The role of the secondary circulation in the tropical transition of Hurricane Ophelia

Abstract. This study investigates the mesoscale dynamic and thermodynamic mechanisms governing the tropical transition (TT) of Hurricane Ophelia (2017). A fundamental aspect of this transition is the co-evolution of the primary and secondary circulations; specifically, the development of the secondary overturning circulation is what drives the structural evolution of the vortex. As the first high-resolution analysis of secondary circulation in a real TT, it broadens the scope of existing diagnostic frameworks, proving that methods originally developed for idealized tropical cyclones are also effective for quantifying the dynamics of transitioning systems. Using high-resolution numerical simulations, advanced energy-budget diagnostics and wind-tendency equations have been computed to assess the evolution of the secondary circulation. Results show that following an initial phase driven by an upper-level potential vorticity intrusion and baroclinic forcing, organized deep convection facilitates vorticity redistribution and core warming. During the transition phase, momentum and thermal forcings contribute nearly equally to the intensification of the secondary circulation. However, once the transition is complete, thermal forcing becomes the dominant mechanism. The equivalent potential temperature budget analysis reveals a fundamental shift in system energetics: while vertical diffusion, associated with surface fluxes and air-sea instability, dominates energy input during the transition, organized vertical advection within the eyewall sustains the system in its mature stage. The study also identifies a period of structural relaxation midway through the process, highlighting the non-linear nature of the tropical transition before achieving self-sustaining convective coupling. By clarifying currently debated TT behavior, this work establishes key signatures that facilitate the non-trivial characterization of these systems.