Hydrodynamics of the Bay of Sept-Îles

Abstract

ABSTRACT Seasonal and tidal variability of temperature, salinity, and currents were measured from spring to fall 2017, and during spring 2018 in the Bay of Sept-Îles (BSI), located in the northwest of the Gulf of St. Lawrence, Canada. Surface drifters were deployed to measure surface current velocities and CTD profiles were recorded regularly at 21 stations across the bay and the archipelago. Repeated passages along a transect at the mouth of the bay were conducted for 12 hours with a towed acoustic Doppler current profiler. During these transects, regular stops were made for CTD profiling. The bay was found to be a two-layer system, with a pycnocline near a depth of 10 m. There was a horizontal temperature and salinity front near the tributary rivers separating them from the surface waters of the Gulf of St. Lawrence; however, this front was outside the bay during the spring freshet. Tide-averaged currents were seaward near the surface (2–10 m) and shoreward in deeper waters (10–30 m), suggesting an estuarine circulation in the BSI. Average and maximum surface drifter speeds were 17.4 cm s and 86.6 cm s. Instantaneous drifter velocity was coarsely ( west of it) but steadily aligned with the direction of the wind, and average surface currents seemed to follow the dominant wind directions (SSE and WNW), since drifters often (in 22/46 cases) shoaled on their leeward beaches. Currents were dominated by tides locally but usually resulted from the complex interaction of tides with estuarine circulation, winds and effects of the Earth's rotation. Usual alignment of currents with coasts and bathymetry emphasized the key role of topographic steering on bay scale flow patterns. The bulk flushing time of water in the BSI was estimated to days.