Moored Observations of the West Greenland Coastal Current along the Southwest Greenland Shelf

Nicholas P. Foukal aWoods Hole Oceanographic Institution, Woods Hole, Massachusetts

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Robert S. Pickart aWoods Hole Oceanographic Institution, Woods Hole, Massachusetts

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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 ≡ 106 m3 s−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 × 10−3 Sv in the WGCC vs 42 × 10−3 Sv 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.

© 2023 American Meteorological Society. This published article is licensed under the terms of the default AMS reuse license. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Nicholas P. Foukal, nfoukal@whoi.edu

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 ≡ 106 m3 s−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 × 10−3 Sv in the WGCC vs 42 × 10−3 Sv 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.

© 2023 American Meteorological Society. This published article is licensed under the terms of the default AMS reuse license. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Nicholas P. Foukal, nfoukal@whoi.edu

1. Introduction

The deep continental shelf around Greenland creates two bathymetric gradients that each support a surface-intensified current: the slope-to-shelf gradient supports a shelfbreak jet and the shelf-to-coast gradient supports a coastal current. Around Greenland, the shelfbreak jet is referred to as the East Greenland Current (EGC) or West Greenland Current (WGC), and these currents are joined by the Irminger Current (IC) that flows cyclonically around the subpolar gyre in the Labrador and Irminger Seas (Le Bras et al. 2018; Pacini et al. 2020). Inshore of the shelfbreak jet is the coastal current which is called the East Greenland Coastal Current (EGCC) or West Greenland Coastal Current (WGCC; Fig. 1). This coastal current system transports the freshest water masses around Greenland, made up of meltwater from a mix of the Greenland Ice Sheet and Arctic pack ice. Many studies have hypothesized that if the fresh, buoyant water masses from the coastal current were mixed offshore, they could stratify the subpolar North Atlantic, reduce water mass transformation, and slow the Atlantic meridional overturning circulation (AMOC; e.g., Rahmstorf et al. 2015; Hansen et al. 2016). Given their potential role in large-scale climate variability, surprisingly little is known about the coastal currents and how this freshwater is introduced to the subpolar North Atlantic, if at all. Mapping the coastal circulation around Greenland—including where the currents are located, how strong they are, and mechanisms of their variability—is the next step in understanding this potential AMOC forcing mechanism.

Fig. 1.
Fig. 1.

(a) Schematic circulation around southern Greenland. The U.S. OSNAP Labrador Sea Mooring Array spans the WGCC, WGC/IC, and deep western boundary current (DWBC). (b) The time-mean velocity vectors from the upward-looking ADCPs (upper 100 m) over the period 2014–20 (blue) and 2018–20 (red). The LSA and LSB moorings were first deployed in 2018. The along-stream and cross-stream coordinate system is shown in black.

Citation: Journal of Physical Oceanography 53, 11; 10.1175/JPO-D-23-0104.1

The EGCC has been observed frequently on the southeast Greenland shelf from surface drifters (Duyck and de Jong 2021) and individual shipboard snapshots (e.g., Bacon et al. 2002; Lin et al. 2018; Benetti et al. 2019). These observations depict a persistent coastal current core on the southeast Greenland shelf with velocities reaching over 1 m s−1 (Pickart et al. 2005). The EGCC is associated with a fresh “wedge” of water banked up against the coastline, which is initially supported by buoyant outflow from the Arctic through Fram Strait (Karpouzoglou et al. 2023), and is maintained down to Cape Farewell at the southern tip of Greenland by a combination of glacial meltwater input to the inshore side of the current, and along-coast, downwelling-favorable barrier winds on the East Greenland shelf (Moore and Renfrew 2005; Sutherland and Pickart 2008; Foukal et al. 2020). The volume transport varies considerably between the hydrographic sections, ranging from about 0.25 to 2 Sverdrups (Sv; 1 Sv ≡ 106 m3 s−1; e.g., Pickart et al. 2005; Sutherland and Pickart 2008), likely due to a mixture of subinertial variability (Gelderloos et al. 2022) and instabilities arising from the strong velocities interacting with the rugged coastline and rough bathymetry (Duyck and de Jong 2021). An analysis of 2 years of continuous mooring data from the Overturning in the Subpolar North Atlantic Program (OSNAP) Cape Farewell line at 60°N confirmed this large variability and found that, in the mean, the EGCC transported 0.86 ± 0.10 Sv southward, and 3 times more liquid freshwater transport referenced to 34.9 than the EGC/IC (Le Bras et al. 2018). The seasonality over this 2-yr period indicates that the EGCC is strongest in October–February and weakest in April–July (Le Bras et al. 2018).

The WGCC has not been as extensively observed as the EGCC. In 2014, a series of sections were occupied in a spoke–wheel pattern around Cape Farewell that cut across the WGCC at three locations. These data indicate that much of the EGCC continues around Cape Farewell on the shelf and feeds directly into the WGCC that shifts toward the shelf break (Lin et al. 2018). An additional section from Benetti et al. (2019) on the southwest Greenland shelf also captured a distinct WGCC, though the peak velocities were relatively weak (∼0.25 m s−1) and no volume transport was reported. An analysis of the different components of the West Greenland Current system across the AR7W line near Cape Desolation (∼61°N) did not detect a distinct coastal current (Rykova et al. 2015), although the station spacing and distance to the coast of the inshore-most station could have missed the WGCC. High-resolution modeling work (1/60° lateral grid spacing) shows that the WGCC is present from Cape Farewell northward to Davis Strait, although its volume and freshwater transports decrease northward as water is diverted into the shelfbreak WGC (Gou et al. 2021).

Variability in the coastal currents is largely set by along-coast winds which regulate the cross-shelf sea surface height (SSH) gradient leading to accelerations/decelerations of the coastal currents (Bacon et al. 2002; Sutherland and Pickart 2008). The winds on the Greenland shelf are strongly influenced by the relief of the continent and ice sheet and thus tend to blow along the coastline. The along-coast winds vary on seasonal and interannual time scales that are largely influenced by the North Atlantic Oscillation (Pickart et al. 2003; Moore et al. 2013), as well as on synoptic time scales in response to the North Atlantic storm track (Våge et al. 2008). Low pressure systems passing south of Greenland along the North Atlantic storm track tend to impinge on the topography of southern Greenland causing forward tip jet events that drive strong westerly winds into the Irminger Sea, and reverse tip jets that drive northeasterly winds in the southeastern Labrador Sea (Moore and Renfrew 2005). These events likely cause significant shelf–basin exchange around Cape Farewell (Schulze Chretien and Frajka-Williams 2018; Duyck et al. 2022; Pacini and Pickart 2023), although their impact on the coastal current is less well known. Similarly, how the coastal current responds to the change from downwelling conditions on the East Greenland shelf to upwelling conditions on the West Greenland shelf as it rounds Cape Farewell is not yet fully understood.

In this paper, we present the first continuous observations of the WGCC using 2 years of moored data along the U.S. OSNAP Labrador Sea array from 2018 to 2020. Previous work with data from this array (Pacini et al. 2020, 2021; Pacini and Pickart 2022, 2023) has focused on the shelfbreak WGC/IC using the first 4 years of data (2014–18). In 2018, two moorings were added to the inshore side of the array that provide an unprecedented look at the WGCC and its variability, which we describe herein.

2. Data and methods

a. Mooring array design

We use temperature, salinity, and velocity data acquired from the U.S. OSNAP Labrador Sea moored array from September 2018 through June 2020. This line of ten moorings crosses the continental rise, slope, and shelf of southwest Greenland and ends near the Kitsissut Islands, an archipelago of islands extending about 15 km from the coast. High-resolution models and surface drifters indicate that there is minimal net flow through the Kitsissut Islands (Gou et al. 2021); thus, we treat the offshore edge of this archipelago as the coastline throughout the paper. As seen in Fig. 2, offshore from the Kitsissut Islands, the bathymetry drops to 120 m within 2 km of the coastline. The inner shelf is broken by a 10-km-wide depression down to 200-m depth; then, the shelf slopes gradually down to the shelf break at a depth of 190 m about 50 km offshore. Beyond the shelf break, the bathymetry drops off to over 2000-m depth in 25 km. This shipboard-based bathymetric section (used in the vertical sections) aligns closely with the BedMachine Greenland v4 bathymetric product (Morlighem et al. 2017) used in the figure maps (e.g., Fig. 1b).

Fig. 2.
Fig. 2.

Mean and composites of the along-stream velocity field. (a) The time-mean velocity field. (b) Composite of 17.7% of the hourly snapshots when no WGCC was identified. (c) Composite of 47.8% of the hourly snapshots when the WGCC was sampled but not bracketed by the mooring array. (d) Composite of 34.5% of the hourly snapshots when the WGCC was bracketed by the mooring array. See the text for details. In all panels, black circles denote the velocity measurements, either from an ADCP bin (upper water column) or a point current meter.

Citation: Journal of Physical Oceanography 53, 11; 10.1175/JPO-D-23-0104.1

To focus on the WGCC, we use the five shelf moorings (LSA–LS3) spanning the 120–190-m isobaths, as well as the upper 350 m of the water column out to LS6, about 90 km from the coast. The inshore-most mooring (LSA) was positioned 17 km offshore of the coast at 120-m depth, LSB was 23 km offshore at 131 m, LS1 was 32 km offshore at 142 m, LS2 was 41 km offshore at 155 m, and LS3 was 50 km offshore at 191 m. Seaward of the shelf, moorings LS3, LS4, and LS5 were spaced 12 km apart, and moorings LS5 and LS6 were separated by 15 km. The five shelf moorings (LSA–LS3) consisted of bottom tripods anchored to the seafloor outfitted with a conductivity–temperature–depth (CTD) instrument, a point current meter, and an upward-looking acoustic Doppler current profiler (ADCP). The ADCPs were either RDI 300-kHz Workhorses with 8-m bins (LSA, LSB, and LS1) or RDI 75-kHz Long Rangers with 10-m bins (LS2 and LS3). The CTDs were Sea-Bird 37SI MicroCATs, and the point current meters were Nortek Aquadopps. Above the tripods, LSA and LSB had a CTD on an ice-expendable tether at 50-m depth, whereas LS1 through LS3 had one at 100-m depth. These tethers were designed to release their flotation via a weak link in case the package was hit by ice, allowing the instruments to remain attached to the tripods, and thus be recovered. LSA and LSB had redundant flotation on the tether at 70 m so that, if the top float were hit, the CTD would drop to 90-m depth. The three tall moorings used in this study (LS4–LS6) had upward-looking RDI Workhorses at 100 m, as well as Nortek Aquadopp current meters at 240, 500, and 600 m (LS4); 280, 540, 790, 1040, and 1440 m (LS5); and 250, 500, 750, 1000, 1400, and 2007 m (LS6). The ADCPs recorded hourly data, the current meters recorded data every 30 min, and the CTDs recorded data every 15 min. This extended configuration of moorings was deployed from 22 September 2018 to 29 June 2020, which is the period considered in the study. Calibrations, quality control, and processing of the ADCP and CTD data were identical to that described by Pacini et al. (2020).

b. Data return and gridding

The data return for the 10 moorings was nearly perfect, although the CTD on the LS3 tripod (190-m depth) flooded and no data were recovered from this instrument. In addition, the CTD on the LSA tether (50 m) lost its flotation on 1900 UTC 20 June 2019, presumably due to contact with an iceberg. The instrument dropped to 90 m and continued returning good data for the remainder of the deployment. At LSB, we detected a sizable drift in the conductivity measured on the tripod CTD (144 m) starting on 15 July 2019, likely due to biofouling. We hypothesized that we could fill in the salinity data after this date data using linear interpolation between the LSA and LS1 tripods due to the positioning of the wedge of freshwater on the shelf. To test this hypothesis, we calculated the correlation between the LSB salinity data from September 2018 to June 2019 and the linearly interpolated salinity between the LSA and LS1 tripod CTDs over the same period. The r2 value was 0.87 at 15-min resolution, indicating that the majority of variability could be explained by this linear interpolation. This high correlation, together with a clear physical reason why the salinities should be related, justified our choice to reconstruct the remainder of the LSB tripod time series using the linear relationship.

The CTD data and current meter data were averaged to 1-h resolution to match the output from the ADCPs. We computed potential temperatures and densities referenced to the sea surface and used practical salinities (no units). The current velocities were detided using a harmonic analysis from T_tide v1.4 (Pawlowicz et al. 2002) and then rotated into along-stream (positive = 312°T) and cross-stream (positive = 222°T) directions (see Fig. 1b). This choice minimizes the mean cross-stream velocities and orients the positive along-stream velocities approximately along the isobaths (Pacini et al. 2020).

We sought to grid the data onto a regularly spaced grid at hourly resolution for further analysis. To do this in a physically consistent way that would limit density inversions or spurious vertical shears, we first extrapolated the temperatures, salinities, and velocities from the topmost valid data point to the surface. The ADCPs typically collected data close to the surface, but intermittent blowdowns of moorings LS4–LS6 from strong currents (Pacini et al. 2021), as well as periods in winter when there were low scatterers in the water column at all moorings, led to some missing data in the upper layer. Both types of gaps were filled by fitting the topmost 50 m of valid data with a linear vertical gradient and extrapolating to the surface from the topmost valid data point. For the temperature and salinity fields, a vertical gradient was calculated between the tripod and tether CTD, and the water column was filled at 10-m increments with this linear interpolation between the two instruments, and extrapolation above the tether. (The lack of CTD data from the LS3 tripod meant we could not calculate a vertical gradient at that site; therefore, we did not interpolate or extrapolate the vertical hydrographic structure there.) The sensitivity of this choice of linear interpolation was tested with respect to shipboard hydrographic data collected during the summer mooring turnaround cruises (Figs. S1–S3 in the online supplemental material). The shipboard data were collected at high vertical (1-m CTD bins and 4-m velocity bins) and horizontal (∼5-km spacing between CTD casts) resolution and extended shoreward to as close as 2 km from the coastline (Kitsissut Islands). While they resolve more detailed structure in the hydrographic and velocity fields, they are limited by the fact that they are snapshots in time and to the summer months (July–September). The shipboard data were gridded onto a 2-km (horizontal) and 10-m (vertical) section for visualization (Fig. S1).

Following the extrapolation to the surface, the temperature, salinity, and velocity fields were put onto a regular grid using a Laplacian spline interpolator at each hourly time step (Smith and Wessel 1990). We first gridded the offshore data LS5–LS8 at 15-km (horizontal) and 200-m (vertical) resolution due to the larger distances between these moorings. We then combined the output of that coarser grid with the instrument data from the inshore moorings (LSA–LS5) and gridded the combination at 2-km (horizontal) and 25-m (vertical) resolution. This procedure, which retains the higher instrument resolution on the shelf while avoiding spurious interpolated structure over the slope, was also used successfully by Pacini et al. (2020) using the U.S. OSNAP Labrador Sea mooring data.

c. Identifying the WGCC

We searched for the presence of the WGCC in each daily gridded velocity field. To isolate the WGCC from the deeper and stronger WGC/IC, we first identified the location of the WGC/IC by vertically summing the gridded velocity field over the top 500 m to yield a single value in units of m2 s−1 at each horizontal distance from 14 to 92 km offshore. The location of the maximum of this transport per unit width at each time step was identified as the location of the WGC/IC. By integrating to 500 m, despite the depth of the shelf being less than 200 m, we ensured that the algorithm would correctly identify the peak transport offshore of the shelf break instead of velocity peaks on the shelf associated with the WGCC. Once the WGC/IC was identified, we then isolated the WGCC by vertically averaging the velocities in the upper 100 m and looking for local maxima inshore of the WGC/IC and LS3 (located 50 km offshore) that exceeded 15 cm s−1. If there were no distinct local maxima on the shelf, we concluded that the WGCC was not present and then proceeded to the next daily time step. If there was a velocity peak on the shelf, the strongest peak was identified as the WGCC. We also further diagnosed the gridded velocity field when the WGCC was detected to determine if the moorings sampled only part of the current or bracketed the entire current. To bracket the WGCC, the inshore-most value of the upper 100-m mean velocity had to be less than 1/e of the strongest peak value on the shelf. If this condition was met, we calculated the WGCC volume transport by multiplying the gridded velocity field by the cross-sectional area of the grid cells (25 m × 2 km) and summing all velocities exceeding 1/e of the maximum WGCC velocity inshore of LS3. If the WGCC velocities did not drop below 1/e of the WGCC core at the inshore-most location, then the snapshot was classified as partially sampling the WGCC, and no transports were calculated.

d. Atmospheric reanalysis data

To assess the physical forcing mechanisms of the WGCC, we use the 10-m winds from the ERA5 atmospheric reanalysis (Hersbach et al. 2020) for the period of the mooring array deployment at hourly resolution from September 2018 through June 2020. The wind fields were spatially averaged over two regions marked in Fig. 7c and Fig. S4: the East Greenland shelf (defined by 60.00°N, 43.25°W; 62.00°N, 42.00°W; 62.00°N, 40.75°W; 60.00°N, 42.00°W) and the vicinity of the west Greenland moorings (defined by 60.19°N, 45.50°W; 59.50°N, 47.30°W; 59.05°N, 46.70°W; 59.74°N, 44.90°W). The winds in each region were rotated into the alongshore and cross-shore components (positive alongshore = 312°T for West Greenland moorings and 255°T for the East Greenland shelf), with positive denoting downwelling-favorable conditions.

3. Results

a. Composite analysis of along-stream velocity and salinity sections

The mean vertical section of the along-stream currents averaged over the 21-month period (Fig. 2a) depicts a WGC/IC with a peak velocity of 65 cm s−1 centered about 20 km offshore of the shelf break, and weaker flow in the 15–40 cm s−1 range on the shelf. The surface currents weaken monotonically from LS4 toward the coast; although at 100–150-m depth, there is a slight upward lift of the 15 and 20 cm s−1 isotachs near LS2. Thus, the WGCC is not fully distinct from the WGC/IC in the mean, although there is some separation at depth. We now present the average velocity structure during the three conditions: when no distinct WGCC was identified (Fig. 2b), when the WGCC was sampled (Fig. 2c), and when the WGCC was bracketed (Fig. 2d). In the no WGCC composite (Fig. 2b), the shelf velocities at LSA and LSB are between 0 and 10 cm s−1 northward, or 15–25 cm s−1 weaker than the mean, whereas the surface velocities are intensified by up to 10 cm s−1 at LS3. The combination of the weakening of the midshelf velocities and strengthening on the outer shelf indicates that the coastal current has likely merged with the WGC/IC during these periods (17.7% of the total hourly snapshots). During the time that the WGCC was sampled (almost 50% of the time, Fig. 2c), a peak surface velocity is present at LSA and a minimum is present at LS1, while the velocity structure on the mid-to-outer shelf (30–50 km from the coast) is very similar to the mean conditions. The core of the WGCC is likely inshore of LSA during these time periods. In the composite when the WGCC is bracketed by the mooring array (34.5% of the total hourly snapshots, Fig. 2d), the peak occurs at LS1 with core velocities exceeding 45 cm s−1. There is also a clear minimum between the WGCC and WGC/IC located about 40–45 km offshore. When the WGCC is bracketed, the velocities on the inner shelf at LSA (15–20 km from the coast) are over 15 cm s−1 weaker than the mean state.

We further diagnose the salinity fields (Fig. 3) during these three conditions of the current. In the mean, salinities as low as 32.6 on the inner shelf at LSA and 34.2 on the outer shelf at LS3 create a strong cross-shelf gradient. This corresponding wedge of fresh, polar water is persistent in all seasons but is freshest in January–March, and contracts shoreward in July–September (not shown). In the composite without a WGCC (Fig. 3b), the outer shelf (25–50 km) is fresher by up to 0.4, indicating that the fresh wedge has moved offshore, in line with the along-stream velocities that indicated the WGCC had merged with the WGC/IC in this composite. When the WGCC was sampled (Fig. 3c), the entire shelf was saltier, indicating that the fresh wedge had migrated shoreward of LSA (Fig. 2c). This is also consistent with the composite velocity field for this condition, indicating that the core of the current was inshore of LSA (Fig. 2c). When the WGCC is bracketed, the inner shelf (15–35 km from the coast) freshened by up to 0.25 near the surface, whereas the outer shelf became slightly saltier (0.05 increase at LS3). The combination of these two effects sharpened the cross-shelf salinity front at LS1, where the WGCC is centered in along-stream velocity (Fig. 2d). The signals in each of these salinity anomaly composites align closely with the along-stream velocities (Fig. 2), and both can be explained by lateral shifts in the location of the WGCC. To summarize, the WGCC was merged with the WGC/IC on the outer shelf roughly a fifth of the time, was located on the central shelf and bracketed by the array roughly a third of the time, and was partially shoreward of the array on the inner shelf roughly half of the time.

Fig. 3.
Fig. 3.

Mean and composite anomalies (calculated relative to the mean) of the salinity field. (a) The time-mean salinity, depicting a wedge of freshwater on the shelf. Note the nonlinear color axis. (b) Salinity anomaly (color) when no WGCC was detected. The black contours are the salinity field of the composite. (c) As in (b), but when the WGCC was sampled. (d) As in (b), but when the WGCC was bracketed. In all panels, white circles denote the mean location of the MicroCATs.

Citation: Journal of Physical Oceanography 53, 11; 10.1175/JPO-D-23-0104.1

To determine the time variability of the WGCC location on the shelf, we calculated the percent of each month in which the WGCC was in each of the three states, plus the combination of sampled and bracketed, defined to be “WGCC present” (Fig. 4). There is considerable month-to-month variability in each of the time series, but no clear seasonality, suggesting that the WGCC moves freely across the shelf regardless of the season. We also considered higher frequencies to determine whether the WGCC preferred one state over the other for prolonged periods of time. Even at hourly time scales, the current switched between locations. The most common duration (mode) of each state of the WGCC (no coastal current, sampled, and bracketed) was 1 h, although the distributions all had long tails. This is especially true of the sampled state, so that the mean durations were 3.8 h (no coastal current), 9.0 h (sampled), and 5.9 h (bracketed). Interestingly, there were two periods extending over a week each in which the WGCC was consistently sampled, indicating that there was a quasi-stable condition in which the WGCC was banked against the coastline, inshore of LSA.

Fig. 4.
Fig. 4.

Percent of each month covered by the three sampling regimes. The “Coastal current present” curve (green) is the sum of “Coastal current sampled” (orange) and “Coastal current bracketed” (red).

Citation: Journal of Physical Oceanography 53, 11; 10.1175/JPO-D-23-0104.1

b. Volume and freshwater transport of the WGCC

We now focus on the period when the WGCC was bracketed to calculate the volume transport, liquid freshwater transport, and flow-weighted mean salinity. These time series are shown in Fig. 5, and the statistics are presented in Table 1. The volume transport is highly variable, ranging from 0.09 to 3.61 Sv, with an overall median of 0.83 Sv. We report median values here because the data are nonnormally distributed with a skew toward large values. The variability in the volume transport is larger in the winter months. In particular, the standard deviation for October–April is 0.52 Sv versus 0.34 Sv for June–September. The 5-day smoothed time series (red curves in Fig. 5) demonstrate that much of this variability is present in the synoptic wind time scale of 3–5 days. We find no clear seasonal cycle in the volume transport with these 21 months of data.

Fig. 5.
Fig. 5.

Time variability of (a) volume transport, (b) liquid freshwater transport calculated relative to 34.9, and (c) flow-weighted salinity. Values are calculated at hourly resolution (black) when the coastal current is bracketed and then smoothed with a 5-day running mean (red).

Citation: Journal of Physical Oceanography 53, 11; 10.1175/JPO-D-23-0104.1

Table 1.

WGCC volume transport, liquid freshwater transport, and flow-weighted salinity statistics.

Table 1.

The median liquid freshwater transport relative to 34.9 is 30.40 × 10−3 Sv. It is positively correlated with the volume transport (r = 0.90, p < 0.01 on 5-day time scales) and exhibits similar seasonality in variability (Fig. 5b). In Table 1, we report the liquid freshwater transport relative to 34.9 and 34.8 to be consistent with previous publications (Sutherland and Pickart 2008; Lique et al. 2009; de Steur et al. 2009; Lin et al. 2018; Le Bras et al. 2018; Gou et al. 2022), while also allowing direct assessment of the sensitivity to this choice of reference salinity. Freshwater transport across a non-mass-conserving section is always sensitive to the choice of reference salinity (Schauer and Losch 2019).

In addition to the freshwater transport, we report the flow-weighted salinity (Fig. 5c). The combination of volume transport, which addresses how strong the current is, and flow-weighted salinity, which addresses how salty the current is, provides a more quantitatively robust metric than freshwater transport that is not sensitive to a choice of reference salinity. The flow-weighted salinity of the WGCC varies between 31.70 and 34.89, with a median value of 33.55. It is weakly negatively correlated to the WGCC volume transport (r = −0.30, p < 0.01 on 5-day time scales). This negative correlation suggests that the WGCC is fresher when it is stronger, two factors that together drive stronger variability in the freshwater transport. Interestingly, the magnitude of the correlation between the flow-weighted salinity and the freshwater transport (r = −0.65, p < 0.01 on 5-day time scales) is weaker than the volume transport, demonstrating that the variability in freshwater transport at 5-day time scales is primarily set by variability in the volume transport, and secondarily by variability in the salinity.

4. Comparison to the EGCC

The volume and freshwater transports reported herein compare quite closely with values reported from the EGCC at the OSNAP East mooring line (Le Bras et al. 2018). The mean EGCC volume transport from 2014 to 2016 was 0.86 Sv, with a liquid freshwater transport of 42 × 10−3 Sv referenced to 34.9, although no flow-weighted salinity was reported. The 95% confidence intervals (∼1.96 × standard error) were given as ±0.10 Sv and 6 × 10−3 Sv, respectively. The similarity in the volume transports of the EGCC from 2014 to 2016 and the WGCC from 2018 to 2020 is likely serendipitous, however, given that these values do not cover the same period and the methods differ. In particular, Le Bras et al. (2018) integrate all velocities inshore of their CF2 mooring (∼25 km from the coast) using extrapolation inshore of their CF1 mooring (∼12 km from the coast), while in the present analysis, we only calculate the volume and freshwater transports when the WGCC is bracketed because of the current’s strong spatial variability. These varying methods likely affect comparisons between the mooring arrays. However, a more consistent approach taken by Lin et al. (2018) using shipboard hydrographic/velocity sections yielded a similar large-scale picture: the volume transport of the coastal current decreased only slightly from 1.00 Sv at OSNAP East to 0.87 Sv at OSNAP West. These shipboard sections have higher spatial resolution and extend closer to the coastline than the mooring arrays; thus, their higher values are likely closer to the truth for these snapshots. However, the mooring records demonstrate the strong temporal variability of the EGCC and WGCC and highlight the potential for temporal aliasing with individual hydrographic sections. Combining these moored records and hydrographic snapshots, we cannot detect a statistically significant decrease in volume transport of the coastal current as it rounds the southern tip of Greenland.

By contrast, the liquid freshwater transport from the two mooring arrays decreases from 42 ± 6 × 10−3 Sv in the EGCC to 30.40 ± 3.67 × 10−3 Sv in the WGCC (both relative to 34.9), indicating that the EGCC is significantly fresher than the WGCC and that freshwater is indeed lost around Cape Farewell. The shipboard sections analyzed by Lin et al. (2018) support this, although their values are higher at both locations: 68.45 × 10−3 Sv at OSNAP East and 54.02 × 10−3 Sv at OSNAP West (referenced to 34.8). Again, this can likely be explained by the higher spatial resolution and coverage of the inner shelf in the shipboard sections. The authors explain this decrease in the liquid freshwater transport around Cape Farewell as a result of a portion of the WGCC diverting offshore toward the shelf break and interacting with the WGC/IC. Our results indicate that the WGCC does move offshore, although its location is highly temporally variable.

One of the most striking results from our analysis of the moorings is how freely and quickly the WGCC moves laterally from the inner shelf, where only a portion of the current was sampled by LSA, to the shelf break where it merges with the WGC/IC. This result suggests that the WGCC may be dominated by eddies and fine-scale structure rather than a well-formed jet like the EGCC. We note, however, that the WGCC is clearly identifiable over 80% of the time (sampled + bracketed) and thus is a consistent feature that is regularly distinguishable from the shelfbreak jet. We interpret this variability as an adjustment of the coastal current to different wind forcing as it rounds Cape Farewell and separates from the coast. On the southeast Greenland shelf, the winds are downwelling favorable, which traps the wedge of freshwater against the coastline. On the southwest Greenland shelf, the winds are upwelling favorable and baroclinic instabilities are prone to develop (Gou et al. 2021; Pacini et al. 2021; Pacini and Pickart 2022); thus, the WGCC appears to move chaotically across the shelf as revealed by our fixed mooring array.

A notable discrepancy between the EGCC reported in Le Bras et al. (2018) and the WGCC presented here pertains to the seasonal cycle in volume transport. Le Bras et al. (2018) found a clear seasonal cycle with a range of 0.46 Sv, peaking in December and at a minimum in June. In the first 12 months of our record (September 2018–August 2019), a similar pattern emerged. However, there was no intensification of the WGCC in the second winter (October 2019–May 2020), and thus, we find no robust seasonality in the volume transport over the 21-month record. We note that weak transport values in January–March 2020 corresponded with a decrease in the percentage of time that the WGCC was bracketed, and an increase in the percent of months when no coastal current was present (with the assumption that it merged with the shelfbreak jet, Fig. 5). This period was characterized by unusually strong upwelling conditions on the West Greenland shelf (Fig. S4) that likely forced the WGCC toward the shelf break and diminished its strength.

In a 1/60° model of the Labrador Sea from 2005 to 2014, Gou et al. (2021) report a mean volume transport of the WGCC across the OSNAP West line of 0.54 Sv and a freshwater transport (relative to 34.8) of 29.4 × 10−3 Sv. These values were updated in Gou et al. (2022), using the same model with different surface forcing, to be 0.47 Sv and 26.9 × 10−3 Sv, respectively. The freshwater transports are similar to what we report here, although the lower volume transports (0.47–0.54 Sv in the model compared to 0.83 Sv from the moorings) indicate that the modeled WGCC was significantly fresher than the observations indicate. The authors also report a robust seasonal cycle in the volume and freshwater transports that we do not find here. One possible reason for these discrepancies is the splitting of the flow in the modeled WGCC just upstream of the OSNAP West moorings (see their Fig. 2). This feature appears due to a small island on the shelf that is not present in other bathymetric products of the region (nor encountered during the OSNAP cruises). A portion of the WGCC may be diverted to the WGC/IC at this island in the model, leading to the lower volume and freshwater transports.

There are more records of the Greenland coastal current volume transport for comparison if one considers a wider geographic range northward along the East Greenland shelf. These are compiled in Fig. 6, with values from shipboard sections shown as black markers and values from moored records shown as solid red/blue markers. Starting north of Denmark Strait on the East Greenland shelf at 72°N, a notable intensification of the EGCC occurs across Denmark Strait, where the volume transport nearly doubles from 0.66 Sv (mean of four sections north of Denmark Strait) to 1.12 Sv (mean of three values at 66°N). Low values near 65.5°N can be explained by the indentation of the coastline near Sermilik Deep that splits the EGCC (Harden et al. 2014; Duyck and de Jong 2021), and sampling that is considerably outdated from 1962 (Wilkinson and Bacon 2005), or did not cover the entire shelf (Harden et al. 2014). Around Cape Farewell, the continuity of volume transports from the moored records can be visualized quite clearly in the figure, and, excluding the two ship-based outliers that exceed 1.5 Sv, the shipboard sections also imply such continuity.

Fig. 6.
Fig. 6.

Compilation of all published values of EGCC and WGCC volume transports, arranged in geographical order starting from 72°N on the East Greenland shelf, proceeding southward through Denmark Strait to Cape Farewell, and then around Cape Farewell to Cape Desolation on the West Greenland shelf. Values from continuous moored observations (colored stars) are available at OSNAP East (Le Bras et al. 2018) and OSNAP West (present study). Figure updated from Lin et al. (2018).

Citation: Journal of Physical Oceanography 53, 11; 10.1175/JPO-D-23-0104.1

The various publications of EGCC and WGCC volume transports compiled in Fig. 6 use a range of methods to define the currents. Here, we have used a velocity-based metric to identify the WGCC, but others have used a fixed geographic box (Le Bras et al. 2018; Gou et al. 2021), isotachs (Sutherland and Pickart 2008; Lin et al. 2018), or a combination of fixed boxes, isohalines, and isotachs (Foukal et al. 2020). The precise definition of these coastal currents could contribute to the variability in Fig. 6, especially because the offshore delineation between the shelfbreak jet and the coastal current is often subtle, and isotachs typically do not outcrop (see Fig. 2d). As such, caution should be taken in interpreting these values, and more weight should be placed on locations with multiple sections using a diversity of methods (e.g., OSNAP East).

As described above, a similar plot of freshwater transport would convey less continuity around Cape Farewell. Attaching significant meaning to a plot of freshwater transport would be difficult because of a range of reference salinities used in the literature, as well as uncertainty in the structure in the gridded salinity from the moorings. In particular, the gridded salinity fields from the OSNAP East and OSNAP West mooring arrays involve differing degrees of extrapolation to arrive at a fully gridded product. In this analysis, we have linearly extrapolated the salinity in the vertical from the topmost instrument to the surface using the gradient between the tether and bottom-mounted CTDs. When compared to shipboard CTD data from summertime cruises, linear interpolation overestimates the salinity in the upper part of the water column because the tethers at LS1–LS3 did not sample into the fresh wedge that was typically in the upper 100 m (Fig. S1). Thus, the summertime flow-weighted salinity is likely overestimated, and the summertime liquid freshwater transport is likely underestimated. To quantify how sensitive our results are to this choice, we calculated the flow-weighted salinities and liquid freshwater transports using a gridded product that extrapolated to the surface with twice the gradient from the top two CTDs. This method matched the summer shipboard data more closely (Fig. S3) and is physically meaningful because it captures the expected halocline structure above the LSA–LS2 tethers. Using this method, the flow-weighted salinity was 33.34, and the freshwater transport relative to 34.9 was 35.89 × 10−3 Sv (33.54 relative to 34.8). Thus, our results are sensitive to how we extrapolate the salinities to the surface, demonstrating that the lack of near-surface salinity measurements in regions with ice is a major contributor to uncertainty in these important high-latitude, freshwater currents. We report the linearly interpolated data because we expect the mixed layers to reach the topmost CTDs (50 m at LSA and LSB; 100 m at LS1–LS3) in the nonsummer months, implying that the linear interpolation is more representative of the year-round conditions.

5. Role of wind forcing on WGCC variability

We now investigate the physical controls on the WGCC volume transport, in particular, the role of wind forcing on synoptic-scale WGCC variability. As noted above, our record of the WGCC is intermittent and the WGCC exhibits large chaotic swings in its location on the shelf. Thus, we only attempt to explain the largest signals in the WGCC volume transport. Our original hypothesis was that local downwelling- (upwelling-) favorable winds would intensify (weaken) the WGCC by raising (lowering) the coastal sea surface height. A first attempt to analyze individual notable events demonstrated that the WGCC volume transport was instead primarily set by nonlocal wind forcing over the East Greenland shelf and secondarily modulated by local winds at the array site. To illustrate the role of nonlocal wind forcing over the East Greenland shelf, we present three events that cover a range of wind forcing: 1) a 6-day period in early December 2018 when the WGCC volume transport reached its maximum over the 21-month record despite weak wind forcing locally over the moorings; 2) a 5-day period in early November 2018 when the strongest downwelling-favorable wind stress over the 21-month record caused a muted response from the WGCC; and 3) a 7-day period in early October 2019 when downwelling-favorable winds on the East Greenland shelf intensified the WGCC despite weak to slightly upwelling-favorable local winds.

The first event is depicted in Fig. 7. At the start of the event (2 December 2018; first vertical dashed line denoted “1” in Figs. 7a,b and corresponding to Figs. 7c,g), upwelling-favorable, northwesterly winds were present over the mooring array, with minimal winds on the East Greenland shelf (Figs. 7a,c). The volume transport of the WGCC was only 0.34 Sv and was consistently measured at less than 0.5 Sv for 12 h (Fig. 7b). The upwelling-favorable winds over the moorings appeared to push the WGCC offshore toward the shelf break, and there were a few instances on 3–4 December in which we could calculate a volume transport of the WGCC (Fig. 7b). Over the course of two days, a reverse tip jet event developed, with downwelling winds over the moorings peaking at 1400 UTC 4 December. The along-stream velocity (Fig. 7h) indicated southward flow on the shelf at this time and little immediate response to these local winds. The WGCC then shifted from the shelf break to inshore of LSA near midnight on 4 December and intensified to 2.34 Sv midday on 5 December and then to 3.61 Sv early on 6 December. This second increase in volume transport demonstrates that the WGCC was continuing to respond more than 30 h after the local winds had peaked, indicating that the local wind forcing could not have caused the strong intensification of the WGCC. Instead, as the low pressure rotated to the east around Cape Farewell, the winds became strongly downwelling favorable on the East Greenland shelf (Fig. 7e), while over the moorings, the along-coast winds weakened to near zero. The WGCC volume transport increased to the highest transport (3.61 Sv) in the record (Fig. 7b) at 0300 UTC 6 December, or 12 h after the winds intensified on the East Greenland shelf. This peak in the volume transport corresponded to WGCC velocities exceeding 100 cm s−1 centered at LS2 and strong northward velocities over the entire shelf to LSA. About 10 h after the winds subsided to near zero to the east of Cape Farewell, the WGCC volume transport relaxed back toward its mean state at 0800 UTC 7 December (Fig. 7j). A weaker WGCC was present centered at LSB at this time step. This time lag from the southeast Greenland shelf to the OSNAP West moorings of about 10–12 h corresponds well to the 11.9-h lag calculated by Pacini et al. (2020) for a coastally trapped barotropic wave propagating this distance.

Fig. 7.
Fig. 7.

Wind event 1 from 0000 UTC 2 Dec 2018 to 2300 UTC 7 Dec 2018. (a) Along-coast wind stress; (b) WGCC volume transport and location on the shelf; (c)–(f) ERA5 10-m winds; and (g)–(j) gridded along-stream velocities. Wind stress in (a) is averaged over the regions outlined in (c) and is positive for downwelling-favorable winds. Dashed vertical lines numbered 1–4 in (a) and (b) indicate the four snapshots shown in (c)–(j). In (b), the locations of the moorings are shown on the right axis for reference. The OSNAP West mooring array (red stars) is shown in (c)–(f). Black contours in (g)–(j) indicate isopycnals (kg m−3).

Citation: Journal of Physical Oceanography 53, 11; 10.1175/JPO-D-23-0104.1

In the second event, the WGCC was initially near its mean state (0.85 Sv at 0800 UTC 20 November 2018) and centered at LS1 (Figs. 8b,g). The winds were cross-shelf from the southwest locally over the moorings and slightly upwelling favorable on the East Greenland shelf (Fig. 8c). The WGCC subsequently weakened to 0.29 Sv at 0600 UTC 21 November and then intensified toward 1 Sv in response to local downwelling-favorable winds that started midday on 21 November (Fig. 8d). The winds peaked in excess of 34 m s−1 (wind stress of 0.97 N m−1) on 0900 UTC 22 November (Figs. 8b,e), the strongest downwelling-favorable wind stress recorded over the 21 months. The along-coast winds on the southeast Greenland shelf were minimal at this time. The WGCC shifted onshore and the volume transport increased to 1.99 Sv. This peak volume transport coincides almost exactly with the peak local winds, indicating a near-immediate intensification of the WGCC. While the response of the WGCC to these local winds is apparent (an intensification from 0.29 to 1.99 Sv over 25 h, or 0.07 Sv h−1), it was on par with the response to weaker local wind forcing less than 48 h later. This peak volume transport of 1.99 Sv just missed the top third percentile in volume transport; thus, the strongest local winds during the 21-month record can only explain an event that did not qualify in the top third of volume transports. Notably, weaker local wind forcing on 23 and 24 November coupled with downwelling on the East Greenland shelf drove an almost identical increase of the WGCC volume transport. In particular, the winds and the WGCC weakened on 23 November and then intensified again along with downwelling to the east of Cape Farewell. Despite the local wind stress being about half that of the previous local forcing, the intensification of the WGCC is similar (0.79–2.09 Sv over 18 h, or 0.07 Sv h−1). Thus, moderate downwelling locally and on the East Greenland shelf seems to be as effective at intensifying the WGCC as intense local downwelling in the absence of wind forcing on the East Greenland shelf. We discuss why nonlocal forcing is so critical after summarizing the third event.

Fig. 8.
Fig. 8.

Wind event 2 from 0600 UTC 20 Nov 2018 to 1200 UTC 24 Nov 2018 displayed as in Fig. 7.

Citation: Journal of Physical Oceanography 53, 11; 10.1175/JPO-D-23-0104.1

The third event demonstrates that prolonged downwelling-favorable conditions on the East Greenland shelf, in the absence of local wind forcing, is also an effective mechanism to intensify the WGCC. This event is characterized by strong northerly winds on the East Greenland shelf and weak-to-upwelling favorable winds locally over the moorings. Despite this lack of forcing on the West Greenland shelf, the WGCC intensified to 2.71 Sv (2 October) and 2.86 (5 October) in response to two peaks in winds over the East Greenland shelf. The WGCC started close to 1 Sv on 1 October, with slightly downwelling-favorable winds on the East Greenland shelf and near-zero along-shelf winds locally over the moorings (not shown). As the winds over the East Greenland shelf intensified, the WGCC shifted onshore almost immediately and strengthened 10–14 h later. Limited bracketing of the WGCC during this period precludes a full evaluation of the WGCC, but sporadic volume transports on 2–3 October (Fig. 9b) and a vertical section on 0400 UTC 2 October (Fig. 9g) show an intensification of the WGCC, which was likely centered inshore of LSA (Fig. 9b). Similarly, as the winds over the East Greenland shelf relaxed and then intensified again on 4–5 October, the WGCC first responded by shifting onshore and then intensified, almost exactly 12 h after the peak in the winds. Strong stratification can be seen at LSA and LSB during this latter snapshot (Fig. 9i), possibly due to very freshwater advecting around Cape Farewell from east to west in response to the initial downwelling, about 3 days prior. This would match the advective time scale if the currents were on the order of 1 m s−1.

Fig. 9.
Fig. 9.

Wind event 3 from 0000 UTC 2 October 2019 to 1800 UTC 7 Oct 2019 displayed as in Fig. 7.

Citation: Journal of Physical Oceanography 53, 11; 10.1175/JPO-D-23-0104.1

These results indicate that the WGCC is more sensitive to remote wind forcing over the East Greenland shelf than local forcing over the West Greenland shelf. We propose two explanations for this which are likely both at work, one that is physical and one that is methodological. The physically meaningful explanation is that the WGCC as measured across this line of moorings is the downstream extension of the EGCC that is primarily forced locally on the East Greenland shelf. On the West Greenland shelf, the local upwelling-favorable winds and the intricate coastline including the Kitsissut Islands are not as conducive to supporting a coastal current as is the southeast Greenland shelf with its relatively strait coastline and strong downwelling-favorable winds. Thus, the WGCC can be considered the continuation of the EGCC that makes its way around Cape Farewell but is not supported by local forcing. In this conceptual model, the WGCC only exists due to the advection of buoyant water masses around Cape Farewell, and the fresh wedge that maintains the EGCC/WGCC erodes in the absence of supportive wind forcing on the southwest Greenland shelf. Surface salinity variability near the mooring array on the West Greenland shelf appears distinct from the rest of the West Greenland shelf as measured by transiting cargo ships (Reverdin et al. 2018), and models indicate that the WGCC diminishes downstream of the OSNAP West moorings (Gou et al. 2021).

The second explanation is that our focus on events with strong volume transport in which the WGCC was located far enough offshore to be bracketed by the mooring array could bias our finding toward more nonlocal forcing. An example of this is that during the initial local downwelling in the first event on 4 December (Fig. 7), the WGCC shifted inshore of LSA, and thus, we would not have been able to detect a response in the volume transport to the local winds. Similarly, strong downwelling from 20 to 24 November 2018 during the second event (Fig. 8) pushed the WGCC inshore of LSA from 22 to 24 November, and our intermittent record of the WGCC volume transport could have underestimated the full response to these local winds. If this were true more generally, the locally forced intensifications of the WGCC may be underrepresented in our analysis. To examine this further, we explored whether the local winds were correlated to the location of the WGCC core on the shelf and found no relation over the 21-month record, likely due to strong chaotic instabilities controlling the WGCC location rather than strong wind forcing. Seemingly only the strongest local wind events can overcome this internal variability and cause the WGCC to shift onshore (e.g., Figs. 7 and 8).

6. Conclusions

We have presented the first-ever continuous measurements of the WGCC on the southwest Greenland shelf. For the 35% of the 21-month record that the moorings bracketed the current, the median WGCC volume transport is 0.83 ± 0.02 Sv with an average flow-weighted salinity of 33.55 ± 0.02. The WGCC is characterized by strong temporal variability in its lateral position on the shelf, as well as its volume and freshwater transports. We found no significant seasonality in its strength or salinity, in part due to the fact that the second winter (2019/20) had unusually strong upwelling-favorable conditions on the southwest Greenland shelf. We suspect that a longer record would reveal generally stronger volume transports in the winter and weaker transports in the summer, consistent with what was seen in the first year of our data as well as in observations of the EGCC (Le Bras et al. 2018) and models of the WGCC (Gou et al. 2021, 2022). Insufficient sampling in the near-surface layer likely leads to underestimates of freshwater transport, especially in the stratified summer conditions, and could compensate for some of this expected seasonality. Due to the chaotic nature of the WGCC shifting across the shelf, our record of the WGCC was not conducive for studying all the variability of the current. Instead, we focused on three significant events in which remote wind forcing over the East Greenland shelf appeared to more strongly impact the WGCC volume transport than local wind forcing over the West Greenland shelf. Comparisons of these results to previous studies of the EGCC and WGCC indicate continuity in the volume transport of the coastal current around Cape Farewell and a loss of liquid freshwater from a salinification of the current from east to west. These results support the idea that this region could be the site of significant loss of freshwater from the shelf to the basin (e.g., Pacini and Pickart 2023). The subsequent deployment of the U.S. OSNAP West array in 2020 included an additional mooring inshore of LSA with the hope that we can obtain a more complete record in which the WGCC is regularly bracketed. Future work will address this.

Acknowledgments.

We thank the captain and crew of the R/V Neil Armstrong for safely navigating the ice-infested waters of the Greenland shelf during the deployment and recovery cruises. We are grateful to John Kemp, Jim Ryder, Brian Hogue, Andrew Davies, Dan Torres, and Rick Trask for the design, fabrication, and deployment of the mooring array. Frank Bahr, Dan, Torres, Jamie Holte, and Leah McRaven processed the mooring and shipboard data used in the study. This work was sponsored by the National Science Foundation under Grants OCE-1948505 and OCE-2047952.

Data availability statement.

Mooring data from the U.S. OSNAP Labrador Sea Array are publicly available at https://repository.gatech.edu/entities/publication/ec69c2b5-e5a1-420a-9007-7107e7c91935. ERA5 10-m winds are available at https://www.ecmwf.int/en/forecasts/dataset/ecmwf-reanalysis-v5.

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  • Morlighem, M., and Coauthors, 2017: BedMachine v3: Complete bed topography and ocean bathymetry mapping of Greenland from multibeam echo sounding combined with mass conservation. Geophys. Res. Lett., 44, 11 05111 061, https://doi.org/10.1002/2017GL074954.

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  • Nilsson, J., G. Björk, B. Rudels, P. Winsor, and D. Torres, 2008: Liquid freshwater transport and polar surface water characteristics in the East Greenland current during the AO-02 Oden expedition. Prog. Oceanogr., 78, 4557, https://doi.org/10.1016/j.pocean.2007.06.002.

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  • Pacini, A., and R. S. Pickart, 2022: Meanders of the West Greenland current near Cape Farewell. Deep-Sea Res. I, 179, 103664, https://doi.org/10.1016/j.dsr.2021.103664.

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  • Pacini, A., and R. S. Pickart, 2023: Wind-forced upwelling along the West Greenland shelfbreak: Implications for Labrador Sea water formation. J. Geophys. Res. Oceans, 128, e2022JC018952, https://doi.org/10.1029/2022JC018952.

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  • Pacini, A., and Coauthors, 2020: Mean conditions and seasonality of the West Greenland boundary current system near Cape Farewell. J. Phys. Oceanogr., 50, 28492871, https://doi.org/10.1175/JPO-D-20-0086.1.

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  • Pacini, A., R. S. Pickart, I. A. Le Bras, F. Straneo, N. P. Holliday, and M. A. Spall, 2021: Cyclonic eddies in the West Greenland boundary current system. J. Phys. Oceanogr., 51, 20872102, https://doi.org/10.1175/JPO-D-20-0255.1.

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  • Pawlowicz, R., B. Beardsley, and S. Lentz, 2002: Classical tidal harmonic analysis including error estimates in MATLAB using T_TIDE. Comput. Geosci., 28, 929937, https://doi.org/10.1016/S0098-3004(02)00013-4.

    • Search Google Scholar
    • Export Citation
  • Pickart, R. S., M. A. Spall, M. H. Ribergaards, G. W. K. Moore, and R. F. Milliff, 2003: Deep convection in the Irminger Sea forced by the Greenland tip jet. Nature, 424, 152156, https://doi.org/10.1038/nature01729.

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  • Pickart, R. S., D. J. Torres, and P. S. Fratantoni, 2005: The East Greenland spill jet. J. Phys. Oceanogr., 35, 10371053, https://doi.org/10.1175/JPO2734.1.

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    • Export Citation
  • Rahmstorf, S., J. E. Box, G. Feulner, M. E. Mann, A. Robinson, S. Rutherford, and E. J. Schaffernicht, 2015: Exceptional twentieth-century slowdown in Atlantic Ocean overturning circulation. Nat. Climate Change, 5, 475480, https://doi.org/10.1038/nclimate2554.

    • Search Google Scholar
    • Export Citation
  • Reverdin, G., and Coauthors, 2018: North Atlantic subpolar gyre along predetermined ship tracks since 1993: A monthly data set of surface temperature, salinity, and density. Earth Syst. Sci. Data, 10, 14031415, https://doi.org/10.5194/essd-10-1403-2018.

    • Search Google Scholar
    • Export Citation
  • Rykova, T., F. Straneo, and A. S. Bower, 2015: Seasonal and interannual variability of the West Greenland current system in the Labrador Sea in 1993–2008. J. Geophys. Res. Oceans, 120, 13181332, https://doi.org/10.1002/2014JC010386.

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    • Export Citation
  • Schauer, U., and M. Losch, 2019: “Freshwater” in the ocean is not a useful parameter in climate research. J. Phys. Oceanogr., 49, 23092321, https://doi.org/10.1175/JPO-D-19-0102.1.

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    • Export Citation
  • Schulze Chretien, L. M., and E. Frajka-Williams, 2018: Wind-driven transport of fresh shelf water into the upper 30 m of the Labrador Sea. Ocean Sci., 14, 12471264, https://doi.org/10.5194/os-14-1247-2018.

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  • Smith, W. H. F., and P. Wessel, 1990: Gridding with continuous curvature splines in tension. Geophysics, 55, 293305, https://doi.org/10.1190/1.1442837.

    • Search Google Scholar
    • Export Citation
  • Sutherland, D. A., and R. S. Pickart, 2008: The East Greenland coastal current: Structure, variability, and forcing. Prog. Oceanogr., 78, 5877, https://doi.org/10.1016/j.pocean.2007.09.006.

    • Search Google Scholar
    • Export Citation
  • Våge, K., R. S. Pickart, G. W. K. Moore, and M. V. Ribergaard, 2008: Winter mixed layer development in the central Irminger Sea: The effect of strong, intermittent wind events. J. Phys. Oceanogr., 38, 541565, https://doi.org/10.1175/2007JPO3678.1.

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    • Export Citation
  • Wilkinson, D., and S. Bacon, 2005: The spatial and temporal variability of the East Greenland Coastal current from historic data. Geophys. Res. Lett., 32, L24618, https://doi.org/10.1029/2005GL024232.

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Supplementary Materials

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  • Karpouzoglou, T., L. de Steur, and P. A. Dodd, 2023: Freshwater transport over the northeast Greenland shelf in Fram Strait. Geophys. Res. Lett., 50, e2022GL101775, https://doi.org/10.1029/2022GL101775.

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  • Le Bras, I. A.-A., F. Straneo, J. Holte, and N. P. Holliday, 2018: Seasonality of freshwater in the East Greenland current system from 2014 to 2016. J. Geophys. Res. Oceans, 123, 88288848, https://doi.org/10.1029/2018JC014511.

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  • Lin, P., R. S. Pickart, D. J. Torres, and A. Pacini, 2018: Evolution of the freshwater coastal current at the southern tip of Greenland. J. Phys. Oceanogr., 48, 21272140, https://doi.org/10.1175/JPO-D-18-0035.1.

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  • Lique, C., A. M. Tregurier, M. Scheinert, and T. Penduff, 2009: A model-based study of ice and freshwater transport variability along both sides of Greenland. Climate Dyn., 33, 685705, https://doi.org/10.1007/s00382-008-0510-7.

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  • Moore, G. W. K., and I. A. Renfrew, 2005: Tip jets and barrier winds: A QuikSCAT climatology of high wind speed events around Greenland. J. Climate, 18, 37133725, https://doi.org/10.1175/JCLI3455.1.

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  • Moore, G. W. K., I. A. Renfrew, and R. S. Pickart, 2013: Multidecadal mobility of the North Atlantic Oscillation. J. Climate, 26, 24532466, https://doi.org/10.1175/JCLI-D-12-00023.1.

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  • Morlighem, M., and Coauthors, 2017: BedMachine v3: Complete bed topography and ocean bathymetry mapping of Greenland from multibeam echo sounding combined with mass conservation. Geophys. Res. Lett., 44, 11 05111 061, https://doi.org/10.1002/2017GL074954.

    • Search Google Scholar
    • Export Citation
  • Nilsson, J., G. Björk, B. Rudels, P. Winsor, and D. Torres, 2008: Liquid freshwater transport and polar surface water characteristics in the East Greenland current during the AO-02 Oden expedition. Prog. Oceanogr., 78, 4557, https://doi.org/10.1016/j.pocean.2007.06.002.

    • Search Google Scholar
    • Export Citation
  • Pacini, A., and R. S. Pickart, 2022: Meanders of the West Greenland current near Cape Farewell. Deep-Sea Res. I, 179, 103664, https://doi.org/10.1016/j.dsr.2021.103664.

    • Search Google Scholar
    • Export Citation
  • Pacini, A., and R. S. Pickart, 2023: Wind-forced upwelling along the West Greenland shelfbreak: Implications for Labrador Sea water formation. J. Geophys. Res. Oceans, 128, e2022JC018952, https://doi.org/10.1029/2022JC018952.

    • Search Google Scholar
    • Export Citation
  • Pacini, A., and Coauthors, 2020: Mean conditions and seasonality of the West Greenland boundary current system near Cape Farewell. J. Phys. Oceanogr., 50, 28492871, https://doi.org/10.1175/JPO-D-20-0086.1.

    • Search Google Scholar
    • Export Citation
  • Pacini, A., R. S. Pickart, I. A. Le Bras, F. Straneo, N. P. Holliday, and M. A. Spall, 2021: Cyclonic eddies in the West Greenland boundary current system. J. Phys. Oceanogr., 51, 20872102, https://doi.org/10.1175/JPO-D-20-0255.1.

    • Search Google Scholar
    • Export Citation
  • Pawlowicz, R., B. Beardsley, and S. Lentz, 2002: Classical tidal harmonic analysis including error estimates in MATLAB using T_TIDE. Comput. Geosci., 28, 929937, https://doi.org/10.1016/S0098-3004(02)00013-4.

    • Search Google Scholar
    • Export Citation
  • Pickart, R. S., M. A. Spall, M. H. Ribergaards, G. W. K. Moore, and R. F. Milliff, 2003: Deep convection in the Irminger Sea forced by the Greenland tip jet. Nature, 424, 152156, https://doi.org/10.1038/nature01729.

    • Search Google Scholar
    • Export Citation
  • Pickart, R. S., D. J. Torres, and P. S. Fratantoni, 2005: The East Greenland spill jet. J. Phys. Oceanogr., 35, 10371053, https://doi.org/10.1175/JPO2734.1.

    • Search Google Scholar
    • Export Citation
  • Rahmstorf, S., J. E. Box, G. Feulner, M. E. Mann, A. Robinson, S. Rutherford, and E. J. Schaffernicht, 2015: Exceptional twentieth-century slowdown in Atlantic Ocean overturning circulation. Nat. Climate Change, 5, 475480, https://doi.org/10.1038/nclimate2554.

    • Search Google Scholar
    • Export Citation
  • Reverdin, G., and Coauthors, 2018: North Atlantic subpolar gyre along predetermined ship tracks since 1993: A monthly data set of surface temperature, salinity, and density. Earth Syst. Sci. Data, 10, 14031415, https://doi.org/10.5194/essd-10-1403-2018.

    • Search Google Scholar
    • Export Citation
  • Rykova, T., F. Straneo, and A. S. Bower, 2015: Seasonal and interannual variability of the West Greenland current system in the Labrador Sea in 1993–2008. J. Geophys. Res. Oceans, 120, 13181332, https://doi.org/10.1002/2014JC010386.

    • Search Google Scholar
    • Export Citation
  • Schauer, U., and M. Losch, 2019: “Freshwater” in the ocean is not a useful parameter in climate research. J. Phys. Oceanogr., 49, 23092321, https://doi.org/10.1175/JPO-D-19-0102.1.

    • Search Google Scholar
    • Export Citation
  • Schulze Chretien, L. M., and E. Frajka-Williams, 2018: Wind-driven transport of fresh shelf water into the upper 30 m of the Labrador Sea. Ocean Sci., 14, 12471264, https://doi.org/10.5194/os-14-1247-2018.

    • Search Google Scholar
    • Export Citation
  • Smith, W. H. F., and P. Wessel, 1990: Gridding with continuous curvature splines in tension. Geophysics, 55, 293305, https://doi.org/10.1190/1.1442837.

    • Search Google Scholar
    • Export Citation
  • Sutherland, D. A., and R. S. Pickart, 2008: The East Greenland coastal current: Structure, variability, and forcing. Prog. Oceanogr., 78, 5877, https://doi.org/10.1016/j.pocean.2007.09.006.

    • Search Google Scholar
    • Export Citation
  • Våge, K., R. S. Pickart, G. W. K. Moore, and M. V. Ribergaard, 2008: Winter mixed layer development in the central Irminger Sea: The effect of strong, intermittent wind events. J. Phys. Oceanogr., 38, 541565, https://doi.org/10.1175/2007JPO3678.1.

    • Search Google Scholar
    • Export Citation
  • Wilkinson, D., and S. Bacon, 2005: The spatial and temporal variability of the East Greenland Coastal current from historic data. Geophys. Res. Lett., 32, L24618, https://doi.org/10.1029/2005GL024232.

    • Search Google Scholar
    • Export Citation
  • Fig. 1.

    (a) Schematic circulation around southern Greenland. The U.S. OSNAP Labrador Sea Mooring Array spans the WGCC, WGC/IC, and deep western boundary current (DWBC). (b) The time-mean velocity vectors from the upward-looking ADCPs (upper 100 m) over the period 2014–20 (blue) and 2018–20 (red). The LSA and LSB moorings were first deployed in 2018. The along-stream and cross-stream coordinate system is shown in black.

  • Fig. 2.

    Mean and composites of the along-stream velocity field. (a) The time-mean velocity field. (b) Composite of 17.7% of the hourly snapshots when no WGCC was identified. (c) Composite of 47.8% of the hourly snapshots when the WGCC was sampled but not bracketed by the mooring array. (d) Composite of 34.5% of the hourly snapshots when the WGCC was bracketed by the mooring array. See the text for details. In all panels, black circles denote the velocity measurements, either from an ADCP bin (upper water column) or a point current meter.

  • Fig. 3.

    Mean and composite anomalies (calculated relative to the mean) of the salinity field. (a) The time-mean salinity, depicting a wedge of freshwater on the shelf. Note the nonlinear color axis. (b) Salinity anomaly (color) when no WGCC was detected. The black contours are the salinity field of the composite. (c) As in (b), but when the WGCC was sampled. (d) As in (b), but when the WGCC was bracketed. In all panels, white circles denote the mean location of the MicroCATs.

  • Fig. 4.

    Percent of each month covered by the three sampling regimes. The “Coastal current present” curve (green) is the sum of “Coastal current sampled” (orange) and “Coastal current bracketed” (red).

  • Fig. 5.

    Time variability of (a) volume transport, (b) liquid freshwater transport calculated relative to 34.9, and (c) flow-weighted salinity. Values are calculated at hourly resolution (black) when the coastal current is bracketed and then smoothed with a 5-day running mean (red).

  • Fig. 6.

    Compilation of all published values of EGCC and WGCC volume transports, arranged in geographical order starting from 72°N on the East Greenland shelf, proceeding southward through Denmark Strait to Cape Farewell, and then around Cape Farewell to Cape Desolation on the West Greenland shelf. Values from continuous moored observations (colored stars) are available at OSNAP East (Le Bras et al. 2018) and OSNAP West (present study). Figure updated from Lin et al. (2018).

  • Fig. 7.

    Wind event 1 from 0000 UTC 2 Dec 2018 to 2300 UTC 7 Dec 2018. (a) Along-coast wind stress; (b) WGCC volume transport and location on the shelf; (c)–(f) ERA5 10-m winds; and (g)–(j) gridded along-stream velocities. Wind stress in (a) is averaged over the regions outlined in (c) and is positive for downwelling-favorable winds. Dashed vertical lines numbered 1–4 in (a) and (b) indicate the four snapshots shown in (c)–(j). In (b), the locations of the moorings are shown on the right axis for reference. The OSNAP West mooring array (red stars) is shown in (c)–(f). Black contours in (g)–(j) indicate isopycnals (kg m−3).

  • Fig. 8.

    Wind event 2 from 0600 UTC 20 Nov 2018 to 1200 UTC 24 Nov 2018 displayed as in Fig. 7.

  • Fig. 9.

    Wind event 3 from 0000 UTC 2 October 2019 to 1800 UTC 7 Oct 2019 displayed as in Fig. 7.

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