North Atlantic Subpolar Mode Water (SPMW) is an important reservoir of heat, carbon, and other tracers, and plays a key role in ocean–atmosphere interaction and climate regulation. It also serves as a precursor water mass for the formation of North Atlantic Deep Water (NADW), thereby influencing the deep limb of the Atlantic Meridional Overturning Circulation (AMOC). In this study, we investigate the decadal variability of North Atlantic SPMW and its driving mechanisms using a 220-year monthly CESM pre-industrial control (pi-control) simulation. SPMW is identified based on density and spiciness. Spectral analysis shows that SPMW volume exhibits pronounced low-frequency variability, with a dominant period of approximately 16–20 years. To investigate the processes responsible for this variability, we construct a volume budget framework bounded by density, spiciness, and the mixed layer, and compare SPMW volume changes with cumulative subduction. The results show that cumulative subduction is strongly correlated with SPMW volume changes (r = 0.82), indicating that subduction is a major driver of SPMW volume variability. We further decompose the kinematic subduction rate into vertical pumping, entrainment, and lateral induction. The entrainment term accounts for the largest fraction of the decadal variability in total subduction (85%), followed by lateral induction (48%), whereas vertical pumping contributes relatively little (−23%). These results suggest that mixed-layer depth variability is the primary process controlling the decadal variability of SPMW formation.