Moored Observations of the Iceland‐Scotland Overflow Plume Along the Eastern Flank of the Reykjanes Ridge

Abstract

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.

Rapid Freshening of Iceland Scotland Overflow Water Driven by Entrainment of a Major Upper Ocean Salinity Anomaly

“…Drifters were released every 3 months from 1992 to 1994 at various depths within the ISOW layer (i.e., at depths below the 27.8 kg/m 3 isopycnal) in the two main branches of ISOW flow at the OSNAP line and their trajectories were computed backwards in time for 24 months. An upper ridge branch and a basin interior branch were identified in the observational studies by Zou et al (2017) and Johns et al (2021). Although FLAME output does not span the OSNAP observational record, sensitivity experiments show that advective pathways are representative of typical ISOW flow and should not affect our major conclusions (Gary et al, 2011).…”

“…Finally, we consider whether the magnitude of the ISOW freshening signal is consistent with the magnitude of the upper ocean freshening event, via the process of entrainment. Past studies show that the final ISOW product contains about 25% of entrained Subpolar Mode Water (Fogelqvist et al, 2003;Johns et al, 2021;Saunders, 1996). If entrainment of the upper ocean anomaly is the main cause of the O(0.01) PSU ISOW freshening, this would imply a salinity decline of 0.04 PSU in the Subpolar Mode Water that is entrained into the overflow.…”

“…The array provides hourly hydrographic measurements of the NAC and ISOW flows from July 2014 to July 2018. Readers are referred to Lozier et al (2019) for a description of the entire OSNAP array, and to Johns et al (2021) for a detailed description of the moored array data used in this study. The eastern side of the array, mainly moorings M3 and M4, sample the northward flowing NAC waters while the western side captures the southward, bottom trapped ISOW (Figure 1).…”

“…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…”

Rapid Communication of Upper‐Ocean Salinity Anomaly to Deep Waters of the Iceland Basin Indicates an AMOC Short‐Cut

“…However, since direct measurements of the AMOC in the SPNA only commenced in 2014 (Lozier et al, 2017), the relationship between freshwater pulses on multiple time scales and AMOC strength is yet far from understood, although there is a consensus that large freshening events, such as the great salinity anomalies (Dickson et al, 1988), play a critical role in causing basin-scale ocean circulation changes (e.g., Zhang and Vallis (2006)). 2021) reports that the Iceland Scotland Overflow Waters (ISOW), a main component of the AMOC lower limb in the Subpolar North Atlantic (Johns et al, 2021;Zou et al, 2017), and which is formed in the first place in the Nordic Seas through various processes (Greenland Sea deep convection, densification on the Arctic shelves, and transformation of Atlantic-origin waters; e.g., Eldevik et al, 2009), has experienced a significant freshening in the Iceland Basin (IB) since 2017 (Figure 1). This was just after the largest upper-ocean freshening event was observed in 2016 in the upper 1000m (Figure 2a) of the eastern SPNA (Holliday et al, 2020), caused by diversion of Arctic-origin freshwater from the Labrador Current into the interior basin as a result of a reorganization of the wind-stress curl (Holliday et al, 2020; their Figure 10).…”