share a focus on annual or multi-decadal mean SST estimates averaged over large ocean areas, such as the NAO Main Development Region (MDR), also referred to as the "Atlantic cyclogenic region," in which most hurricanes develop (here assumed to denote the region within 6° to 18° N latitude by 20° to 60° W longitude). Relatively elevated MDR SST during 1995 was hypothesized to increase the frequency of hurricane formation by transferring a relatively greater amount of heat to the usual number of easterly waves of wind (typically ~60) that enter the MDR each year between May and November after leaving the west coast of Africa, each acting to stimulate shallow cumulus convection that may evolve into cyclonic behavior (Saunders & Harris, 1997).The SST hypothesis has extended this frequency-specific observation, based on NAO data gathered in a single year, to pertain generally to cyclone intensity.However, the physical basis for this hypothesized extension involving local SST effects is dubious, in view of the great distances typically traversed by these storms.Over the last century, local NAO SST has varied in a large and predictable way along individual hurricane tracks, which typically extend many thousands of kilometers (km) from their points of origin (Fig. 1).The magnitude of this variation is sufficient to allow historical hurricane-activity records to serve as "natural experiments" that can be used to perform a direct test of the SST hypothesis, by examining the historical correlation between estimated hurricane intensities, and corresponding estimated historical changes in local SST (Smith & Reynolds, 2003, 2004), along individual hurricane tracks.To perform this test, we examined paired differences (Δv max ) in estimated local hurricane intensity v max , and corresponding differences (ΔSST) in local (i.e., year-, month-, latitude-, and longitudespecific) SST, over various non-overlapping intervals Δt = t i+1t i over time t for adjacent positions (indexed by i) along each hurricane track.Over all such intervals that occurred prior to attainment of each peak hurricane intensity, Max(v max ), the causal SST hypothesis requires that any detectable correlation between Δv max and ΔSST be positive.Below we describe methods and historical data on individual hurricane tracks that we used to implement this direct test of the SST hypothesis, and results we obtained that indicate that variation in SST is not causally linked to variation in hurricane intensities.If SST variations do not explain the historically observed pattern of hurricane intensities, what other factor(s) might explain this pattern?Statistical and trend analysis for hurricane activity traditionally has focused on annual frequency, landfall likelihood, storm duration, storm intensity (maximum sustained storm-specific wind speed v max ), or Saffir-Simpsonscale wind-intensity category (Fisher,
18