Since 2014, an array of current meters deployed in the Iceland Basin as part of the Overturning in the Subpolar North Atlantic Program has provided new measurements of the southward flow of Iceland‐Scotland Overflow Water (ISOW) along the eastern flank of the Reykjanes Ridge. The location of the array, near 58–59°N, captures the ISOW plume at the farthest downstream location in the Iceland Basin before significant amounts of ISOW can flow into the Irminger Basin through deep fractures in the Reykjanes Ridge. The net transport of the ISOW plume at this location—approximately 5.3 Sv based on the first 4 years of observations—is significantly larger than previous values obtained farther north in the Iceland Basin, suggesting that either previous measurements did not fully capture the plume transport or that additional entrainment into the ISOW plume occurs as it approaches the southern tip of the Reykjanes Ridge. A detailed water mass analysis of the plume from continuous temperature/salinity observations shows that about 50% of the plume transport (2.6 Sv) is derived from dense waters flowing over the Nordic Sea sills into the Iceland Basin, while the remainder is made up of nearly equal parts of entrained Atlantic thermocline water and modified Labrador Sea Water. The overall results from this study suggest that the ISOW plume approximately doubles its transport through entrainment, similar to that of the Denmark Strait overflow plume in the Irminger Sea that forms the other major overflow source of North Atlantic Deep Water.

Iceland Scotland Overflow Water (ISOW) is a major constituent of the Atlantic Meridional Overturning Circulation's (AMOC) southward, abyssal flow. ISOW is formed from warm, salty upper ocean waters delivered to the North Atlantic Subpolar Gyre and Nordic Seas via the North Atlantic Current (NAC) (Hansen & Østerhus, 2007). After its formation, ISOW is exported out of the Iceland Basin where it mixes with several other deep water masses to form North Atlantic Deep Water, the dominant water mass of the AMOC's lower limb. Understanding the drivers of variability in ISOW is therefore critical to understanding variability in the whole AMOC system. Hydrographic variability within the abyssal ISOW layer is linked to its source water masses. Two distinct processes converge to form ISOW: convection in the Nordic Seas that forms a dense overflow across the Iceland Scotland Ridge (ISR), and entrainment of upper ocean waters into this overflow as it descends into the northern Iceland Basin (Beaird et al., 2013;Chafik et al., 2020;García-Ibáñez et al., 2015;Semper et al., 2020). The majority of the Nordic Seas overflow crossing the ISR is funneled through the Faroe Bank Channel (FBC) before spilling into the Iceland Basin. Entrainment occurs as the flow spills out of FBC and descends into the abyssal layer as a gravity current, with the bulk of entrainment and watermass transformation concentrated within a few 100 km of the FBC sill (Beaird et al., 2013;Fer et al., 2010;Girton et al., 2006). This process mixes warm, salty upper ocean waters into the overflowing waters, creating the final ISOW water mass. Cumulatively across the entire ISR, the entrainment process nearly doubles the total transport from ∼3 Sv of the original Nordic Seas overflow crossing the ISR to 5.3 Sv (Johns et al., 2021). The

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