Variability in the annual cycle of the Rio Atuel streamflows and its relationship with tropospheric circulation

摘要

Climate-induced changes in the annual regime of snow-fed rivers have serious implications for water resource management. In the Central Andes (CA, 28 degrees-36 degrees S) of Argentina-Chile, the snow accumulated in high-elevation mountains in winter is the dominant component of streamflows during the spring-summer melting season. Although topography introduces complexity in snowpack responses to the annual temperature cycle, streamflow series over a century in length make the CA particularly suitable for identification of long-term hydrological changes. Principal component (PC) analysis of Rio Atuel annual hydrographs from 1906 to 2012 discriminates between precipitation- and temperature-related components associated with variations in snow accumulation (49% of variance) and advances/delays of the streamflow annual peak (21% of variance), respectively. The temporal evolution of PC1 loadings reveals a predominant negative period from 1917 to 1976 and from 1988 to present, suggesting the propensity to undergo long periods with reduced flows. In turn, the PC2 pattern is predominantly positive from 1948 to the present, revealing a tendency to more frequent peak flows in late spring since the mid-20th century. Above-average streamflows related to abundant snowfalls in the CA are associated with northward shifts in stormtracks that are remotely induced by above-average sea surface temperatures in the equatorial Pacific. On the other hand, earlier streamflow peaks in November-December are concurrent with above-average temperatures across the Atuel basin induced by enhanced meridional circulation from the Tropics due to the strengthening of the South Atlantic anticyclone. These circulation anomalies are linked to the persistent positive phase of the Southern Annular Mode during the last decades. Additionally, years with reduced streamflows in January, and proportionally larger flow contributions in November-December, are associated with anomalous air cooling at high levels induced by low pressure centres over the region as part of a quasi-zonal stationary Rossby wave train that extends from Australia to the South American-South Atlantic sector.