CYCLOP Revegoca: The First Documented Cyclone Formed From Locally Originating Potential Intensity Over the Southwestern South Atlantic

Abstract On 9 December 2025, an atypical synoptic cyclone developed near the borders of Paraguay, Argentina, and southern Brazil, triggering severe weather across south‐southeastern Brazil. Unlike typical South Atlantic extratropical cyclones, which intensify near the coast, this system originated inland and underwent explosive cyclogenesis without a classical frontal structure. This study investigates its unique characteristics through synoptic and large‐scale analyses, a potential vorticity (PV) perspective, potential intensity (PI), Cyclone Phase Space (CPS), and Lorenz's Energy Cycle (LEC). A Rossby wave break over the eastern Pacific generated a cutoff low that crossed the Andes and induced low‐level perturbations, initiating cyclogenesis. The cyclone was associated with a strong upper‐level negative PV anomaly linked to a downward stratospheric intrusion, and its development was consistent with cyclogenesis driven by locally originating PI. Diabatic heating subsequently produced a compact, barotropic PV structure resembling the “PV tower” typical of tropical cyclones. Accordingly, CPS classified the system as tropical between 10 and 12 December. LEC analysis showed that peak kinetic energy resulted from the combined effects of available potential energy conversion, associated with warm air ascent and latent heat release, and barotropic conversion from mean flow shear. Given the persistence of upper‐level forcing during the acquisition of tropical characteristics and the presence of locally generated PI, the system is classified as a CYCLOP, representing a distinct tropical cyclogenesis pathway. Named Revegoca, this cyclone highlights the need to investigate non‐classical South America/South Atlantic systems, where land‐based, complex cyclogenesis remains poorly documented.