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1. Structure and Forcing of Observed Exchanges across the Greenland–Scotland Ridge

   https://doi.org/10.1175/JCLI-D-17-0889.1  

   Bringedal, Carina; Eldevik, Tor; Skagseth, Øystein; Spall, Michael A.; Østerhus, Svein (December 2018, Journal of Climate)

   null (Ed.)

   The Atlantic meridional overturning circulation and associated poleward heat transport are balanced by northern heat loss to the atmosphere and corresponding water-mass transformation. The circulation of northward-flowing Atlantic Water at the surface and returning overflow water at depth is particularly manifested—and observed—at the Greenland–Scotland Ridge where the water masses are guided through narrow straits. There is, however, a rich variability in the exchange of water masses across the ridge on all time scales. Focusing on seasonal and interannual time scales, and particularly the gateways of the Denmark Strait and between the Faroe Islands and Shetland, we specifically assess to what extent the exchanges of water masses across the Greenland–Scotland Ridge relate to wind forcing. On seasonal time scales, the variance explained of the observed exchanges can largely be related to large-scale wind patterns, and a conceptual model shows how this wind forcing can manifest via a barotropic, cyclonic circulation. On interannual time scales, the wind stress impact is less direct as baroclinic mechanisms gain importance and observations indicate a shift in the overflows from being more barotropically to more baroclinically forced during the observation period. Overall, the observed Greenland–Scotland Ridge exchanges reflect a horizontal (cyclonic) circulation on seasonal time scales, while the interannual variability more represents an overturning circulation. 

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2. Directly Measured Currents and Estimated Transport Pathways of Atlantic Water between 59.5oN and the Iceland-Faroes-Scotland Ridge

   Childers, K. H. (October 2015, Tellus. Series A, Dynamic meteorology and oceanography)

   Using vessel-mounted acoustic Doppler current profiler data from four different routes between Scotland, Iceland and Greenland, we map out the mean flow of water in the top 400 m of the northeastern North Atlantic. The poleward transport east of the Reykjanes Ridge (RR) decreases from 8.5 to 10 Sv (1 Sverdrup 106 m3 s1) at 59.58N to 618N to 6 Sv crossing the IcelandFaroesScotland Ridge. The two longest 1200 km transport integrals have 1.40.94 Sv uncertainty, respectively. The overall decrease in transport can in large measure be accounted for by a 1.5 Sv flow across the RR into the Irminger Sea north of 59.58N and by a 0.5 Sv overflow of dense water along the IcelandFaroes Ridge. A remaining 0.5 Sv flux divergence is at the edge of detectability, but if real could be accounted for through wintertime convection to 400 m and densification of upper ocean water. The topography of the Iceland Basin and the banks west of Scotland play a fundamental role in controlling flow pathways towards and past Iceland, the Faroes and Scotland. Most water flows north unimpeded through the Iceland Basin, some in the centre of the basin along the Maury Channel, and some along Hatton Bank, turning east along the northern slopes of George Bligh Bank, Lousy Bank and Bill Bailey’s Bank, whereupon the flow splits with 3 Sv turning northwest towards the IcelandFaroes Ridge and the remainder continuing east towards and north of the Wyville-Thomson Ridge (WTR) to the Scotland slope thereby increasing the Slope Current transport from 1.5 Sv south of the WTR to 3.5 Sv in the FaroesShetland Channel 

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3. Transport and Evolution of the East Reykjanes Ridge Current

   https://doi.org/10.1029/2020JC016377  

   Koman, G.; Johns, W. E.; Houk, A. (October 2020, Journal of Geophysical Research: Oceans)

   Abstract This study of the first continuous multiyear observations of the East Reykjanes Ridge Current (ERRC) reveals a highly variable, mostly barotropic southwestward flow with a mean transport of 10–13 Sv. The ERRC effectively acts as a western boundary current in the Iceland Basin on the eastern flank of the Reykjanes Ridge. As part of the Overturning in the Subpolar North Atlantic Program (OSNAP), continuous measurements of the ERRC have been maintained for the first time using acoustic Doppler current profilers, current meters, and dynamic height moorings at six mooring sites near 58°N since 2014. Together with satellite altimetry and Argo profile and drift data, the mean transport, synoptic variability, water mass properties, and upstream and downstream pathways of the ERRC are examined. Results show that the ERRC forms in the northeastern Iceland Basin at the convergence of surface waters from the North Atlantic Current and deeper Icelandic Slope Water formed along the Iceland‐Faroe Ridge. The ERRC becomes denser as it cools and freshens along the northern and western topography of the Basin before retroflecting over the Reykjanes Ridge near 59°N into the Irminger Current. Analysis of the flow‐weighted density changes along the ERRC's path reveals that it is responsible for about one third of the net potential density change of waters circulating around the rim of the subpolar gyre. 

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4. The decreasing transport of the Deep Western Boundary Current in the subpolar Atlantic

   https://doi.org/10.21203/rs.3.rs-3232952/v1  

   Koman, Gregory; Bower, Amy; Holliday, N Penny; Furey, Heather; Fu, Yao; Biló, Tiago (September 2023, Research Square)

   Abstract The Deep Western Boundary Current (DWBC) – the primary component of the lower limb of the Atlantic Meridional Overturning Circulation – flows along the eastern flank of Greenland from a combination of Denmark Strait Overflow Water and Iceland Scotland Overflow Water. The Overturning in the Subpolar North Atlantic Program (OSNAP) has continuously measured the DWBC since 2014 using current meters, temperature/salinity sensors, and acoustic doppler current profilers. This mooring array located near Cape Farewell also incorporates data from the Ocean Observatories Initiative’s Global Irminger Sea Array to create the longest continuous observations of the DWBC closest to where Iceland Scotland Overflow Water and Denmark Strait Overflow water first merge. This study reveals that the DWBC has decreased by 26% over the first six years of OSNAP observations primarily due to a thinning of the traditionally defined DWBC layer (σθ > 27.8 kg m-3) due to a known freshening signal moving through the subpolar region. Despite this decrease, the Atlantic Meridional Overturning Circulation as calculated by OSNAP has remained relatively steady over the same period. Ultimately, the reason for this difference is due to the methods used to define these two circulations. Finding such notably different trends for two seemingly dependent circulations raises the question of how to best define these transports. 

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5. Propagation and Transformation of Upper North Atlantic Deep Water From the Subpolar Gyre to 26.5°N

   https://doi.org/10.1029/2023JC019726  

   Petit, T; Lozier, M S; Rühs, S; Handmann, P; Biastoch, A (August 2023, Journal of Geophysical Research: Oceans)

   Abstract Because new observations have revealed that the Labrador Sea is not the primary source for waters in the lower limb of the Atlantic Meridional Overturning Circulation (AMOC) during the Overturning in the Subpolar North Atlantic Programme (OSNAP) period, it seems timely to re‐examine the traditional interpretation of pathways and property variability for the AMOC lower limb from the subpolar gyre to 26.5°N. In order to better understand these connections, Lagrangian experiments were conducted within an eddy‐rich ocean model to track upper North Atlantic Deep Water (uNADW), defined by density, between the OSNAP line and 26.5°N as well as within the Labrador Sea. The experiments reveal that 77% of uNADW at 26.5°N is directly advected from the OSNAP West section along the boundary current and interior pathways west of the Mid‐Atlantic Ridge. More precisely, the Labrador Sea is a main gateway for uNADW sourced from the Irminger Sea, while particles connecting OSNAP East to 26.5°N are exclusively advected from the Iceland Basin and Rockall Trough along the eastern flank of the Mid‐Atlantic Ridge. Although the pathways between OSNAP West and 26.5°N are only associated with a net formation of 1.1 Sv into the uNADW layer, they show large density changes within the layer. Similarly, as the particles transit through the Labrador Sea, they undergo substantial freshening and cooling that contributes to further densification within the uNADW layer. 

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