Modelling of the circulation in the Estuary and Gulf of St. Lawrence in response to variations in fresh water runoff and winds
We examine recent results issued from observations and numerical simulations of the general circulation and climate of the St. Lawrence Estuary and Gulf. We focus this study on the intensity of the estuarine circulation. The approach is presented with an emphasis on the integration, using numerical simulations, of key processes such as turbulent mixing associated with tides at the head of the Laurentian Channel and air-sea exchanges over the Lower Estuary and Gulf. The numerical simulations allow us to follow the evolution of circulation and climate over periods of a few hours to several years. We first examine the tidal variability of the circulation at the head of the Laurentian Channel and the Upper Estuary, illustrating the importance of processes that largely govern the estuarine circulation. The most important processes are the dynamical instabilities associated with tidal straining and internal hydraulic jumps near the sills. On the other hand, the wind stress dominates the variability of freshwater export from the Lower Estuary and Gulf to the Atlantic Ocean. Using sensitivity experiments, the effects of freshwater runoff and wind intensity on the estuarine circulation are isolated. The results show that the St. Lawrence model responds to the external forcings like a classical two-layer estuary during periods of higher stratification (spring and summer), when the freshwater drives more efficient withdrawal of the cold intermediate layer of the Gulf to a depth of approximately 150 m. On the other hand, in periods of lower stratification (fall and winter), the wind stress drives the deep water circulation (more than 150 m depth) and an excess freshwater input partly inhibits this ventilation, as in a marginal northern sea. These results suggest that an eventual reduction of freshwater runoff from the St. Lawrence River, associated with climate change or development, could reduce the estuarine circulation during spring and summer. During winter, such a reduction could favour the withdrawal of deep waters from the Laurentian Channel.