Decadal trends have been noted in the leading mode of boreal winter variability. Given that this mode is thought to be an internal mode of the atmosphere it remains unclear as to what is responsible for interannual to interdecadal oscillations of this mode. We demonstrate that continental-scale snow cover varies at the same multi-year time periods as the atmosphere but leads the atmosphere by several months through their mutual oscillations. Therefore we propose snow cover as a potential contributor to the interannual variability of the leading boreal winter mode of the atmosphere.