How momentum advection schemes influence current-topography interactions at eddy permitting resolution
Résumé
Recent studies have shown that the use of an enstrophy-and-energy-conserving momentum advection scheme substantially reduces widespread biases of mean currents in the global 1/4° DRAKKAR model. This paper investigates the origin of these improvements. A series of sensitivity simulations with different momentum advection schemes is performed with the North Atlantic 1/4° DRAKKAR model. Three second order momentum advection schemes conserving, respectively, enstrophy (ens), energy (efx) and both quantities (een) are tested and their impact on the model solution are compared. The mean kinetic energy vertical profile is found to change up to 10% depending on the chosen scheme. This sensitivity is maximum in bottom layers. The analysis of the vorticity tendency due to horizontal momentum advection reveals that the three schemes differ mostly in bottom layers as well. The average magnitude of this term is enhanced with the efx scheme and reduced with the een scheme. These differences are found to be consistent with the instantaneous tendency of each scheme. In addition, we show that the differences between the schemes are related to the grid-scale irregularity of the velocity field. Both the grid scale irregularity and the differences between the schemes are found to be enhanced in bottom layers. We conclude that the model solution depends crucially on the ability of the momentum advection scheme to handle under-resolved flows close to the bottom topography. This work emphasizes the critical influence of topography in eddy-active regions on mean circulation features such as the position of the North-Atlantic current or the Gulf Stream separation.