Assessment of a Tropical Transition over the Southwestern South Atlantic Ocean: The Case of Cyclone Akará

Abstract

Tropical cyclones are rare in the South Atlantic Ocean. Hurricane Catarina (2004), developed from a tropical transition, was the first documented case, followed by Iba (2019), which had a purely tropical genesis. In February 2024, the southeastern South Atlantic recorded its third tropical cyclone, Akará, initially a subtropical system. Due to the specific conditions required for tropical cyclones to develop in this ocean basin, the main purpose of this study is to describe the physical mechanisms that triggered the genesis of Akará’s precursor and its tropical transition. Data from various sources and methodologies, including the cyclone phase space diagram, are used in this study. Results show that the passage of a cold front created an environment with horizontal wind shear, contributing to most of the cyclonic relative vorticity in the genesis region. This was the primary driver of cyclogenesis at 1200 UTC on 15 February, along with other secondary processes. The tropical transition occurred as the vertical shear weakened, and turbulent heat fluxes from the ocean to the atmosphere increased, enhancing diabatic processes that warmed the atmosphere. This led to the tropical transition at 0600 UTC on 17 February.

Keywords

• tropical transition
• cyclone phase space
• vertical wind shear
• horizontal wind shear vorticity
• South Atlantic Ocean