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Decorrelation between s‐wave and bulk sound velocities in the lowermost mantle and explicit density models based on seismic tomography give evidence for non‐thermal lateral density variations in the lowermost mantle. Here we implement such variations in a numerical model of mantle flow driven by density anomalies which are derived from seismic tomography. In the lowermost ≈300 km, we either use both s‐wave and bulk sound velocities to infer both thermal mantle density anomalies and non‐thermal heterogeneities, or we assume that regions with s‐wave speed anomalies below −1% have an additional non‐thermal density anomaly. We predefine the shape of the viscosity profile in the upper mantle, transition zone and lower mantle, but absolute values of viscosity in the lithosphere, upper mantle, transition zone and lower mantle are free parameters. We use geoid, radial heat flux profile, viscosity …