Increase in Atlantic Hurricane Numbers Likely Due to Improvements in Monitoring

August 11, 2009

Records of Atlantic hurricanes seem to show an increase in storm frequency since the late 19th century, but research published this week reveals that the increase in tropical storm and hurricane numbers is likely due to better observations of short-lived storms.   Improvements in observational tools and analysis techniques have resulted in more complete storm monitoring and recording systems. 

A sampling methodology developed by GFDL scientists Gabriel Vecchi and Thomas Knutson takes into account the possibility of "missing" storms not measured by ships over the open Atlantic Ocean.  Previously published research by Vecchi and Knutson (2008, Journal of Climate) demonstrated that a substantial number of storms would not have been observed or recorded during the era  when satellites were not available, and ships were used to record storms (from the late 1800s through the 1950s).

In this new study, lead author Chris Landsea (NOAA’s National Weather Service), coauthors Vecchi, Knutson, and Lennart Bengtsson (University of Reading, UK), analyzed the increase in storm frequency since the 19th century and show that any increase arises mainly from very short-lived storms.  Moderate duration storms (lasting more than two days) have not shown a long-term increase.  The increase in very short-lived storms is likely due to enhanced monitoring, the authors conclude. 

The resulting time series, with the very short-lived cyclones removed and estimated number of missed longer-lived cyclones added in, shows no significant long-term trend.  Although there is no significant increase in tropical cyclone frequency, their paper, "Impact of Duration Thresholds on Atlantic Cyclone Counts" (in American Meteorological Society’s Journal of Climate) does not consider the possible changes in hurricane strength that may occur in the future, due to global warming. 

Coauthors Gabriel Vecchi and Thomas Knutson discuss this research in an audio podcast.  See a related web page with more information on Historical Changes in Atlantic Hurricane and Tropical Storms.

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