Climatic Impacts of “Friagens” in Forested and Deforested Areas of the Amazon Basin

Abstract

Meteorological observations from the Anglo?Brazilian Amazonian Climate Observation Study (ABRACOS), together with the global reanalysis from the National Center for Environmental Prediction (NCEP) and satellite images, have been used to study the spatial extent and intensity of cold surges (known locally as ?friagens?) in the Amazon basin. Case studies are presented of two of the strongest events of the 1994 winter season: 26 June and 10 July 1994. In both events, daily minimum temperatures in southeastern Brazil dropped to near or below 0°C, while at the same time minimum temperatures in southern Amazonia (Ji-Paraná site) were almost 8°C below average. Air temperature and humidity also fell in central and western Amazonia (Manaus and Marabá sites, respectively), although the fact that these reductions were less substantial than those farther to the south indicates that the cold air is greatly modified as it moves across Amazonia. In Ji-Paraná the largest drops in minimum temperature coincided with strong winds from the south, implying that cold advection was the main mechanism for the falling temperatures. In contrast, there was no increase in wind speed at Manaus and Marabá during the days with reduced temperatures. At these sites, cooling was due to a reduction of the maximum temperature caused, at least partially, by increased cloudiness rather than by a lowering of minimum temperatures as at Ji-Paraná. With regard to the depth of the atmospheric boundary layer (ABL), it is observed that during the passage of the cold air in southern Amazonia, the ABL was cooler and shallower than during the pre- and postfriagem days. The friagens presented are of 5- to 6-day duration, including the passage of the cold front, but the period with cold temperatures lasts between 2 and 3 days.