Resumo:
While in the South Atlantic Ocean (OAS) there are records of several occurrences of subtropical cyclones, in the Southeast Pacific Ocean (OPS) there is only news in the media since 2015. In order to promote the understanding of the physical processes associated with subtropical cyclones, in this study, expenses are two cyclones: one with pure subtropical genesis, which was Cyclone Guará which occurred in December 2017 at OAS, and another with subtropical transition, which was Cyclone Lexi, which occurred in May 2018 at OPS. In addition, the study aims to define the climatological characteristics in the region of the oceans adjacent to South America in order to indicate the regions favorable to these events. To this end, data from the 1989 to 2019 period of the ERA5 reanalysis are used, which is considered state-of-the-art in terms of reanalysis. One of the main results obtained in the climatological analysis on the Genesis Potential Index (IPG). Although this index indicates regions favorable to tropical cyclogenesis, it also serves as an indicator of favorable conditions for subtropical cyclones, since there are many similarities between these two types of systems. There is potential for the genesis of tropical/subtropical systems in OAS along the Brazilian coast from October to March, however, the signal is most intense between February and March. In the OPS, some factors, in isolation, showed a propensity to cyclogenesis of cyclones with a warm core. However, the IPG's climatology shows little or no favoring the formation of tropical cyclones and CS. In relation to the two cyclones studied, a subtropical transition from Lexi was influenced by the interaction with a cutoff low, which favored an environment with weak vertical wind shear, as it was coupled with the extratropical cyclone on the surface. The subtropical cyclone Guará was formed associated with both dynamic processes, such as horizontal wind shear with cyclonic signal and weak vertical wind shear, and thermodynamic processes, with an impact of turbulent heat fluxes at the sea-air interface. The use of heat balance and vorticity equations contributed to confirm the results.
