A few preliminary notes on this year’s hurricane season

1. A few preliminary notes on this year’s hurricane season Klaus Wolter NOAA-Earth System Research Lab & University of Colorado at Boulder-CDC klaus.wolter@noaa.gov • 2010 – active, but a bit underwhelming • A testable hypothesis • ENSO link to late season activity Boulder, 23 November 2010

2. 2010 Hurricane Season As expected, this was a hyper-active season – both in terms of number of named storms (19) and hurricanes (12) - each tied at second since 1948; In terms of major hurricanes, this one produced five, tied in 9th-14th place, similar to ACE which is the 12th highest since 1948; this is quite high (top 20% or so), but maybe a little less than expected; In terms of Cat 5 hurricanes, there was none, dropping the ranking down below those 20 out of 63 years with such hurricanes; more unusually, landfalling hurricanes in the U.S. were conspicuously absent: this is only the 15th time out of 63 years that we did not have a landfalling hurricane. In fact, this is the record-highest ACE season without having a landfalling hurricane ( 1951 is a distant second ranked season of this type).

3. A testable hypothesis Why did we not get our ‘fair share’ of landfalling hurricanes, as widely expected in the light of a very high forecast number of storms? If you compare the number of named tropical storms in different parts of the North Atlantic vs. the number of landfalling storms, it is obvious that storms that form in the Gulf of Mexico have a much better chance of hitting the U.S. coastline than storms that form immediately off the coast of West Africa. In fact, of the 138 named systems from 1948-2009 that formed in the Gulf of Mexico, a full 100 ended up making landfall in the U.S. (72.5%). This compares to storms in the Main Development Region (281) of which 67 made landfall (23.8%) in the U.S., and only one out of 7 such storms that formed to the east (<20°W) of the MDR (14.3%). 2010 ended up with 12 hurricanes, half of which formed as tropical storms to the east of 60°W. Only two storms formed in the Gulf of Mexico, both failing to attain hurricane status. Could the lop-sided distribution of tropical storm formation have been a factor in 2010? Was this driven by unusually warm SST off the coast of West Africa, or was it more a function of unusual atmospheric circulation features?

4. ENSO link strongest with mid-late season Summer-to-fall ENSO conditions (MEI) relate best to the hurricane count after September 1st. However, it is mostly the depression of such hurricanes in El Niño situations that stands out (i.e., the early end of the season) rather than a strong La Niña signal.