Decadal to interdecadal variability of the North Atlantic oscillation: a paleoclimatic index reconstruction
The study aims to extend the wintertime index of the North Atlantic Oscillation (NAO) using paleoclimatic data. The primary interest lies in the decadal and interdecadal variability. Isotopic concentrations and accumulation rates measured in ice cores, and tree-ring widths chronologies form the proxy basis for the NAO index reconstruction. This multiproxy data set includes proxies from Greenland, Europe, North Africa and North America. The statistical treatment of the reconstruction problem is discussed in detail. Two different reconstruction strategies are presented for the decadal to interdecadal time scales: the pre- and post-filtering concepts. The decadal (interdecadal) variability is reconstructed over the period 1700-1978 (1623-1966). The comparison with two independent reconstructions of other research groups hint at an anomalous pattern of atmospheric variability in the North Atlantic sector in the early 18th century. The magnitude of the variance was probably increased by solar or volcanic forcing during this period. On interdecadal time scales (32-64 years), an increased variability was reconstructed over the 17th and early 18th century. If further indicators supported these findings, then the recent trend in the NAO index, from negative values in the 1960-70s to prolonged positive values since the 1980s, would be in the range of natural variability.