Volcanically Forced Cooling over the last 600 years in Northern Patagonia

Abstract. We present the first tree-ring (TR) based summer surface temperature reconstruction (1382–2017) for the Southern Hemisphere that expresses strong volcanically forced cooling. The Northern Patagonia (NPAT) reconstruction is based on ring-width (RW) and Blue Intensity (BI) parameters measured from Araucaria araucana trees sampled from 6 locations across the middle to southern end of the species' range in Argentina. This multi-TR-parameter reconstruction explains 53 % of the summer surface temperature variance (1903–2017), which is on par with the fidelity of density or mixed TR parameter-based reconstructions from the Northern Hemisphere. The reconstruction coheres strongly with surface mean air temperatures for a large region in South America, including sea surface temperatures well into the southeastern Pacific. The warmest 10-year period is 2008–2017, corresponding to the last ten years of the reconstruction, while the coldest period is 1455–1464. The coldest reconstructed austral summer is 1971, followed by 1460. For both the NPAT reconstruction and a range of model simulations, superposed epoch analysis, using significant tropical eruptions since the 1400s, indicates a significant post-eruption T+1 mean surface cooling of ca. 0.4–1.0 °C, depending on which volcanic events are used. The degree of relative cooling is on par, or even stronger, with the cooling represented by individual TR records used in the Northern Hemisphere N-TREND reconstruction suggesting that the volcanic response in northern Patagonia over the last 6 centuries is equivalent, or even more extreme, to what is observed in many Northern Hemisphere locations. Our results indicate that the use of ring-density parameters is of paramount importance for assessing past volcanically forced cooling in the Southern Hemisphere, but the dating and seasonality of the eruptions as well as the continentality and mid-latitude location of the woodland sites may also be important factors for capturing the signal of volcanic cooling.