafter, the coastal regions from Odisha to Tamil Nadu have seen a number of super cyclones making landfall with devastating effect.
In October 1999, Odisha was devastated by the Odisha Super cyclone that left over 15,000 dead, followed by December 2011 (Thane) that hit Cuddalore.
On October 12, 2013, the North Coastal Andhra Pradesh and Odisha were struck by Phailin, which was a category-5 extremely severe cyclonic storm and the wind speed touched 260 kmph. In the same year, Helen made its landfall at Machilipatnam on November 21 and Lehar made its second landfall again at Machilipatnam, a week later.
But the most devastating in the recent times was Hudhud, when it made its landfall with wind speed touching 215 to 240 kmph in Visakhapatnam on October 12, 2014.
"If the pattern is noticed, then most of the devastating category-level cyclones, be it on the eastern or western coast, made landfall between October and November. But since 2011, Thane and now Phethai impacted the coasts in December. This shows that there is a shift in the cyclonic season and its intensity," said Prof. Ramakrishna.
Increase in Bay temperature
According to R.R. Rao, retired scientist from Naval Physical and Oceanographic Laboratory (NPOL), the warming of surface temperature of the oceans is a general phenomenon across the globe and it is one of the major reasons for generation of more cyclonic systems over the oceans.
"Over the last 100 years, the surface temperature of the oceans has increased by about 10 C, which is a sign of global warming," he said.
Higher the ocean surface temperature, more will be the evaporation, more will be the moisture in the air, which will lead to low pressure and increase in instability. These collaborated with other factors such as vertical wind shear, initial vortex, relative humidity and degree of convective instability, will trigger a tropical cyclone, added Prof. Ramakrishna.
Why Bay of Bengal
North Indian Ocean is the most potential source for formation cyclonic storms, primarily it is because that the ocean is a land locked one.
It has two arms — the Bay of Bengal and the Arabian Sea. Compared to the both, the recurrence of tropical cyclones is more over Bay of Bengal. "This is primarily because the Arabian Sea temperature is relatively cooler and Bay is warmer," said Dr. R.R. Rao.
According to him the water stratification is higher in the Bay of Bengal. "The flow of fresh river water and rain water is much higher in the Bay of Bengal and due to which the vertical mixing of cooler lower water with the warmer surface water is weaker. And due to weak mixing, the top surface remains warmer and the upper air column becomes buoyant. With the supply of moisture, the atmosphere becomes unstable, which leads to formation of such systems," explained the former scientist from NPOL.
