Over the weekend, the a tropical cyclone developed in the Bay of Bengal - the second of the season so far. Cyclone Aila made landfall near the the border of India and Bangladesh, bringing torrential rains, hurricane force winds, and storm surge. Presently, there are over 200 fatalities from Aila, with over 1000 additional people unaccounted for.
Cyclone Aila formed in a manner not often seen in the Atlantic or the eastern Pacific. Most tropical storms and hurricanes in these regions form from tropical waves, subtle disturbances that occasionally spark convection and then grow and organize. In the northern Indian Ocean, often the progenitor of tropical cyclones is the monsoon trough, a large zone of converging air that migrates from the equator to the Indian subcontinent as part of the seasonal monsoons. Perturbations along the monsoon trough can acquire rotation, becoming large monsoon depressions, and occasionally these monsoon depressions can organize into tropical cyclones. It is during the months when the monsoon trough is migrating (May and November, give or take) that tropical cyclones are most often seen in the northern Indian Ocean.
The image at the start of this entry shows Cyclone Aila at its peak strength, with winds estimated at 75 mph. Note Also that Aila is a large system - not as big as last year's Hurricane Ike, but still expansive nonetheless. Compare that to the image at left, which is the monsoon depression that evolved into Aila, but two days earlier (click to open a full-size version in a new window, which will be helpful to get an idea of the scale of the system). The monsoon depression covers almost the entire Bay of Bengal (a body of water comparable in size to the Gulf of Mexico) with bands and clusters of strong but disorganized convection. The overall system has a hint of a broad cyclonic turning, but not much in the way of a clearly defined circulation.
There is some speculation that tropical cyclones in the northern Indian Ocean, which are closely tied to the monsoon trough and the formation of monsoon depressions, are somewhat more predictable than their cousins in other basins. This would make sense in that the monsoon depression, being a large feature, could be better resolved by most operational computer forecast models. Still, the transition from the monsoon depression to the tropical cyclone would be difficult to anticipate. A similar situation has been seen anecdotally in the Atlantic for systems that have a subtropical origin. Often times the forecast model can pick up on the larger scale upper low or occluded synoptic scale low days in advance, although it is difficult to pin down the subtropical and tropical transitions with significant lead time. Such was the case with 2007's Tropical Storm Barry. The GFS forecast model suggested the possibility of subtropical development 6-8 days in advance, but the model was not much help in determining when the transition to a tropical system would occur. Unfortunately the situation is not as cut-and-dried as that single example would indicate, and although forecast skill in anticipating the formation of tropical cyclones has improved, there is still much more work to be done.
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