The Relationship Between Upper-Tropospheric Disturbances and Subtropical Cyclone Life Cycles in the North Atlantic Basin

Oceanic cyclones exhibiting properties of both tropical and extratropical systems have been categorized as subtropical cyclones (STCs) since the early 1950s. 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. This study investigates the roles of baroclinic and diabatic processes during the evolution of STCs by calculating three PV metrics from the National Centers for Environmental Prediction Climate Forecast System Reanalysis 0.5° gridded dataset. The three PV metrics quantify the relative contributions of lower-tropospheric baroclinic processes, midtropospheric diabatic heating, and upper-tropospheric dynamical processes during the evolution of individual cyclones. Quantification of these three contributions reveals the changing PV structure of an individual cyclone, indicates fluctuations in the dominant energy source of the cyclone, and aids in categorizing the cyclone. An evaluation of the relative contributions of lower-tropospheric baroclinic processes, midtropospheric diabatic heating, and upper-tropospheric dynamical processes during the evolution of individual cyclones, in addition to an evaluation of the sign of upper-tropospheric thermal vorticity, will be used to identify North Atlantic STCs and construct a 1979-2010 climatology. North Atlantic STCs included in the climatology are associated with an upper-tropospheric disturbance, positive upper-tropospheric thermal vorticity, and varying degrees of midtropospheric diabatic heating and lower-tropospheric baroclinicity. A cyclone-relative composite analysis performed on subjectively constructed clusters of North Atlantic STCs that represent the most common upper-tropospheric features linked to STC formation will be presented to document the structure, motion, and evolution of these features during STC life cycles. It is hypothesized that STCs forming in association with similar upper-tropospheric features will exhibit characteristic PV evolutions during their life cycles, which will be diagnosed in terms of the three aforementioned PV metrics. Case studies also will be presented to illustrate the characteristic PV evolutions of STCs during their life cycles for each cluster.