The occurrence of cut-off lows (COLs) over South Pacific can be seen on different synoptic maps and on satellite imageries. However, the formation and maintenance mechanisms of the COLs are not well understood. An energetics analysis of a COL case through the eddy kinetic energy (EKE) equation is the aim of this study. The main terms analysed were ageostrophic flux convergence (AFC), baroclinic (BRC) and barotropic (BRT) conversions. This analysis shows that the formation mechanism of the COL was associated with BRC (COL western side) and AFC (COL eastern side). After the maximum intensity period, the BRC term was negative and the AFC positive.

Brazil and consequently causes the temperature decrease. The energetics shows that the cold events kinetic energy maxima are more intense than those of cool events. For the cold events three maxima are observed, the first (K1) and the third (K3) maxima are developed by baroclinic conversion and ageostrophic flux convergence and the second one (K2) by ageostrophic flux convergence. For the cool events two maxima are found, the first maximum (K4) developed by baroclinic conversion and the second one by ageostrophic flux convergence.

The energetics and behavior of midtropospheric troughs over the Southern Hemisphere and their re lationship 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 com posites. 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 signifi cantly 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.

Even though mesoscale convective systems (MCS) are of great importance in precipitation regimes besides being related to severe weather events, they are still not easy to predict. This study builds an objective index for South American MCS based on synoptic features present before the initiation of the MCS between 2005 and 2010. The National Centers for Environmental Prediction (NCEP) Climate Forecast System Reanalysis (CFSR) product is used to access environmental conditions near the MCS-initiation centroid and in unorganized convective environments to obtain a South American MCS index (SA-MCS index). Upper and middle atmospheric levels presented the main patterns used to identify the MCS environments and unorganized convection, with the jet streak downstream of a short trough at medium levels frequently present in the convectively organized environment. The SA-MCS index is shown to depend on a vertical wind shear between 0 and 6 km high, temperature advection at 775 hPa, lifted index (LI) and vertical velocity omega at 800 hPa. When compared with the MCS index developed through the United States MCS climatology, the SA-MCS index was able to predict more than twice as many MCS at a distance of less than 1 ∘ from the point of maximum intensity of the index.

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