Since the austral summer of 2014 southeastern Brazil has been experiencing one of the most severe droughts in decades. This rainfall deficiency has generated water shortages and a water crisis that have affected population and local economies in the metropolitan region of Sao Paulo, the largest megacity in South America. By January 2015, main reservoirs had reached storage levels of only 5% of their 1.3 billion m 3 capacity. The meteorological causes of the drought situation were linked to changes in the regional circulation, characterized by a mid-troposphere blocking high that lasted 45 days during the summer of 2014 over southeastern Brazil, something not seen in five decades. The water crisis was aggravated by a combination of lack of rainfall and higher temperatures, the summer of 2014 being the warmest and driest over the Cantareira reservoir system since 1951. Increasing population and water consumption increased vulnerability in the region, and while human-induced warming may not have generated the atmospheric conditions behind the 2014 and 2015 summer droughts in Southeast Brazil, it is more likely that the warm temperatures have affected the severity of the drought and exacerbated the impacts on the population. KeywordsDrought, Sao Paulo, Water Crisis, Vulnerability C. A. Nobre et al. 253

The Pantanal region in South America is one of the world's largest wetlands. Since 2019, the Pantanal has suffered a prolonged drought that has spelled disaster for the region, and subsequent fires have engulfed hundreds of thousands of hectares. The lack of rainfall during the summers of 2019 and 2020 was caused by reduced transport of warm and humid summer air from Amazonia into the Pantanal. Instead, a predominance of warmer and drier air masses from subtropical latitudes contributed to a scarcity of summer rainfall at the peak of the monsoon season. This led to prolonged extreme drought conditions across the region. This drought had severe impacts on the hydrology of the Pantanal. Hydrometric levels fell all along the Paraguay River. In 2020, river levels reached extremely low values, and in some sections of this river, transportation had to be restricted. Very low river levels affected the mobility of people and shipping of soybeans and minerals to the Atlantic Ocean by the Hidrovia -Paraná-Paraguai (Paraná-Paraguay Waterway). This study is directed to better understand the hydroclimatic aspects of the current drought in the Brazilian Pantanal and their impacts on natural and human systems. As a consequence of the drought, fires spread and affected natural biodiversity as well as the agribusiness and cattle ranching sectors. While fires had serious socioecological and economic consequences, we do not intend to investigate the effect of the downstream low-level waters on the Pantanal ecosystems or the drought in the risk of fire.

A seca e a crise hídrica de 2014-2015

A low pressure system [known as the northwestern Argentinean low (NAL)] is commonly observed over northwestern Argentina near the Andean slopes. This study describes two NAL episodes for summer and winter, with emphasis on the characterization of their three-dimensional structure and temporal variability. With the aid of a high-resolution regional model [Eta/Centro de Previsão de Tempo e Estudos Climáticos (CPTEC)], the main mechanisms involved in the NAL life cycle were studied in order to examine how the thermal topographical processes influenced the system behavior. Surface pressure changes in the NAL are mostly dominated by the 600-900-hPa thickness variability, suggesting its thermal character. Based on this result, the temperature tendency equation has been used to quantify all the contributions to thermal changes. The summer NAL has a significant diurnal cycle that has been dominated by surface warming. This single mechanism can explain the low pressure system formation by itself, suggesting that the NAL could always be present during this season unless an adverse process counteracts the positive contribution by the surface sensible heat flux. Other favoring processes found in the analyzed cases were the Foehn effect (warming by subsidence) and the latent heat release. The intermittent behavior of the NAL is associated with a suppression of net warming in the 600-900-hPa layer, due to a cold-air outbreak. In the winter case, the Foehn effect has been mainly responsible for the NAL development. This mechanism acts in connection with an upper-level cyclonic disturbance approaching the Andes, indicating that the thermal response is triggered by a dynamical forcing. As the Foehn effect (locally known as Zonda wind) is a frequent winter phenomenon, the NAL intermittence during this season could be related to transient baroclinic activity, which modulates both the intensification and the decay stages. The NAL has been regarded as a thermal-orographic system. This study suggests that the analyzed NALs behave as an almost ''pure'' surface thermally driven low in summer, while dynamical-orographic forcing is the organizing mechanism in winter.

Drought is a natural and recurrent phenomenon. It is considered 'a natural disaster' whenever it occurs in an intensive manner in highly populated regions, resulting in significant damage (material and human) and loss (socioeconomic). This paper presents the efforts developed to monitor the impact of drought in the semiarid region of Northeast Brazil. In this scope, information from different sources is compiled to support the evaluation and identification of impacted municipalities, with the main objective of supporting emergency actions to mitigate their impact. In the semiarid region of Brazil there are frequent occurrences of dry periods during the rainy season, which, depending on the intensity and duration, can cause significant damage to family-farmed crops, with a farming system characterized by low productivity indices. However, rain-fed agriculture has great economic expression and high social importance due to the region is densely occupied, and contributes to the establishment of communities in the countryside. Specifically, in the present study, the methodology adopted to monitor the impact of agricultural droughts, including an analysis of the hydrological year 2015-2016, is presented, considering different water stress indicators for the identification of the affected municipalities and assessment of the methods and tools developed.

Meridional transport of air masses between the tropics and midlatitudes in South America are the most intense in the entire Southern Hemisphere, mainly due to the presence of the Andes. The incursions of tropical air into midlatitudes occur on the eastern side of the Andes in two preferred regions. The first is located in the tropical latitudes, close to the mountains between 20°and 30°S, and the second is a function of the position of the South Atlantic Convergence Zone (SACZ). All year long, the two preferred regions maintain their behaviour, with only small variations of their position or relative importance in different seasons of the year. The variability of the meridional transport is larger on the eastern side of the Andes, due to the presence of the mountain barrier, which favours baroclinic activity and allows an active exchange of air masses in both senses, especially during winter. The importance of the air mass transport is evident in the precipitation and surface temperature fields. During summertime, the Chaco Low (25°S and 65°W) intensifies due to the positive net radiation, favouring the transport of tropical air masses towards the south and the presence of strong convective activity, which is fed by moisture from tropical regions. During winter, the penetration of tropical air towards higher latitudes is more sporadic. The displacement of midlatitude air towards tropical latitudes occurs on both sides of the Andes. On the western side, the air associated with the subtropical Pacific anticyclone flows northward channelled by the Andes. On the eastern side, incursions of polar air towards lower latitudes are linked to cold fronts whose trajectory and movement is also favoured by the presence of the Andes. In particular, during wintertime the cold fronts are more intense and faster, and sometimes even reach tropical and equatorial latitudes which produces freezes in subtropical regions, such as the coffee growing areas in southeastern Brazil. In contrast, the incursions of cold air are notably weaker and less frequent in summer, and during these events the active cold fronts move northwards merging with the SACZ, which becomes more intense.

The zonda is a warm and extremely dry wind that occurs east of the Andes Cordillera in the extratropical latitudes of South America. Its orographic origin is similar to the foehn that blows in Germany and Austria and the chinook that occurs east of the Rocky Mountains. Three typical zonda events of different categories (surface and elevated) are described, through observational and Eta-Centro de Previsão de Tempo e Estudos Climáticos (CPTEC) model output. During the events the temperature rises sharply by 10Њ-15ЊC and the dewpoint temperature drops by 15Њ-20ЊC in an interval of a few hours. The sustained wind strength at the surface increases to 30 kt, with gusts of more than 40 kt. The episodes generally start around midday and last for about 10 h.The Eta-CPTEC model was able to forecast several aspects of the three analyzed zonda cases, such as wind strength, temperature, and humidity changes, and their starting and ending times. Some relationships between the intensity of the windward static stability and the zonda occurrence were observed. The synoptic pressure configuration resulting from frontal passages associated with depressions that moved over lower than normal latitudes preceded the zonda occurrence. Sensitivity experiments showed that if the Andes were lower, more frequent but less intense zonda episodes could be expected.

Heavy rains frequently occur over the Serra do Mar, southeast coastal mountain region in Brazil, and cause landslides and life losses especially during summer season. The objective of this work is to produce a synoptic climatology of heavy rainfall episodes of the period from November to April using ten-year re-analyses data. The identification of the synoptic pattern of these events should provide guidance to the forecasters. The landslides events were mostly related to the South Atlantic Convergence Zone (SACZ) and cold front passages. These systems differed from climatology by exhibiting more intense characteristics for the frontal situations and a blocking circulation configuration for the SACZ situations. In both cases, the composite fields showed the 250-hPa mass divergence was strikingly more intense than climatology and had a preferred location to be positioned in the 24 hours before the landslide events. Anomalies of this ten-year event climatology showed above normal moisture anomalies, which are more evident in the SACZ cases.