found to be part of a large-scale teleconnection wave train linked with the subsidence branch of the Walker circulation in the tropical east Pacific, which in turn was generated by an anomalous tropical heat source in north/northeastern Australia. A regional Hadley circulation with an ascending branch to the south of the subsidence branch of the Walker circulation in the tropical east Pacific was identified as an important component connecting the tropical and extratropical circulation. The ascending branch of this Hadley circulation in the south Pacific coincided with an identified Rossby wave source region, which contributed to establishing the extratropical component of the large-scale wave train connecting the south Pacific and the Atlantic region surrounding southeast Brazil. This connection between the Pacific and the Atlantic was confirmed with Rossby ray tracing analyses. The local circulation response was associated to downward air motion (subsidence) over Southeast Brazil, contributing to the expressive negative precipitation anomalies observed during summer 2014, and leading to a major drought event in the historical context. The analysis of atmospheric and oceanic patterns of this event helped defining a schematic framework leading to the observed drought conditions in southeast Brazil, including the involved teleconnections, blocking high pressure, radiative and humidity transport effects.

ABSTRACT:The differences between 1948-1976 and 1977-2002 periods in the interannual relationships of South American precipitation with sea surface temperature (SST) indices in selected oceanic sectors are documented using correlation analyses. For the total correlation between the eastern equatorial Pacific (EEP) and tropical South Atlantic (TSA) indices and the precipitation, the differences between the two periods reflect, in part, the El Niño-Southern Oscillation (ENSO) teleconnections over South America, with the Pacific decadal oscillation (PDO) background acting to enhance (weaken) the correlations when ENSO and PDO are in the same (opposite) phase. For the total correlation between the tropical North Atlantic (TNA) and southwest South Atlantic (SWA) indices and precipitation, the results here suggest that the differences between the two periods might be reflected as variations in the Bolivian high-pressure system. Both the TNA and TSA indices may act to weaken the positive ENSO relations with precipitation over central and eastern Brazil during the cold PDO phase. These analyses provide indications that the ENSO relationship with the TSA shows differences between the two analysed periods.

forest ecosystems sequester large amounts of atmospheric co 2 , and the contribution from seasonally dry tropical forests is not negligible. thus, the objective of this study was to quantify and evaluate the seasonal and annual patterns of co 2 exchanges in the Caatinga biome, as well as to evaluate the ecosystem condition as carbon sink or source during years. in addition, we analyzed the climatic factors that control the seasonal variability of gross primary production (Gpp), ecosystem respiration (R eco) and net ecosystem co 2 exchange (nee). Results showed that the dynamics of the components of the co 2 fluxes varied depending on the magnitude and distribution of rainfall and, as a consequence, on the variability of the vegetation state. Annual cumulative NEE was significantly higher (p < 0.01) in 2014 (−169.0 g C m −2) when compared to 2015 (−145.0 g C m −2) and annual NEP/GPP ratio was 0.41 in 2014 and 0.43 in 2015. Global radiation, air and soil temperature were the main factors associated with the diurnal variability of carbon fluxes. Even during the dry season, the NEE was at equilibrium and the Caatinga acted as an atmospheric carbon sink during the years 2014 and 2015. CO 2 concentration has a high interannual variability due to its absorption by terrestrial ecosystems (carbon sinks) 1-5. However, despite this variability, data show a systematic increase in CO 2 throughout the years 6,7. In South America, the Amazon forest is an example of a terrestrial carbon sink (considering its 20-year mean behavior), although it has occasionally behaved as CO 2-neutral or even a carbon source in the last years 8. Interannual variability and trends in CO 2 sinks are controlled by different biogeographic regions. The annual mean behavior of sinks is controlled mainly by highly productive lands, such as wet tropical forests (i.e. the Amazon forest) 5. On the other hand, semiarid environments control the global scale trends observed in the last few decades 9,10. Despite its prominent role, there is still much to be studied and investigated regarding CO 2 exchanges in these regions, which are still much less understood than wet forests or croplands 5,10. According to the literature 10 , gaps in understanding CO 2 exchanges in these environments have limited our ability to understand and predict interannual and decadal variations on global scale carbon cycle. There are a few inherent difficulties when quantifying CO 2 exchanges in semiarid environments, such as the rapid expansion of some of its areas due to climate change and anthropic activities 11,12. Studies show that some regions in South America are becoming more arid, such as the Amazon 13,14 ; the Brazilian semiarid region, dominated by the Caatinga biome, which is a seasonally dry tropical forest (SDTF) 15-17 and the Cerrado, which is a Brazilian savanna-type vegetation 18 .

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Understanding the long-term behavior of rainfall and potential evapotranspiration (PET) over watersheds is crucial for the monitoring of hydrometeorological processes and climate change at the regional scale. The São Francisco watershed (SFW) in Brazil is an important hydrological system that transports water from humid regions throughout the Brazilian semiarid region. However, long-term, gapless meteorological data with good spatial coverage in the region are not available. Thus, gridded datasets, such as the Climate Research Unit TimeSeries (CRU TS), can be used as alternative sources of information, if carefully validated beforehand. The objective of this study was to assess CRU TS (v4.02) rainfall and PET data over the SFW, and to evaluate their long-term (1942–2016) climatological aspects. Point-based measurements retrieved from rain gauges and meteorological stations of national agencies were used for validation. Overall, rainfall and PET gridded data correlated well with point-based observations (r = 0.87 and r = 0.89), with a poorer performance in the lower (semiarid) portion of the SFW (r ranging from 0.50 to 0.70 in individual stations). Increasing PET trends throughout the entire SFW and decreasing rainfall trends in areas surrounding the semiarid SFW were detected in both gridded (smoother slopes) and observational (steeper slopes) datasets. This study provides users with prior information on the accuracy of long-term CRU TS rainfall and PET estimates over the SFW.