In this study we determined characteristic temporal modes of atmospheric variability at thedecadal and interdecadal timescales. This was done on the basis of 1000 year long integrationsof a global coupled atmosphere-ocean climate model of moderate complexity including thetroposphere, stratosphere, and mesosphere. The applied model resolves explicitely the basicfeatures of the large-scale long-term atmospheric and oceanic variables. The synoptic-scaleprocesses are described in terms of autocorrelation and crosscorrelation functions. The paperincludes an extended description and validation of the model as well as the results of analysesof two 1000 year long model integrations. One model run has been performed with the fullycoupled model of the atmosphere-ocean system. The performed time-frequency analyses ofatmospheric fields reveal strong decadal and interdecadal modes with periods of about 9, 18,and 30 years. To quantify the influence of the ocean on atmospheric variations an additionalrun with seasonally varying prescribed sea surface temperatures has been carried out, whichis characterized by strong decadal modes with periods of about 9 years. The comparison of bothruns suggests that decadal variability can be understood as an inherent atmospheric mode dueto the nonlinear dynamics of the large-scale atmospheric circulation patterns whereasinterdecadal climate variability has to be regarded as coupled atmosphere-ocean modes.