Quantifying the related precipitation and moisture sources in the lifecycle of subtropical cyclones in the South Atlantic basin

Abstract

This study applies a Lagrangian approach to identify the origin of moisture for subtropical cyclones (SCs) along their tracks in the South Atlantic Ocean (SAO) basin from 1980 to 2015. The analysis shows that the local evaporation cannot fully explain the moisture gained by SCs, highlighting the role of external sources in moisture support, mainly through the northeasterly winds associated with the South Atlantic Subtropical Anticyclone. Overall, the northwestern SAO was the principal moisture source for SCs. It was noticeable that its contribution (~40%–45%) remained quasi‐constant during the year. Conversely, the moisture supplied from the northeastern SAO, southwestern SAO and the terrestrial source southeastern Brazil exhibited a marked seasonal variability. While contributions from the southwestern SAO intensify (weaken) in austral winter (summer), the southeastern Brazil source is more intense (weak) in summer (winter). In addition, SCs predominantly gain atmospheric humidity from sources close to their position, which reduces the mean water vapor residence time to ~3.2 days. Interestingly, we also found that ~42%–47% of the moisture that reached the SCs precipitates. These findings constitute a further step toward improving our knowledge of mechanisms underlying SC activity in the SAO basin.