Deep through-flow in the Bight Fracture Zone

Abstract. Iceland–Scotland Overflow Water (ISOW) is exported from the Nordic Seas into the Iceland Basin to feed the lower limb of the Meridional Overturning Circulation. The Bight Fracture Zone (BFZ) is known to be a major route for ISOW toward the Irminger Sea, but the role of this gateway in the evolution of ISOW properties over the subpolar gyre is unclear. A combination of ship-based and Deep-Argo data gathered between 2015 and 2018 allows us to investigate the pathways and hydrographic evolution of ISOW as it flows through the BFZ, as well as its influence on the North Atlantic Deep Water (NADW) properties in the Irminger Sea. The ISOW flow through the BFZ amounts to 0.8 ± 0.2 Sv and is mainly fed by the lighter part of the ISOW layer flowing west of 29–30∘ W as part of the East Reykjanes Ridge Current in the Iceland Basin. In the rift valley of the BFZ, between an eastern and a western sill, the bathymetry of the BFZ shapes a cyclonic circulation along which the ISOW layer is homogenized. The largest changes in ISOW properties are however observed downstream of the western sill, at the exit of the BFZ. There, ISOW is mixed isopycnally with comparatively fresher NADW circulating in the Irminger Sea. Hence, our analysis reveals the key role of the BFZ through-flow in the salinification of the NADW in the Irminger Current.