Influences of Topography on the Modeling of Abyssal Water Masses. Part II: Spurious Extrema

Abstract

Unphysical extrema are found in ocean simulations employing space-centered advection even when horizontal mixing coefficients are sufficient to suppress checkerboard behavior. They are mostly found as isolated spurious temperature minima (density maxima) occurring in close association with topography and in places of concentrated upwelling or overflowing adjacent to the margins of deep basins, where velocities considerably exceed the vertical, or sometimes horizontal, grid Péclet condition. Temperature depressions may be as large as the expected temperature difference between levels and are capable of contaminating upstream water mass properties through horizontal diffusion and convective adjustment. Examples are discussed with reference to the finite difference solution for a stream of variable speed and diffusivity. Increased vertical resolution disproportionately reduces the magnitude but not necessarily the incidence of depressions. Flux correction eliminates the extrema and the contamination, but it can be very diffusive in the deeper layers and cannot overcome problems due to failure to resolve circulations.