Upper-Level Precursors Associated with Subtropical Cyclone Formation in the North Atlantic Basin

Oceanic cyclones exhibiting properties of both tropical and extratropical systems have been categorized as “subtropical” since the early 1950's. The development of these so-called subtropical cyclones (STCs), sometimes called hybrid cyclones in the current literature, requires the existence of a baroclinically unstable environment, quasigeostrophic forcing for ascent, and the production of lower-to-midtropospheric potential vorticity (PV) from diabatic heating. Previous studies have established that STCs are associated with weak low-level baroclinicity and significant lower-to-midtropospheric PV during their formation. The hybrid nature of STCs makes them likely candidates to become tropical cyclones (TCs) via the tropical transition process.

The opportunity to investigate the relationship between STCs, TC activity, and high-impact weather events motivates this presentation. We will show the results of a North Atlantic STC climatology constructed for 1979–2010 using the NCEP Climate Forecast System Reanalysis 0.5° gridded dataset. The STCs included in the climatology are defined as a subset of North Atlantic cyclones in which: 1) the ratio of PV associated with low-level baroclinicity to PV associated with midlevel latent heat release is small, and 2) tropospheric-deep wind shear values exceed 12.5 m s^-1 during the initial development of each cyclone. Intraseasonal and interannual variability in STC frequency, track, and intensity will be documented for this climatology.

A cyclone-relative composite analysis of the upper-level features linked to the formation of the North Atlantic STCs identified in the climatology will be presented to document the structure, motion, and evolution of these features. Synoptic-scale anticyclonic wave breaking (AWB) events in the midlatitudes are known to produce intrusions of relatively cold upper-tropospheric air into the subtropics that are associated with PV streamers. These upper-level features are hypothesized to be important precursors to STC formation, which motivates their emphasis in the composite analysis. A synoptic overview of STC Sean (2011), which formed beneath the fractured equatorward end of an elongated PV streamer on the equatorward side of an antecedent AWB event, will be presented as an illustrative case of STC formation. In this case, the intrusion of relatively cold upper-tropospheric air associated with the fractured PV streamer helped to destabilize the subtropical troposphere and facilitated the development of the deep convection that served as a catalyst for STC formation.