A Dynamically Based Climatology of Subtropical Cyclones that Undergo Tropical Transition in the North Atlantic Basin

Subtropical cyclones (STCs) derive a considerable portion of their energy from baroclinic and diabatic processes. The opportunity to investigate the roles of baroclinic and diabatic processes during the evolution of STCs from a potential vorticity (PV) perspective motivates this study. The roles of baroclinic and diabatic processes during the evolution of STCs are determined by calculating three PV metrics from the 0.5° NCEP Climate Forecast System Reanalysis dataset. The three PV metrics quantify the relative contributions of lower-tropospheric baroclinic processes, midtropospheric latent heat release, and upper-tropospheric dynamical processes during the evolution of individual cyclones. An evaluation of the three PV metrics, as well as the sign of the upper-tropospheric thermal vorticity, during the evolution of individual cyclones is used to devise an objective STC identification technique and construct a 1979–2010 climatology of North Atlantic (NATL) STCs that undergo tropical transition. An investigation of the intraseasonal variability associated with the location and frequency of STCs identified in the 1979–2010 climatology shows that STCs typically form over the southern Gulf of Mexico and western NATL during April–June; over the northern Gulf of Mexico and western NATL during July–September; and over the western, central, and eastern NATL during October–December. STC formation occurs most frequently during September, when baroclinic and convectively driven forcings overlap across portions of the NATL. The frequency of STC formation is sensitive to the phase of ENSO and is maximized during periods of anomalously low SSTs in the eastern equatorial Pacific.