Coarse grid Ocean General Circulation Models (OGCMs) take into account the effect of unresolved mesoscale eddies by means of a rotated mixing operator which diffuses tracers (such as temperature or salinity) along surfaces of constant potential density called isopycnals. In spite of its profitable physical aspects, the discrete version of the isopycnal mixing parameterization can produce oscillations in the tracer fields, which disagrees with the well-known properties of diffusion operators. The causes of this non-monotonic behaviour of a diffusion operator are highlighted. The location and magnitude of these over-/under-shootings are examined in the results of an OGCM.
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