A Study of the Gulf of St. Lawrence Water Masses in Response to Atmospheric Forcing and Model Resolution Using Quasi-Operational Canadian Coastal Ice-Ocean Prediction Systems

Francois Roy1, Dr. Jean-Philippe Paquin, PhD2, Gregory Smith2, Audrey-Anne Gauthier3, Sarah MacDermid4, Simon St-Onge Drouin5, Simon Senneville6, Frederic Dupont7 and Jerome Chanut8, (1)Environment and Climate Change Canada, Meteorological Research Division, Dorval, QC, Canada, (2)Meteorological Research Division, Environment and Climate Change Canada, Dorval, QC, Canada, (3)McGill University, Montreal, QC, Canada, (4)Meteorological Service of Canada, Environment and Climate Change Canada, Halifax, NS, Canada, (5)Institut Maurice-Lamontagne, Fisheries and Oceans Canada, Mont-Joli, QC, Canada, (6)UQAR-ISMER, Rimouski, QC, Canada, (7)Meteorological Service of Canada, Environment and Climate Change Canada, Dorval, QC, Canada, (8)Mercator Ocean International, Ramonville Saint-Agne, France
Abstract:
The Gulf of St. Lawrence (GSL) water masses evolve under a complex estuarine circulation. Continental freshwaters travel downstream at the ocean surface and mix with some of the cold Atlantic waters entering the GSL through Belle-Isle Strait (Labrador Current), and with some of the warmer and saltier Atlantic waters entering through Cabot Strait at depth in the Laurentian channel that crosses the continental shelf outside the GSL and connects to the Gulf Stream area. The density driven circulation is modulated by strong tidal flows interacting with a complex topography to mix water masses, and by the effect of an important seasonal cycle. Extreme weather events and a strong heat loss in fall and winter lead to the formation of a sea ice cover and a thickening of the cold intermediate layer (CIL, ~30-150m). We study how the formation, circulation and mixing of these water masses are represented in two ice-ocean prediction systems based on the NEMO-CICE modeling framework, compared to available observations. Two configurations are examined, a northwest Atlantic 1/36 degree (~2 km) resolution domain and a GSL 500 m resolution domain. The first domain covers the Gulf Stream region and the Canadian east coast including mainly the Grand Banks, the Labrador and Scotian Shelves, and the GSL. The second domain covers the GSL, bounded by Cabot and Belle-Isle straits. Both systems have explicit tides, storm surge forcing and a general length scale scheme for vertical mixing. We test different sets of atmospheric forcing (from 33 km to 2.5 km resolution) and examine the importance of wind channeling (intensity) in the Upper and Lower Estuary in controlling stratification and the CIL formation and penetration. In this respect, the effect of ocean resolution is also studied. We quantify how extreme winter events and their intensity modulate exchanges through vertical sections of the GSL and the resulting water mass volumes.
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