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

Iceland-Scotland Overflow Water (ISOW), a dense water mass formed in the Nordic Seas, spills into the Iceland Basin through the Iceland-Faroe Ridge and Faroe Shetland Channel before propagating southwestward along the eastern flank of the Reykjanes Ridge as a deep boundary current, contributing to the lower limb of the Atlantic Meridional Overturning Circulation. Recent work has demonstrated that pathways of ISOW out of the Iceland Basin are complex and variability has not been explored on interannual and longer timescales. In this study, we use the basin-scale, eddy-rich (1/12°), 40-year Atlantic Hybrid Coordinate Ocean Model (HYCOM) simulation to investigate the variability in ISOW export pathways. Simulated Lagrangian particles are released within the ISOW layer ( σ 0 > 27.80 kg m −3 ) at two locations along the eastern flank of the Reykjanes Ridge and classified according to their export pathway: over the Reykjanes Ridge, through the Charlie Gibbs Fracture Zone (CGFZ) or into the Western European Basin (WEB). We show that export over the Reykjanes Ridge exhibits variability on timescales from 2.5 years to decadal and is strongly correlated with mid-depth northward transport into the Iceland Basin forced by fluctuations in the North Atlantic Current (NAC), suggesting a link to large-scale gyre dynamics. Export through the CGFZ is characterized by variability of 2.5–5 years and is influenced by the meridional position of the eastward-flowing NAC within the CGFZ, which can block ISOW export and divert particles into the WEB. Export pathways are shown to exhibit a significant depth-dependence such that shallower layers are more strongly impacted by the surface circulation while deeper layers are more subject to topographic steering, results that are corroborated by utilizing observations from RAFOS floats deployed during the OSNAP campaign. Together, these findings illustrate the high-degree of variability in pathways of ISOW export and contributes to the growing body of evidence that challenges the notion of a single, dominant pathway out of the Iceland Basin. • Export pathways of Iceland-Scotland Overflow Water from the Iceland Basin exhibit strong depth dependence, with shallower layers more influenced by surface circulation and deeper layers steered by topography. • Variability in Iceland-Scotland Overflow Water export, particularly over the Reykjanes Ridge, is closely tied to fluctuations in the North Atlantic Current, linking overflow pathways to large-scale dynamics of the subpolar gyre. • The traditional pathway of Iceland-Scotland Overflow Water, through the Charlie Gibbs Fracture Zone and northward along the western flank of the Reykjanes Ridge, exports only about 10% of all ISOW and is highly variable, suggesting it is a secondary pathway limited to shallower ISOW layers.