Annular Hurricane

A relatively new term used by the U.S. National Hurricane Center (NHC), an annular (or annular-type) hurricane is a mature-stage tropical cyclone that possesses meteorological characteristics that make them unique as compared to other tropical systems of similar size, intensity, or location. First identified in 2002, the annular hurricane generally contains a large eye of between 25 miles and 45 miles (40 km–72 km) in diameter, with a symmetrical eyewall configuration, and convective activity that is evenly spread throughout the eyewall structure.

An annular hurricane tends to survive and even intensify in environments that are not often considered favorable for tropical cyclone development, such as those with mild wind shear and below-average seasurface temperatures (SSTs). Meteorologists believe that an annular hurricane is not as susceptible to eyewall rejuvenation cycles, and for this reason is able to maintain a certain level of intensity for much longer periods than statistical and other analogs would predict.

In seeking to better define and understand annular hurricanes, meteorologists and climatologists have studied past tropical cyclones in the North Atlantic and eastern North Pacific Oceans, including 1996’s Hurricane Edouard and 2003’s Hurricane Isabel in the North Atlantic; and Hurricane Daniel in the eastern North Pacific Ocean in July 2006. At the time that Daniel exhibited annular characteristics on July 20, the system was of Category 4 intensity, with a central barometric pressure reading (estimated) of 27.99 inches (948 mb) and sustained winds of 125 MPH (205 km/h). Not all annular hurricanes are this powerful, however. In the example of Hurricane Epsilon in 2005, longevity rather than intensity, characterized the annular nature of this North Atlantic tropical system. It is possible that further research into the annular hurricane will indicate that Cyclone Caterina, which surprisingly appeared in the South Atlantic Ocean in March 2004 and made landfall in Brazil, was of the annular type.