The 500 and 1000 hPa weather type circulations and their relationship with some extreme climatic conditions over southern South America

• Published in: International Journal of Climatology
• Main Authors: Bischoff, Susana A., Vargas, Walter M.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2003
• Subjects: Pacific; South Atlantic Ocean
• Online Access: http://dx.doi.org/10.1002/joc.894; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1002%2Fjoc.894; https://rmets.onlinelibrary.wiley.com/doi/pdf/10.1002/joc.894

Abstract 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 ...