Quantifying Effective Radiative Forcing due to aerosol-cloud interactions (ERF_ACI) remains a largely uncertain process, and the magnitude remains unconstrained in general circulation models. Previous studies focus on the magnitude of ERF_ACI arising from all cloud types, or examine it in the framework of dynamical regimes. Aerosol forcing due to aerosol-cloud interactions in the HadGEM3-GA7.1 global climate model is decomposed into several global observational cloud regimes. Regimes are allocated to model gridboxes and forcing due to aerosol-cloud interactions is calculated on a regime-by-regime basis with a 20-year meaning period. Patterns of regime occurrence are in good agreement with satellite observations. ERF_ACI is then further decomposed into three terms, representing radiative changes within a given regime, transitions between different cloud regimes, and nonlinear effects. The total global mean ERF_ACI is -1.8 Wm^-2. When decomposed, simulated ERF_ACI is greatest in the stratocumulus regime (-0.75 Wm^-2).