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Ann. Geophys., 25, 1049-1069, 2007
www.ann-geophys.net/25/1049/2007/
© European Geosciences Union 2007
Warm and Dry Spells (WDS) in Austral Winter over Central South America
P. Satyamurty, M. da Silva Teixeira, and C. Klug Padilha
Centro de Previsão de Tempo e Estudos Climáticos (CPTEC), Instituto Nacional de Pesquisas Espaciais (INPE), Av. Astronautas, No. 1758, São José dos Campos, SP, CEP: 12.245-970, Brazil
Abstract. The horizontal and vertical structure of unusually warm and dry spells (WDS)
over the central parts of South America during the winter and post-winter
months (JJAS) are studied. During WDS the mean temperature and humidity
anomalies over central Brazil are about +4.1°C and −13.2%,
respectively. The mean duration of WDS is 11 days and their mean frequency
is less than one per year during the months of JJAS. Apparently, WDS have no
preference for the phase of ENSO. Widespread and persistent subsidence in
the middle troposphere is observed in tropical Brazil during WDS, which
renders the lower tropospheric air warm and dry. The negative anomalies of
the specific humidity are observed to be associated with the subsidence
regions. A strong, slow moving ridge in the eastern South Pacific and a
low-pressure center in northern Argentina are important surface
characteristics during the WDS. A more detailed investigation of two
specific WDS events, a strong event (August–September 1999) and a moderate one (June
2002), shows a blocking-like situation in the 500-hPa geopotential and
surface pressure fields in the Pacific. The South Atlantic subtropical high
somewhat approaches the continent. Strong northerlies over the central and
eastern parts of Brazil are also observed in the lower troposphere. During
WDS the regional circulation acquires summertime characteristics, except
for the absence of precipitation, and the circulation in the meridional
plane is in the opposite sense from the Hadley circulation. A frontal
system, supported by a 500-hPa trough, advances into central Brazil, causing
the dissipation of the anomalous situation.
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