A NUMERICAL STUDY OF THE FORTNIGHTLY MODULATION OF BASIN-OCEAN WATER EXCHANGE ACROSS A TIDAL MIXING ZONE

The authors examine the effect of fortnightly modulated tidal mixing o ver a sill on a bidirectional estuarine shear Row through it. The stud y proceeds numerically by using two sets of vertically two-dimensional nonhydrostatic numerical models. In the first numerical model, spatia l and temporal resolutions are high enough to reproduce internal wave generation by strong tide-sill interactions. After higher frequency fl uctuations such as semidiurnal tides are filtered out, the density fie ld demonstrates that basic stratification is significantly disturbed o ver the sill during spring tides producing horizontal density gradient s. This causes the secondary Rows consisting of the convergent horizon tal currents near the surface and bottom and divergent horizontal curr ents near the thermocline, accompanied by upwelling and downwelling in the lower and upper layers within the mixing zone, respectively. Exce pt in the proximity of the sill, these secondary Rows are successfully reproduced in the second numerical model where tidal mixing over the sill is modeled simply by assuming fortnightly varying coefficient of vertical eddy diffusivity that reaches of the order of 0.01 m(2) s(-1) at spring tides. The secondary flows thus induced are superposed on t he background exchange flow such that, at spring tides, seaward flow a t the surface is significantly reduced on the seaward side of the mixi ng zone and a considerable fraction of the bottom water flowing landwa rd into the mixing zone refluxes back seaward near the thermocline, As neap tide is approached and hence tidal mixing intensity decreases, t he density field returns to the undisturbed state so that the bottom i nflow from outside and surface outflow from inside continue landward a nd seaward, respectively. This model reproduces observations of deep-w ater replacement in coastal basins showing that basin-ocean exchange p rocesses are developed mainly during periods of weak tidal currents.