Cyclone Vardah has probably been the most powerful cyclone to hit the city of Chennai and northern coastal districts of Tamil Nadu in recent years, say observers of storm systems in India.

The cyclone, which made landfall on December 12, killed 10 people in Tamil Nadu, uprooted more than 250 trees, badly damaged fishermen’s boats and carried away tin roofs and solar panels from the top of buildings.

Cyclone Vardah came just a week after Cyclone Nada – which had formed as a depression in the Bay of Bengal, gathered strength and achieved cyclone status with wind speeds up to 75 kmph – weakened and made landfall near Karaikal in Puducherry as a depression on December 2.

Vardah, which originated as a depression over the South East Bay of Bengal in the afternoon of December 6, intensified into a cyclonic storm on December 8, hovering about 1,040 km east-southeast of Visakhapatnam. Over December 9, 10 and 11, the system grew to a very severe cyclonic storm and made its way westward towards Chennai, in what meteorologists call a recurving path. By noon on December 12, about the time it made landfall, Vardah had wind speeds of between 120 kmph and 130 kmph, and gusts up to 140 kmph.

Image: IMD

The questions everyone is asking in the aftermath of the two storm systems include: “How many more Vardah’s can Chennai expect?” and, “Is this the new norm for weather in India’s south east coast?”

Weather patterns are changing across the world, with scientists attributing many of the changes partly to climate change. A very common prediction based on climate change models is its impact on tropical cyclones.

Models run by international teams of climate scientists indicate that greenhouse warming results in a shift towards stronger tropical cyclonic storms. These scientists predict that by the year 2100, the global average intensity of tropical storms will be between 2% and 10% stronger. At the same time the models also predict that the global frequency of these storms will reduce by 6% to 34%. Therefore, we can expect to see fewer, but more severe, cyclones around the world.

These projections rely on the basic assumptions that oceans are likely to absorb increasing quantities of heat from the atmosphere due to global warming, resulting in increases in sea surface temperatures. This, in turn, is expected to result in increased evaporation, precipitation, formation of low pressure areas and stronger winds, all which lead to increased cyclonic activity.

And indeed, the Bay of Bengal has seen a reduction in the number of cyclonic systems over the last 50 years.

“Over the North Indian Ocean, the frequency of cyclonic storms and severe cyclone storms are decreasing,” said Dr Mrutyunjay Mohapatra, head of the Regional Specialised Meteorological Centre of the Indian Meteorological Department. “This is the analysis of the recent satellite period from 1960 to 2010.”

However, a 2014 study from the University of Allahabad that analysed cyclonic data from the North Indian Ocean over a period of 122 years between 1891 and 2013 points out that a larger number of cyclonic disturbances are intensifying into tropical cyclones in the post-monsoon period, especially in November. What this seems to suggest is that although the total number of cyclonic systems is decreasing, a larger percentage of these systems are developing into severe cyclonic storms.

Moreover, the path of cyclones in the Bay of Bengal seems to have been shifting. “Right now tracks are shifting southward towards Tamil Nadu,” said Ashutosh Mishra, research fellow at the University of Allahabad and lead author of the study.

“Before 1980, cyclones were hitting in large numbers towards West Bengal, Odisha and Andhra coasts,” said Mishra. “But after 1980 they have been hitting the lower Andhra and Tamil Nadu coasts.”

Mishra, who has been analysing climate data across India, contends that the lower eastern Indian coast is getting warmer. This excessive heat is creating a pressure gradient that pulls cyclonic storms towards the coast and increases the odds of a storm making landfall.

The El Niño Southern Oscillation

In the Bay of Bengal specifically, there are other factors that affect cyclone formation and intensification.

“Instead of the long-term trend due to climate change the inter-annual variability due to large-scale process like the El Niño Southern Oscillation is more determinant for cyclones in the North Indian Ocean,” said Mohapatra.

The El Niño Southern Oscillation is a periodic fluctuation in sea surface temperature and air pressure in the equatorial Pacific Ocean that affects much of the tropics and subtropics. The warming phase is known as El Niño and the cooling phase as La Niña.

Yet another complicating factor is the presence of the Indian Ocean Dipole, a temperature gradient like the El Niño Southern Oscillation but in the Indian Ocean.

During a La Niña year, Mohapatra explained, cyclones follow a northerly track and more cyclones hit Odisha, West Bengal and Bangladesh. In an El Niño year the cyclones head towards Tamil Nadu and southern Andhra Pradesh. Secondly, the frequency and intensity of cyclones is greater in a La Niña year for the post-monsoon period between October and December. In an El Niño year the frequency, intensity, track length and likelihood of cyclones are all lower.

Mohapatra observed that cyclonic activity had been subdued this year in October and up till mid-November, after which it increased significantly and generated cyclones Nada and Vardah in quick succession.

“El Nino conditions were strong last year and now this is the year following strong El Niño conditions,” said Mohapatra. “Right now after about November 15, there have been La Niña conditions and so the frequency and intensity of cyclones are increasing.”

What all this suggests is that there is no quick answer to whether India’s East coast will be battered by more severe storms in the coming months and years. But if Cyclone Vardah and 2014’s Cyclone Hudhud are anything to go by, cities like Chennai and Visakhapatnam, and coastal areas adjoining them, had best learn to adapt and start building storm-proof infrastructure to save lives and livelihoods.