ATLANTIC SEA-SURFACE TEMPERATURES AND TROPICAL CYCLONE FORMATION

It has long been accepted that interannual fluctuations in sea surface temperature (SST) in the Atlantic are associated with fluctuations in seasonal Atlantic basin tropical cyclone frequency. To isolate the ph ysical mechanism responsible for this relationship. a singular value d ecomposition (SVD) is used to establish the dominant covarying modes o f tropospheric wind shear and SST as well as horizontal SST gradients. The dominant SVD mode of covarying vertical shear and SST gradients, which comprises equatorially confined near-zonal vertical wind shear f luctuations across the Atlantic basin, is highly correlated with both equatorial eastern Pacific SST anomalies (associated with El Nino) and West African Sahel rainfall. While this mode is strongly related to t ropical storm, hurricanes, and major hurricane frequency in the Atlant ic, it is not associated with any appreciable Atlantic SST signal. By contrast, the second SVD mode of covarying vertical shear and horizont al SST gradient variability, which is effectively uncorrelated with th e dominant mode, is associated with SST fluctuations concentrated in t he main tropical cyclone development region between 10 degrees and 20 degrees N. This mode is significantly correlated with tropical storm a nd hurricane frequency but not with major hurricane frequency. Statist ical tests confirm the robustness of the mode, and lag correlations an d physical reasoning demonstrate that the SST anomalies are not due to the developing tropical cyclones themselves. Anomalies of SST and ver tical shear during years where the mode has substantial amplitude conf irm the resemblance of the individual fields to the modal structure, a s well as the association of hurricane development with the warmer SST s. Although SSTs are of secondary importance to vertical shear in modu lating hurricane formation, explaining only similar to 10% of the inte rannual variability in hurricane frequency over the similar to 50% exp lained by vertical shear, the results support the conclusion that warm er SSTs directly enhance development. The lack of correlation with maj or hurricanes implies that the underlying SSTs are not a significant f actor in the development of these stronger systems.