Bay of Bengal

Roughly situated between the equator and the tropic of Cancer (23.5 degrees North), the expansive, V-shaped Bay of Bengal stands as the northeasternmost extension of the Indian Ocean. Bordered to the southwest by the island of Sri Lanka, to the west and northwest by India, to the north by Bangladesh, and to the north-northeast by Myanmar (Burma), the Bay of Bengal’s position within the intertropical convergence zone (ITCZ) has long made it one of the most active spots on Earth for cyclone generation. On average, eight percent of the world’s tropical cyclones originate over the warm waters of the bay in any given year. Between 1737 and 2006, some 742 recorded cyclones were formed; this represents an average of three storms annually. While cyclones have occurred in the Bay of Bengal during every month of the year, statistics indicate that peak periods for cyclone generation come in July and September, or during those periods when the Asiatic monsoon begins its advance and retreat. Furthermore, a number of these storms have been of significant intensity and duration, posing a grave hazard both to the shipping that regularly plies the bay and to those nations—in particular India and Bangladesh—that ring it.

Nearly three centuries of comprehensive study into the character of the Bay of Bengal’s frequent cyclones have increasingly revealed a cyclical connection between such storms and the ever-changing seasons of the monsoon. Prompted by the sharp temperature and pressure differences between the landmass of the Indian subcontinent and the large bodies of water that surround it, the early summer arrival of the monsoon is characterized by both an abrupt reversal in the direction of the prevailing winds and the rapid formation of tropical cyclones. For this reason, cyclone generation in the Bay of Bengal tends to be most common during the change of seasons, or when the prevailing winds of May and June first shift to the north and flow up from the Indian Ocean and into the hot, humid regions of Bengal and northwest India.

Similarly, the retreat of the monsoon, which generally occurs between September and November, brings with it another peak in cyclone origination, one noted for its predilection toward violent cyclone strikes. Indeed, some of the most destructive cyclones on record have come ashore from the Bay of Bengal during September, October, and November, including the False Point Cyclone of September 21, 1885, the Hooghly River Cyclone of October 7, 1737, the Great Calcutta Cyclone of October 5, 1864, the Backergunge Cyclone of October 31, 1876, and the Great Cyclone of 1970.

Although death-toll estimates from these storms vary from source to source, their cumulative losses can be accurately measured in terms of hundreds of thousands of people either left dead or missing. A majority of early season Bay of Bengal cyclones tend to originate in the northern half of the bay during the months of June and July. These storms, which are usually of smaller size and intensity than those late-season cyclones that develop south of the 16th parallel, almost always move in a northwesterly direction, a course that takes them ashore on India’s northeast coast or in the vicinity of the major port cities of Calcutta and Visakhapatnam.

Although early season Bay of Bengal cyclones do not as a rule prove as destructive as their late-season cousins, they do remain better organized for a longer period of time, bringing huge precipitation counts and gusty winds to the inland regions of the Ganges River Valley. On the other hand, late-season cyclones in the Bay of Bengal, those storms that form between September and November, are generally of greater size and intensity at landfall because of the long distances over which they must travel. 

Such cyclones, which originate in either the south or southeast quadrants of the bay, are steadily carried to the north or northwest by the vast influx of moist air that sweeps up from the west-southwest as part of the retreating summer monsoon. Powered by both their energy-laden steering currents and the warm 86°F (30°C) waters over which they pass, these Bay of Bengal cyclones often grow into storms of enormous destructive potential. With central barometric pressures below 27.91 inches (945 mb), winds in excess of 155 MPH (250 km/h), and record-breaking storm surges, these cyclones frequently slam into India’s southeast beaches or inundate the low-lying south coast of Bangladesh, with catastrophic results for the indigenous populations who make their lives along the shores of the Bay of Bengal.