Patterns of atmospheric circulation associated with cold outbreaks in southern Amazonia

This study is based on climatological records (1979-2008) of cold outbreak (friagem) events that occurred in the southern Amazon region during the austral winter months (May to September). The friagem events were identified with an objective method that used climatological data from the surface station of the Department of Airspace Control (DCEA) in Vilhena city, including the minimum temperature (T-min), the pressure reduced to mean sea level (P-msl) and the decrease in T-min (DTmin). The purpose of this study was to investigate the association between atmospheric circulation and friagem events using Climate Forecast System Reanalysis (CFSR) and a composite method. One hundred and forty-four friagem events were identified with the proposed methodology. These events produced large changes in weather conditions in Amazonia, including an abrupt decrease in air temperature, an increase in the amount of dry air, and an increase in surface pressure. The main atmospheric features associated with friagem events are an intensification of high post-frontal surface pressure that is caused by the intensification of a ridge at middle and high levels when air approaches the Andes and strong southerly winds at the surface that carry cold, dry air to tropical latitudes.