The 500 and 1000 hPa weather type circulations and their relationship with some extreme climatic conditions over southern South America
Authors: Bischoff Susana A. , Vargas Walter M. 
Published: April 2003

Different circulation types for southern South America are derived from the circulation anomaly maps at 500 hPa corresponding to a 9 year period. The series of daily maps was obtained from the ECWMF reanalysis. These data are particularly useful, since real data, especially the radiosonde network, are sparse in this region. The properties to be studied are selected in such a way that they describe different flow conditions over the region, in order to obtain a statistical diagnosis useful for modelling an objective forecast.

The circulation types were obtained by a correlation–map‐based pattern classification technique. Lund's method is used in this paper to identify the most frequent circulation patterns.

The classification method was applied to daily Z anomalies at 500 hPa over the whole record (1980–88). This allows one to analyse the evolution and presence of a particular type over different months and years. The most outstanding circulation‐type structures, represented by only eight types, explain about 63% of the total number of cases in the sample. The most frequent type, Type 1 (Z), shows an almost zonal circulation with a strong meridional gradient, associated with a trough in the west of the region. Type 2 (LCE) is represented by a low‐pressure system in the centre of the region (approximately at 37° S, 65° W). In general, this type is related to the occurrence of blocking situations in the South Atlantic Ocean and the passage of cold fronts over the region. Types 3 (SW) and 4 (WNW) show a SW and WNW atmospheric circulation over the whole region with a meridional gradient lower than in Type 1. Type 5 (TNS) shows an NW–SE trough axis over the continent. Type 6 (WW) has an intense SW flow over the southern part of the continent. Type 7 (NW) shows a very deep trough to the west of the continent, located over the Pacific Ocean at 80° W and 34° S. The continent is affected by a NW atmospheric circulation. Type 8 (R) represents a ridge over the continent with an intense NW flow in the south. The frequency of Types 1 to 8 is about 63%. None of the circulation types mentioned accounts for more than 12% of the sample. There is a considerable variability in monthly circulation type distributions; the first six types only account for approximately 50% of the variability. This suggests that the use of probability models related to circulation type occurrence in the middle troposphere should be analysed on a monthly basis.

The results attempt to summarize the region's synoptic regime through a small number of circulation types at 500 hPa and the corresponding 1000 hPa maps. The frequency distribution of each type during warm, neutral and cold phases in the equatorial Pacific Ocean was analysed. These results partially explain the best dynamic conditions (cyclonic vorticity), for the Argentine humid and semi‐humid region, southern Brazil, Paraguay and Uruguay, that lead to the harvest precipitation events in spring (October–December 1982) with SOI < 0. In winter these circulation types are more important during cold phases with advection of cold and dry air (July–September 1982).

The circulation types at 500 hPa and the corresponding maps at 1000 hPa associated with mean maximum and minimum monthly temperature and precipitation in the region are studied. The monthly maximum and minimum precipitation recorded in stations near the Uruguay River basin in southeastern South America and the mean monthly maximum and minimum temperatures from three stations in Argentina during 1980–88 are used in the analysis. Some of the daily patterns are strongly associated with the occurrence of maximum and minimum monthly temperature and precipitation in different places of the country. In reference to the maximum and minimum precipitation records, the behaviour of the variables under study is observed to be physically consistent with the associated circulation patterns. Copyright © 2003 Royal Meteorological Society