Reply to comment by K. Emanuel on “Sea-surface temperatures and tropical cyclones in the Atlantic basin”

Abstract

[1] Most recent studies of tropical cyclones and their relationship to changing environmental conditions, including sea surface temperatures (SST), have examined regionalscale seasonal averages in comparison with basin-wide seasonal tropical cyclone characteristics [e.g., Emanuel, 2005; Webster et al., 2005; Hoyos et al., 2006]. In our recent paper [Michaels et al., 2006], we took a different approach. We examined the local SST (weekly averages contained in 1 1 degree gridcells) associated with each named tropical cyclone as it traversed the North Atlantic Ocean during the period 1982–2005. Aggregating the SST associated with the maximum storm intensity (as measured by wind speed) attained in each storm, we found that while overall (across the full range of SST) there exists a positive relationship between maximum storm intensity and SST, this relationship breaks down above 28.25C. We identified this temperature (28.25C) as both a threshold necessary to be reached for a tropical cyclone to reach major hurricane status (category 3 or higher on the Saffir-Simpson hurricane scale, sustained surface winds of at least 50 m/s), as well as the SST beyond which there is no longer a statistically significant relationship between maximum storm intensity and local SST in the collection of Atlantic tropical cyclones that make up our data set. We proposed that in a warming climate, higher SST would lead to a modest increase in mean maximum tropical cyclone wind speeds as an increasing number of storms encountered SST necessary to develop into major hurricanes. We noted that while we expected an increase in the number of major hurricanes, we did not expect to see an increase in the intensity of these storms. [2] In his comment to our paper, Emanuel [2007] has concerns as to whether spatial patterns of storm tracks may have led us to overestimate the SST/intensity relationship at low SST, and that the available sample size may be insufficient to allow us to detect a significant relationship between SST and storm intensity at higher SST. New analysis presented here, along with the discussion and analysis provided by Emanuel, have led to a better understanding of these issues.