Summertime Precipitation Variability over South America: Role of the Large-Scale Circulation

The observed large-scale circulation mechanisms associated with summertime precipitation variability over South America are investigated. Particular attention is paid to the Altiplano where a close relationship has been observed between rainfall and the position and intensity of the Bolivian high. Empirical orthogonal function (EOF), correlation, and composite analyses suggest that on intraseasonal timescales (typically 5–20 days), rainy periods on the Altiplano are associated with at least three types of circulation anomalies, involving either extratropical cyclones, cold-core lows, or the westward enhancement of the South Atlantic high. In each instance, the primary support for high rainfall rates is a moist, poleward flow at low levels along the eastern flank of the central Andes in association with the South Atlantic convergence zone (SACZ). The warm, low-level flow along the SACZ also inflates the overlying atmospheric column, resulting in an intensification and southward shift of the Bolivian high. Thus, the position of the SACZ (and associated frontal activity) plays a crucial role in the variability of both the Bolivian high and Altiplano rainfall. On longer timescales, the Bolivian high also shifts southward and intensifies during wet periods on the Altiplano. The seasonal transition from December to the wetter month of January is accompanied by a westward enhancement of the SACZ and northwesterly flow along the central Andes. The transition to the drier month of February is accompanied by weakening northwesterly flow and cooler, drier conditions along the Altiplano. Interannual precipitation variability on the Altiplano is strongly correlated with the amplitude of the fifth EOF of the 200-mb height field, the “dry phase”, which is much like the anomalous conditions during February. A case study of the dry conditions during the 1987 El Niño associates reduced convection on the Altiplano with the presence of a strong cold front over eastern South America and cold, dry air to the west. The characteristic eastward shift of the South Pacific convergence zone during El Niño may be responsible for this enhanced frontal activity in the SACZ (through teleconnections) and, therefore, the cool, dry, convectively unfavorable conditions in the central Andes.