INSTITUTO ARGENTINO DE NIVOLOGIA, GLACIOLOGIA Y CIENCIAS AMBIENTALES
Unidad Ejecutora - UE
congresos y reuniones científicas
ENSO, SAM and Rossby waves as forcings of Río Atuel hydrological cycle variability
ARANEO D.; VILLALBA R.
Conferencia; WCRP Conference for Latin America and the Caribbean: Developing, linking and applying climate knowledge; 2014
The main patterns of variability in the hydrological cycle of Río Atuel were successfully identified by applying Principal Component (PC) Analysis to the annual hydrographs from 1906 to 2012. The first and second PCs explain 49% and 21% of the total variance in the annual hydrological cycle, respectively. Whereas PC1 is largely related to interannual variability of the total runoff, PC2 is associated with seasonal shifts of the monthly streamflow peak occurring late in spring (direct mode) or late in summer (indirect mode). Years with summer streamflows above (below) the normal are related to abundant (reduced) winter snowfalls in the high Central Andes (CA) in response to a northward (southward) shift of the stormtracks remotely induced by Sea Surface Temperature anomalies in the equatorial Pacific (El Niño / Southern Oscillation, ENSO). On the other hand, the propensity to increase (to reduce) the streamflow peak in November-December is related to above-average (below-average) temperatures over the CA, which are induced by the strengthening (weakening) of the South Atlantic anticyclone which in turn enhances the tropical (sub-Antarctic) meridional circulation over Río Atuel latitudes. These anomalies are, in turn, linked to the positive (negative) phase of the Southern Annular Mode (SAM). Additionally, years with a propensity to reduce (to increase) the streamflow peak in January are associated with anomalous air cooling (warming) at high levels induced by low (high) pressure anomaly centers as part of a quasi-zonal stationary Rossby wave train extended from Australia to the South Atlantic. Interannual variations in PC1 loadings (PCL) were compared with an index of June-November regional precipitation from Santiago, Curicó and Concepción stations. For 1917-2010, a significant correlation (r = 0.66, p < 0.99) between PC1L and the regional precipitation is consistent with the fact that the amount of snow accumulated in winter in the upper mountains is the principal driver of interannual variability in Río Atuel discharge. We also compared interannual variations of PC2 loadings with temperature records from San Luis, Santa Rosa and San Rafael stations. For 1960-2010, variations in December minus January temperatures are significantly correlated with PC2L (r = 0.60, p < 0.99). Consequently, the variability of Río Atuel annual cycle can be decomposed into a snow-dependent component in winter (mainly associated with ENSO variability), which determines the above/below normal runoff variability (PC1), and into a thermal-dependent component of spring-summer (related to SAM and quasi-stationary Rossby waves), which determines the seasonal shift of the monthly streamflow peak (PC2).