Interdecadal climatic variations in millennial temperature reconstructions from southern South America

Abstract

Two millennial reconstructions of mean summer temperature departures from the eastern and western slopes of the southern Andes (1120 and 3620 years, respectively) have been spectrally analyzed to determine the most significant modes of climate variations in southern South America during the last millennia. Blackman-Tukey and Maximum Entropy spectral analyses of these reconstructions identify temperature oscillations with periods of 35.8, 23.9, 9.5, 6.0–6.9, 5.4–5.7, 4.5–4.8, and 3.4–3.7 years. Because of the relatively flexible standardization used to derive the temperature reconstructions, a large part of the variance (related to the oscillations of summer temperature) is concentrated in short-term periods. A new set of eight long tree-ring chronologies from Fitzroya cupressoides (ranging from 1120 to 3620 years in length), has been used to develop a regional record of tree-ring variations during recent millennia in the southern Andes. A more conservative detrending than that used for developing the temperature reconstructions has been employed here to preserve the low frequency components in the tree-ring series. Maximum Entropy spectral analyses for different filter orders indicate prominent peaks at around 250, 77, 50, 33, 21, 17, 15, and 11 years. Some power is also seen at 4–7 years, probably associated with El Niño-Southern Oscillation events. The 11, 21 and 77-year terms may be related to long-term solar variations or long-term circulation of the Southern Oceans. Ocean-atmosphere model predictions suggest the possibility of using the South American tree-ring records to reconstruct past changes of the thermohaline circulation in the Atlantic Ocean.