The Energetics and Tidally Induced Reverse Renewal in a Two-Silled Fjord

Abstract

A laterally integrated, two-dimensional numerical model is used to examine the influence of the M2 tide on the circulation in the Saguenay Fjord, a two-silled fjord (with a “large” inner and a “small” outer basin) located on the north shore of the St. Lawrence Estuary. It is found that the M2 tide is more vigorous in the outer than in the inner basin and that more vertical mixing occurs in the outer basin. Therefore, the density at depth in the outer basin decreases faster than it does in the inner basin, and the resulting horizontal pressure gradient causes a bottom flow of water from the inner to the outer basin across the inner sill. This “reverse renewal” is evident in both the available observations and the simulation. According to the model, much of the M2 energy withdrawn from the surface tide is fed into the internal tide. Significant tidal energy is also advected by the mean flow velocity. Approximately 25% of the net energy flux into the fjord is dissipated within 2 km of the outer sill. Because of the baroclinic pressure (i.e., the total pressure but with the influence of the surface displacement removed) the subtidal circulation is associated with very large energy fluxes within the fjord. These fluxes are much larger than the net rate at which tidal energy enters the fjord and they are greater than the subtidal, advective energy flux. They represent a large redistribution of energy within the fjord and their horizontal divergence along the fjord is almost in balance with the time rate of change of potential energy. When a flux Richardson number is calculated for the inner basin below sill depth, a value of 0.075 is obtained, which is close to values found for other fjords.