The southern westerly winds influence weather patterns and water resources across the southern high-latitude regions, with important socioeconomic impacts. The strengthening and poleward migration of these winds since the late 20th century also have implications for regional environmental change, including drought, wildfire, and sea-ice loss. However, it is challenging to recognize the natural variability of the westerlies and predict their future behavior, as those recent changes have been influenced by anthropogenic factors. We present a 4200-yr-long record from a southern Patagonian peatland in a location that is sensitive to changes in the position and/or strength of the westerlies. Our δ13C record shows a 6‰ increasing trend from 4200 to 1200 cal. yr B.P., indicating a progressive, millennial-scale increase in peatland moisture. This long-term trend is attributed to an increase in moisture induced by strengthening southern westerly winds associated with a change in the mean state of the El Niño–Southern Oscillation (ENSO) system. Superimposed on this millennial trend, centennial-scale shifts in hydroclimate persist into modern times. We suggest that a “paleo”–Southern Annular Mode, which is linked to tropical Pacific climate, with dry events contemporaneous with positive phases and La Niña–like conditions, is responsible for this enhanced hydroclimate variability. Overall, our results point to millennial- and centennial-scale changes in hydroclimate during the late Holocene that link tropical Pacific climate variability with the Southern Annular Mode and the southern westerlies, with far-reaching implications for future changes in the southern high latitudes, including CO2 ventilation from the Southern Ocean.

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