In this study we have run a number of numerical simulations to examine the ‘Veronis effect’ in an ocean general circulation model. This effect is characterized by anomalous interior downwelling, east of the western boundary current of an ocean basin. The impact of varying the horizontal diffusivity and the use of an isopycnal mixing parameterization are examined. Several diagnostics are used. The net volume transport for a sector in the southern part of the domain, east of the western boundary is found to be the clearest indicator of the strength of the Veronis effect. This effect is found to depend crucially on the horizontal diffusivity. The use of an isopycnal mixing parameterization significantly mitigates the interior downwelling problem but is constrained by the required use of a background horizontal diffusivity. Thus, in a geopotential coordinate model an isopycnal mixing parameterization enables the use of a significantly reduced horizontal diffusivity while emphasizing the mixing along isopycnals.