Extended Abstract (220K)6.5
Climatology of the South American cold fronts
Iracema F. A. Cavalcanti, CPTEC/INPE, Cachoeira Paulista, SP, Brazil; and V. E. Kousky
Frontal systems are seen to affect the weather over South America during all seasons of the year. They are easilly visualized in satellite images as they advance from southwest to northeast over the continent. In this study, NCEP/NCAR reanalysis data are used to compute a climatology of cold fronts over South America during the period of 1979 to 2000 and to determine the associated atmospheric circulation features. The analysis is performed seasonally and monthly, using combined criteria for temperature, pressure and wind. Possible frost cases are identified and the main atmospheric characteristics and evolution are discussed. The results show two main tracks for frontal systems over the continent, one in the interior, extending from northern Argentina northward to the western Amazon region and the other one along the east coast of Brazil. There is a gradual increase in the number of systems that reach lower latitudes, from DJF to JJA. Over southern Brazil the number increases in September and October and the minimum number is found in January and February. The main features associated with potential frost cases are an amplified wave pattern with a ridge over the eastern Pacific and a trough over the Atlantic and southern Brazil. At the surface, a strong high pressure system crosses the Andes favoring northward flow of cold air over subtropical South America, which sometimes reaches as far north as the equator. Composites of cases over South America show that the ridge/trough pattern is part of a wavetrain which extends from the western South Pacific eastward to South America that can be identified as much as 4 days prior to the cold air outbreak over South America. Time-longitude sections of 500 hPa geopotencial anomalies indicate Rossby wave propagation and energy dispersion, that at times link cases of cold fronts over South America to later occurrences of cold air incursions over southern Africa.
Session 6, Weather and Forecasting I
Tuesday, 25 March 2003, 1:30 PM-5:00 PM
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