A novel ocean mixed layer heat budget analysis methodology is developed and used to investigate the physicalprocesses which determine Subpolar North Atlantic (SPNA) SST and OHC variability on decadal-multidecadaltimescales using the state-of-the-art coupled climate model HadGEM3-GC2. Contributions to OHC and SSTvariability from three sources are evaluated i) the net ocean-atmosphere heat flux, ii) divergence of the oceanichorizontal temperature transport and iii) entrainment between the ocean surface mixed layer and the layer beneath.Anomalies in OHC and SST tendency propagate anticlockwise around the SPNA on multidecadal timescaleswith a clear relationship to the Phase of the AMOC. AMOC anomalies lead SST tendencies which in turn leadOHC tendencies. This result is common to both eastern and western SPNA, although the primary driver of OHCvariability differs with surface fluxes dominant in the west and advection-diffusion in the east. The reciprocal ofthe mixed layer depth is found to be an important variable which causes SST to respond faster to AMOC changesthan OHC. The implications of these new results for interpretation of lower resolution coupled models, includingmany of those used in the CMIP6 assessment are discussed.