Actas de congresos
Moisture sources for subtropical cyclogenesis over Southwestern South Atlantic
Fecha
2014Registro en:
EGU General Assembly, 2014. Vienna.
1029-7006
Autor
Gozzo, Luiz Felippe
Gimeno, Luis
Rocha, Rosmeri Porfirio da
Institución
Resumen
Subtropical cyclones are non-frontal low pressure systems that present a hybrid thermal structure, with a low
tropospheric warm core and an upper level cold core. These systems ideally develop in a tropical-like environment,
with weak vertical wind shears and high sea surface temperature; however, subtropical cyclogenesis is possible
even when these conditions are not met. Over the soubtropical South Atlantic Ocean, near the eastern coast of
South America, these cyclones generally occur over relatively cool waters (around 23oC). As the local source of
heat and moisture associated to surface turbulent fluxes is reduced, remote source regions may play an important
role in the development of such systems. To investigate this hypothesis, a composite analysis of moisture source
and sink regions was carried out for 112 cyclogeneses days, in the period of 1979-2008, using the Lagragian
FLEXPART model and data from ERA-Interim reanalysis. The cyclones were tracked over the RG1 cyclogenetic
area (30.5oS-21oS, 49.5oW-35.5oW) using an algorithm based on relative vorticity of horizontal wind field at
925 hPa, and the subtropical ones were selected by applying the Cyclone Phase Space (CPS) methodology. The
Lagrangean analysis shows that most of the moisture available for these subtropical cyclogeneses in summer (DJF),
autumn (MAM) and spring (SON) originates in the South Atlantic around the latitude 15oS and is advected to the
RG1 by the South Atlantic Subtropical High flow. During autumn, an important contribution is seen from the
region to the south of RG1, due to enhanced transient anticyclones activity. The moisture transported by the South
Atlantic Low Level Jet does not contribute significantly for the subtropical cyclogeneses, as the air parcels lose
most of its moisture content by precipitation over the continent, before reaching RG1. As expected, the local source
of moisture presents a negative anomaly in all season’s composites. A case study of the Hurricane Catarina (2004),
the only documented tropical transition in this basin, indicates the Atlantic Ocean region over the northern branch
of the Subtropical High again as an important remote source region, but as the system transitions from extratropical
to subtropical, the local contribution increases. This enhanced input energy may have helped the system to undergo
tropical transition, unlike other subtropical cyclones.