Western boundary disturbances detected in the first multicentury integration of the National Center for Atmospheric Research Climate System Model are analyzed, and the role they play in the model interdecadal variability is investigated. The boundary signals propagate southwestward along the coast of North America. At depth they reach the equator and propagate along the equator but with decreasing amplitude away from the western boundary. Their southwestward propagation is associated with the evolution of the western boundary current system (the Gulf Stream system in the upper part of the water column and the Deep Western Boundary Current at depth) from weaker than average to stronger than average conditions. Thus the boundary disturbances seem to play an important role in the model thermohaline circulation variability at interdecadal timescales. They show similarities with the boundary “waves” found in ocean‐only models exhibiting interdecadal variability, and their role in the model's response to changes in the rate of sinking appears to be in agreement with the theory of the spin‐up of the deep ocean circulation. The main result of this study is that the similarities with ocean‐only results and theories support the idea that interdecadal timescales of variability originate from the ocean