Case Studies of Extreme Climatic Events in the Amazon Basin

Abstract

The mechanisms of climate anomalies in the Amazon basin were explored from surface climatological and hydrological series, upper-air, and satellite observations. The paper is focused on the March?April rainy season peak in the northern portion of Amazonia. Case studies for the moderately wet year 1986 (WET), showed a relatively far-southerly location of the Atlantic near-equatorial trough, and embedded intertropical convergence zone (ITCZ); strong ascending motion and vigorous convection over the Amazon basin, contrasting with pronounced subsidence off the west coast of South America,, and weak subtropical westerly jets (STWJ) in both hemispheres. In contrast, the extremely dry El Niño year 1983 (DRY), featured a more northward located ITCZ; subsidence over the Amazon basin; ascending motion and convective rainfall to the west of the Andes; and strong STWJ. In synthesis from these analyses, some major mechanisms of extreme rainfall events in northern Amazonia stand out, but only for the late austral summer, when the ITCZ in the tropical Atlantic-South American sector attains its southernmost position, as the intense summertime convection over Amazonia is an important component of the ITCZ. Thus, ascending motion over the northern part of the Amazon basin with an anomalously far-southward displaced ITCZ appears compatible with subsidence to the west of the Andes during the high phase of the Southern Oscillation (SO), which is defined by anomalously high/low pressure at Tahiti/Darwin. In contrast, ascending motion and convection over the easternmost equatorial Pacific, as is common during extreme events of the low-SO phase, require compensatory subsidence, and this may interfere with convection to the east of the Andes. However, hydrometeorological anomalies in Amazonia are not prevailingly related to the SO.