The surface field from the reference SFT simulations at t = 18 months (top) and t = 72 months (bottom) are shown in black. The results of the FTD simulation with a vertical field outer boundary and an anisotropic near-surface diffusivity (250 km² s^-1 in the horizontal directions and 25 km² s^-1 in the vertical direction). The vertical pumping is based on the vertical diffusivity gradient with k = 20 (red), 10 (blue), 5 (green), and 0 (yellow).