Upper-Tropospheric Precursors to the Formation of Subtropical Cyclones that Undergo Tropical Transition in the North Atlantic Basin

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.