A wavelet characterization of the North Atlantic oscillation variation and its relationship to the North Atlantic sea surface temperature

In this study, Multiresolution Fourier Transform (MFT) spectral analysis is employed to resolve the temporal structure of the variation of the North Atlantic Oscillation (NAO) in terms of various frequency components. The NAO index displays fluctuations on multiple timescales, resulting in an MFT spectrum characterized by the occurrences of distinct frequency bands at certain times in the record. These frequency bands are examined within the context of annual to interdecadal spectral components. The relative contribution by each of these spectral components toward the overall NAO variability varies with time. Phase analysis of the spectrum demonstrates that, in addition to some apparent 'random' fluctuations, the phase of each of the spectral components on occasions exhibits relatively constant value over a period of time (from a few years to a decade). In addition, a significant coherent relationship between the NAO and the North Atlantic sea surface temperature (SST) is found at interannual and interdecadal timescales. Some evidence has also been found to suggest that the phase of the interdecadal component of the NAO is itself modulated by the North Atlantic SST at a timescale of about 60 years. Copyright (C) 1999 Royal Meteorological Society.