Synoptic- and Frontal-Scale Influences on Tropical Transition Events in the Atlantic Basin. Part I: A Six-Case Survey

Abstract

Abstract The process by which a baroclinic, vertically sheared, extratropical cyclone is transformed into a warm-core, vertically stacked tropical cyclone is known as tropical transition. Six recent tropical transitions of strong extratropical precursors in the subtropical North Atlantic are compared to better understand the manner by which some of the canonical structures and dynamical processes of extratropical cyclones serve to precondition the cyclone for transition. All six transitions resulted from the interaction between a surface baroclinic zone and an upper-level trough. During the extratropical cyclogenesis of each storm, a period of intense near-surface frontogenesis along a bent-back warm front occurred to the northwest of each sea level pressure minimum. Within the resultant circulation, diabatic redistribution of potential vorticity (PV) promoted the growth of a low-level PV maximum near the western end of the warm front. Concurrently, the upper-level PV anomaly associated with each trough was deformed into the treble clef structure characteristic of extratropical occlusion. Thus, by the end of the transitioning process and just prior to its becoming fully tropical, each cyclone was directly beneath a weakened upper-level trough in a column with weak vertical shear and weak thermal contrasts. The presence of convection to the west and southwest of the surface cyclone at the time of frontogenesis and upper-level PV deformation suggests that diabatic heating contributes significantly to the process of tropical transition in a manner that is consistent with its role in extratropical occlusion. Thus, it is suggested that tropical transition is encouraged whenever extratropical occlusion occurs over a sufficiently warm ocean surface.