The Subpolar North Atlantic (SPNA) plays a key role in global ocean heat content (OHC) due to its profound heat storage capacity. While the SPNA OHC is projected to undergo significant changes under global warming, how its decadal variability will respond and the underlying physical mechanisms remain poorly understood. This study is based on high-resolution Earth System Model simulations (CESM-HR), including the pre-industrial control experiments (PI-CTRL), historical experiments (Historical), and high-emission Representative Concentration Pathway 8.5 simulations (RCP8.5), and employs a heave–spice decomposition method to systematically investigate the characteristics and physical mechanisms of OHC decadal variability in this region. The results show that under the future high-emission scenario, the decadal variability of SPNA OHC is significantly weakened, with both Heave and Spice components exhibiting a basin-wide decline. Furthermore, variance decomposition reveals a clear regime shift in the SPNA, transitioning from a historical state of comparable Heave and Spice contributions to a high-emission state dominated by the Spice component. These findings highlight the modulation of internal OHC variabilities by external forcing, providing key physical insights to improve decadal predictability of North Atlantic climate variability.