Decadal ocean heat content (OHC) variability modulates the global climate system. However, the response of decadal OHC variability in the North Atlantic to global warming, and the underlying mechanisms, remain poorly understood. Using observational reanalysis and CMIP6 model results, we find a robust reduction in the  decadal OHC variability during the future SSP585 period compared to the historical period in the North Atlantic and particularly at higher latitudes. A variance decomposition shows that ocean’s meridional heat transport (MHT) convergence/divergence dominates the OHC variance consistently during the historical period (68%) and SSP585 (59%), and accounts for most (97%) of the variance reduction in the decadal OHC variability between the two periods. The changes in the MHT convergence/divergence are attributed to a reduction in the MHT strength and increasing synchronization of the MHT meridional coherence. Our findings elucidate the intricate coupling between overturning circulation and temperature in governing decadal OHC variability under climate change.