Environmentally risky unseasonal warm and dry spells in the subtropics of Brazil

Forest and bush fires increase during the prolonged warm and dry spells in the winter semester (April through September) in the subtropics of Brazil. As a consequence smoke concentration increases causing discomfort and respiratory problems to the population. The mean characteristics of these episodes and the synoptic situations responsible for their occurrence are obtained. The episodes are defined in terms of the mean temperature anomaly with respect to the climatology over the target area enclosed between 15°S - 25°S and 45°W - 60°W. If the anomaly is greater than 2.5°C for more than six consecutive days the episode is designated 'little summer' (LS). A total of 37 LS were identified in the 18-year period 1981 through 1999. The duration of the LS varied from 7 days to 22 days. The mean positive temperature anomaly was 3.0 C and the mean negative anomaly of the relative humidity was 14%, over the target area for the duration of the episodes. There was little or no rainfall over the target area during the LS. The LS are not apparently related to the ENSO phase. The LS, especially the severest ones, are associated with the blocking situations in the eastern South Pacific. A displacement of the South Atlantic subtropical high pressure center to the west closer to the South American continent and the formation of an anomalous upper anticyclone over the subtropics of South America (similar to the Bolivian High) are observed in the formative and maintenance stages of the LS. The warming associated with adiabatic subsidence contributed for the positive temperature tendency. A frontal band incursion into the area brought moisture and cooled the temperatures during the dissipative stage of the LS.