Atmospheric circulation patterns associated with long-term variability in the streamflow annual cycle of Río Atuel

ARANEO D. C.; VILLALBA R.

Uspallata

Encuentro; IAI CRN 2047 Science Meeting: Documenting, understanding and projecting changes in the hydrological cycle in the American Cordillera.; 2013

IAI

Río Atuel, draining from the eastern side of the Central Andes, is the most important irrigation source for the southern oases in Mendoza. The present study determines the main atmospheric circulation patterns associated with long-term variability in the annual streamflow cycle of Río Atuel. Principal Component Analysis (PCA) was applied to the annual hydrological cycle to extract the main features in the annual regime. Correlation fields between the PC factors and atmospheric-oceanic variables were also examined to establish potential links between the variability in the annual streamflow regime and tropospheric circulation. The first (PC1) and second (PC2) Principal Components explain 48% and 22% of the total variance in the annual cycle, respectively. The direct (indirect) mode of PC1 corresponds to annual cycles characterized by streamflows above (below) the mean. The PC2 represents seasonal lags of the streamflow peak shifted to late spring (direct mode) or early autumn (indirect mode). The PC1 streamflow pattern is significantly related to the winter-spring tropospheric circulation conditions. Direct (indirect) PC1 conditions are associated with significant positive (negative) pressure anomalies over the southeastern Pacific Ocean concurrent with warmer (cooler) sea surface temperature (SST) anomalies in the central equatorial Pacific. A northward (southward) shift of the storm tracks is induced by the anomalies in tropospheric circulation over the southeastern Pacific in response to SST anomalies in the equatorial Pacific (El Niño/La Niña events). The PC2 streamflow pattern is significantly correlated with late-spring and middle-to-late summer tropospheric circulation. Streamflow peaks in the late spring (middle-to-late summer) are associated with anomalous warming (cooling) over the subtropical Andes due to the strengthening (weakening) of the South Atlantic anticyclone and the enhanced (reduced) meridional circulation over subtropical latitudes in November-December. In addition, years with a tendency to experience streamflow peaks in the late spring (the middle-to-late summer) also occurred associated with anomalous cooling (warming) at high levels in the atmosphere over the Rio Atuel basin during January, related to a low pressure anomaly center linked to a stationary quasi-zonal wave train from Australia across the Pacific Ocean.The temporal evolution of the PC factor loadings reveals that the PC1 pattern was predominantly indirect during the periods 1917-1976 and 1988-1999, suggesting a tendency to undergo extended periods of streamflow deficit with relatively shorter (1906-1916 and 1977-1987) periods of above-average runoff. In turn, the PC2 pattern is predominantly indirect from 1906 to 1926 but direct from 1948 to 2002, suggesting a tendency to a dominance of streamflow peaks during late spring during most of the second half of the 20th century. Relationships between the temporal evolution of the annual cycle of Río Atuel and large-scale circulation indexes are discussed.