Spurious numerical mixing under strong tidal forcing: a case study in the South East Asian Seas using the Symphonie model (v3.1.2)

Abstract. The role of mixing between layers of different density is key to how the ocean works and interacts with other components of the Earth System. Accounting correctly for its effect in numerical simulations is therefore of utmost importance. However, numerical models are still plagued with spurious sources of mixing, originating mostly from the vertical advection schemes in the case of fixed coordinates models. As the number of phenomenon explicitly resolved by models increases, so does the amplitude of resolved vertical motions and the amount of spurious numerical mixing; and regional models are no exception to this. This papers provides a clear illustration of this phenomenon in the context of simulation of the South East Asian Seas is provided, along with a simple way to reduce it. This region is known for its particularly strong internal tides and the fundamental role they play in the dynamic of the region. Using the Symphonie ocean model, simulations including and excluding tides and using a pseudo third-order upwind advection scheme on the vertical are compared to several reference datasets, and the impact on water masses in assessed. The high diffusivity of this advection scheme is demonstrated, along with the importance of accounting for tidal mixing for a correct representation of water masses. Simultaneously, an improvement of this advection scheme to make it more suitable for use on the vertical is provided. Simulations with the new formulation are added in the comparison. We conclude that the use of a higher order numerical diffusion operator greatly improves the overall performance of the model.