The role of ocean dynamics for low-frequency fluctuations of the NAO in a coupled ocean-atmosphere GCM

Variability at all timescales, including low-frequency variability, is foun d in the North Atlantic sector in a 300-yr control integration of the coupl ed ocean-atmosphere general circulation model (CGCM) ECHAM4/OPYC3. The atmo spheric variability is dominated by the North Atlantic Oscillation (NAO). O nly rather weak spectral peaks are superimposed on the "white noise" power spectrum of sea level pressure and on the essentially "red noise" spectrum of SST in highly active regions. Replacing the full ocean model with a 50-m fixed-depth mixed layer ocean (MLO) and coupling it to the atmosphere yiel ds qualitatively and quantitatively very similar power spectra of the NAO i ndex. Net surface heat fluxes, describing the coupling between the ocean and the atmosphere For the long-term variations (>10 yr) of the NAO are much weaker in the MLO model, but show general agreement in both simulations regarding spatial distributions. This spatial agreement with respect to NAO variabil ity occurs even though the associated SST anomaly pattern in the CGCM is sh ifted northward by about 10 degrees relative to its position in the run wit hout the dynamical ocean. This fact is mainly attributed to advection in th e full ocean model. There is evidence for the existence of ocean-cryosphere-atmosphere coupling in the CGCM. From the fact that we found only weak spectral peaks it appea rs that the role of a fully coupled ocean with respect to long-term NAO var iability is limited to a shift in SST variability and to a moderate increas e of the atmosphere's long-term variability over most parr of the domain. I n view of the subordinate relevance of ocean-atmosphere coupling for the NA O it is suggested that the CGCM presented in this study mainly follows the stochastic climate model concept, that is, the ocean integrates over the ch aotic forcing imposed by the atmosphere, leaving the NAO rather unpredictab le on decadal and longer timescales.