Subtropical Cyclones over the Southwestern South Atlantic: Climatological Aspects and Case Study

Abstract

Hurricane Catarina (2004) and subtropical storm Anita (2010) called attention to the development of subtropical cyclones (SCs) over the South Atlantic basin. Besides strong and organized storms, a large number of weaker, shallower cyclones with both extratropical and tropical characteristics form in the region, impacting the South American coast. The main focus of this study is to simulate a climatology of subtropical cyclones and their synoptic pattern over the South Atlantic, proposing a broader definition of these systems. In addition, a case study is presented to discuss the main characteristics of one weak SC. The Interim ECMWF Re-Analysis (ERAInterim) and NCEP-NCAR reanalysis are used to construct the 33-yr (1979-2011) climatology, and a comparison between them is established. Both reanalyses show good agreement in the SCs' intensity, geographical distribution, and seasonal variability, but the interannual variability is poorly correlated. Anomaly composites for austral summer show that subtropical cyclogenesis occurs under a dipole-blocking pattern in upper levels. Upward motion is enhanced by the vertical temperature gradient between a midtropospheric cold cutoff low/trough and the intense low-level warm air advection by the South Atlantic subtropical high. Turbulent fluxes in the cyclone region are not above average during cyclogenesis, but the subtropical high flow advects great amounts of moisture from distant regions to fuel the convective activity. Although most of the SCs develop during austral summer (December-February), it is in autumn (March-May) that the most ''tropical'' environment is found (stronger surface fluxes and weaker vertical wind shear), leading to the most intense episodes.

Climatology and numerical case study of moisture sources associated with subtropical cyclogenesis over the southwestern Atlantic Ocean

“…Flexpart model was used to find source regions for the atmospheric moisture present over the southeastern coast of Brazil (30.58–21.8°S and 49.58–35.58°W, hereafter RG1), during SC events. These events were selected by the criteria and tracking methods described in Gozzo et al [], which have identified the RG1 as the region of western South Atlantic where most of the SCs have their genesis. In a period of 36 years (January 1980 to December 2015) a total set of 150 SCs was used to identify source regions of the moisture present in RG1 in subtropical cyclogenesis time (defined as 1200 UTC of the day when the cyclone was first identified by the tracking algorithm).…”

“…Subtropical cyclones are nonfrontal low‐pressure systems with a thermally hybrid core (warm at low levels and cold at upper levels) and intense convective activity that develop mostly over the oceanic areas. Their occurrence over South America, more specifically in the southwestern South Atlantic, is documented in a number of case studies [ McTaggart‐Cowan et al , ; Veiga et al , ; Pereira‐filho et al , ; Vianna et al , ; Dias Pinto et al , ], and the climatology of such systems is presented in Evans and Braun [] and Gozzo et al []. This is still an incipient research area, and a better comprehension of subtropical cyclone (SC) development is important in a theoretical point of view and also to improve operational weather forecast.…”

“…Evans and Braun [], in their 50 year climatology of SCs over the South Atlantic, found that the median SST coincident with subtropical genesis is 18°C, while the median North Atlantic SST is 26°C. Composite analysis in the 33 year climatology by Gozzo et al [] showed that during SC events the mean SST in South Atlantic does not exceed 25°C, and the surface latent and sensible heat fluxes are close to the climatological value.…”

“…Nevertheless, SCs in southwestern South Atlantic occurs where there is large moisture content in lower levels of the troposphere [ Dias Pinto et al , ; Gozzo et al , ]. If local evaporation during these events is not unusually intense, the transport of water vapor from distant regions by the atmospheric circulation should be a vital moisture source.…”

“…A numerical case study using the WRF model was carried out to assess the relative importance of local and remote moisture sources during SC development. This work is a complement of the results presented in Gozzo et al [], bringing analysis for a slightly larger period and set of SCs.…”