Mean atmospheric circulation leading to generalized frosts in central southern South America

The purpose of this study is to analyze the atmospheric circulation patterns associated to extreme frost episodes which affect the extensive region known as the Wet Pampas in the center-south of South America during the 1961-1990 period. The years with an extreme frequency of generalized frost are identified by selecting the cases beyond one standard deviation above and below the average of the corresponding period. Two groups were formed: one for the years with events above the average (+s) and the other with those below the average (-s). The years of generalized frosts were separated into the periods from May to September, June to August and individual months. From the comparison between the composite of the two groups (+s and -s), it was possible to determine that the anomaly fields are opposite in almost all the periods studied. In the seasonal composites of generalized frosts below average, the circulation field presented a barotropic structure with an anomalous cyclonic persistence over the south-west of South America and a weak subtropical jet over the continent. On the other hand, the structure and evolution of the systems that produce generalized frosts in extremely cold winters were analyzed by calculating the composites of the daily fields from these episodes. The results showed that the incursion of cold air in the lower levels begins with a migratory anticyclone moving from the south-east Pacific to the south of the continent and a deepening of an anomalous low pressure center over the south-east of the Argentine Atlantic coast. Both systems cause cold, dry air advection from the south, with a reduction in temperature over the center and eastern parts of the country. The development of a mid-latitude wave was observed at higher levels with a large-amplitude trough over South America extending to tropical latitudes and a progressive amplification from another trough located upstream in the Pacific Ocean. The analysis of the different periods confirmed the relationship between the intensification of the subtropical jet in South America and the higher frequency of generalized frosts. The intensification of the jet may be related to an amplification of the pressure gradient in the region due to the increase in Rossby wave activity.