Tidal mixing versus thermal stratification in the Bay of Fundy and gulf of Maine

Abstract A simple energetic argument (Simpson and Hunter, 1974) shows that the boundary between well‐mixed and stratified areas of a shallow sea in summer should correspond to a critical value of BH/U3 , where B is the buoyancy JEUX due to solar heating, H the mean water depth and U the amplitude of the tidal current. We demonstrate the importance of this parameter in a simple model of vertical mixing, and discuss the role of many other factors affecting stratification. Examination of hydrographic data from the Bay of Fundy and Gulf of Maine, together with estimates of tidal dissipation (proportional to U 3) from Greenberg's (1978) numerical model, shows a transition from well‐mixed to stratified conditions, in July and August, for H/U3 = 70 m‐2s3. This corresponds to a mixing efficiency of only 0.26%. Predictions are made of the changes in extent of well‐mixed areas that would be caused by tidal power development. Some stratification, due to both solar heating and freshwater input, is possible in previously mixed areas which would be the headponds for two schemes. Outside the barriers the changes are less dramatic, although the merging of mixed areas over Georges Bank and Nantucket Shoals is predicted.