Effect of numerical diffusion on the water mass transformation in eddy-resolving models

• We investigate an effect of numerical diffusion on water mass transformation.
• We investigate a dependency of a numerical diffusion effect on explicit diffusivity.
• Numerical diffusion seems to be inevitable for current eddy-resolving models.
• We must adequately evaluate an effect of numerical diffusion.

This study investigates the effect of numerical diffusion associated with advection schemes on water mass transformation in an eddy-resolving model. The effect of numerical diffusion is evaluated as a residual between the total water mass transformation and the explicit water mass transformation: the former is calculated as the sum of meridional streamfunction and the temporal change rate of an isopycnal surface depth, and the latter is directly calculated with the use of the tendency equation of density. This method is used for investigating a dependency of numerical diffusion on explicit diffusivity. It is found that idealized channel experiments are categorized into three regimes according to a magnitude of explicit diffusivity: numerical diffusion, transitional, and explicit diffusion regimes. The numerical diffusion regime is defined as the regime where explicit diffusion changes do not significantly impact the solution. The magnitude of numerical diffusion is independent of the explicit diffusivity there. In the transitional regime, explicit (numerical) diffusion works more (less) with higher explicit diffusivity. Explicit and numerical diffusions are comparably important there. The explicit diffusion becomes significantly large and the numerical diffusion is almost negligible in the explicit diffusion regime. The total diffusion effect on water mass transformation there is considerably larger than those in the two other regimes.Two experiments are conducted with a Southern Ocean model under a realistic configuration. These belong to the numerical diffusion and transitional regimes. The model becomes a little too diffusive in the latter experiment. This result and results of channel experiments indicate that it is not an adequate option for a realistic Southern Ocean simulation that we adopt a diffusion coefficient in the explicit diffusion regime in order to reduce levels of numerical diffusion. It indicates that numerical diffusion is inevitable for eddy-resolving models with horizontal resolution around 0.1° and we must adequately evaluate its effect on model results when analyzing outputs of such high resolution models. The method proposed in the current study for assessing numerical diffusion will be useful for investigating an eddying ocean with numerical models.