Hurricane clusters in the vicinity of Florida

Models that predict annual U.S. hurricane activity assume a Poisson distribution for the counts. This assumption leads to a forecast that under predicts both the number of years without hurricanes and the number of years with three or more Florida hurricanes. The under dispersion in forecast counts arises from a tendency for hurricanes to arrive in groups along this part of the U.S. coastline. We recently developed an extension to our earlier Poisson model that assumes the rate of hurricane clusters follows a Poisson distribution with cluster size capped at two hurricanes.

Hindcasts from the cluster model better fit the distribution of Florida hurricanes conditional on the climate covariates including the NAO and SOI. Results are similar to models that parameterize the extra-Poisson variation in the observed counts including the negative binomial and the Poisson inverse Gaussian, but we argue that the cluster model is physically consistent with the way Florida hurricanes tend to arrive in groups.

This research is done in collaboration with Thomas H. Jagger (Climatek) and is supported by the Risk Prediction Initiative. It is currently in review with the AMS Journal of Applied Meteorology and Climatology.

Frequency and Intensity Changes

Today 20% of the strongest cyclones exceed 49 m/s on average globally. With a 1C rise in SST, 20% of the strongest cyclones could exceed 51 m/s according to Elsner et al. (2008). Thus the 80th percentile increases from 49 to 51 m/s. Today, on average, 17 cyclones/yr exceed 49 m/s and 13 exceed 51 m/s. If 51 m/s is the new 80th percentile (after a 1C warming) then, without a change in the overall number of cyclones, 13 becomes 17.