Southern oscillation signal in South American palaeoclimatic data of the last 7000 years

During strong El Niño events, rainfall anomalies and changes in wind patterns are observed in different regions of South America. Along the central Brazilian coast, during the 1983 El Niño year, the frontal systems were blocked to the south, provoking a reversal of the longshore sand transport. Long-duration reversals of longshore transport were also recorded in Holocene beach-ridge terraces from the Rio Doce coastal plain. This led to the formulation of a model relating these reversals of longshore transport to El Niño-like conditions. El Niño-like conditions are past average climate situations that generate the same perturbations as the strong El Niño events observed during the last decade. They are likely to correspond to the long-duration low phase of the Southern Oscillation. To confirm this hypothesis we compared the Holocene beach-ridge record with other palaeoenvironmental records from regions where strong El Niño events would have a substancial signal as well: (1) water-level fluctuations of Lake Titicaca, (2) a pollen and sediment record in an eastern Amazonian lake, (3) changes of the Rio Xingu discharge in eastern Amazonia, and (4) variations of sand supply at the Rio Piura and Rio Chira outlets in the Sechura Desert. The occurrences of El Niño-like conditions were numerous before 3900-3600 yr B.P., absent between 3900-3600 and 2800-2500 yr B.P., and infrequent after 2800-2500 yr B.P. © 1993 University of Washington

The evolution of the continental and coastal environments during the last climatic cycle in Brazil (120 KY. B.P. to present)

Studies done in Brazil, under the CNPq (Brazil) and ORSTOM (France) agreement, allowed us to envisage the general trend of climatic changes that occurred during the last 60,000 years. At first, examination of successive accumulation and erosion phases, recorded within Central Brazilian valley deposits, provided evidence of importam changes in slope vegetation cover and in the hydraulic regime of water courses, and hence characteristics of precipitation and climate regimes. Studies of Tamanduá river valley deposits in the town of São Simão (State of São Paulo) showed that between 32 and 21 ky. B.P. the climate was humid, whereas between 17 and 11 ky. B.P. it was dry with very scarce and strong rains. From 10 ky to 8.5 ky. B.P. the climate was humid too. Finally, after 7.5 ky. B.P. the deposits revealed the existence of several dry episodes within a globally humid climate. Palynological and sedimentological analyses of lacustrine deposits sampled by vibrocorers in the Eastern Amazon (Carajás, State of Pará) and in Central Brazil (Salitre, State of Minas Gerais) permitted us to outline the evolutionary history of the vegetation cover within these regions during the last 60,000 years for Carajás and 30,000 years for Salitre and thus to have a preliminary idea about their past climates. Then, it was possible to show that at Carajás regression of the evergreen rainforest occurred four times, at about 60, 40, between 23-11 and 7-4 ky. B.P. Moreover, apparently the last rainforest regression episode was quite different from the previous three episodes. At Salitre, the most characteristic is the indication of the existence of Araucária forest between 12 and 8.5 ky. B.P. This Araucária forest location is clearly much further north than at present, which suggests that during that time the climate was wetter and colder. Finally, studies developed along the central Brazilian coast allowed us to show evidence of wind-/driven change in littoral dynamics during the last 5 ky. A detailed examination of Doce river (State of Espírito Santo) mouth beach-ridge geometry has shown inversions in littoral drift direction as a consequence of change in wave and then in wind regimes, reflecting characteristics of atmospheric circulation during the last 5 ky.Estudos feitos no Brasil, segundo um acordo entre o CNPq (Brasil) e ORSTOM (França), permitiram-nos vislumbrar a tendência geral de mudanças climáticas que ocorreram durante os últimos 60.000 anos. Primeiramente, o estudo de sucessivas fases de erosão e sedimentação, registradas nos depósitos de vale do Brasil Central, forneceram evidências de importantes mudanças na cobertura vegetal de vertentes e no regime hidráulico de cursos fluviais e então chegando-se às características dos regimes pluviométricos e climáticos. Estudos de depósitos do vale do rio Tamanduá na cidade de São Simão (SP) mostraram que entre 32 e 21 ka. A.P. o clima foi úmido, enquanto entre 17 e 11 ka. A.P. ele foi seco com chuvas esparsas e fortes. De 10 ka. a 8,5 ka. A.P. o clima foi também úmido. Finalmente, após 7,5 ka. A.P. os depósitos revelaram a existência de vários episódios secos através do clima globalmente úmido. Análises palinológicas e sedimentológicas de depósitos lacustres amostrados por vibrotestemunhador na Amazônia Oriental (Carajás, PA) e no Brasil Central (Salitre, MG) permitiram delinear a história evolutiva da cobertura vegetal nessas regiões, durante os últimos 60.000 anos para Carajás e 30.000 anos para Salitre, e então ter uma idéia preliminar sobre seus paleoclimas. Então foi possível mostrar que em Carajás por quatro vezes, há cerca de 60, 40, entre 23-11 e 7-4 Ka. A.P., ocorreu a regressão da floresta pluvial perene. Além disso, aparentemente o último episódio de regressão da floresta pluvial foi bem diferente dos três episódios anteriores. Em Salitre, mais característica é a indicação da existência de floresta de Araucária entre 12 e 8,5 Ka. A.P. A localização desta floresta de Araucária está claramente mais ao norte que atualmente, sugerindo que durante aquela época o clima era mais úmido e mais frio. Finalmente, estudos desenvolvidos na costa do Brasil Central mostraram evidências de mudanças na dinâmica costeira comandadas pelo vento durante os últimos 5 Ka. Um estudo detalhado da geometria das cristas praiais de desembocadura do rio Doce (ES) mostrou inversões no sentido da deriva litorânea como conseqüência de mudanças nos regimes da onda e do vento, permitindo conhecer as características da circulação atmosférica durante os últimos 5 Ka