Redrawing the Iceland-Scotland Overflow Water Pathways in the North Atlantic

The spreading pathway of the Iceland-Scotland Overflow Water (ISOW), an important component of the global Atlantic Meridional Overturning Circulation, has been historically depicted as a deep boundary current flowing counter-clockwise around the subpolar North Atlantic. Specifically, after passing through the Charlie-Gibbs Fracture Zone (CGFZ) from the Iceland Basin, it has been assumed, based on sparse observations, to turn sharply northward to parallel the western flank of the Reykjanes Ridge into the Irminger Sea. However, this boundary-following northward pathway has been challenged by recent hydrographic and modeling studies that reveal a significant westward spreading of ISOW after passing through the CGFZ. Here we present new Lagrangian observations and model results that support the emerging view of alternative ISOW spreading pathways in the western subpolar region. We show that a significant portion of ISOW follows a west-northwestward interior pathway that extends from the CGFZ to the central Labrador/Irminger Sea. In addition, we describe a newly observed southward pathway along the western flank of the Mid-Atlantic Ridge. This southward pathway potentially serves as a “fast track” for the export of some ISOW to the subtropical region. Finally, the partitioning of these alternative pathways is shown to be influenced by deep-reaching eddies and meanders of the eastward-flowing North Atlantic Current. Our results, in tandem with a handful of previous studies, call for a revision in the standard paradigm of how ISOW spreads throughout the North Atlantic.