On the Functioning of the Southern Oscillation in the South American Sector. Part II: Upper-Air Circulation

Abstract

The functioning of the Southern Oscillation (SO) in the South American sector is analyzed with particular emphasis on the upper-air circulation anomalies. Throughout the year, but especially during austral summer, the negative SO-phase (defined as anomalously low/high pressure at Tahiti/Darwin) is typically associated with a relatively warm tropical troposphere, while negative temperature anomalies prevail during the positive SO-phase. Outside the tropics, upper-air circulation anomalies are most pronounced during the winter. Thus, in boreal winter the negative SO phase is associated with reduced 500 and 200 mb heights at 20-30 degrees N, but positive height departures southward of 20 degrees N. The implied steepening of the meridional thickness gradient is consistent with anomalously intense upper-air westerlies over the Gulf of Mexico and Caribbean region. In austral winter during the negative SO phase, relatively cold tropospheric conditions prevail over the southern portion of South America. Case studies for extremes of the SO during January-February reveal broadly opposite anomaly patterns during the positive SO phase compared to those during the negative SO phase.