Climatology of tropical cyclogenesis in the North Atlantic (1948-2004)

The threat posed to North America by Atlantic Ocean tropical cyclones (TCs) was highlighted by a series of intense landfalling storms that occurred during the record-setting 2005 hurricane season. However, the ability to understand-and therefore the ability to predict-tropical cyclogenesis remains limited, despite recent field studies and numerical experiments that have led to the development of conceptual models describing pathways for tropical vortex initiation. This study addresses the issue of TC spinup by developing a dynamically based classification scheme built on a diagnosis of North Atlantic hurricanes between 1948 and 2004. A pair of metrics is presented that describes TC development from the perspective of external forcings in the local environment. These discriminants are indicative of quasigeostrophic forcing for ascent and lower-level baroclinicity and are computed for the 36 h leading up to TC initiation. A latent trajectory model is used to classify the evolution of the metrics for 496 storms, and a physical synthesis of the results yields six identifiable categories of tropical cyclogenesis events. The nonbaroclinic category accounts for 40% of Atlantic TCs, while events displaying perturbations from this archetype make up the remaining 60% of storms. A geographical clustering of the groups suggests that the classification scheme is identifying fundamentally different categories of tropical cyclogenesis. Moreover, significant differences between the postinitiation attributes of the classes indicate that the evolution of TCs may be sensitive to the pathway taken during development.