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Climate researcher explains dynamics behind cyclones
Ajit Patowary
 GUWAHATI, May 11 - Keeping a tab on the weather, it is observed that cyclones hit the Indian coasts on two occasions, pre-monsoon (April-May) and post-monsoon (October and November) respectively.

The month of November is considered the primary peak for cyclones, while May is the time for secondary peak. Again, the east coast of India is most frequented by cyclones which wreak most damage, said Dr Rahul Mahanta, coordinator of the Interdisciplinary Climate Research Centre (ICRC) of Cotton University here. He is well known as a leading climate researcher.

Talking to this newspaper, Dr Mahanta said analysis of cyclones during AD 1891-2000 shows that while 308 cyclones (103 of them being severe) affected the east coast of India, only 48 tropical cyclones (24 of them being severe) crossed the west coast.

Of all the cyclones developing over the Bay of Bengal, over 58 per cent made landfall on Indian coasts in October and November. The corresponding number for cyclones originating on the Arabian Sea is 25 per cent, he said.

One may ask as to why cyclones are so severe in the east. The answer to this question lies in the differing nature of the Bay of Bengal and the Arabian Sea. Most of the Indian coasts lie in the tropical region. Tropical cyclones need a temperature of around 25-27 degree Celsius. Greater the temperature over the sea, more powerful is the tropical cyclone.

The Arabian Sea is relatively cooler than the temperature range which the Bay of Bengal offers. This is why Tamil Nadu, Andhra Pradesh, Odisha and West Bengal face more cyclones than Kerala, Karnataka, Goa, Maharashtra and Gujarat.

Greater frequency of Bay of Bengal cyclones, besides more strength come from a foreign source. The neighbouring Pacific Ocean is more prone to cyclones. Typhoons originating in nearby Philippines, China, Thailand and Malaysia enter the Andaman Sea and Bay of Bengal after they weaken in their native regions.

Most cyclones in the Arabian Sea are local. They collapse quickly after making landfall as there is no back-up supply.

However, the recent Ockhi cyclone was an exception as it remained strong for sometime even after hitting Maharashtra and the Gujarat coasts.

Also, the hills along the eastern coasts are not high enough to stop cyclones making much inroad into the coastal states, while the Western Ghats run almost the entire distance of the western coast and prevent cyclones from entering the hinterland.

Since both sea surface temperature and humidity are directly linked to the chance of cyclone formation, the Bay of Bengal is a more likely target because it gets higher rainfall, and because the sluggish winds around it keep temperatures relatively high, i.e., about 28 degrees round the year. Warm air currents enhance this surface temperature and facilitate formation of cyclones.

Moreover, the Bay of Bengal receives higher rainfall and constant inflow of fresh water from the Ganga and the Brahmaputra, thus making it impossible for the Bay’s warm surface water to mix with the cooler water below. This is an ideal condition for a depression.

Conversely, the Arabian Sea receives stronger winds that help dissipate the heat, and the lack of constant fresh water supply helps the warm water mix with the cool water, reducing the temperature.

Since high water and air temperatures are crucial to the formation and intensification of cyclones, they are most commonly reported in summer. However, the Bay of Bengal witnesses cyclones in both pre-monsoon and post-monsoon periods.

The post-monsoon period sees a higher number of cyclones than the pre-monsoon period. Summers and pre-monsoons see dry and hot air moving from north-western India towards the Bay. This blocks the rise of air from the water, and the subsequent formation of clouds, preventing cyclone-friendly conditions. Absence of this air movement in the post-monsoon phase increases the chances of cyclones.

All these factors have made the Bay of Bengal a highly-charged theatre when it comes to cyclones, said Dr Mahanta.

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