Reconstruction of the North Atlantic Oscillation, 1429-1983

The North Atlantic Oscillation (NAO) is considered to be the dominant mode of winter atmospheric variability in the Northern Hemisphere (Barnston AG, Livezey RE. 1987. Classification, seasonality and persistence of low frequency atmospheric circulation patterns. Monthly Weather Review 115: 1083 - 1126), especially in the North Atlantic region. A better understanding of its recent variability in the context of pre-instrumental period variations is critical for prediction purposes. A 555-year (1429-1983) multi-proxy reconstruction of the cool season NAO. calibrated against the Lisbon-Iceland (LISJHI) NAO, is presented. Predictor variables include tree-ring chronologies from Morocco and Finland, GISP2 delta O-18 annual series. and a GISP2 snow accumulation record. Although the reconstructed values are generally lower than the instrumental values during the calibration period (1863-1983), the final reconstruction does capture the low frequency of the instrumental NAO. The reconstruction compares favourably with existing shorter NAO reconstructions and with the instrumental NAO. The variability in the reconstructed NAO is also discussed within the context of lengthy regional climate records. Results suggest that the occurrence and length of the recent persistently high phase of the NAO are not unusual over the 555-year period of time. but that the magnitude of some of the instrumental values may, in fact, be unique. Copyright (C) 2001 Royal Meteorological Society.