Moored observations of the Iceland-Scotland Overflow plume along the eastern flank of the Reykjanes Ridge

Since 2014, an array of current meters deployed in the Iceland Basin as part of the OSNAP trans-basin observing system 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 transport of the ISOW plume at this location - approximately 6 Sv based on the first four 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. Most of the plume transport (4.5 Sv) is carried in a main branch of the plume along the upper ridge crest in depths from 1400-2200 m. A secondary branch in depths of 2400-2800 m along the lower ridge crest carries about 1.6 Sv. The total plume transport is highly variable, with monthly-averaged transports varying from 2-10 Sv, and both plume branches vary independently. A detailed water mass analysis of the plume from continuous temperature/salinity observations shows that about 50% of the plume transport (2.7-3.0 Sv) is derived from Norwegian Sea Overflow waters (NSOW) - consistent with the amount of NSOW known to be 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.