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Abstract A mooring array has been maintained across the West Greenland shelf and slope since 2014 as part of the Overturning in the Subpolar North Atlantic Program (OSNAP). Here, we use the first 8 years of data to investigate the interannual variability of the two overflow water components of the deep western boundary current (DWBC): the Denmark Strait Overflow Water (DSOW) and the Northeast Atlantic Deep Water (NEADW). While the velocity structure has remained similar throughout the record, both water masses have freshened considerably, especially the NEADW salinity core. Using revised density criteria to define these two components, their transports decreased significantly between 2014 and 2022: from 6.2 to 3.8 Sv (1 Sv ≡ 10⁶m³s⁻¹) (−0.33 Sv yr⁻¹) for the DSOW and from 5.4 to 4.1 Sv (−0.19 Sv yr⁻¹) for the NEADW. Since the overflows across the Denmark Strait and the Faroe Bank Channel have remained steady over this period, this points to decreased entrainment downstream of the sills as a possible mechanism for the observed transport reduction south of Greenland. Using shipboard and mooring data from the two sills, and a hydrographic database for the surrounding region, we predict the downstream transport of the two DWBC components via the framework of a streamtube model. The predicted transport explains 94% of the observed DSOW trend and 63% of the observed NEADW trend. This implies that further entrainment of the NEADW must occur during its long pathlength, which would also help explain the fresher-than-predicted NEADW salinity observed at the OSNAP array. 

Abstract Variability of the Atlantic Meridional Overturning Circulation (MOC) has drawn extensive attention due to its impact on the global redistribution of heat and freshwater. Here we present the latest time series (2014–2022) of the Overturning in the Subpolar North Atlantic Program and characterize MOC interannual variability. We find that any single boundary current captures ∼30% of subpolar MOC interannual variability. However, to fully resolve MOC variability, a wide swath across the eastern subpolar basin is needed; in the Labrador Sea both boundaries are needed. Through a volume budget analysis for the subpolar basins' lower limbs, we estimate the magnitude of unresolved processes (e.g., diapycnal mixing) required to close the mean budget (∼2 Sv). We find that in the eastern subpolar basin surface‐forced transformation variability is linked to lower limb volume variability, which translates to MOC changes within the same year. In contrast, this linkage is weak in the Labrador Sea. 

Abstract This study presents the first continuous observations of Iceland Scotland Overflow Water (ISOW) passing through the Bight Fracture Zone (BFZ), the northernmost deep bathymetric channel across the Reykjanes Ridge between the Iceland and Irminger Basins in the subpolar North Atlantic. Data from two 2‐year moorings, measuring temperature, salinity, and current velocity from 2015 to 2017, along with a set of deep ISOW‐embedded RAFOS floats, are used to investigate ISOW transport and water property variability through the BFZ, as well as advective pathways between the Iceland and Irminger Basins. The mooring‐derived record‐mean ISOW transport through the BFZ was −0.59 ± 0.27 × 1e6 m³/s (westward) and varied seasonally with weaker transport in winter and stronger transport in summer. Flow direction of ISOW through the BFZ was consistently westward except in winter, when week‐long flow reversals were frequently observed. The previously reported subpolar North Atlantic freshening event of the 2010s is evident in the BFZ mooring records beginning about January 2017. About one‐quarter of floats deployed in ISOW at 1800‐m depth upstream in the Iceland Basin show a direct advective pathway into the BFZ that appears to be primarily determined by bathymetry. Another quarter of the floats crossed over the ridge to the Irminger Sea through other gaps prior to reaching the Charlie‐Gibbs Fracture Zone. 

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. 

Abstract We present the first continuous mooring records of the West Greenland Coastal Current (WGCC), a conduit of fresh, buoyant outflow from the Arctic Ocean and the Greenland Ice Sheet. Nearly two years of temperature, salinity, and velocity data from 2018 to 2020 demonstrate that the WGCC on the southwest Greenland shelf is a well-formed current distinct from the shelfbreak jet but exhibits strong chaotic variability in its lateral position on the shelf, ranging from the coastline to the shelf break (50 km offshore). We calculate the WGCC volume and freshwater transports during the 35% of the time when the mooring array fully bracketed the current. During these periods, the WGCC remains as strong (0.83 ± 0.02 Sverdrups; 1 Sv ≡ 10⁶m³s⁻¹) as the East Greenland Coastal Current (EGCC) on the southeast Greenland shelf (0.86 ± 0.05 Sv) but is saltier than the EGCC and thus transports less liquid freshwater (30 × 10⁻³Sv in the WGCC vs 42 × 10⁻³Sv in the EGCC). These results indicate that a significant portion of the liquid freshwater in the EGCC is diverted from the coastal current as it rounds Cape Farewell. We interpret the dominant spatial variability of the WGCC as an adjustment to upwelling-favorable wind forcing on the West Greenland shelf and a separation from the coastal bathymetric gradient. An analysis of the winds near southern Greenland supports this interpretation, with nonlocal winds on the southeast Greenland shelf impacting the WGCC volume transport more strongly than local winds over the southwest Greenland shelf. 

Abstract The Greenland Ice Sheet is losing mass at an accelerating pace, increasing its contribution to the freshwater input into the Nordic Seas and the subpolar North Atlantic. It has been proposed that this increased freshwater may impact the Atlantic Meridional Overturning Circulation by affecting the stratification of the convective regions of the North Atlantic and Nordic Seas. Observations of the transformation and pathways of meltwater from the Greenland Ice Sheet on the continental shelf and in the gyre interior, however, are lacking. Here, we report on noble gas derived observations of submarine meltwater distribution and transports in the East and West Greenland Current Systems of southern Greenland and around Cape Farewell. In southeast Greenland, submarine meltwater is concentrated in the East Greenland Coastal Current core with maximum concentrations of 0.8%, thus significantly diluted relative to fjord observations. It is found in water with density ranges from 1,024 to 1027.2 kg m⁻³and salinity from 30.6 to 34, which extends as deep as 250 m and as far offshore as 60 km on the Greenland shelf. Submarine meltwater transport on the shelf averages 5.0 ± 1.6 mSv which, if representative of the mean annual transport, represents 60%–80% of the total solid ice discharge from East Greenland and suggests relatively little offshore export of meltwater east and upstream of Cape Farewell. The location of the meltwater transport maximum shifts toward the shelfbreak around Cape Farewell, positioning the meltwater for offshore flux in regions of known cross‐shelf exchange along the West Greenland coast. 

Abstract Understanding the variability of the Atlantic Meridional Overturning Circulation is essential for better predictions of our changing climate. Here we present an updated time series (August 2014 to June 2020) from the Overturning in the Subpolar North Atlantic Program. The 6-year time series allows us to observe the seasonality of the subpolar overturning and meridional heat and freshwater transports. The overturning peaks in late spring and reaches a minimum in early winter, with a peak-to-trough range of 9.0 Sv. The overturning seasonal timing can be explained by winter transformation and the export of dense water, modulated by a seasonally varying Ekman transport. Furthermore, over 55% of the total meridional freshwater transport variability can be explained by its seasonality, largely owing to overturning dynamics. Our results provide the first observational analysis of seasonality in the subpolar North Atlantic overturning and highlight its important contribution to the total overturning variability observed to date. 

Abstract Arctic‐origin and Greenland meltwaters circulate cyclonically in the boundary current system encircling the Labrador Sea. The ability of this freshwater to penetrate the interior basin has important consequences for dense water formation and the lower limb of the Atlantic Meridional Overturning Circulation. However, the precise mechanisms by which the freshwater is transported offshore, and the magnitude of this flux, remain uncertain. Here, we investigate wind‐driven upwelling northwest of Cape Farewell using 4 years of high‐resolution data from the Overturning in the Subpolar North Atlantic Program west Greenland mooring array, deployed from September 2014–2018, along with Argo, shipboard, and atmospheric reanalysis data. A total of 49 upwelling events were identified corresponding to enhanced northwesterly winds, followed by reduced along‐stream flow of the boundary current and anomalously dense water present on the outer shelf. The events occur during the development stage of forward Greenland tip jets. During the storms, a cross‐stream Ekman cell develops that transports freshwater offshore in the surface layer and warm, saline, Atlantic‐origin waters onshore at depth. The net fluxes of heat and freshwater for a representative storm are computed. Using a one‐dimensional mixing model, it is shown that the freshwater input resulting from the locus of winter storms could significantly limit the wintertime development of the mixed layer and hence the production of Labrador Sea Water in the southeastern part of the basin. 

Abstract Recent mooring measurements from the Overturning in the Subpolar North Atlantic Program have revealed abundant cyclonic eddies at both sides of Cape Farewell, the southern tip of Greenland. In this study, we present further observational evidence, from both Eulerian and Lagrangian perspectives, of deep cyclonic eddies with intense rotation (ζ/f> 1) around southern Greenland and into the Labrador Sea. Most of the observed cyclones exhibit strongest rotation below the surface at 700–1000 dbar, where maximum azimuthal velocities are ~30 cm s⁻¹at radii of ~10 km, with rotational periods of 2–3 days. The cyclonic rotation can extend to the deep overflow water layer (below 1800 dbar), albeit with weaker azimuthal velocities (~10 cm s⁻¹) and longer rotational periods of about one week. Within the middepth rotation cores, the cyclones are in near solid-body rotation and have the potential to trap and transport water. The first high-resolution hydrographic transect across such a cyclone indicates that it is characterized by a local (both vertically and horizontally) potential vorticity maximum in its middepth core and cold, fresh anomalies in the deep overflow water layer, suggesting its source as the Denmark Strait outflow. Additionally, the propagation and evolution of the cyclonic eddies are illustrated with deep Lagrangian floats, including their detachments from the boundary currents to the basin interior. Taken together, the combined Eulerian and Lagrangian observations have provided new insights on the boundary current variability and boundary–interior exchange over a geographically large scale near southern Greenland, calling for further investigations on the (sub)mesoscale dynamics in the region.