Under global warming, the continued poleward transport of Atlantic Water drives the ongoing Atlantification of the Arctic Ocean and influences the Subpolar–Arctic ocean dynamical system as well as the evolution of the Atlantic Meridional Overturning Circulation (AMOC). Using a global ocean–sea ice coupled model and sensitivity experiments, with thermodynamic and dynamical diagnostics, we investigate the multiscale impacts of Atlantic inflow on the Subarctic–Arctic ocean dynamical system under climate warming. Results show that the AMOC weakens under climate warming, primarily due to the decline of deep convection in the North Atlantic. Meanwhile, the northward extension of Atlantic Water into the intermediate and deep Arctic Ocean enhances horizontal density gradients and baroclinic instability, thereby promoting the conversion of mean available potential energy into eddy kinetic energy and significantly intensifying mesoscale eddy activity along the Eurasian Basin continental slope. Enhanced mesoscale dynamics and sea ice retreat further modify water-mass transformation and transport pathways in density space, leading to a northward shift of the dynamical core of the overturning circulation. These results demonstrate that Atlantic inflow influences AMOC evolution through multiscale processes involving thermodynamic modification, enhanced mesoscale activity and overturning circulation adjustment, providing insights into future changes in high-latitude ocean circulation and their climatic impacts.