A quasi-geostrophic, hemispheric three-level atmospheric modelwith horizontal T21 resolution is driven by Northern Hemisphere's T21topography, whereas its thermal and surface forcingare determined by an automated, iterativetuning procedure.The zonal parts of the forcings are tuned toproduce a realistic zonal wind profile for northern winter, while non-zonalthermal forcings are adjusted to obtain time-averaged non-zonal diabaticheating fields equal to wintertime observations. The perpetual wintermodel simulation reproduces observed wintertime climatology and patterns oflow-frequency variability with accuracy. The model exhibits two significantcirculation regimes which correspond to the positive and negative phase ofthe Arctic Oscillation (AO), respectively.An explanationof the dynamical structures underlying the model's regime behaviour issuggested by the results of a series of model experiments, in which thetuning procedure is repeated using lower values of surface friction.The weaker surface friction is, the moredistant and pronounced the two AO regimes become, indicated by increasinggeopotential standard deviation at polar latitudes and alsoby the AO index distribution, the bimodality of which is becoming more andmore extreme. The regime persistence, but also themodel's sensitivity with respect to forcing changes dramatically increase.Due to this sensitivity, the tuning procedure fails to reproduce the observedzonal climate if the strength of the surface friction is below some criticalvalue. Rather, the model's climate flips between the two extreme AO phasesfrom one tuning iteration to another, but still allows for rare jumpsto the other regime, respectively.Based on these results, the two regimes observed in the control simulationare interpreted as a feature of the attractor's large-scale geometry in phasespace, not necessarily requiring the existence of steady states embedded in theattractor. In the case of very low surface friction, the attractor evidentlyconsists of two lobes connected by a thin channel, a structurequalitatively similar to Lorenz' attractor. The almost-intransitivity of thesystem generates the persistence of the regimes, the irregularity of thetransitions and ultra-low frequency variability. Increase of the surfacefriction makes the two lobes approach each other and strengthens theconnection between them, thus shortening the lifetime of regimes andincreasing the frequency of transitions.