Though the Atlantic Meridional Overturning Circulation (AMOC) is an essential element of the global climate system, the source of its variability on timescales longer than those observed remains unsettled. We conduct a broad study of that variability by examining transport on different timescales at different densities within the AMOC lower limb using climate simulations and OSNAP observations in the subpolar North Atlantic. We find that overturning in the Irminger and Iceland basins predominantly drives AMOC variability on monthly to multidecadal scales. Due to the cyclonic circulation of the subpolar basin, the Labrador Sea essentially inherits this variability via advection. Though water mass formation in the Labrador Sea can amplify that variability, the deep waters exiting this basin derive their signature from waters entering the basin. Coherent propagation of transport anomalies is a robust feature presented in OSNAP observations and the climate models studied herein. This study underscores the efficacy of using climate models in conjunction with observations to interrogate AMOC variability.