The rainfall regime over the South American Altiplano has a well defined annual cycle, with a marked maximum during the austral summer (DJF), when rainfall episodes occur in association with development of deep, moist convection over the central Andes and the adjacent Amazon basin. Consequently, most studies have focused on the mechanisms and variability of summertime rainfall. Nevertheless, episodes of precipitation during winter (May to September) account for a minor fraction of the annual total. Furthermore, some of these events may cause major disruptions in agriculture and terrestrial transportation due to snow accumulation and low temperatures. Proper understanding of the wintertime precipitation over the Altiplano and its variability is also important in the context of past climate studies.

Using rainfall observations at few stations and OLR data, we identified 35 episodes of wintertime precipitation. There is a marked interannual variability in the number of episodes, with no clear relationship with ENSO. Preliminary synoptic analyses indicate two recurrent patterns. About two thirds of the episodes are associated with mid- and upper-tropospheric cutoff lows or trough moving from the west at abnormally low latitudes. We noticed that in most cases, these synoptic-scale episodes occur when a blocking high is placed to the west of southern South America. The rest of the episodes are associated with cold fronts ahead of cold air incursions into tropical South America to the east of the Andes, with an upper-level ridge over the subtropical Andes. Cold incursions are frequently observed in this region, but only those deep enough can cause precipitation over the Altiplano during winter.