Future climate trends of subtropical cyclones in the South Atlantic basin in an ensemble of global and regional projections

Abstract

The South Atlantic Ocean (SAO) is characterized by the development of different types of synoptic scale cyclones, which affect the weather and climate of South America. For the first time, we obtained the long term trend of subtropical cyclones (SCs) climatology over the SAO through two ensembles under RCP8.5 scenario. Regional Climate Model version 4 (RegCM4) projections were driven by three global climate models (GCMs) from CMIP5. SCs are obtained by applying three algorithms: (1) for tracking all cyclones based on relative vorticity; (2) to describe the thermal structure of the cyclones; and (3) for selecting only the SCs. Ensemble means are able to capture the main SCs characteristics shown by ERA-Interim reanalysis in the present climate (1979–2005), such as the main region of formation (near the southeastern Brazilian coast), track density, seasonality (higher frequency in austral summer) and lifetime (~ 3 days). The RegCM4 and GCMs ensembles project a negative and statistically significant trend in the frequency of SCs in the future climate (2050–2080) near the southeastern coast of Brazil. The projections also indicate a greater negative trend of SCs than for all cyclones. This would be a response to the future increase in the mean sea level pressure (expansion of South Atlantic subtropical anticyclone), which in turn leads to a change in the low-level circulation acting to decrease the moisture transport to the main region of SCs development. Though the SCs frequency will decrease in the future, they are projected to be more intense due to stronger convective forcing.