| Summary: | The Labrador Current transports cold, relatively fresh, and well-oxygenated waters within the subpolar North Atlantic and along the western American continental shelf. The contribution to both regions is determined by the strength of the eastward retroflection of the Labrador Current at the Grand Banks. A good understanding of the pathways of the Labrador Current and of their underlying drivers is thus crucial to improve our ability to predict physical and biological changes in the northwestern Atlantic. Here, we investigate the pathways of the Labrador Current using a Machine Learning unsupervised k-means++ clustering method applied to a large set of Lagrangian trajectories. The trajectories are that of virtual particles advected by the velocity field of the GLORYS12V1 ocean reanalysis model. The Labrador Current mainly follows a westward-flowing and an eastward retroflecting pathway (20% and 50% of the flow, respectively) that compensate each other through time in a see-saw behaviour. We develop a retroflection index to investigate the drivers of the retroflection of the Labrador Current. Our analyses reveal that strong retroflection generally occurs when a large-scale circulation adjustment, related to the subpolar gyre, accelerates the Labrador Current and shifts the Gulf Stream northward, partly driven by a northward shift of the zero-wind-stress-curl line in the western North Atlantic. Starting in 2008, a particularly strong northward shift of the Gulf Stream dominates the other drivers. Monitoring winds and circulation around the Grand Banks could thus help predict consequences on marine life in the northwestern Atlantic and to set fishing quotas.
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