Nearly eight lakh.

That’s the number of people who have died across the globe during the total 1,942 tropical cyclones over the last five decades, according to the World Meteorological Organization. They cause huge economic losses, running into thousands of billions of dollars. And about seven percent of them are recorded in the North Indian Ocean..

Despite being the least active ocean region in terms of cyclone activities, NIO continues to experience some of the most intense cyclones which strike very densely populated and low-lying coastal regions and leave in their wake many deaths and destruction of properties. But why? Before we try to understand the reasons with scientific literature, let’s take a look at the different types of cyclones.

How is a tropical cyclone different from a hurricane?

The terms depend on the location. It’s called a “cyclone” in the Bay of Bengal and Arabian Sea, a “tropical cyclone” in the southwest Indian ocean, and a “severe tropical cyclone” in the southeast Indian and south Pacific Ocean. Meteorologists deploy the generic term “tropical cyclones” for rapid rotating storms that develop over the tropical and subtropical waters. Weaker phenomena are called tropical storms and tropical depressions. A hurricane is a tropical cyclone that develops over the tropical waters of the Atlantic Ocean. Tropical storms that develop over the northern Pacific Ocean are called typhoons.

Is the frequency of tropical cyclones consistent across the NIO?

It isn’t. The NIO region includes over 7,500 km of coastline of the Indian peninsular mainland, Lakshadweep and Andaman and Nicobar Islands. It is divided into two sub-basins – the Bay of Bengal and Arabian Sea.

There is a 52 percent increase and an eight percent drop in the number of cyclonic storms over the Arabian Sea and the Bay of Bengal, respectively, in the recent epoch (2001-2019) as compared to the previous epoch (1982-2000). There has been a global surge in intense cyclonic storms, and the intensity and duration of these events have also risen over the Arabian Sea.

Four recent major super cyclonic storms – Kyarr, Vayu, Hikka and Maha – formed over the Arabian Sea during 2019. While Chapala and Megha had grabbed headlines in 2015, Kyarr was the first super cyclonic storm over the north Indian Ocean, in the Arabian Sea basin, since Gonu in 2007. Causing huge crop damage and loss of life with heavy rainfall and flash floods in western India, Kyarr had also affected major cities in West Asia.

Even in 2021, the extremely severe cyclonic storm Tauktae, which started off the coast of Kerala as a tropical depression, made landfall in the Saurashtra region between Diu and Una. It was recorded as the strongest cyclone to make landfall in Gujarat since the 1998 cyclone.

The spotlight is also on the Bay of Bengal, which is an outlier, recording a slight decrease as compared to its previous record even when there is a surge in the number of such events across the globe. However, it continues to record a higher dominance of these events as compared to the Arabian Sea, in terms of both frequency as well as intensity.

Why is it different for the Bay of Bengal compared to the Arabian Sea? 

The formation of tropical cyclones and their intensification are governed by various geophysical and thermodynamic parameters such as sea surface temperatures, sea level pressure, tropical cyclone heat potential, convective available potential energy, air temperature at height levels, relative humidity at mid-tropospheric levels and vertical wind shear – that is how winds can change their speed from the surface to the top of the atmosphere.

Of these, the parameters linked to global warming are sea surface temperatures and ocean heat content (oceans absorb nearly 93 percent of additional heat due to human activities). 

Several long-term data sets indicate that sea surface temperature trends are higher over the NIO as compared to the other oceans – a rise of 0.15o to 0.2oC per decade as compared to 0.11oC in the latter. Within these trends, the values are higher for the Arabian Sea than what is observed in the Bay of Bengal. This means that the Arabian Sea is warming at a higher rate as compared to the Bay of Bengal and has the potential of giving rise to more tropical storms in future.

One of the factors behind the slight decrease in the duration of tropical cyclones in Bay of Bengal is the long-term reduction in atmospheric humidity, which is rising at a greater rate over the Arabian Sea like its surface temperature.

According to a recent study published in Science Advances, statistical models suggest that the risk of strong tropical cyclones is expected to more than double across the globe by 2050 except for the Bay of Bengal and the Gulf of Mexico.

Is there a period of peak activity?

There are two peak periods for cyclonic activity in the two sub-basins – the primary peak during the post-monsoon season, from October to December, and the secondary in the pre-monsoon season, from April to June. During the spring or pre-monsoon season, tropical cyclones are more often carried towards the northeast or east of the country – towards Bangladesh or Myanmar. There is little cyclonic activity between these two periods.

What’s the economic cost?

The World Meteorological Organization pegs the economic impact of tropical cyclones over the last 50 years at 1.4 trillion dollars.

In India, since the last decade, there has been an increase in the frequency of pre-monsoon intense Arabian Sea tropical cyclones. These have mostly shown north-eastward propagation, causing more devastation over the western or southwestern coast, hitting a major portion of the exclusive economic zone.

Since this zone is key for several energy resources, oil, minerals, commercial fishing, international trade, and even national security, an increase in such weather events poses a greater risk to all these sectors.

The Science Desk is a collaborative effort between Newslaundry’s subscribers and its editorial team.