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Climatic variations in Southern Brazil263The near-decadal cycle revealed by thewavelet analysis consistently lags SOI by 2 years.The study of Robertson & Mechoso (1998) cited theassociation of streamflow with SSTa south ofGreenland, which would be a possible link to theNAO. Cross wavelet and wavelet coherence betweenML water level and the transformed NAO indexindicate a sharp region of possible association nearthe 8-year band around 1960. Hurrel et al. (2003)suggested that the 1960’s were characterized byanomalously high surface pressures and severewinters from Greenland across northern Europe,during a negative NAO regime. Although the resultsdescribed here do not reject the hypothesis ofassociation between the NAO and SSA surfaceclimate, the overall signal on ML basin is punctualand barely significant. This may result from the factthat NAO impacts climate mostly during northernwinter (austral summer) and this is the dry season insouthern Brazil. Decadal variations of the basic stateduring this season may not be as important as theworld-wide ENSO impact on the 7 year timescales.Whether the positive trend identified here isa consequence of natural oscillations of the climatesystem or a result of anthropogenic forcing is stillunclear. Recently, Church et al. (2008) suggestedthat volcanic activity may be related to a globalcooling of the upper ocean and the offset of theincreasing rate of sea-level rise between 1963 and1991. A combination of a robust ENSO periodduring the 1950’s and the subsequent period of highvolcanic activity might led to the regimeconfiguration of positive mean and low variancefrom 1958/1963 to the end of the water level recordanalyzed here.Considering human impact around MLbasin, surrounding regions in Uruguay have beensubjected to intensive cattle and sheep grazing sincethe early 1940’s (García-Rodríguez et al. 2002).Overbeck et al. (2007) estimated a 25% decrease innatural grassland area in southern Brazil since thelate 1970’s due to the expansion of agriculturalactivities (mainly grazing, irrigated rice crops andmore recently Eucalyptus sp. plantations). Marqueset al. (2004) reported that nearly 89% of Uruguayanirrigated rice is cultivated on ML basin. On the otherhand, Baldi & Paruelo (2008) used satellite data todemonstrate that ML region presented a small rate ofchange from grass to cropland when two periods inthe last 30 years were compared (1985-89 and 2002-04), suggesting that major changes may haveoccurred before 1985. Nonetheless, Gautreau (2010)showed evidences that forest loss on the Uruguayanbanks of ML could be explained by rice cropextension but improved conditions for forest growthin Uruguay during the last century could be aconsequence of increased rainfall. Medeanic et al.(2010) used algal palynomorphs from anothercoastal lagoon in southern Brazil to demonstrate atendency of increasingly humid conditions duringthe last century, with a marked anthropogenicimpact detected after the 1970’s. All thesereferences lead to the conclusion that human impactis important. However, the increase in water level isremarkably high, especially considering that waterdiverted from ML to irrigated rice productionnegatively impacts the water balance of the lagoon.The positive trend in ML is consistent withthe trends found by Genta et al. (1998) for fourmajor rivers in SSA. A positive trend in rainfall overa large region in SSA is presented by Haylock et al.(2006) at least between 1960 and 2000. In theAmazon basin, Marengo et al. (1998) found noevidences of changes in the 20 th century, whereasslow increases in rainfall were found fornortheastern Brazil. Collinschon et al. (2001)analyzed river flow and rainfall from 1900 to 1995on the Paraguay River basin. They detectedincreased river flow since 1970 associated withchanges in rainfall patterns (increased frequency ofrainfall events) and suggested that at least part of therunoff increase should be due to deforestation.Moreover, the authors showed that, in Africa, theCongo River flow presented the exactly oppositebehavior of Paraguay River throughout the lastcentury, indicating a possible large-scale climaticconnection between the continents.A modeling study by Cook et al. (2004)pointed out that the West African Monsoon, inboreal summer, generates a Walker-type circulationwith low-level convergence and wet conditions overAfrica and divergence and drier conditions overnortheastern South America and tropical Atlantic.Thus, a weakening of the African Monsoon wouldlead to higher precipitation over northeastern Brazil.In fact, the African Monsoon presented a reductionin precipitation during the second half of the lastcentury and numerical modeling experimentsindicate Atlantic SST variability as the main driverof the observed rainfall decline (Paeth & Hense2004). Janicot et al. (1998) showed that divergentanomalies over the tropical Atlantic, associated withEl Niño events, may lead to a weaker AfricanMonsoon especially if there are positive SSTanomalies over the eastern tropical Atlantic as well.Moron et al. (1995) showed evidences of stronger ElNiño impact over the West African Monsoon after1970. On the other hand, Diaz et al. (1998)demonstrated that the Atlantic Ocean may impactrainfall over southern Brazil and Uruguay and thatPan-American Journal of Aquatic Sciences (2010), 5(2): 254-266