North Atlantic Oscillation: Diagnosis and simulation of decadal variability and its long-period evolution

Two 1000-year numerical experiments based on the IFA RAN global climate mod el, the first with completely interacting atmosphere and ocean and the seco nd with a fixed climatic mean annual cycle of sea surface temperature, are analyzed. In both cases, a quasi-decadal cyclicity (QDC), but with substant ially different amplitude-frequency characteristics, is detected for the No rth Atlantic Oscillation (NAO) in winter. Significant changes in the QDC re gimes from one century to another are observed in the model. A comparison o f the numerical results with empirical data and reconstructions reveal a fa irly good agreement of the QDC amplitude and periods for winter NAO regimes in the model with completely interacting atmosphere and ocean for individu al model subperiods on the order of a century. The model results suggest th at interdecadal NAO variations of natural origin can be noticeably strength ened in the climate system without any influence of external, in particular , anthropogenic factors. In the case of a fixed annual cycle of SST, the QD C amplitudes are underestimated several times by the model, and no positive correlation is observed between the amplitudes and periods of the NAO QDC in contrast to the empirical data, reconstructions, and the model with comp letely interacting atmosphere and ocean.