The North Atlantic plays a central role in regulating regional and global climate through heat storage and transport. While variability in ocean heat content (OHC) has been increasingly observed, the mechanisms governing its seasonal-to-interannual changes remain insufficiently constrained due to the lack of coherent, multi-source observational synthesis. Here we analyze OHC changes in the extratropical North Atlantic from 2014 to 2020, combining satellite-derived data, RAPID-MOCHA and OSNAP mooring observations, hydrographic profiles, and atmospheric reanalysis products within a box inverse model framework. The inverse model enforces integral constraints on mass, heat, and freshwater conservation across a multi-product ensemble, with satellite surface geostrophic velocity as an additional constraint, achieving basin-scale heat budget closure between OSNAP and RAPID. Seasonal OHC variability is mainly driven by net air-sea heat fluxes, whereas interannual changes reflect basin-scale imbalances between advective heat convergence and net surface cooling. The advective heat convergence is further decomposed into overturning and gyre components at both bounding sections, revealing their respective roles in driving interannual OHC variability. This observation-based framework provides new constraints on the mechanisms linking North Atlantic heat content changes to large-scale circulation.