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mike Meehan

Depressions Or Cyclones

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I have suddenly thought, we all know that north of equator depressions revolve in an anti-clockwise manner and south in a clockwise manner but what happens when it crosses from one hemisphere to another; does it retain its original rotation or does something else happen?

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It's very rare for depressions to form near the equator because the the lack of Coriolis force to impart rotation.

It can happen though, and the rotation remains the same direction as long as the Coriolis force is not great enough to impart the opposite spin.

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Cyclone Agni of 2004 briefly crossed from north to south while in its formative stages. The circulation would have started out with a weak anticlockwise vorticity in the northern hemisphere, which then slowed down to nothing whilst drifting across the equator before developing a clockwise rotation in the southern hemisphere.

Tropical cyclone formation within 5 degrees of the equator is very unusual due to lack of Coriolis force. An event like Typhoon Vamei (2001), which formed near Singapore, is believed to have a return rate of between 100-400 years (and that didn't even cross the equator). From Wikipedia:

"Vamei formed and reached tropical storm strength at 1.5º N, only 156 km (97 mi) from the equator.[3] This broke the previous record of Typhoon Sarah in the 1956 Pacific typhoon season, which reached tropical storm strength at 2.2º N.[2] Due to a lack of Coriolis effect near the equator, the formation of Vamei was previously considered impossible.[2] However, a study by the Naval Postgraduate School indicated that the probability for a similar equatorial development was at least once every four centuries.[1]

Vamei developed in a vortex that appears every winter along the northwest coast of Borneo and is maintained by the interaction between monsoonal winds and the local topography. Often, the vortex remains near the coastline, and in an analysis of 51 winters, only six reported the vortex as being over the equatorial waters for four days or more. As the area in the South China Sea between Borneo and Singapore is only 665 km (415 mi) wide, a vortex needs to move slowly to develop. A persistent northerly wind surge for more than five days, which is needed to enhance the vortex, is present, on average, nine days each winter. The probability for a pre-existing tropical disturbance to develop into a tropical cyclone is between 10–30 percent. Thus, the conditions which resulted in the formation of Vamei are believed to occur once every 100–400 years.[1]"

http://en.wikipedia....cal_Storm_Vamei

More in-depth source: http://www.weather.nps.navy.mil/cpchang/papers/vamei/vamei.htm

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