It is now recognized that between 1/4 and 1/2 of all tropical cyclones in the Atlantic basin are initiated by extratropical disturbances that traverse sufficiently warm water. Many of these disturbances have a baroclinic origin and are considered cold-core systems. A fundamental dynamic and thermodynamic transformation of such disturbances is required to create a warm-core tropical cyclone. We refer to this process as tropical transition (TT), to be contrasted with extratropical transition (ET), the latter resulting in an extratropical disturbance given a tropical cyclone.

In this paper, we review what is known about tropical transition and highlight the process in two contrasting Atlantic cases: Michael (2000) and Humberto (2001). Michael featured a strong frontal cyclone which produced a mesoscale core capable of initiating WISHE. Humberto was initiated through convection organized by two mobile upper-tropospheric vortices. The convection organization appears to have been aided by finite vertical wind shear. Simulations using the Weather Research and Forecast model reveal this organization and the subsequent development of an intense mesoscale vortex in the lower troposphere.