The variability in the number of major sudden stratospheric warmings (SSWs) is analyzed in a multi-century simulation under constant forcing using a stratosphere resolving atmosphere-ocean general circulation model. A wavelet-analysis of the SSW time series identifies significantly enhanced power at a period of 52 years. The coherency of this signal with tropospheric and oceanic parameters is investigated. The strongest coherence is found with the North Atlantic ocean-atmosphere heat-flux from November to January. Here, an enhanced heat-flux from the ocean into the atmosphere is related to an increase in the number of SSWs. Furthermore, a correlation is found with Eurasian snow cover in October and the number of blockings in October/November. These results suggest that the multi-decadal variability is generated within the ocean-troposphere-stratosphere system. A two-way interaction of the North Atlantic and the atmosphere buffers and amplifies stratospheric anomalies, leading to a coupled multi-decadal mode.