Variability of the circulation in the western North Atlantic and its impact on deep-water oxygen concentrations in the St. Lawrence Estuary on the western continental shelf: evidence from observations

Oxygen concentrations in the deep waters of the Lower St. Lawrence Estuary, in eastern Canada, have decreased by 50% over the past century, reaching hypoxic levels. To study the causes of this deoxygenation, we applied a mixing model (an extended multi-parameter analysis - eOMP) to data collected in the St. Lawrence Estuary since the 1970s and from the late 1990s to 2018. This method accounts for diapycnal mixing and can distinguish between the physical and biogeochemical causes of deoxygenation. The eOMP reveals that, in recent years, most of the deoxygenation of deep waters of the St. Lawrence Estuary is due to a change in the circulation pattern in the western North Atlantic. Since 2008, the Slope Sea and the deep waters of the St. Lawrence Estuary are fed by an increasing amount of oxygen-poor North Atlantic Central Waters (NACW), transported by the Gulf Stream, at the expense of oxygen-rich Labrador Current Waters (LCW). The oxygenation level of the St. Lawrence Estuary therefore reflects what is happening in the western North Atlantic. In contrast, the eOMP shows that, from the 1970s to the late 1990s, biogeochemical changes such as local eutrophication and variations in oxygen consumption rates in the North Atlantic dominated the deoxygenation. Further analyses suggest that the variability in the LCW:NACW ratio in the Slope Waters is mainly controlled by the Scotian Shelf-break Current, an extension of the Labrador Current, and not by the position or strength of the Gulf Stream, as often suggested. When the Labrador Current is strong, little of the southward flowing Labrador Current waters follow the coast all the way to the Scotian Shelf, and most of these waters are deviated east towards the North Atlantic. The opposite is true when the Labrador Current is weak. We will present some analysis of LCW trajectories in different conditions and discuss their potential drivers, based on a high resolution model. Overall, our results highlight the primary role of the Labrador Current in determining (i) the oxygen concentration and other water properties on the western North Atlantic continental shelf and slope, and (ii) the advection of fresh Labrador Current Water into the subpolar North Atlantic, with possible implications on the thermohaline and gyre circulation.

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