Presenter: Mathilde Jutras (McGill University)

Description:
The Labrador Current (LC) flows south along the coasts of Labrador and Newfoundland until it reaches the tip of the Grand Banks. At that location, part of the current is retroflected to the east, feeding the subpolar North Atlantic (SPNA), whereas another part flows westwards along the coast, feeding the Slope Sea. The volume of LC water that follows each path impacts the North Atlantic freshwater content, as well as the salinity and oxygenation level of the northern American coastal waters. No mechanism has yet been given to explain the retroflection of the LC. In a previous study, we proposed that the strength of the LC, rather than the position or strength of the Gulf Stream, determines which path is favored. To further study what controls the variability in the retroflection of the LC, we used Lagrangian trajectories of virtual particles released in the 1/12° ocean reanalysis GLORYS12. We quantify the retroflection of the LC with two indices: one based on counting particles crossing hydrographic sections, and one based on a machine learning technique that classifies trajectories in clusters following their shape. Our results indicate that a stronger current along the Labrador Shelf is associated with a stronger retroflection towards the SPNA. Correlation maps suggest that a stronger current is promoted by a negative anomaly in the wind stress curl over the Labrador Shelf. These results imply that the variability in the retroflection may be controlled upstream of the retroflection point. Analyses of local interactions between Gulf Stream eddies or meanders and the LC reveal that, whereas these interactions sometimes divert the LC waters towards the east, they do not cause the retroflection.  On the basis of these results, we propose a physical mechanism that explains the retroflection of the LC based on the conservation of vorticity at the tip of the Grand Banks.