On the Functioning of the Southern Oscillation in the South American Sector. Part I: Surface Climate

The functioning of the Southern Oscillation (SO) over South America and the surrounding tropical Atlantic and eastern Pacific Oceans is studied through analyses of bimonthly patterns of correlation between an SO index and the fields of pressure, wind, temperature and rainfall; hydrometeorological anomalies being further ascertained from river discharge and lake level records and measurements of outgoing longwave radiation. Most pronounced is the anomalously low pressure over the eastern Pacific during the negative SO phase (defined by anomalously low/high pressure at Tahiti/Darwin). Consistently positive SO-pressure correlations over the eastern tropical Pacific extend eastward into the North Atlantic region and southern South America during the respective winter semester. During the austral winter the tendency for anomalously abundant rainfall in central Chile during the negative SO phase appears related to an anomalously weak and northward displaced South Pacific subtropical high, while relatively dry conditions in the northwestern portion of the continent seem related to enhanced pressure in the Caribbean. During the austral summer, SO-related climate anomalies grow most pronounced, particularly conspicuous being the following three anomaly regimes of the negative SO phase: (i) The excessive rainfall along the Ecuador-Peru littoral and the adjacent open equatorial Pacific is consistent with an intensified and southward displaced new-equatorial trough; (ii) In the tropical North Atlantic during the latter past of the austral summer, the near-equatorial trough is displaced northward, the meridional pressure gradient is reduced, the northeast trades are weak, the surface waters are anomalously warm, and consistent with the large-scale circulation relatively dry conditions prevail in Northeast Brazil and part of the Amazon basin; (iii) Contrasting with concurrent negative rainfall anomalies in a large portion of tropical South America, a weak tendency for relatively wet conditions is apparent in the Paraná river basin. This feature, most pronounced in the early part of the austral summer, appears associated with intensified frontal activity.