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Abstract

Tropical cyclones (TCs) are extremely rare over the South Atlantic Ocean (SATL) due to predominant unfavourable conditions, i.e., cool sea surface temperature and strong vertical wind shear. Nevertheless, unusual conditions over SATL associated with a blocking system promoted the formation of Hurricane Catarina from an extratropical precursor in late March 2004, which produced heavy rainfall over the states of Santa Catarina and the Rio Grande do Sul in south Brazil. This work identifies the moisture sources for the precipitation produced by Catarina along its 6-hourly trajectory through a Lagrangian moisture tracking method. We extracted the pathways of precipitating air parcels within the cyclone’s outer radius from the global outputs of the FLEXPART model forced with the ERA-Interim reanalysis. Our findings revealed the terrestrial source in south-southeastern Brazil (SEB) and the oceanic source limited by the box between 20-40°S and 30-50°W over SATL (WSATL) as the principal moisture sources, with the overall support of ~27% and ~66%, respectively. However, their contribution varied according to the development phase of Catarina. While the moisture uptake from SEB decreased from ~75% during the extratropical phase to ~8% during the hybrid stage, the moisture contribution from WSATL notable increased from ~20% to 87%, respectively. Likewise, the contributions from SEB and WSATL during the tropical phase of the cyclone accounted for ~13% and ~85%, respectively. The tracked air parcels achieved the high water content in a short period before the precipitation, leading to a reduction of the mean water vapour residence time to ~3.1 days. Additionally, the precipitating moisture uptake along the Catarina trajectory was noticeably higher than the climatological value.