The 2009 Atlantic Hurricane Season will be remembered as a below-average season. There were nine named storms, including three hurricanes. Two of the hurricanes were major (Category 3 or higher on the Saffir-Simpson Hurricane Wind Scale). Only two tropical storms made landfall in the U.S.: Claudette in the Florida panhandle in August and Ida near Dauphin Island, Alabama in November. No hurricanes made landfall in the U.S. in 2009, although Hurricane Bill caused widespread coastal flooding from surge and waves along much of the Atlantic coast. Some coastal infrastructure damage resulted from the high waves at the coast.
The weak to moderate El Niño contributed, in part, to the reduced tropical cyclone activity through stronger-than-average vertical wind shear. It should be noted that five of the tropical cyclones dissipated over the open Atlantic. This is unusual and is a direct result of the increased shear.
Six of the tropical storms (Ana, Claudette, Danny, Erika, Grace and Henri) lasted only one to three days. I have always been a fan of the Accumulated Cyclone Energy (ACE) Index which is a more robust measure of the tropical cyclone activity. The ACE Index is defined as the sum of the square of a named storm’s maximum wind speed for each 6-hour period of its existence. The ACE Index accounts for the number, intensity and duration of the tropical storms and hurricanes. A long-lived major hurricane will have a much higher ACE vaue than several short-lived tropical storms.
The above figure (kindly provided by Local 10’s Aliana Perez) shows the ACE values going back to 1995. Data were provided by the National Hurriane Center (NHC). The ACE value for the 2009 Atantic Hurricane Season is 52 (although this could change slightly after the NHC completes the best tracks). Since 1995, only 1997 (also an El Niño year) had a lower ACE value than 2009. To me, the ACE index is a much better parameter to use to characterize the overall activity of a hurricane season than simply referencing the number of storms or hurricanes.
While most folks, including me, celebrate November 30th as the end of the 2009 hurricane season, I like to remind myself that this date also marks the beginning of the six month period that we have to prepare for the 2010 hurricane season.