Mixing and stratification in a tropical tidal embayment subject to a distributed freshwater source

Abstract

The Bay of Banten provides an example of a semicircular shallow tidal embayment in a tropical ROFI (Region Of Freshwater Influence), subject to a mixed,mainly diurnal tidal regimeand amonsoon-driven residual circulation. A partially inactivated delta shapes the east coast of the bay and constitutes a distributed source of freshwater during the wet season. Measurements of flow velocity, salinity and temperature were taken at two contrasting sites in the bay, to investigate stratification dynamics throughout a spring–neap cycle. Bulk Richardson numbers are shownto vary both at tidal and subtidal frequencies, and indicate that thewater column occasionally remains stratified all through a tidal cycle. About half of the variance of the subtidal flow can be explained from wind forcing. The wind-driven flowvaries between 1 and 5% of the wind speed,with a pronounced difference between the twolocations under study. Stratification was further analyzed by estimating the contributions of advection by the depth–mean flow, tidal straining, vertical advection and heating, to variation in potential energy anomaly (PEA). To do so, horizontal gradients in density and PEAwere estimated frommulti-linear regression analysis. The obtained results show that spatial gradients of density and PEA can reverse sign frequently in the absence of strong mixing, associated to pulses of freshwater discharge. The freshwater discharge, in turn, is distributed over small streams. The contribution of vertical mixing was found to be relatively small, and none of the remaining terms were clearly dominant. Estimates of the boundary layer thickness of clockwise and anticlockwise rotary components of the tidal flow show the impact of baroclinic processes on velocity profiles.