A weeklong workshop in Brazil in August 2004 provided the opportunity for 28 scientists from southern South America to examine daily rainfall observations to determine changes in both total and extreme rainfall. Twelve annual indices of daily rainfall were calculated over the period 1960 to 2000, examining changes to both the entire distribution as well as the extremes. Maps of trends in the 12 rainfall indices showed large regions of coherent change, with many stations showing statistically significant changes in some of the indices. The pattern of trends for the extremes was generally the same as that for total annual rainfall, with a change to wetter conditions in Ecuador and northern Peru and the region of southern Brazil, Paraguay, Uruguay, and northern and central Argentina. A decrease was observed in southern Peru and southern Chile, with the latter showing significant decreases in many indices. A canonical correlation analysis between each of the indices and sea surface temperatures (SSTs) revealed two large-scale patterns that have contributed to the observed trends in the rainfall indices. A coupled pattern with ENSO-like SST loadings and rainfall loadings showing similarities with the pattern of the observed trend reveals that the change to a generally more negative Southern Oscillation index (SOI) has had an important effect on regional rainfall trends. A significant decrease in many of the rainfall indices at several stations in southern Chile and Argentina can be explained by a canonical pattern reflecting a weakening of the continental trough leading to a southward shift in storm tracks. This latter signal is a change that has been seen at similar latitudes in other parts of the Southern Hemisphere. A similar analysis was carried out for eastern Brazil using gridded indices calculated from 354 stations from the Global Historical Climatology Network (GHCN) database. The observed trend toward wetter conditions in the southwest and drier conditions in the northeast could again be explained by changes in ENSO.

Para citar este documentoRabelo da Rocha Repinaldo, C.., Müller, G. V., Martins Andrade, K.. (2017). Patrones atmosfericos simulados en el clima presente y futuro asociados al descenso de temperatura en el sudeste de Sudamerica. Boletín geográfico, 39, 13-34. ResumenLas características atmosféricas asociadas a eventos extremos fríos, identificados a partir del descenso de la temperatura en el invierno en tres regiones en el sudeste de Sudamérica, son analizadas con datos de reanálisis NCEP/NCAR y simulaciones de los modelos HadCM3 y GFDL-CM2.0 en la versión acoplada océano-atmósfera, para el clima presente y el escenario futuro más crítico A2 del CMIP3. Para las simulaciones del clima presente, el modelo que mejor representó las características observadas en el conjunto del reanálisis fue el GFDL-CM2.0, presentándose más coherente con relación a las posiciones de las altas pos frontales y de las isotermas de 0°C y 10°C. Para el futuro, el modelo GFDL-CM2.0 proyecta un debilitamiento de las anomalías negativas de temperatura y los eventos extremos de caída de temperatura con menos avance en dirección al Ecuador, mientras que, según el modelo HadCM3, la simulación para el futuro

Seasonal linear trends of precipitation from South American station data, which have been averaged onto grids, are examined, with emphasis on the central continent. In the period 1976-99, the largest trend south of 20ЊS occurs during the January-March season, is positive, and is centered over southern Brazil. From 1948 to 1975 the trend is also positive, but with less than half the slope. The trend is not due to a systematic change in the timing of the rainy season, which almost always starts before January and usually ends after March, but rather results from an increase in the percent of rainy days, and an increase in the rainy day average. The dynamic causes of the trend are not obvious. It does not appear to be accounted for by an increase in synoptic wave activity in the region. The precipitation trend is related to a positive sea surface temperature trend in the nearby Atlantic Ocean, but apparently not causally. The trend in the Atlantic seems to result from a decrease in mechanical stirring and coastal upwelling associated with a decrease in the strength of the western edge of the circulation associated with the South Atlantic high.

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