Robertson, A. W., and C. R. Mechoso, 1998: Interannual and Decadal Cycles in River Flows of Southeastern South America. J. Climate, in press.
The time series of annual streamflow of four rivers in southeastern and south-central South America (the Negro, Paraguay, Paraná and Uruguay Rivers) for the period 1911-1993 are analyzed. Application of the multi-taper method shows that the following features are significant at the 95% level: (1) a nonlinear trend, (2) a near-decadal component, and (3) interannual peaks with El Niño-Southern Oscillation (ENSO) time scales. The trend and near-decadal components are most marked in the two more central rivers, the Paraguay and Paraná, with ENSO time-scale variability most pronounced in the Negro and Uruguay rivers in the southeast. Composites of sea surface temperature (SST) are made for each of the statistically-significant oscillatory components of river flow, by reconstructing each component using singular spectrum analysis. These composites confirm the influence of ENSO on the streamflow variability of the Negro and Uruguay rivers, with El Niño associated with enhanced streamflow. On the decadal time scale, high river runoff is associated with anomalously cool SSTs over the tropical North Atlantic. A very similar near-decadal oscillation in SST over this region is identified separately from a rotated empirical orthogonal function analysis of gridded annual mean SSTs. The near-decadal component of the Paraguay and Paraná rivers is strongest in the austral summer.