The North Atlantic subpolar gyre is a critical region for global climate, yet the mechanisms driving its multidecadal circulation variability under anthropogenic forcing remain poorly understood. This study investigates the physical processes underlying large multidecadal shifts in density overturning strength at 55°N using a 100-member ensemble of the Community Earth System Model version 2 (CESM2). Using change point and composite analyses, we identify three distinct categories of circulation shifts: strengthening events, and two types of weakening events separated by a 1985 regime shift. Strengthening shifts are driven by an internal positive feedback where enhanced surface heat loss triggers deep convection and strengthened horizontal gyre circulation, subsequently increasing northward heat and salt transport that sustains the anomaly. Weakening shifts before 1985 are primarily driven by internal density-gradient adjustments between the subpolar and subtropical gyres. In contrast, post-1985 weakening events are characterized by basin-wide thermodynamic changes, where greenhouse gas-induced warming and reduced surface buoyancy loss suppress convection across the Irminger Sea and eastern subpolar North Atlantic. Our results reveal that these shifts are non-linear and asymmetric, reflecting a transition from a salinity-dominated internal variability regime to a forced, temperature-driven regime. These findings suggest that the North Atlantic circulation is undergoing a fundamental change in its governing dynamics, highlighting the increasing influence of anthropogenic forcing on the stability of the Atlantic Meridional Overturning Circulation.