Climate variability and change in western Patagonia

Rene Garreaud, Universidad de Chile, Santiago, Chile

Weather systems embedded in the SH westerly winds belt arrive year round to western Patagonia causing copious rain in the coast that further increases over the Andes. Such hyper-humid regime is disrupted by the El Niño Southern Oscillation (ENSO) and the Southern Annular mode (SAM). Both of these modes can produce positive/negative pressure anomalies over the austral Pacific thus weakening/strengthening the westerly flow at midlatitudes and hence decreasing/increasing precipitation over western Patagonia. ENSO is a natural mode of variability and alternate between its positive (El Niño) and negative (La Niña) phases. So does SAM, fluctuating at monthly time scale between its positive and negative phases, but it also exhibits a significant trend towards its positive polarity during austral summer/fall over the last 3-4 decades. This trend has been attributed to stratospheric ozone (O3) depletion and the increase in greenhouse gas (GHG) atmospheric concentrations mediated by changes in the intensity of the upper-level polar vortex. The SAM tendency towards its positive polarity seems to drive a drying trend since the mid 1970’s across Patagonia, which stands out against paleorecords thus emphasizing the role of anthropogenic forcing. The superposition of natural variability upon the drying trend results in extremely dry conditions like the ones observed in recent years thus leading to severe environmental disruption over Patagonia. The ongoing recovery of the stratospheric O3 will be offset by the continuous rise of GHG so that climate models consistently predict a continuation of the drying trend during the rest of the 21st century.