Why North Indian Ocean hotbed of cyclones
25 MAY 2020 - Source: the pioneer - According to NASA, the tropical cyclones are the most violent weather incidents on the earth. Each year, in the Northern Indian Ocean, half a dozen cyclones develop and sustain to hit the coastal boundaries of Myanmar, Bangladesh, India and Sri Lanka. The Bay of Bengal, located in the northeastern part of the Indian Ocean, is the largest bay in the world having around 500 million people staying close to the coastal belt. For last few decades, cyclones have become a frequent affair in the bay than in the Arabian Sea, north-western part of the Indian Ocean.
Cyclones form due to very low air pressure above the warm tropical waters of an ocean. From thorough research it is verified that, in the Bay of Bengal, ocean-atmosphere interact with all favoured conditions falling in place for development of cyclones more often than in the Arabian Sea. The perennial Himalayan rivers like the Ganges, Brahmaputra and most of peninsular rivers like Godavari, Krishna, Cauvery, Mahanadi, Irrawaddy discharge huge fresh water into the Bengal basin while the Arabian basin receives scanty fresh water. During the monsoon, the rainfall over the Bay of Bengal is more than the Arabian Sea. Thus much higher monsoonal rainfall along with fresh water input from the landmass over the bay decreases the surface water salinity making it one of the freshest tropical oceans ever known.
Winds over the Bay of Bengal are sluggish unlike relatively very strong winds over the Arabian Sea during the summer. The weak wind forcing in the bay offsets vertical mixing of water allowing a well stratified sea with warmer surface water persisting for a long time producing huge amount of moisture via convective activities. In the Arabian Sea, strong wind forcing encourages high evaporation, surface cooling and hence cause overturning of water always forming a weakly stratified sea than the Bay of Bengal. Although both the basins receive similar amount of incoming solar radiation (Insolation), the sea surface temperature (SST) in the Bay of Bengal remain favourable at 28°C and above along with high relative humidity which are prime reasons for formation and sustenance of cyclones.
The year 2019 was a hyperactive year recording nine cyclonic storms in the North Indian Ocean and its adjoining seas. Data from the Indian Meteorological Department (IMD) shows, in the year 2019, five cyclones churned up the waters of Arabian Sea equalling a 117 -year -old record. Vayu in June and Hikka in September were initially recognised as "very severe cyclonic storm", further weakened into depression and cyclonic storm before doing moderate damage to the coast of Gujarat and Oman respectively. In late October, cyclones Kyarr and Maha formed in the Arabian Sea dissipated in the coast of Somalia. Cyclone Pawan brewed in December in the Arabian Sea fizzled out into a well marked low pressure causing heavy rainfall to the coast of Somalia and adjacent areas.
The strongest-ever tropical cyclone in last several years in the Arabian Sea was cyclone Gonu. The cyclone with heavy rainfall and peak gust of 180km/hr left a trail of destruction in low lying eastern Oman by killing 50 people, the worst natural disaster for the country till date. However, Cyclone Ockhi which was less intense than the cyclone Gonu left 270 people dead with heavy infrastructural damage. Ockhi intensified at southwest Bay of Bengal from a deep depression to a cyclonic storm in a span of six hours. It followed a clockwise recurving path towards the Arabian Sea, devastated the coasts of Kerala, Tamil Nadu and Gujarat. It is considered to be one of the worst cyclones to have struck the western coastline of India.
Meteorologists have pointed out that post monsoon (i.e. in late October onwards) cyclonic storms in the Arabian Sea are very rare events. These unusual occurrences of cyclonic storms were attributed to strongest-ever Indian Ocean Dipole (IOD), a phenomenon associated with warming of Arabian Sea waters. However, the overall cyclonic storms in the Arabian Sea are not of high intensity very often as strong wind shear and cold waters near to the coasts weaken them.
Compared to the Arabian Sea, the Bay of Bengal is considered the hotbed of tropical cyclones with dreadful storms. Historical record says eight out of ten deadliest tropical cyclones originated in the Bay of Bengal only. The 1999 Super Cyclone that had hit Odisha remains the most powerful-ever cyclone recorded in India and one of the strongest anywhere in the world in last 120 years. Even 36 hours after its landfall at the Paradip coast, huge pressure drop increased the destructive capabilities with wind speeds of 250-260 kmph, storm surge having wave heights as much as 40 feet killing more than 10,000 people and over 1 lakh livestock. Extremely severe tropical cyclone Nargis struck Myanmar which caused widespread catastrophic destructions killing nearly 1.38 lakh people. Nargis is considered to be the biggest natural disaster ever. In 2019, Fani, which intensified as severe cyclonic storm in the bay, gathered moisture and intensified to an extremely severe cyclonic storm. It hit the Puri coast. Fani behaved unconventionally as it shifted direction frequently, gained speed in last stretch and made landfall quickly than its predicted time. It was the second deadly cyclone after the 1999 Super Cyclone with a gusting wind speed of about 210 kmph wreaking havoc in 14 districts with a death toll of 41 people and 2.2 million domestic animals.
Just a few days back, the fiercest cyclone to hit West Bengal in last 100 years was cyclone Amphan, which took 80 lives and made thousands homeless. Before hitting WB, it has caused severe damages to the Baleswar, Bhadrak and Kendrapada districts too. The Indian Meteorological Department (IMD) played a pivotal role saving many lives in the two States by doing accurate forecast about the trajectory and area of landfall of Amphan.
In the Bay of Bengal, every now and then, the coastal areas have come under the wrath of cyclones like Hudhud, Bulbul, Phailin, Titli and Amphan.
New studies are indicating that overheated oceans caused by global warming are increasing frequency of cyclones in the Bay of Bengal and Arabian Sea as well. However, over the past few decades, our understanding on tropical cyclones progressed rapidly because the phenomena occur relatively often.
Meteorologists, oceanographers and geologists have made significant progress to explain, document and simulate the oceanic and atmospheric parameters of formation of cyclones far away from the coasts.