How momentum advection schemes influence current-topography interactionsat eddy permitting resolution

摘要

Recent studies have shown that the use of an enstrophy-and-energy-conserving momentum advectionscheme 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 differ-ent momentum advection schemes is performed with the North Atlantic 1/4° DRAKKAR model. Three sec-ond order momentum advection schemes conserving, respectively, enstrophy (ens), energy (efx) and bothquantities (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 horizontalmomentum advection reveals that the three schemes differ mostly in bottom layers as well. The averagemagnitude of this term is enhanced with the efx scheme and reduced with the een scheme. These differ-ences 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 irregularityof the velocity field. Both the grid scale irregularity and the differences between the schemes are found tobe enhanced in bottom layers. We conclude that the model solution depends crucially on the ability ofthe momentum advection scheme to handle under-resolved flows close to the bottom topography. Thiswork emphasizes the critical influence of topography in eddy-active regions on mean circulation featuressuch as the position of the North-Atlantic current or the Gulf Stream separation.