Weather Almanac for November 2008

THE DEADLY CYCLONES OF BANGLADESH

In November 1970, famed Indian sitarist Ravi Shankar became distressed over the human misery following a severe tropical cyclone that struck the Chittagong Coast and Ganges Delta region of East Pakistan. The storm took an estimated 300,000 lives, the greatest weather disaster of the 20th Century. Following the natural tragedy, millions more were affected as civil war broke out across the region when East Pakistan broke away from West Pakistan, eventually forming the new nation of Bangladesh.

Shankar would soon be asked to perform some benefit concerts to raise money to aid those suffering. Shankar sought assistance from his friend George Harrison of the Beatles, The result was the first of the great benefit concerts, the Concert for Bangladesh, held in New York City on 1 August 1972. The concert, which included the two key organizers, Bob Dylan, Ringo Starr, Eric Clapton, and Billy Preston, raised nearly a quarter million dollars for UNICEF relief in the region. Royalties from the record and movie produced from the concert raised additional funds.

Fast-forward just over two decades to 1991 when another tropical cyclone strikes the Chittagong coast. Pushed by 225 km/h (140 mph) winds, the resulting storm surge and following flood took nearly 140,000 lives. In between these two storms, a May 1985 cyclone struck the region with 4200 reported dead, and a November 1988 storm took nearly 1600 lives.

Tropical storm warnings for the region were still in their infancy at the time of the 1991 storm, and the consulting company I worked for proposed the development of a storm-warning system, a project I dubbed BEWARES for Bay of Bengal Early Warning System. While the initial proposal circulated around international agencies, I left the company but have kept the region in the corner of my weather eye.

Improved tropical storm warnings now exist for the region, but the local topography continues to pose a grave hazard to the population during tropical cyclones. The coastal region of the nation, which includes the delta for the Ganges, Brahmaputra and Meghna Rivers, extends for approximately 700 km (440 miles) and is generally less than 12 metres (39 ft) above sea level. (Flooding from the rivers can also have deadly consequences in this very rural nation.) Between 1980 and 2000, approximately 60 percent of the global deaths due to tropical storms occurred in Bangladesh, over a million people from 1970 to the end of the century.

The last major storm to directly hit the Bangladesh coast did so on 15 November 2007. Tropical Storm Sidr, designated 06B and also known as Super Cyclonic Storm Sidr, boasted peak sustained winds of 260 km/h (160 mp/h), a Category-5 equivalent tropical cyclone on the Saffir-Simpson Scale. The human toll due to Sidr has been officially pegged at 3447, but Save the Children estimated the number of deaths between 5,000 and 10,000, while the Red Crescent Society reported that the number of deaths could be as high as 10,000. The storm also devastated the World Heritage Site of the Sunderbans. Nearly a quarter of the vegetation in the Sundarbans' large mangrove forest was destroyed, about ten percent of the trees irreparably damaged.

Early this year (late April 2008), a deadly tropical cyclone would form in the Bay of Bengal but would not directly affect Bangladesh. Tropical Cyclone Nargis struck the Myanmar (formerly Burma) coast on 2 May with deadly force. With sustained winds of 208 km/h (130 mph) and gusts reaching 256 km/h (160 mph) — the equivalent of a strong Category 3 or minimal Category 4 hurricane on the Saffir-Simpson scale — Nargis made landfall near Cape Negrais and continued across the Ayeyarwady Delta on its way towards the capital city of Yangon (Rangoon). The initial official death count was 77,738 confirmed dead with 55,917 missing and 19,359 injured. The United Nations estimated that as many as 2 million people were affected by the storm. Full details and an updated death count are yet unavailable due to the reluctance of the Myanmar government to communicate with the outside world. And as I wrote this piece, Cyclone Reshmi struck the Bangladesh coast on 27 October 2008 with initial accounts reporting the loss of ten lives and scores missing.

Tragic as the Myanmar storm was (as well as others striking India and Myanmar/Burma), in this piece I focus on the events in the region of the Bay of Bengal north coast known today as Bangladesh.

The Geography of Bangladesh

Today's nation of Bangladesh lies within the traditional region of Bengal, sharing it with the Indian State of West Bengal. Its current borders were set in 1947 with the partition of Bengal from India and became the eastern portion of the new nation of Pakistan. It became independent from Pakistan in 1971. Bangladesh's 150 million-plus inhabitants live on 147,570 square km (55,599 sq mi) ranking it the 11th densest populated nation, though most nations ahead of it in population density are small city-states. Dhaka is the capital and largest city of Bangladesh. Other major cities include Chittagong, Khulna, Rajshahi, and Barisal.

Map of northern Bay of Bengal coastline
showing Bangladesh and surrounding nations.

Bangladesh is located in South Asia along the northern shore of the Bay of Bengal, having about 580 km (360 miles) of coastline, with India and Myanmar on its borders. Geographically, the country surrounds the fertile Ganges-Brahmaputra-Meghna Delta which empties into the Bay of Bengal. The soil deposited by these rivers has created some of the most fertile land in the world. The highest point in Bangladesh is found in the Mowdok range in the Chittagong Hill Tracts of southeast Bangladesh: 1230 metres (4032 ft). However, most of Bangladesh stands 12 metres (39 ft) or less above sea level, and about 50% of the land is less than a metre (3 ft) asl.

A large portion of the nation's coastline comprises a marshy jungle known as the Sunderbans. This World Heritage Site contains the world's largest mangrove forest and a diverse flora and fauna population that includes the endangered Royal Bengal Tiger.

Bangladesh straddles the Tropic of Cancer and has a monsoonal tropical climate. A mild winter lasting from October to March is followed by a hot, humid summer from March to June. The remaining months (June to October) are warm, humid and wet. During this period, most of the country's rain falls. Bangladesh is beset by annual natural hazards including tropical cyclones with storm surges, tornadoes, and severe flooding, the latter exacerbated by widespread deforestation. In September 1998 flooding put two-thirds of Bangladesh was underwater. The flood destroyed 300,000 houses leaving 30 million homeless. One thousand people died along with 135,000 cattle. Over 50 square kilometres (19 square miles) of land were destroyed along with 9,700 km (6,000 miles) of road.

Tropical Storms in the Bay of Bengal

Tropical storm that move across the Bay of Bengal form in the North Indian Ocean basin which essentially comprises the Bay of Bengal and the Arabian Sea. For a tropical storm to form, sea surface temperatures must exceed 27^oC, and this requirement is met throughout the year in some regions of the Bay of Bengal. Storms most often develop near the Andaman Islands in the south-central Bay and then generally move initially northwest before curving eastwards.

Composite of Cyclone Tracks Crossing
the Bay of Bengal (1970 to 2005)

Source: Wikimedia Commons

The North India Ocean is unique in that it is the only ocean basin in the world where there are two distinct tropical cyclone seasons. The first season peaks in April and May, and the second in late October/early November. The reason for the split is that the summer monsoon (running from June to October) is so strong that it makes tropical cyclone development very unlikely. During the monsoon, the large low pressure trough, within which tropical storms may form when over the ocean, moves inland, and its upper-level winds blow across the ocean waters at jet-stream strength (80-130 km / 50-80 mph) which inhibits the organization of a tropical storm. The two cyclone seasons thus occur before and after the summer monsoon when the trough lies over the ocean waters and upper-level winds are weak.

Cyclones thus are rare in the Bay of Bengal from January to March. Those that do form do so in the southern reaches of the Bay and move west-northwestwards toward the Tamil Nadu and Sri Lanka coasts. Cyclones arising in April and May, are usually born in the South Bay and adjoining Central Bay. They generally move initially northwestward, then north, and finally recurve to the northeast striking the Arakan coast in April and Andhra-Orissa-West Bengal-Bangladesh coasts in May.

Cyclones Tracking Over Bangladesh During the Twentieth Century
Click Here for larger image.
Source: Banglapedia

Any cyclones that might form during the monsoon (June–September) do so in the Central and the North Bay and move west-northwestwards to affect the Andhra-Orissa-West Bengal coasts. Post monsoon (October-December) cyclones usually develop in the south and the central Bay and move northward recurving between 15 and 18 degrees N to hit the Tamil Nadu-Andhra Orissa-West Bengal-Bangladesh coasts.

Tropical storms forming in the pre and post-monsoon periods are usually more violent than the storms of the monsoon season. The life span of a severe cyclonic storm in the region averages about 4 days from the time it forms at sea until the time it moves over land where it quickly dissipates.

The terminology for tropical cyclones in the region is different from those in other basins. They are, in order of increasing intensity:

The super cyclonic storm is the equivalent of a Safir-Simpson Category 5 hurricane.

On average, the North Indian Ocean experiences about 5 tropical storms of which two are cyclone storms (what we call hurricanes in the Atlantic basin). On average, 1.4 cyclonic storms form in the Bay of Bengal annually. A recent climatology of landfalling tropical cyclones for Bangladesh covering a 124-year period (1877-2001) found a total of 133 tropical cyclones hit the coast of Bangladesh. Of these, 42 were tropical depressions, 57 were tropical storms and 34 reached cyclonic storm (hurricane) intensity. The rate of tropical cyclones hitting the Bangladesh coast during the past century (1900-2001) was 1.06 per year. However, for the last 51 years (1950-2001) the rate increased to 1.26 per year. Most (74%) of the cyclonic storms struck during October and May with only a few occurring in January, February and March.

The study found that the landfalls are not evenly distributed along coastal segments. During the 1877-2001 period, 33 percent of the total landfalling cyclones hit the Khulna coast in the southwest of Bangladesh, while the Cox's Bazar coast in the southeast took only 7% of the total landfalls. The remaining struck the central Bangladesh coast.

Cyclones in the Bay of Bengal typically produce sustained wind speeds of 150 km/h (94 mph) and push storm surges which are 3-9 metres (10-30 ft) above the normal astronomical tide. Due to Bangladesh's flat terrain, a severe surge can push inland as far as 200 km (120 miles).

Coastal Damage due to Cyclone Sidr.
Courtesy U.S. Navy, photo taken by
Mass Communication Specialist Seaman Christopher Lange

Storm surges are the more dangerous elements of cyclones in Bangladesh, killing many people and livestock and destroying houses and crops as they push inland. The funnel-shaped northern coast of the Bay of Bengal with apex along the Bangladesh coast and the shallow coastal waters accentuates the surge, pushing sea water far up the delta. Effects of the storm surge are greatest when the storm hits at high tide. In 1997, a very severe cyclonic storm hit during low tides, but caused relatively less damage than other similar storms that hit at higher tides.

Traditionally, tropical cyclones in the North Indian Ocean have been identified by a basin location code — BOB for those in the Bay of Bengal and ARB for the Arabian Sea — followed by a four-digit number. The number is made up of the last two digits of the year followed by a two-digit sequential number, starting with 01 each year. For example, the second tropical cyclone of 2004 in the Bay of Bengal will be identified as BOB 0402. It is only in the last few years that cyclones were given names.

Beginning in September 2004, the Panel on Tropical Cyclones for the Bay of Bengal and the Arabian Sea implemented its own popular name list, the last basin to do so. The names are followed by the traditional identifier, e.g., Angi ARB 0404. The lists of popular names for use for the period from mid-2004 until 2009 was produced by the India Meteorological Department from suggestions from Bangladesh, India, the Maldives, Myanmar, Oman, Pakistan, Thailand and Sri Lanka. The names are ordered by the English alphabetical order of country names. The lists, which began with Onil in 2004, will be used sequentially and will only be used once. The first name for a given year is the one immediately following the last name used in the previous year. (I assume a whole new list will be produced for the years following 2009.)

For a discussion of significant cyclones in Bangladesh, go to:
Significant Severe Cyclones Striking Bangladesh

For an album of images: Cyclones of Bangladesh: Images

Learn More From These Relevant Books
Chosen by The Weather Doctor

• Burt, Christopher C.: Extreme Weather: A Guide and Record Book, 2007, W. W. Norton & Company, ISBN 039333015X, paperback.
• Heidorn, Keith C.:And Now...The Weather, 2005, Fifth House, ISBN 1894856651
• Williams, Jack: The Weather Book, 1997, Vintage Books, ISBN 0-679-77665-6.

Written by
Keith C. Heidorn, PhD, THE WEATHER DOCTOR,
November 1, 2008

The Weather Doctor's Weather Almanac: The Deadly Cyclones of Bangladesh
©2008, Keith C. Heidorn, PhD. All Rights Reserved.
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