Recent studies, using data from the OSNAP observational campaign and from numerical ocean models, suggest that surface buoyancy losses over the Iceland Basin and the Irminger Sea may, in contradiction to the established consensus, be more significant than those over the Labrador Sea, and that these former regions are in fact the dominant sites for formation of upper North Atlantic Deep Water), with the Labrador Sea acting mainly as a region of further densification as the dense waters flow around the gyre. Here we present a set of hindcast integrations of a global 1/4° NEMO ocean configuration from 1958 until nearly the present day, forced with three standard surface forcing datasets. We use the surface-forced streamfunction, estimated from surface buoyancy fluxes, along with the overturning streamfunction, similarly defined in potential density space, to investigate the causal link between surface forcing and decadal variability in the strength of the Atlantic meridional overturning circulation (AMOC). A scalar metric based on the surface forced streamfunction, evaluated in critical density and latitude classes, and accumulated in time, is found to be a good predictor of changes in the overturning strength, and the surface heat loss from the Irminger Sea is confirmed to be the dominant mechanism for decadal AMOC variability. We use the streamfunctions to demonstrate that the watermasses in the simulations are transformed to higher densities as they propagate around the subpolar gyre from their formation locations in the north-east Atlantic and the Irminger Sea, consistent with the picture emerging from observations.