Interannual-to-interdecadal variability of the South Atlantic Convergence Zone and riverflow predictability

During the southern summer, rainfall over South America is dominated by the monsoon over Amazonia and the South Atlantic Convergence Zone (SACZ). Variations in position and intensity of the SACZ strongly influence precipitation; their predictability is, therefore, of major regional importance.

We examine interannual-and-longer variations of the summertime atmospheric circulation over subtropical South America using EOF analysis of the NCEP/NCAR Reanalysis data (1958-97). A deep anomalous cyclonic circulation-shown to be characteristic of a stationary Rossby wave-is found to accompany an amplified SACZ with intensified descent to the southwest. On longer time scales, we find a 15-17 year oscillatory component in the strength of the SACZ Rossby wave, which we are able to corroborate from longer records of SSTs over the southwest Atlantic, and river flows. Using singular spectrum analysis, all show near-leading oscillatory components with very similar period and phase. When the SACZ is intensified, the Paraná and Paraguay rivers in southern Brazil are found to tend to swell, while the Uruguay and Negro rivers beneath the region of anomalous descent to the south tend to ebb.

A quantitative assessment of streamflow predictability is made based on the 15-17 year cycle, together with ENSO and 9-year oscillatory components found previously; each accounts for typically 15-20% of the variance of seasonal means. Autoregressive models are fitted to the oscillatory components, which are then extrapolated forward in time to make a prediction. Using contingency tables, we demonstrate marked changes in the conditional probability of above and below normal monthly streamflow, according to the phase of the slow oscillatory components. Some hindcast skill for probabilistic categorical predictions is demonstrated for the Paraná river up to 10 years in advance.