Did 2008’s hurricanes meet expectations? Professor Mark Saunders of TSR reviews a stormy year.
Mark Saunders provides a retrospective on an active and well predicted storm season.
2008 was an active, damaging and deadly North Atlantic hurricane season. The season saw Atlantic basin tropical storm activity about 50% above the long-term (1950-2007) norm, US landfalling storm and hurricane activity 100% above-norm, and a basin total damage bill estimated at over US $ 40bn. Tropical storms in 2008 also caused over 800 fatalities, mostly on Haiti. The year continued the current active hurricane era which has seen 10 of the last 14 years witness above-norm hurricane activity.
2008 saw 16 tropical storms, 8 hurricanes and 5 major hurricanes (Cat 3+) (Figure 1). This compares to long-term norm values of 10.3, 6.2 and 2.7 respectively. Six tropical storms (Dolly, Edouard, Fay, Gustav, Hanna and Ike) struck the U.S. with three of these (Dolly, Gustav and Ike) striking at hurricane strength. These strike numbers compare to long-term norm values of 3.1 and 1.5 U.S. strikes per year respectively. In terms of historical precedent, 2008 ranks - depending on which activity measure is used - between the 4th and 15th most active hurricane season since comprehensive records began in 1944.
The 2008 North Atlantic and U.S. landfalling hurricane seasons were predicted to have 'high activity' (i.e. within the top one third of years historically) to high (65-70%) probability from early December 2007 (see www.tropicalstormrisk.com; http://tropical.atmos.colostate.edu). By early August 2008 Tropical Storm Risk had raised these probabilities to 97% certainty that Atlantic basin hurricane activity would be in the top third of years historically and to 91% that US landfalling hurricane activity would be in the top third of years historically. The latter forecast employed the model published in the leading scientific journal Nature (Saunders, M. A. and A. S. Lea, Seasonal prediction of hurricane activity reaching the coast of the United States, Nature, 434, 1005-1008, 2005). A very active 2008 hurricane season was also predicted by Colorado State University and by NOAA.
Noteworthy and unusual features of the 2008 hurricane season include: (a) Bertha being the first major hurricane to form east of 60°W before 1st August since reliable records began; (b) Bertha being a tropical cyclone for 17 days (3-20 July) making it the longest-lived July storm on record in the Atlantic basin; (c) The U.S. being struck by six consecutive named tropical storms, the first time this has ever happened; (d) Cuba being struck by a record three major hurricanes (Gustav, Ike and Polama); (e) Paloma being the second strongest November hurricane on record with peak winds of 145mph; (f) The occurrence of intra-seasonal variability in storm incidence linked, it is thought, to the 40-50 day period of the atmospheric phenomenon called the Madden Julian Oscillation.
Why was 2008 so active for hurricanes? The reasons will vary somewhat depending upon who is asked as each seasonal forecast model is based upon different physics. However, the following physical factors may all have contributed: weaker than normal trade winds over the tropical North Atlantic (2008 had the 2nd most favourable (after 1995) such winds since 1950); warmer than normal sea surface temperatures (SSTs) in the tropical North Atlantic (2008 had the 6th highest SSTs since 1950); an ongoing multidecadal signal of favourable ocean and atmospheric conditions; lingering effects of the 2007/8 La Nina helping to decrease vertical wind shear.
The first extended outlooks for the 2009 hurricane season have just been announced by Tropical Storm Risk (TSR) and Colorado State University (CSU). Both predict another active hurricane season in 2009 (albeit slightly less active than 2008). TSR predicts that activity will be 35% above the 1950-2008 norm, and CSU predicts activity will be 30% above the 1950-2000 norm. However, users should be aware that the skill of these extended range forecasts for Atlantic hurricane activity over the last 25 years, while positive, is low.
Mark Saunders is Professor of Climate Prediction in the Department of Space and Climate Physics at University College London, UK, and lead scientist on the TropicalStormRisk.com forecasting venture.