The properties and genesis environments of South Atlantic cyclones

Abstract

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

A potential vorticity perspective on cyclones over South America

“…Argentina, Uruguay and SEBrazil) agree qualitatively with earlier studies using different methods and datasets to identify cyclones (Taljard, 1967;Gan and Rao, 1991;Hoskins and Hodges, 2005;Reboita et al, , 2018Neu et al, 2013;Gramcianinov et al, 2019). is very active during all seasons with a slightly higher frequency in austral summer (Figures 4.2a,4.3), which is consistent with previous studies (Hoskins and Hodges, 2005;Reboita et al, , 2018Gramcianinov et al, 2019).…”

“…As discussed in section 5.1 and shown in Figure 5.18c, low pressure anomalies are stronger in winter, which was already identified in other studies over SA Gramcianinov et al, 2019).…”

“…From Figure 4.1b the RV and SLP tracks present four main concurrent hot spots for cyclogenesis, where three of them are already known and studied (the coastal regions) and the fourth is over the continent, at lower latitudes and in the lee side of the Andes (20°S). This region, for instance, is not seen in Reboita et al ( , 2018, Mendes et al (2007), or in Gramcianinov et al (2019), even though the latter considered the continent for the tracking.…”

“…Nevertheless, the cyclonic activity near the eastern coast of SA and southwestern South Atlantic significantly influences weather and climate conditions over the South American continent. Previous climatologies have identified three most cyclogenetic regions in eastern SA: south/southeast of Brazil, east of Uruguay, and southern Argentina (Sinclair, 1995;Hoskins and Hodges, 2005;Reboita et al, , 2018Krüger et al, 2012;Gramcianinov et al, 2019). The main dynamical mechanism for the formation of these cyclones is mainly baroclinic instability of the westerly flow.…”

Climate-induced variability in South Atlantic wave direction over the past three millennia

“…2 a). The high-energy SSE swells from this region are commonly generated by long and intense wind fetches resulting from extratropical cyclones 21 – 23 migrating around the northern edge of the Southern Ocean, and sometimes moving northwards along the southern end of South America, deviating eastward into the South Atlantic around 35–40°S 27 , 28 . This component of the mean effective energy flux is observed throughout the year, but it dominates from the austral autumn to winter 21 , 22 (Fig.…”