Influence of St. Lawrence River discharge on the circulation and hydrography in Canadian Atlantic waters

Abstract

This study examines the effects of freshwater discharge from the St. Lawrence River (SLR) on the three-dimensional circulation and hydrography in Canada's Atlantic waters using a regional shelf circulation model based on the Princeton Ocean Model. The study region comprises the Gulf of St. Lawrence, the Scotian Shelf, and the Gulf of Maine. The model forcing includes atmospheric forcing at the sea surface; tidal elevation and currents at the lateral open boundaries; and freshwater discharge from the SLR and other major rivers in the region. The forcing at the lateral open boundaries also includes non-tidal currents as well as temperature and salinity fields produced by a coarse-resolution circulation model for the northwest Atlantic. The shelf circulation model is integrated for 5 years (1999–2003); the simulated hydrography and circulation during the last 4 years are compared to observations to assess the model's performance. Three additional model runs are made, called the climate, wet, and dry runs, in which the SLR discharge values specified in the model are respectively the long-term mean and the long-term mean plus or minus 3.5 standard deviations. Model results from the climate run demonstrate that the variability of freshwater flux in the St. Lawrence Estuary takes ∼100 days to reach Cabot Strait; ∼160 days to reach the central Scotian Shelf; and ∼220 days to reach the eastern Gulf of Maine. Within the Gulf of St. Lawrence, an increase in the SLR discharge results in a stronger Gaspé Current (up to ∼0.1ms−1 stronger in the wet run than in the dry run) as well as lower salinity in the western part of the gulf. In the winter, the effect of a higher SLR discharge extends along the Scotian Shelf and into the Gulf of Maine, with salinity in both areas lower by ∼0.1psu in the wet run than in the dry run.