Tidal circulation and buoyancy effects in the St. Lawrence Estuary

The action of tides on density-driven circulation, internal gravity waves, and mixing was investigated in the St. Lawrence Estuary between Rimouski an d Quebec City. Time-varying fields of water level, currents and density wer e computed under typical summer conditions using a three-dimensional hydros tatic coastal ocean model that incorporates a second order turbulence closu re submodel. These results are compared with current meter records and othe r observations The model and the observations reveal buoyancy effects produ ced by tidal forcing. The semi-diurnal tide raises the isopycnals over the sills at the head of the Laurentian Trough and English Bank, producing inte rnal tides radiating seaward. Relatively dense intermediate waters rise fro m below 75-m depth to the near surface over the sills setting rip gravity c urrents on the inner slopes. Internal hydraulic controls develop over the o uter sills; during flood, surface flow separation occurs at the entrances o f the Saguenay Fjord and the upper estuary west of Ilet Rouge Bank. Early d uring ebb flow (restratification), the surface layer. deepens to encompass the tops of the sills. As the ebb current intensifies, the model predicts t he formation of seaward internal jumps over the outer sills, which were con firmed from acoustic reflection observations. As the internal Froude number increases further; flow separation migrates up to sill height As a result of these transitions internal bores emanate from the head region one to two hours before low water We find that the mixing of oceanic and surface,wate rs near the sills is driven by the vertical shear produced during ebb in th e channel south of Ilet Rouge, the shear produced in the bottom gravity flo od currents, and, to a lesser extent the processes over the sills.