The Atlantic Meridional Overturning Circulation (AMOC) exhibits pronounced variability on decadal timescales. In the subpolar North Atlantic, studies further suggest that the AMOC variability is influenced by water mass transformation with a potential role of the horizontal gyre circulation. Here, we use a high-resolution simulation from the Community Earth System Model (CESM) to investigate the underlying processes with a focus on how changes in the horizontal circulation may have contributed to AMOC variability on decadal time scales. We will analyze water mass transformation to evaluate the role of buoyancy-driven processes and examine its potential interactions with wind-driven gyre adjustments. We will also discuss their relative contributions and the possible dependence on timescales.