In 2005, large sections of southwestern Amazonia experienced one of the most intense droughts of the last hundred years. The drought severely affected human population along the main channel of the Amazon River and its western and southwestern tributaries, the Solimões (also known as the Amazon River in the other Amazon countries) and the Madeira Rivers, respectively. The river levels fell to historic low levels and navigation along these rivers had to be suspended. The drought did not affect central or eastern Amazonia, a pattern different from the El Niño-related droughts in 1926, 1983, and 1998. The choice of rainfall data used influenced the detection of the drought. While most datasets (station or gridded data) showed negative departures from mean rainfall, one dataset exhibited above-normal rainfall in western Amazonia.The causes of the drought were not related to El Niño but to (i) the anomalously warm tropical North Atlantic, (ii) the reduced intensity in northeast trade wind moisture transport into southern Amazonia during the peak summertime season, and (iii) the weakened upward motion over this section of Amazonia, resulting in reduced convective development and rainfall. The drought conditions were intensified during the dry season into September 2005 when humidity was lower than normal and air temperatures were 3°-5°C warmer than normal. Because of the extended dry season in the region, forest fires affected part of southwestern Amazonia.

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.

[1] The year 2010 featured a widespread drought in the Amazon rain forest, which was more severe than the "once-in-a-century" drought of 2005. Water levels of major Amazon tributaries fell drastically to unprecedented low values, and isolated the floodplain population whose transportation depends upon on local streams which completely dried up. The drought of 2010 in Amazonia started in early austral summer during El Niño and then was intensified as a consequence of the warming of the tropical North Atlantic. An observed tendency for an increase in dry and very dry events, particularly in southern Amazonia during the dry season, is concomitant with an increase in the length of the dry season. Our results suggest that it is by means of a longer dry season that warming in the tropical North Atlantic affects the hydrology of the Amazon Rivers at the end of the recession period (austral spring). This process is, sometimes, further aggravated by deficient rainfall in the previous wet season.

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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

Using the PRECIS regional climate modeling system this study analyses the distribution of extremes of temperature and precipitation in South America in the recent past and in a future (2071-2100) climate under the IPCC SRES A2 and B2 emissions scenarios. The results show that for the present climate the model simulates well the spatial distribution of extreme temperature and rainfall events when compared with observations, with temperature the more realistic. The observations over the region are far from comprehensive which compromises the assessment of model quality. In all the future climate scenarios considered all parts of the region would experience significant and often different changes in rainfall and temperature extremes. In the future, the occurrence of warm nights is projected to be more frequent in the entire tropical South America while the occurrence of cold night events is likely to decrease. Significant changes in rainfall extremes and dry spells are also projected. These include increased intensity of extreme precipitation events over most of Southeastern South America and western Amazonia consistent with projected increasing trends in total rainfall in these regions. In Northeast Brazil and eastern Amazonia smaller or no changes are seen in projected rainfall intensity though significant changes are seen in the frequency of consecutive dry days.

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This study discusses the climatological aspects of the most severe drought ever recorded in the semiarid region Northeast Brazil. Droughts are recurrent in the region and while El Nino has driven some of these events others are more dependent on the tropical North Atlantic sea surface temperature fields. The drought affecting this region during the last 5 years shows an intensity and impact not seen in several decades in the regional economy and society. The analysis of this event using drought indicators as well as meteorological fields shows that since the middle 1990s to 2016, 16 out of 25 years experienced rainfall below normal. This suggests that the recent drought may have in fact started in the middle-late 1990s, with the intense droughts of 1993 and 1998, and then the sequence of dry years (interrupted by relatively wet years in 2007, 2008, 2009 and 2011) after that may have affected the levels of reservoirs in the region, leading to a real water crisis that was magnified by the negative rainfall anomalies since 2010.