The processes of semi-enclosed basin–ocean water exchange across a tidal mixing zone

Abstract

To assess each of the two possible mechanisms responsible for the fortnightly modulation of semi-enclosed basin–ocean water exchange (‘density tides’), a set of numerical experiments is carried out using a vertically two-dimensional numerical model with realistic situations in Puget Sound in mind. It is found that, although the localized vertical viscosity (or the localized vertical diffusivity) enhanced in the entrance sill region primarily controls the bottom-water transport (or the bottom-water density) during spring tides, it does not lead to any appreciable variations of bottom-water density (or bottom-water transport). This indicates that the fortnightly modulation of vertical viscosity and that of vertical diffusivity both play important roles in creating density tides. In the real ocean, the vertical viscosity and diffusivity are enhanced simultaneously during spring tides, so that it is difficult to discriminate between both effects on density tides. This causes the widespread misunderstanding that density tides are mainly caused by the decreased advection of dense bottom-water from outside due to the enhanced vertical viscosity during spring tides.