“Friagem” Events in the Brazilian Amazon, Changes and Variability in Its Climatological Patterns

Friagem events in the southwestern Brazilian Amazon including their changes, variability and climatological features during the 1979–2020 period were examined. The incursion of polar origin cold air mass into the region during austral autumn and winter, which leads to the abrupt drop of the air temperature, characterizes the friagem event. Sixty-five friagem events were identified during the analysis period. These events are more frequent in July and August, with a decreasing trend in both frequency and duration over the years. The average intensity was 14.8 °C, representing a 5°C drop in relation to the average minimum air temperature in the study domain. Additionally, the most intense events occurred in La Niña years and during the positive phase of the Antarctic Oscillation. On the global scale, friagem event is associated with the zonal wavenumber 3-4 pattern in the 500 hPa geopotential anomaly field, with a northwest-southeast oriented anomalous anticyclone extending from the southeast Pacific, across southern South America to southwestern Atlantic, which is flanked to the northeast by an anomalous cyclone over southern Brazil and the adjacent Atlantic. Friagem is also associated with negative sea surface temperature (SST) anomalies in the central and eastern tropical Pacific resembling the La Niña pattern, and negative SST anomalies in the South Atlantic off southeast and south Brazil. This study contributes to improving our knowledge of the friagem events in the Amazon, highlighting the importance of monitoring in the context of climate change and interannual climate variability associated with El Niño and La Niña.