Role of tidally periodic density stratification in the creation of estuarine subtidal circulation

Tidally periodic stratification is explored as a mechanism for estuarine subtidal flow creation through the use of a numerical water column model. Using a two equation turbulence closure, a tidally periodic steady state is established and the subtidal circulation is calculated. The results demonstrate that the subtidal circulation caused by this forcing is of the same vertical structure as the expected estuarine exchange flow. Further, the subtidal exchange is of similar magnitude to that given by typical scalings of the baroclinic circulation driven by the longitudinal salinity gradient, but is strongly dependent upon the timing of the stratification event within the tidal cycle, even reversing sign for certain phases. The dependence of this subtidal flow mechanism on the local depth, the tidal forcing and the magnitude of the stratification is explored through the use of a bulk Richardson number, for which the subtidal circulation is maximal at intermediate values, and is reduced for both large and small bulk Richardson numbers. This dependence of the subtidal circulation on the bulk Richardson number is demonstrated to be consistent with an analysis of the energetics of mixing and buoyancy on the tidal timescale.