Comments on the parameterization of barotropic tide-induced bottom friction mixing in ocean circulation models

Results from an eddy-admitting and a non-eddy admitting ocean circulation model with parameterizations for barotropic tide-induced bottom friction mixing are compared to two observation data sets. The use of tide-induced vertical viscosity and diffusivity in addition to other large-scale drivers of ocean mixing processes such as wind and current shear leads to moderate improvements and more realistic upper-ocean temperature structures on larger scales. However, parameterized tide-induced mixing in the eddy-admitting model causes too strong mixing in shelf sea areas such as the British Channel/Irish Sea and the East China/Yellow Seas. The overall result from both models is that care must be taken when tidal mixing parameterizations are applied to ocean circulation models. Each individual use of a tidal mixing parameterization requires careful fine-tuning; and its impact on model skill should be considered together with other relevant parameterizations. We discuss possible reasons for the excessive vertical mixing and propose options which should lead to improved simulations with barotropic tidal mixing parameterizations.