Period 7.8 Years Oscillatory Modes in the Monthly NAO Index and Temperature Records and Their Possible Phase Synchronization

Recently, Gamiz-Fortis et al. (J. Geophys. Res. 107(D23) 4685, 2002) have detected, using the Monte Carlo Singular System Analysis (MCSSA), an oscillatory mode with the period around 7.7 years in the winter North Atlantic Oscillation (NAO) index. Palus and Novotna (Phys. Lett. A 248 191-202, 1998) proposed an enhancement of MCSSA in which, in addition to the signal covariance structure, a complexity measure of the SSA modes is tested against surrogate data; and were able to detect a period 7.8 years oscillatory mode in long-term monthly near-surface air temperature records from several European locations. We show that using the enhanced MCSSA the period 7.8 yr oscillatory mode can be detected in the monthly NAO index, too. These modes are extracted from the raw data (expressed in the SSA/EOF basis), their instantaneous phases are computed and their relations are studied. In particular, a possibility of phase synchronization between the NAO and the temperature oscillatory modes is tested for.