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The Southern Hemisphere Westerlies (SHW) play a crucial role in large-scale ocean circulation and global carbon cycling. Accordingly, the reconstruction of how the latitudinal position and intensity of the SHW belt changed during the last glacial termination is essential for understanding global climatic fluctuations. The southernmost part of the South American continent is the only continental mass intersecting a large part of the SHW belt. However, due to the scarcity of suitable palaeoclimate archives continuous proxy records back to the last glacial are rare in southern Patagonia. Here, we show an oxygen isotope record from cellulose and purified bulk organic matter of submerged aquatic moss shoots from Laguna Potrok Aike (52 degrees S, 70 degrees W), a deep maar lake located in semi-arid, extra-Andean Patagonia, covering the last glacial-interglacial transition (26 000 to 8500 cal BP). Based on the highly significant correlation between oxygen isotope values of modern aquatic mosses and their host waters and abundant well-preserved moss remains in the sediment record a high-resolution reconstruction of the lake water oxygen isotope (delta O-18(lw-corr)) composition is presented. The reconstructed delta O-18(lw-corr) values for the last glacial are ca. 3 parts per thousand lower than modern values, which can best be explained by generally cooler air temperatures and changes in the moisture source area, together with the occurrence of permafrost leading to a prolonged lake water residence time. Thus, the overall glacial delta O-18(lw-corr) level until 21 000 cal BP is consistent with a scenario of weakened or absent SHW at 52 degrees S compared to the present. During the last deglaciation, reconstructed delta O-18(lw-corr) values reveal a significant two-step rise describing the detailed response of the lake’s hydrological balance to this fundamental climatic shift. Rapid warming is seen as the cause of the first rise of ca. 2 parts per thousand in delta O-18(lw-corr) during the first two millennia of deglaciation (17 600 to 15 600 cal BP) owing to more O-18 enriched precipitation and increasing temperature-induced evaporation. Following this interpretation, an early strengthening of the SHW would not be necessary. The subsequent decrease in delta O-18(lw-corr) by up to 0.7 parts per thousand marks a millennial-scale transition period between 15 600 and 14 600 cal BP interpreted as the transition from a system driven by temperature-induced evaporation to a system more dominated by wind-induced evaporation. The delta O-18(lw-corr) record resumes its pronounced increase around 14 600 cal BP. This further cumulative enrichment in O-18 of lake water could be interpreted as response to strengthened wind-driven evaporation as induced by the intensification and establishment of the SHW at the latitude of Laguna Potrok Aike (52 degrees S) since 14 600 cal BP. delta O-18(lw-corr) approaching modern values around 8500 cal BP reflect that the SHW exerted their full influence on the lake water balance at that time provoking a prevailing more arid steppe climate in the Laguna Potrok Aike region.