The properties and genesis environments of South Atlantic cyclones

A new climatology of South Atlantic cyclones is produced to provide new insights into the conditions leading to genesis in different regions of the domain. Cyclones are identified and tracked based on the relative vorticity at 850 hPa computed from the NCEP-CFSR winds. The characteristics of the cyclones are obtained by diagnostic variables sampled within a radial distance from the cyclone centers to produce the spatial distribution of cyclone properties at the time of genesis. Also, cyclone centered composites are used to analyze the cyclone structure and evolution during their genesis. There are four main cyclogenesis regions in the South Atlantic Ocean: the Southern Brazilian coast (SE-BR, 30. S), over the continent near the La Plata river discharge region (LA PLATA, 35. S), the southeastern coast of Argentina (ARG, 40.S- 55. S) and the Southeastern Atlantic (SE-SAO, centered at 45. S and 10. W). We found that cyclogenesis northward of 35. S occurs mainly due to low-level forcing associated with moisture transport in the summer, and is associated with upper-level forcing in the winter due to a strong baroclinic environment. Southward of 35.S, cyclones develop in a high baroclinic environment throughout the year with only a small influence from moist processes. The cyclone composites reveal that SE-BR and SE-SAO cyclones are associated with secondary development, the LA PLATA cyclones development is influenced by an orographic low in their early stages, and ARG cyclones are influenced by thermal advection as an essential mechanism in the reduction of static stability.