Improving Tidal Mixing Parameterizations

Tidal motions over rough topography can lead to turbulence and mixing in the ocean. Simmons et al. (2004) have pioneered the development, implementation and testing of a parameterization of this process for global ocean models. Here we present two improvements to the Simmons et al. scheme by considering (1) sub-grid scale bathymetry and (2) separately 4 tidal constituents: the two main semi-diurnal tides M2 and S2 as well as the diurnal K1 and O1 tides, including their variability due to the 18.6 year lunar nodal cycle (LNC). We show that including sub-grid scale bathymetry is important for coarse resolution models that do not resolve topographic features such as narrow island chains. In these models mixing occurs too deep in the water column with underestimated effects on the circulation. The sub-grid scale parameterization is tested in the UVic model and is shown to lead to more mixing at shallower depth and increased flow of North Atlantic Deep Water and Antarctic Bottom Water. Including the LNC leads to oscillations of SSTs of ~0.3°C in the North Pacific regions of strong diurnal tides (Sea of Okhotsk, Kuril and Aleutian Islands). We speculate that these oscillations may be important in modulating decadal climate variability in the North Pacific. REFERENCES Simmons, H. L., S. R. Jayne, L. C. St Laurent, and A. J. Weaver (2004), Tidally driven mixing in a numerical model of the ocean general circulation, Ocean Model, 6(3-4), 245-263.