Quantifying numerical mixing in a tidally forced global eddy-permitting ocean model

An ensemble of experiments based on a ¼° global NEMO configuration is presented, including tidally forced and non-tidal simulations, and using both the default z* geopotential vertical coordinate and the z∼ filtered Arbitrary Lagrangian-Eulerian coordinate, the latter being known to reduce numerical mixing. This is used to investigate the sensitivity of numerical mixing, and the resulting model drifts and biases, to both tidal forcing and the choice of vertical coordinate. The model is found to simulate an acceptably realistic external tide, and the first-mode internal tide has a spatial distribution consistent with estimates from observations and high-resolution tidal models, with vertical velocities in the internal tide of over 50 metres per day. Tidal forcing with the z* coordinate increases numerical mixing in the upper ocean between 30°S and 30°N where strong internal tides occur, while the z∼ coordinate substantially reduces numerical mixing and biases in tidal simulations to levels below those in the z* non-tidal control. The implications for the next generation of climate models are discussed.