Revisiting The Rare Transition of a South Atlantic Cyclone to Tropical Storm Akará: Energy Cycle and Stratosphere-Troposphere Interaction

Cyclone Akará was the third documented tropical cyclone in the South Atlantic, undergoing a rare subtropical-to-tropical transition in February 2024. This study investigates the dynamical, thermodynamical, and energetic evolution of Akará using a diagnostic framework combining the Lorenz Energy Cycle (LEC) and heat and vorticity budgets. Akará originated in a post-frontal, weakly baroclinic environment characterized by warm sea surface temperatures (>28$\circ C$), strong ocean-atmosphere thermal contrast, and low vertical wind shear. These conditions favored convective activity and led to the development of a symmetric warm core, initially supported by latent heat release and interaction with an upper-level cutoff low. As the system intensified, it transitioned into a tropical cyclone, exhibiting a deep warm core and organized convection. Stratospheric air intrusions were identified during and after the tropical transition, contributing to upper-tropospheric warming and enhancing cyclone intensification. Heat and vorticity budget analyses revealed how vertical motion, latent heat release, and vortex stretching shaped the storm’s three-dimensional structure and contributed to its deepening. During the mature stage, Akará reached peak intensity while moving over cooler waters. Although convective activity decreased, the warm core was sustained—likely through continued stratosphere-troposphere interactions and strong barotropic energy conversions at low and mid levels. The combined use of LEC and budget diagnostics proved effective in disentangling the physical mechanisms behind Akará's evolution, offering valuable insight into the dynamics of rare South Atlantic tropical transitions. Expanding such analyses to other events is essential to improve our understanding and forecasting of tropical cyclogenesis in this basin.