Subject: A6) What is a sub-tropical cyclone?
Contributed by Chris Landsea (NHC) and Sandy Delgado (NHC)
A sub-tropical cyclone is a low-pressure system existing in the tropical or subtropical latitudes (anywhere from the equator to about 50°N) that has characteristics of both tropical cyclones and mid-latitude (or extratropical) cyclones. Therefore, many of these cyclones exist in a weak to moderate horizontal temperature gradient region (like mid-latitude cyclones), but also receive much of their energy from convective clouds (like tropical cyclones). Often, these storms have a radius of maximum winds which is farther out (on the order of 100-200 km [60-125 miles] from the center) than what is observed for purely "tropical" systems. Additionally, the maximum sustained winds for sub-tropical cyclones have not been observed to be stronger than about 33 m/s (64 kts, 74 mph)).
Many times these subtropical storms transform into true tropical cyclones. A recent example is the Atlantic basin's Hurricane Otto in October 2010 which began as a subtropical cyclone before becoming fully tropical. Note there has been at least one occurrence of tropical cyclones transforming into a subtropical storm (e.g. Atlantic basin storm Allison in 2001).
Subtropical cyclones in the Atlantic basin are classified by the maximum sustained surface winds:
For more information see Penn State University's write up on the Subtropical Cyclones.
Last Revised : May 18, 2011
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