Interannual variability and depth-dependence in pathways of Iceland-Scotland Overflow Waters exiting the Iceland Basin

In the subpolar North Atlantic, the strength of the Meridional Overturning Circulation is linked to rates of North Atlantic Deep Water formation, a water mass partially composed of Nordic Seas Overflow Waters. While Denmark Strait Overflow Water takes a relatively direct route out of the Irminger basin via the cyclonic boundary current, exit pathways of Iceland-Scotland Overflow Water (ISOW) from the Iceland Basin are less understood and more complex. Here, ISOW pathways and their interannual variability are explored in a Lagrangian framework using particles seeded within the 45-year 1/12° eddy-resolving North Atlantic HYCOM simulation. Our analysis reveals significant depth-dependent variability in ISOW pathways. Upper layers preferentially cross into the Irminger Basin through gaps in the Reykjanes Ridge while deeper layers take the more traditional route to the Charlie-Gibbs Fracture Zone (CGFZ). At the CGFZ, we observe a strong anticorrelation in the percentage of particles that end up in the western vs. eastern basin which varies on a timescale of ~2.5 years and is likely associated with the position of the North Atlantic Current (NAC). This anticorrelation however is much stronger in the upper layers as the influence of the NAC appears to decrease with depth. Of the approximately 55% of particles that translate through the CGFZ, those in the upper layers are more likely to follow the cyclonic boundary current while lower layer particles diffuse northwestward towards the Labrador Sea. These depth-dependent patterns, identified from simulated particle trajectories, are corroborated by observations from RAFOS floats deployed during the OSNAP campaign. These findings illustrate the importance of depth-dependent dynamics and interannual variability of the NAC in shaping ISOW pathways, with implications for deep circulation patterns in the subpolar North Atlantic and the rate of large-scale overturning.