Extremely dangerous Cyclone Sidr bears down on Bangladesh and India

Tropical Cyclone Sidr, a powerful Category 4 storm with 140 mph winds, is bearing down on the densely populated coasts of Bangladesh and India. Landfall is expected along a low-lying stretch of coast just east of Calcutta (Kolkata), India in just 36 hours. Sidr (the Arabic word for the the jujube tree) is only the second major (Category 3 or higher) tropical cyclone to affect the Bay of Bengal this decade. The other was Tropical Cyclone Mala, which hit Myanmar as a Category 3 storm on April 28, 2006, killing 22 people.


Figure 1. Image of Tropical Cyclone Sidr as a Category 4 storm (933 mb) taken at 2:47am EST 11/14/07. Image credit: Navy Research Lab.

Sidr weakened briefly yesterday, as its inner eyewall collapsed and was replaced by a new outer eyewall with a larger diameter. Now that this process is complete, Sidr has taken advantage of the low wind shear and high heat content oceanic waters it find itself over, and was intensified. As the storm continues northwards over the next 24 hours, the storm should be able to maintain its intensity. However, in the 12 hours prior to landfall, there will be a sharp increase in wind shear associated with a trough of low pressure. Ocean heat content will be on the decline as well, and the best guess is that Sidr will be a Category 1 or 2 cyclone when it hits land.


Figure 2.Deadliest tropical cyclones of world history.

A Category 1 or 2 cyclone hitting the low-lying, densely populated coasts of Bangladesh could still be devastating. The triangular shape of the Bay of Bengal acts to funnel storm surge waters into Bangladesh, and the very shallow bottom of the bay allows the surge to pile up to very high heights. A list of the 13 deadliest cyclones in world history (Figure 2) shows that nine of these have occurred in the Bay of Bengal. The big killer in all of these cyclones was the storm surge. The only known cyclone of Category 5 strength to hit Bangladesh, the April 1991 cyclone, brought a 30 foot storm surge to the coast near Chittagong. Surge height up to 41 feet are possible along some regions of the coast (Figure 3).


Figure 3.Maximum surge height along Bangladesh's Bay of Bengal coast as computed by Islam (2006).

The big question is, how much of the storm surge that Sidr is currently piling up with its Category 4 winds will make it to the coast, if the expected weakening occurs? We know from our experience with Hurricane Katrina that a weakened Category 3 hurricane at landfall can still push a storm surge characteristic of a Category 5 storm to the coast. This occurs because once a hurricane sets a large mass of water spinning, the angular momentum of that spinning water takes a long time to relax. The ocean carries the memory of how strong a hurricane was to the coast, in the form of a higher storm surge. Thus, I would expect a storm surge at least one category higher than what one might expect based on its landfall strength. One positive note is that Sidr is a much smaller storm than Katrina. Hurricane force winds extend outwards about 60 miles from Sidr's center, which is half of what Katrina's winds did. Thus, Katrina's hurricane force winds blew over an area of ocean four times greater than Sidr's.

methaz.org is currently predicting that a storm surge of 12-15 feet will affect the Bangladesh coast.

Links to watch:
Calcutta (Kolkata), India
Barisal, Bangladesh
Cox's Bazaar, Bangladesh

References:

Islam, T., "Integrated Approach to Cyclone Wind Analysis and disaster planning for the Bangladesh coast," Ph.D. Dissertation, Texas Tech University, December 2006.

I'll have an update Thursday.

Jeff Masters