INTERANNUAL AND INTERDECADAL VARIABILITY IN STREAM FLOW FROM THE ARGENTINE ANDES

Abstract

The associations between mean annual stream flow records from 15 rivers draining the Argentine Andes, ranging from the Altiplano to Patagonia, and the Southern Oscillation Index (SOI), as established through simple lag cross-correlation, are examined. The results indicate that rivers in the Cuyo region (30° to 40°S) are negatively associated with the SOI but that the response is lagged by about one year through the regional winter maximum of precipitation and subsequent snowmelt. Stream flow in the area also has a strong positive association with winter precipitation totals in Central Chile. A statistical analysis of recent synoptic variability indicates that the difference between regional zonal and meridional flow patterns is responsible for observed fluctuations of winter precipitation. This is in keeping with the general observation of greater meridional activity globally during warm phases of El Niño-Southern Oscillation (ENSO), and a dominance of zonal flows during cold phases. Both regional stream flow and a record of historic El Niño events suggest that some lower frequency control exists in both of these series. Three periods—1908 to 1935, 1936 to 1975, and 1976 to present—are identified in the 90-year gauged record, and the distributions of annual flows and ENSO frequency are shown to vary between these, perhaps in concordance with periods of increasing global temperatures. Indications of similar associations, both with ENSO and at lower frequencies, over a longer time period are found in historic and proxy precipitation series of Central Chile. Wetter 5-year periods generally correspond to times of increasing temperatures and extended dry periods to declining temperatures, suggesting stronger El Niño events during periods of increasing temperatures. The implications for human activity of such extended wet and dry periods in regions known to currently be effected by ENSO are tentatively drawn and paralleled by archaeological evidence in northern Peru, the Bolivian Altiplano, and Easter Island.