Impacts of the extremes of the Southern Oscillation in Amazonia. Part 1. Hydrometeorology
Alice M. Grimm, Jose A. Marengo, Paulo Zaratini
Abstract
The influence of the extremes of the Southern Oscillation (SO) in the precipitation over Amazonia, is assessed through the analysis of data from 125 rainfall stations and river discharge of 12 gauging sites across the basin. The spatial and temporal distribution of this influence along the cycles of these events is studied. Part 1 deals with hydrometeorology aspects. The low/high phase of the SO, indicative of El Niño/La Niña does not show a homogeneous signal in rainfall in the whole Northern Brazil region. However, northern and central Amazonia experienced anomalous dry summers during the mature phase of the strong El Niño events, such as 1982/93, 1986/87, and this situation is repeating during the current El Niño 1997/98, where forest fires developed and rivers reached their lowest levels in years. During La Niña, there is a tendency for positive rainfall anomalies, even though the signal is less homogenous than during El Niño. On the other hand, positive rainfall anomalies were also observed during El Niño 1972/73. In Southern Amazonia, it has been observed that some stations in Amazonia show abundant rainfall on the year before the peak of El Niño, while others show large negative departures on the beginning of summer rainy season. In this case, large positive rainfall anomalies are observed during the end of the rainy season in late winter and fall of the El Niño year. Thus, the signal of the extremes of SO in northern Amazonia is stronger as compared to the southern part of the basin.
Strong interannual variability in northern Amazonia is linked to the tropical Pacific, while an interdecadal variability typical from the Atlantic is identified in the rainfall and river records in Amazonia, as well as in Northeast Brazil and the Parana River basin in southern Brazil. Regarding the recent 1997/98 El Niño event, rainfall, over central and northern Amazonia exhibited negative departures, since the beginning of the rainy season in September, to the present time where the peak should be occurring. Drought and the presence of intense dry periods, have determined anomalously low levels of the main rivers in the northern and central portions of Amazonia. But in general, based on the intensity of rainfall anomalies and the space distribution of the SO signal, one can affirm that hydrometeorological anomalies in Amazonia are not prevailingly related to the SO.
Key words: Southern Oscillation, rainfall, El Niño, discharges, Amazonia