Patterns leading to extreme events in Argentina:: partial and generalized frosts

This paper studies the atmospheric circulation associated with frosts in the central region of Argentina with the objective of improving the prediction capacity for these events. Frosts are selected according to a spatial principle, which takes into consideration those events affecting an area above 25% of the weather stations located in the region known as Wet Pampas. Partial frosts (PF, less than 75% of the stations) and generalized frosts (GF, above 75% of the stations) constitute two different groups: one of them given by PF + GF and the other one only by the PFs. The analysis comprises the period between 1961 and 1990, where two extremes are selected: those years with a number of frosts one standard deviation above and below the mean (+sigma and -sigma respectively). In each extreme group (i.e. PF + GF +sigma, and -sigma; PF +sigma and -sigma), the average number of frosts varies according to the different periods of the year. Therefore, the following periods - selected independently - are studied: Annual of frosts (from April through October), seasonal (from May through September and from June through August), and each of the involved months individually. The mean circulation patterns analyzed from the composites of different meteorological variables explain the temperature anomalies observed in the region under study. In this way, the extreme occurrence of frosts is a result of the prevailing circulation during the different periods under consideration. Within each group (PF + GF and PF), the composites of the extremes +sigma and -sigma show opposite anomaly patterns for the different periods. Results for +sigma (-sigma) are distinguished by negative (positive) temperature and humidity anomalies in the Wet Pampas for the composites of PF + GF and PF. At low levels, the wind anomalies show the prevalence of winds from the south (north) in the southern section of the continent. At upper levels, this pattern is accompanied by anomalies of the prevailing westerly (easterly) wind for PF + GF and PF. This means a subtropical jet over South America more (less) intense than normal is associated with a higher (lower) frequency of frost occurrence in the seasonal composites and the Annual. However, there are variations in wind anomalies at 250 hPa within the same extreme +sigma (-sigma) when comparing the groups' PF with the previously studied group (formed only by the GF). Composites of group GF +sigma showed a maximum anomaly of the westerly zonal wind located at the jet entrance region into South America, over the Andes. On the other hand, the composites for PF +sigma show a component from the northwest located leeward the Andes, extending toward the southeast up to the Atlantic Ocean. The same anomaly for the extreme -sigma extends from the north of Argentina up to tropical latitudes, and it is more intense for groups PF + GF and PF than for group GF. Additionally, an analysis of the daily evolution of PF events during winters' GF +sigma shows that the PFs are caused by a cyclonic anomaly moving toward the east of the continent, causing cold air advection from the south. The same analysis for the GFs shows an anticyclonic anomaly dominating the southern area of South America, i.e. the anti cyclogenes is that causes the GFs does not occur in the PFs. Copyright (C) 2007 Royal Meteorological Society.