The Labrador Current plays a crucial role in the North Atlantic's climate and ecosystem. It transports cold, fresh, and oxygenated waters from the Arctic and West Greenland Current, influencing water properties on both the subpolar North Atlantic and the eastern American continental shelf. The extent to which these waters contribute to each region is determined by the Labrador Current's retroflection at the Grand Banks of Newfoundland. Recent decades have witnessed concerning trends in the Slope Sea and northeastern American continental shelf, including rising water temperatures and declining oxygen concentrations. These changes extend to connected water bodies like the St. Lawrence Estuary and the Gulf of Maine, severely impacting marine ecosystems and fisheries. A significant freshening event in the subpolar North Atlantic from 2012 to 2016 further complicates the situation, potentially impacting the Atlantic Meridional Overturning Circulation (AMOC). Both deoxygenation and temperature increases on the shelf, as well as subpolar Atlantic freshening, are linked to increased Labrador Current Water transport within the subpolar North Atlantic, at the expense of the Slope Sea and eastern American continental shelf. Understanding the Labrador Current's retroflection is therefore vital for predicting future changes in water properties and their ecological consequences. The Labrador Current's pathway near the Grand Banks is complex, with a split into eastward-retroflecting and westward-flowing branches. While the retroflection's importance is acknowledged, its driving mechanisms remain poorly understood, with various proposed factors including wind patterns, current strength, the North Atlantic Oscillation (NAO), the AMOC, and interactions with Gulf Stream/North Atlantic Current (NAC) eddies and meanders. This study aims to synthesize these different perspectives and provide a comprehensive understanding of the Labrador Current's retroflection and its large-scale controls.

Previous research has proposed several drivers for the retroflection of the Labrador Current. Wind patterns over the Labrador Shelf have been implicated, along with the strength of the Labrador Current itself. Some studies link a weak retroflection to a strong NAO, while others associate it with a weak NAO or a strong AMOC. A strong retroflection has been observed concurrently with a northward shift of the Gulf Stream, suggesting interactions between the two currents as a potential driver. The role of Gulf Stream/NAC eddies and meanders in diverting the Labrador Current offshore or blocking its inflow towards the Scotian Shelf has also been investigated. Seasonal stratification in the Grand Banks region is another factor considered to affect freshwater export from the shelf. However, a unified understanding of these varied perspectives has been lacking. Existing studies offer fragmented insights, highlighting the need for a comprehensive analysis that integrates multiple potential drivers and provides a clearer picture of the mechanisms involved in the retroflection process.

This study employs a novel approach to investigate the retroflection of the Labrador Current using Lagrangian tracking experiments with virtual particles. The daily horizontal and reconstructed vertical velocity fields from the GLORYS12V1 ocean reanalysis (covering 1993-2018) are used to track the particles. GLORYS12V1 is a global 1/12° ocean physical reanalysis from the Copernicus Marine Service (CMS), based on the NEMO system and LIM2 sea ice model, forced with ERA-Interim atmospheric reanalysis. The model assimilates various observational data, including sea surface temperature (SST), sea level anomaly (SLA), temperature and salinity profiles, and sea-ice concentrations. Virtual particles are seeded along a line across the Labrador Current, and their trajectories reveal the dominant pathways of the current. A retroflection index is defined based on the number of particles crossing specific hydrographic sections, capturing the magnitude of the retroflection. The index's variability is examined, and its relationship with various environmental factors is investigated through correlation and composite analyses. These factors include sea surface height (SSH) anomalies, Labrador Current volume transport, wind stress curl, sea level pressure, and climate indices (NAO, AO, and AMOC). The study also analyzes the interaction of the particles with eddies and meanders at the tip of the Grand Banks using an eddy detection algorithm based on the Okubo-Weiss parameter. This allows the researchers to determine if the interactions with the Gulf Stream features influence the retroflection. To validate the results from the Lagrangian analysis, the researchers compare the virtual particle trajectories with those of Argo floats, RAFOS/SOFAR floats, and surface drifters, providing a multi-platform observational comparison to strengthen the findings.

The Lagrangian analysis reveals a seesawing system with westward and eastward branches of the Labrador Current downstream of the Grand Banks. Approximately 60% of the transport is attributed to the eastward (retroflected) branch. Retroflection predominantly occurs between Flemish Cap and the tip of the Grand Banks. The retroflection index, strongly correlated with temperature and salinity anomalies in the subpolar North Atlantic, Slope Sea, and northeastern American shelf, confirms the seesawing nature of the system and the Labrador Current's influence on these regions. A strong (weak) retroflection correlates with positive (negative) salinity and temperature anomalies in the Slope Sea and Scotian Shelf, and negative (positive) salinity anomalies in the subpolar North Atlantic. The index shows strong multiannual variability with a positive trend, consistent with observations of increased Labrador Sea origin waters reaching the eastern North Atlantic since 2008. Multiple forcing mechanisms are identified. Strong retroflection correlates with positive SSH anomalies near the Grand Banks, indicating a northward shift of the Gulf Stream/NAC. While a northward shift enhances retroflection, it's not a necessary condition, as strong retroflection occurred when the Gulf Stream was further south. Strong retroflection is also associated with a strong Labrador Current, indicated by converging barotropic velocity streamlines and positive correlation between Labrador Shelf volume transport and the retroflection index. The subpolar gyre dynamics plays a crucial role, with strong retroflection linked to a contracted western and expanded eastern basin. Wind stress curl anomalies over the Labrador Shelf further influence retroflection, with negative anomalies (stronger zonal winds) associated with stronger retroflection. While a relationship between retroflection and climate indices (NAO, AO) is found, no lagged correlation with AMOC strength is detected. At the Grand Banks, while an increased presence of Gulf Stream eddies and meanders accompanies strong retroflection, these features don't seem necessary for retroflection, which is rather influenced by cyclonic meanders of the Labrador Current. However, the Gulf Stream's proximity after 2008 likely enhanced retroflection by squeezing the Labrador Current's meanders. A dynamical argument suggests that a stronger Labrador Current detaches more easily from the continental slope due to higher inertia, resulting in a stronger β-compensation effect and enhanced retroflection.

The findings highlight the dominant role of large-scale forcing, through winds and gyre dynamics, in controlling the Labrador Current's retroflection. The study refines the understanding of the Gulf Stream's influence, emphasizing its role within the context of larger-scale subpolar gyre dynamics, rather than solely as a physical blocker. The Labrador Current's strength and wind patterns directly contribute to retroflection, with a time lag of a few months. This suggests that these variables could serve as predictors of Labrador water export towards the subpolar and coastal North Atlantic. Monitoring these factors using satellite and mooring data can help anticipate impacts on marine life and inform fishing quotas.

This study provides a comprehensive understanding of Labrador Current retroflection, emphasizing the importance of large-scale forcing through winds and subpolar gyre dynamics. While the Gulf Stream's proximity plays a role, particularly in recent years, the Labrador Current's strength and wind patterns are the primary drivers. The time lag between these variables and retroflection allows for predictive monitoring, benefiting marine ecosystem management and fisheries.

The study primarily relies on the GLORYS12V1 reanalysis, which has known limitations, including a slight overestimation of western boundary current intensity and a slight underestimation of the Labrador shelf-break jet velocity. While the observational data supports the findings, the limited spatial and temporal coverage of some datasets restricts the complete validation of the results. Future studies could benefit from higher resolution models and more extensive observational data to further refine the understanding of the retroflection mechanism.