A Numerical Study of the Fortnightly Modulation of Basin–Ocean Water Exchange across a Tidal Mixing Zone

Abstract

The authors examine the effect of fortnightly modulated tidal mixing over a sill on a bidirectional estuarine shear flow through it. The study proceeds numerically by using two sets of vertically two-dimensional nonhydrostatic numerical models. In the first numerical model, spatial and temporal resolutions are high enough to reproduce internal wave generation by strong tide?sill interactions. After higher frequency fluctuations such as semidiurnal tides are filtered out, the density field demonstrates that basic stratification is significantly disturbed over the sill during spring tides producing horizontal density gradients. This causes the secondary flows consisting of the convergent horizontal currents near the surface and bottom and divergent horizontal currents near the thermocline, accompanied by upwelling and downwelling in the lower and upper layers within the mixing zone, respectively. Except in the proximity of the sill, these secondary flows 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 m2 s?1 at spring tides. The secondary flows thus induced are superposed on the background exchange flow such that, at spring tides, seaward flow at the surface is significantly reduced on the seaward side of the mixing zone and a considerable fraction of the bottom water flowing landward into the mixing zone refluxes back seaward near the thermocline. As neap tide is approached and hence tidal mixing intensity decreases, the density field returns to the undisturbed state so that the bottom inflow from outside and surface outflow from inside continue landward and seaward, respectively. This model reproduces observations of deep-water replacement in coastal basins showing that basin?ocean exchange processes are developed mainly during periods of weak tidal currents.