Improving Global Barotropic Tides With Sub-Grid Scale Topography

In recent years, efforts have been made to include tides in both operational ocean models as well as climate and earth system models. The accuracy of the barotropic tides is often limited by the model topography, which is in turn limited by model horizontal resolution. In this work, we explore the reduction of barotropic tidal errors in an ocean general circulation model (Modular Ocean Model version 6; MOM6) using sub-grid scale topography representation. We follow the methodology from Adcroft (2013, ), which utilizes statistics from finer resolution topographic data sets to represent sub-grid scale features with a light computational cost in a structured finite volume formulation. The geometric effect from sub-grid scale topography can be introduced to the model with only a few parameters at each grid cell. The porous barriers, which are implemented at the walls of the grid cells, are used to modify transport between grid cells. Our results show that the globally averaged tidal error in lower-resolution simulations is significantly reduced with the use of porous barriers. We argue this method is a potentially useful tool to improve simulations of tides (and other flows) in low-resolution simulations. In recent years, significant attention has been given to simulating tides in diverse ocean and climate models, because of the roles of tides in impacting sea levels. In order to better predict future sea level changes, it is therefore essential to improve the accuracy of surface tides simulated in numerical ocean models. One known source of errors that could impact the accuracy of modeled tides stems from the accuracy of ocean topography in the models, which relies on how many grid boxes (i.e., model resolution) are used to represent the complicated seafloor elevations. The model's resolution, however, is often constrained by the efficiency of computations. Our objective in this study is to enhance the portrayal of ocean topography at a given resolution to minimize tidal errors. To achieve this, we employ a technique known as "porous barriers," which uses approximations to mimic the effects of ocean topography that would be omitted in the models, without imposing significant computational burdens. Our findings demonstrate that this technique of porous barriers can substantially reduce tidal errors in our ocean model. We also expect a wider application of this technique to numerous other physical processes within the ocean. The accuracy of tides in ocean models is affected by the representation of topography, which is constrained by model horizontal resolution The geometric effect of the sub-grid scale topographic features can be approximated in the model with the porous barriers The porous barriers can significantly reduce globally averaged errors for barotropic tides