Holocene variations in the southern westerly winds at centennial and millennial timescales over the last 10,000 years

Abstract:

We present a paleovegetation and paleofire record from Lago Cipreses (51°S), SW Patagonia, to explore millennial and centennial-scale hydroclimate changes over the last 10,000 years. Local and regional precipitation exhibits a positive correlation with the southern westerly wind (SWW) intensity, supporting rainforests and icefields along the western slopes of the Andes. Recent studies along western Patagonia detected an association between tree-ring-dated paleofires and negative anomalies in precipitation/positive anomalies in temperature associated with positive phases of the Southern Annular Mode (SAM). Hence, hydrologic inferences based on paleovegetation, past lake-level changes and paleofire-regime shifts from southwestern Patagonia constitute a valuable source of information for reconstructing past variations of the SWW and SAM. The Lago Cipreses record shows dominance of Magellanic forests with varying degrees of openness in response to temperature and precipitation changes. Closed-canopy forests dominated between 10-9.3 and 7.5-6 ka, indicating cool/wet phases, alternating with an open forest interval between 9.3-7.5 ka during the warm/dry Early Holocene, and followed by a highly variable phase over the last 6000 years. Within the latter we detect centennial-scale variations in forest cover, paleofires, and lake-level change we interpret as SAM-like variability.

Because precipitation in the study area is positively correlated to the SWW and this relationship extends over a broad swath of the southern mid-latitudes, we can infer the Holocene behavior of the SWW at a zonal scale. We detect a conspicuous warm/dry multi-millennial phase between 9.3-7.5 ka, implying reduced SWW influence at 51°S, followed by stronger SWW influence starting at 7.5 ka and the onset of SAM-like variability at centennial timescales at 6 ka. We note that the onset of SAM-like variability in SW Patagonia was nearly contemporaneous with the onset neoglaciations in Patagonia, the commencement of ENSO-like variability in the tropics and subtropics, and a rising trend in the atmospheric CO₂ concentrations, suggesting that stronger and more variable SWW might drive enhanced degassing of the Southern Ocean and influence the intensity of Hadley and Walker circulation over the Pacific Ocean.