No abstract

The opportunity to examine the structure and evolution of the various upper-tropospheric precursors to the formation of North Atlantic (NATL) subtropical cyclones (STCs) that undergo tropical transition (TT) motivates this study. Intraseasonal variability associated with the location and frequency of NATL STCs forming in the presence of similar upper-tropospheric features, as well as similarities and differences in the various upper-tropospheric precursors to the formation of NATL STCs that undergo TT, are examined. NATL STCs that undergo TT are categorized according to the upper-tropospheric features associated with their formation during 1979–2010 using the 0.5° NCEP Climate Forecast System Reanalysis dataset. This categorization allows for the documentation of the location and frequency of STCs forming in the presence of similar upper-tropospheric features and for the construction of cyclone-relative composites during the five days prior to STC formation. NATL STCs that undergo TT are separated into one of three categories based on the upper-tropospheric features associated with their formation: 1) cutoff lows, 2) meridional troughs, and 3) zonal troughs. STCs included in the cutoff low and meridional trough categories typically develop poleward of ~25°N over the western, central, and eastern NATL during September–November and August–November, respectively. In contrast, STCs included in the zonal trough category typically develop equatorward of ~30°N over the western NATL during June–September. Cyclone-relative composites reveal that ~61% of the categorized NATL STCs that undergo TT form in association with an upper-tropospheric feature whose structure and evolution are linked to anticyclonic wave breaking.

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.