Extreme Climatic Events in the Amazon Basin and Their Associations with the Circulation of the Global Tropics

The mechanisms of climate anomalies in the Amazon basin were explored in three complementary lines of investigations, drawing on surface climatological and hydrological series, upper-air and satellite observations, as well as the general circulation model (GCM) of the Goddard Institute for Space Studies (GISS). Work focused on the March-April rainy season peak in the northern portion of the Amazon basin (NA). Correlation analyses indicate that rainfall in NA tends to be more abundant with a stronger North Atlantic high, accelerated Northeast trades, colder North and warmer South Atlantic waters, and during the high phase of the Southern Oscillation (SO; defined by anomalously high/low pressure at Tahiti/Darwin). GISS GCM simulations using globally observed sea surface temperature (SST) during 1979-86 reproduced major features of circulation and rainfall over tropical South America, but most satisfactorily for years with extreme SST conditions. Detailed case studies for the moderately WET year 1986 and the extremely DRY year 1983 showed during WET pronounced subsidence off the West coast of South America, contrasting with strong ascending motion and vigorous convection to the East of the Andes, and weak STWJ's in both hemispheres; while DRY featured ascending motion and convective rainfall over the Pacific coast of South America, subsidence over the Amazon basin, and strong STWJ's. In synthesis from these three lines of investigations, some major mechanisms of extreme rainfall events over NA stand out, but only for the late austral summer. The near -equatorial low pressure trough and embedded Intertropical Convergence Zone (ITCZ) attain their southernmost position, the intense summertime convection over Amazonia being 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 SO phase. By contrast, ascending motion and convection over the easternmost equatorial Pacific, as common during extreme events of the low SO phase, require compensatory subsidence, and this may interfere with convection to the East of the Andes. Finally, the study indicates that hydrometeorological anomalies in Amazonia are not prevailingly related to the SO.