Abstract:
A method is presented to reconstruct decadal variations of the North Atlantic
Oscillation (NAO). The spectral characteristics of the NAO on time scales of
decades and longer are of particular interest for the understanding of North
Atlantic ocean's atmosphere interactions. The reconstruction is based on a
transfer model calibration that uses bandpass-filtered time series. The maximum
overlap
... discrete wavelet transform (MODWT) is applied for decomposing the time
series variance into different time scales. A total of 43 proxies, including
Greenland ice cores and European tree-ring chronologies, are selected and
regionally grouped providing four independent reconstructions for the period
1700-1978. The mean reconstruction agrees well with two recently published
reconstructions during most of the time period. However, there are considerable
differences in the earliest part before 1750. Running correlations between the
reconstructions indicate that time-dependent relations exist among the different
NAO reconstructions. The results suggest that the geographical distribution of
proxies strongly affects the reconstruction and could explain some of the apparent
discrepancies among the reconstructions recently published in literature. In the
early eighteenth century, external forcing (solar, volcanic) seems to mask the
NAO signature within the proxies. 
