Energetics of Winter Troughs Entering South America

Abstract

The energetics and behavior of midtropospheric troughs over the Southern Hemisphere and their relationship with South America surface cyclogenesis were studied during the winters of 1999?2003. All surface cyclogenesis situations over Uruguay and adjacent areas associated with 500-hPa troughs were analyzed. The atmospheric circulation associated with type-B and type-C cyclones form the basis for two composites: composite B (with 25 cases) and composite C (with 13 cases). The results showed that the midtropospheric troughs were more intense in composite C than in composite B before the surface cyclogenesis and that the opposite occurred during the surface cyclogenesis. The baroclinic conversion was dominant in both composites. In composite B, the ageostrophic flux convergence (AFC) contributed positively to the intensification of the surface cyclone since it imported energy into the area before the cyclogenesis started. But in composite C, the AFC served as a sink because it exported energy. Based on these results, it can be concluded that (i) the trough was crucial for the cyclogenesis; (ii) the variables in the mid- and upper levels did not differ significantly from one composite to another; (iii) the northerly heat and moisture flow acted as a preconditioning for the cyclogenesis, mainly for composite C; (iv) the baroclinic conversion dominated the energetics; and (v) the AFC had only a secondary role, contributing negatively to the development of the cyclone in composite C and positively to the initial development in composite B.