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 19792010 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.