Norwegian Atlantic Slope Current along the Lofoten Escarpment

. Observations from moored instruments are analyzed to describe the Norwegian Atlantic Slope Current at the Lofoten Escarpment. The data set covers a 14-month period from June 2016 to September 2017, and resolves the core of the current from 200 to 650 m depth, between the 650 m and 1500 m isobaths. The along-slope current, vertically averaged between 200 and 600 m depth has an annual cycle amplitude of 0.1 m s − 1 with strongest currents in winter, and a temporal average of 0.15 m s − 1 . Higher frequency variability is characterized by ﬂuctuations that reach 0.8 m s − 1 , lasting for 1 to 2 weeks, and extend as 5 deep as 600 m. In contrast to observations in Svinøy, the slope current is not barotropic and varies strongly with depth (a shear of 0.05 to 0.1 m s − 1 per 100 m in all seasons). Within the limitations of the data, the average volume transport is estimated at 2.8 ± 1.8 Sv (1 Sv = 10 6 m 3 s − 1 ), with summer and winter averages of 2.3 and 4.0 Sv, respectively. The largest transport is associated with the high temperature classes ( > 7 ◦ C) in all seasons, with the largest values of both transport and temperature in winter. Calculations of the barotropic and baroclinic conversion rates using the moorings are supplemented by results from a 10 high resolution numerical model. While the conversion from mean to eddy kinetic energy (e.g. barotropic instability) is likely negligible over the Lofoten Escarpment, the baroclinic conversion from mean potential energy into eddy kinetic energy (e.g. baroclinic instability), can be substantial with volume-averaged values of (1 − 2) × 10 − 4 W m − 3 .

, and the orientation of the coordinate system (along-slope, x, and across-slope, y). Blue isobaths are drawn every 500 m. The inset is a location map with domains of (a) and (b) marked in red and green, respectively. The monitoring location for the Svinøy section is shown by the red star. NO: Norway, SV: Svalbard, IC: Iceland, GR: Greenland.
2 https://doi.org/10.5194/os-2020-15 Preprint. Discussion started: 27 February 2020 c Author(s) 2020. CC BY 4.0 License. Sv = 10 6 m 3 s −1 ) (Orvik et al., 2001); however, the total geostrophic transport from repeated Seaglider transects reached 6.8 Sv (Høydalsvik et al., 2013), implying a large barotropic contribution. Farther north, detailed glider observations of the front current over the Mohn Ridge confirm large transport rates giving 4.5 Sv annual average with approximately 2 Sv barotropic contribution . 30 University of Bergen, Norway, has monitored the slope current transport at the Svinøy section (star in the inset of Fig. 1b), south of the Vøring Plateau, with continuous measurements since 1995 (Orvik et al., 2001). The slope current there is about 40 km wide, between the 200 and 900 m isobaths, with an annual mean of 0.3 m s −1 . The average annual transport of this barotropic branch is 4.4 Sv (Orvik et al., 2001;Orvik and Skagseth, 2003). The slope current accelerates along steep topography off the Lofoten Escarpment near the Lofoten Islands. The Norwegian Coastal Current (blue arrow Fig 1), carries relatively fresh 35 water over the shelf and as the shelf gets narrow near the Lofoten Escarpment, there might interactions with the slope current.
Here, there are no published moored current meter records, but surface drifters indicate velocities reaching 1 m s −1 (Andersson et al., 2011). The transport and variability of the slope current in this region is not known. It is hypothesized that the current becomes increasingly unstable near this topographic steepening. Using time-averaged fields of an eddy-resolving numerical ocean simulation, Isachsen (2015) showed that the steep Lofoten Escarpment exhibits enhanced unstable baroclinic growth 40 rates and large velocity variability, suggesting high lateral diffusion rates. The structure and transport of the slope current at the Lofoten Escarpment is the focus of this study.
The Lofoten Basin is affected by the Atlantic Water (AW) transport, and becomes a major heat reservoir that is exposed to large surface heat losses (Rossby et al., 2009b;Dugstad et al., 2019a) and substantial water mass transformations (Rossby et al., 2009a;Bosse et al., 2018). The AW enters the basin both as a broad slab in the upper layers between the two branches (Rossby 45 et al., 2009b;Dugstad et al., 2019a) and by eddies detached from the unstable slope current (Köhl, 2007;Isachsen, 2015;Volkov et al., 2015). The eddy-induced lateral heat fluxes distribute the heat in the basin (Spall, 2010;Isachsen et al., 2012;Dugstad et al., 2019a). The region is energized, manifested in the average geostrophic eddy kinetic energy (EKE g , see Sect. 2) map showing two maxima ( Fig. 1a): one in the center, associated with a permanent, energetic eddy (Ivanov and Korablev, 1995;Søiland and Rossby, 2013;Fer et al., 2018;, and a secondary maximum closer to the slope, likely 50 associated with the variability induced by the slope current. The energetics and the variability of the slope current remain to be constrained by observations. The study was conducted as a part of the "Water mass transformation processes and vortex dynamics in the Lofoten Basin of the Norwegian Sea" (PROVOLO) project. The overall objective of PROVOLO was to describe and quantify the processes and pathways of energy transfer and mixing in the Lofoten Basin and their role in water mass transformation. Observations from 55 multiple cruises, gliders and RAFOS floats were analyzed and reported elsewhere with focus on AW tranformation , the permanent Lofoten Basin Eddy  and the frontal structure across the Mohn Ridge . The mooring component concentrated on the slope current. Here we report the first observations of the volume transport rates, energetics and their variability from weekly to seasonal time scales. A set of 4 moorings was deployed across the continental slope of the eastern Lofoten Basin (Fig.1). A deployment and recovery summary is listed in Table 1, and full details are provided with the documentation following the data set (Fer, 2020). Mooring name convention is Mooring North (MN), South (MS), West (MW) and Basin (MB). MB was located at the secondary geostrophic EKE maximum (Fig. 1a) to address the mesoscale variability in the basin. Data from this mooring will be analysed for a separate study and are not reported here. The observations cover a 14-month period from June 2016 to September 2017.
The arrangement of the three moorings on the slope (Fig. 1b)  Microcat), and a water column line with distributed conductivity-temperature-depth (CTD) sensors. This approach mitigated the high risk due to fisheries activities. The ADCP bottom unit and mooring line pairs were deployed close to each other at approximately the same isobath (within 5 m), and within 250 m horizontally, and will be treated as a single mooring.
Unfortunately both water column mooring lines at MN and MS were damaged by fishing boats. The MS line was lost with no data return. The MN line was cut after 3 months. The drifting line together with the sensors were recovered, giving 3 (summer) 75 months of temperature and salinity data in the water column. The current profile and the near-bottom CTD data from the bottom units were successfully recovered and cover the whole study period.
The moorings were densely instrumented and sampled at a hourly rate or faster, covering a large fraction of the water column. The instrument target depths can be seen on the vertical axis of Fig. 4, introduced later. Currents were measured using ADCPs, mainly RDI 75kHz Sentinel Workhorse for the moorings reported here, and point current meters (Anderaa recorders (Microcat,SBE37). Detailed instrument distribution on moorings can be found in the data set documentation (Fer, 2020). Current measurements were corrected for magnetic declination. After all moorings were recovered, a calibration CTD 85 cast was made with all mooring SBE sensors attached to the ship's CTD frame. The temperature and salinity measurements were corrected to be internally consistent, and also against the calibration cast and the profiles taken when the moorings were in water. A data set was prepared after correcting for mooring knock downs caused by intense currents. The initial accuracy of the SBE sensors are ±2 × 10 −3 • C for temperature, ±3 × 10 −4 S m −1 for conductivity, and ±1 dbar for pressure (drift over 1 year is comparable to initial accuracy for temperature and pressure, and 10 times the initial accuracy for conductivity). For 90 the deployment setup used, the ADCPs have a single ping (profile) statistical error of 2.5 cm s −1 , which reduces to 0.4 cm s −1 for the ensemble average profile with 35 pings. The compass direction is accurate to ±2 • . Conservative error estimates are ±1 cm s −1 for velocity, ±10 −2 • C for temperature and ±10 −2 for practical salinity.
Hourly-averaged data set was filtered using a 14 day low-pass filter for background fields, and 35 h to 14 day band-pass filter for eddy covariance and conversion rate calculations. In both cases a 3rd order Butterworth filter was used.

95
We rotated the coordinate system 42 • from East, with x-axis pointing along-slope and y-axis cross-slope toward deeper water (see Fig. 1b). Mean orientation of the slope was calculated using isobaths from ETOPO1 near the slope moorings. Current components are along-slope, u, and across-slope, v. (In the figures, we explicitly use the notation u a and u x , respectively.) Atmospheric forcing was obtained from ECMWF's ERA-Interim (Dee et al., 2011)  that the range of current measurements sufficiently covered the AW layer and the dynamical core at the slope identified by sloping isotherms.
A summary of the environmental forcing during the measurement period shows that the net surface flux was typical of the long-term average, with an event of strong heat loss exceeding 1 standard deviation (std) envelope from mid February to early We averaged the EKE g from satellite altimetry in a 30-km radius at the basin mooring location, MB (a EKE g maximum region, see Fig. 1a), and at MW (Fig. 3c). The EKE g records confirm that MB is 2 to 5 times more energetic in general, except in summer when both locations were relatively quiescent. In winter, the slope was as energetic as the basin.

Temporal variability
The currents measured at moorings MW and MN were highly variable (Fig. 5). The 14-day low-passed currents were strongest in the fall and winter (Fig. 5a-d). The annual cycle of the 200-300 m vertically averaged u a at MN had an amplitude of 0.10 m s −1 and explains 20% of the variance, obtained using a sinusoidal fit to daily data (not shown). These figures are similar 140 for MW for 300-400 m averaged currents (depth ranges are chosen to ensure continuous time series, unaffected by mooring knock-downs). The cross-slope components show a less pronounced seasonality with 1-2 cm s −1 (5-15% variance explained) at both moorings. Temperature record at MW also shows strong seasonality. The amplitude of annual sinusoidal fit to the temperature time series, increases from 0.6 • C at 200-300 m to 1 • C at 500-600 m, accounting for 60-70% of the variance, and rapidly decays deeper.

145
The largest along-slope currents reach 0.8 m s −1 at both moorings, last for 1 to 2 weeks and extend as deep as 600 m. In periods with strong u a , the cross-slope velocity is also energized. These energetic periods also correspond with the peaks in EKE g obtained from satellite altimetry at the MW location (Fig. 3c). Isotherms (available only at MW for the entire mooring record) show vertical displacements of order 100 m, consistent with mesoscale meandering of the slope current.

Transport
Transport calculations were made using daily-averages of the 14 day low-passed current and temperature fields from moorings MW and MN, using the along-slope component of the current. Positive (northeastward, Q p ) and negative (southwestward, 175 Q n ) transport densities (i.e. transport in a water column with 1 m width) were calculated by integrating vertically between 50 m and 650 m depth, roughly corresponding to the AW layer. Average transports over the entire record, over summer months and winter months are listed in Table 2. The net transport (Q p + Q n ) was also computed in 1 • C temperature classes, with the Atlantic Water transport (Q) estimated as the net transport of water warmer than 5 • C. A total transport was estimated by assigning a 14 km width (justified below) for each mooring. Results are summarized in Fig. 6.

180
The moorings are separated by approximately 6 km (horizontal distance between the locations), and when projected onto the cross-slope section to their respective isobaths, the distance is about 8 km. We assume velocity measured at each mooring is representative for the half-width (4 km) to the next mooring. We further extend the width of MW 10 km off-slope (distance to the 2500 m isobath) and MN 10 km on-shore (distance to the 250 m isobath), hence assign a 14 km effective width of water column to each mooring. These choices are motivated by the coverage of the dynamic AW core at Gimsøy section (see 185 Fig. 2). The outer edge corresponds to the location where the 5 • C isotherm is shallowest, and covers the relatively steep lateral isopycnal gradient toward the slope.
The choice of a 28-km total width for the transport calculation is consistent with the lateral structure of the depth-integrated geostrophic current inferred from the Gimsøy hydrographic section. From the 4 occupations of the Gimsøy section, we calculated the relative geostrophic transport. Depth-integrated geostrophic current peak at an isobath between 500 and 750 m,  suggesting that MN and MS are positioned near the maximum velocities of the slope current. The lateral structure of the depthintegrated relative geostrophic current was fairly symmetric and reduced to 20% of its maximum over a total width of 25-30 km. As a result we find that the choice of 28 km width for transport calculations is justified. The relative geostrophic transport for AW calculated in the Gimsøy section between 50-650 m and 50-1500 m were identical to within 0.1 Sv, hence the limited vertical range of our calculation does not introduce additional errors in the baroclinic contribution.

195
There is large variability in Q with 1 to 4 Sv oscillations at 2 to 4 weeks time scale (Fig. 6a). Transport maxima were observed in winter. The transport approached zero at the trough of the oscillations, but the flow reversal was negligible. Total AW transport was typically northward. Monthly averaged transport of AW increased three-fold in fall and winter with a monthlyaverage maximum of 5 Sv in December, from about 1.5-2 Sv in summer (Fig. 6b). The transport in temperature classes is shown in Fig. 7. When averaged over summer and winter months, separately, transport in high temperature classes (7-9 • C) 200 was stronger in winter whilst the low temperature classes (3-7 • C) were stronger in summer. This is because the maximum AWlayer averaged temperatures occurred in winter (e.g., compare the winter and summer temperature profiles at MW, Fig. 4a), when the transport was also large (Fig. 6). In winter, the vertical mixing of warm surface layer resulted in a low stratified Two moorings closely spaced over the slope cannot capture the full dynamics of the slope boundary current. However, as described above, the CTD sections collected at different months along the standard Gimsøy Section, in close vicinity of the moorings, suggest that the mean slope of the isopycnals can be approximated from the mooring records. The sections also suggest that the bulk of the AW is in the upper 650 m, which is resolved by our moorings.

210
AW transports are not sensitive to the definition of the AW temperature and vertical integration limits; however, the crude estimate of the width of the slope current must be treated with caution. Recalculating the transport using water with T ≥ 3 (instead of T ≥ 5) increases Q by 0.1 Sv. Including the top 50 m increases the total mean transport by 0.4 Sv (from 2.8 Sv). The sensitivity to the choice of mooring width is approximately linear. A total current width of 10 km (i.e. assigning 5 km to each mooring instead of 14 km) reduces the mean transport from 2.8 to 1 Sv.

Energetics
The kinetic energy content and variability of the slope current, and conversion rates associated with barotropic and baroclinic instability of the current are presently unconstrained by observations. Using our limited mooring records, we attempt to quantify the energetics of the slope current at the Lofoten Escarpment. For the following analysis, we obtained the fluctuations, denoted by primes, by band-pass filtering the hourly data with cutoff frequencies corresponding to 14 day and 35 hours.

220
We start with the variability in depth-averaged along and cross slope currents, the horizontal eddy kinetic energy density, EKE, and their relation to wind forcing. The EKE in units of J kg −1 or m 2 s −2 , is The along-slope current variability and the evolution of EKE were partly forced by the along-slope wind modulating the geostrophic shear by cross-front Ekman transport (Fig. 8a-c). The annual average wind speed (  ρ 0 is a reference density, and g is gravitational acceleration, and an overbar denotes temporal averaging. We used CHECK! NOT 1 MONTH? 14 day moving averaging. The mean isopycnal slope, ∂z/∂y, was calculated as ∂ρ/∂y/∂ρ/∂z. The barotropic conversion rate can be approximated by 14 https://doi.org/10.5194/os-2020-15 Preprint. Discussion started: 27 February 2020 c Author(s) 2020. CC BY 4.0 License.
The 8-km cross-slope separation of the moorings may be too small to characterize the lateral shear, and the resulting BT can be an underestimate. To derive an upper limit, and likely closer to a more realistic value for BT, we assume that u decreases from the largest value of the observation at MW and MN to 0 over a decay length scale of 15 km, and use the largest of the Reynolds stress (u v ) from the two moorings.  Fig. 2) where we can obtain vertical and lateral gradients, but only for 3 months into the record. We obtained the vertical gradient at 400 m at MW using the records at 300 and 500 m. We obtained the lateral gradient from the records at 400 m. Final conversion rates were obtained by moving averaging over 30 days. Whilst the baroclinic conversion rate time series is limited only to three months, the barotropic conversion rate can be calculated for the entire duration. We computed BT at 300, 400, 500 and 600 m depths. Results are summarized in Fig. 8d. The conversion rates calculated from two moorings may not 260 be representative of the volume-averaged conversion rates over the slope region and must be interpreted with caution. This is discussed in detail in the Appendix, using high-resolution numerical model fields. with values in the range of (1−2)×10 −4 W m −3 , and cannot be captured by the calculations from a single level at 400 m depth.
Based on this discussion, we propose that the conversion rates on the Lofoten Escarpment are likely dominated by baroclinic instability of the slope current.

285
The Norwegian Atlantic Slope Current at the Lofoten Escarpment is described using 14-month long mooring records in the

295
The average volume transport is 2.8 ± 1.8 Sv, with summer and winter averages of 2.3 and 4.0 Sv, respectively. The largest transport is associated with warm water in all seasons, and the water temperatures are the highest in winter.
Calculations of the barotropic and baroclinic conversion rates using the moorings are supplemented by high resolution numerical model. While the conversion from mean kinetic energy into eddy energy (e.g. barotropic instability) is likely negligible over the Lofoten Escarpment, the baroclinic conversion from mean potential energy into eddy kinetic energy (e.g. baroclinic 300 instability), can be substantial with volume-averaged values on the order of 10 −4 W m −3 . Eddy kinetic energy and conversion rates in the slope current are comparable to the published results from the West Spitsbergen Current and the East Greenland Current.
Fishing activity in the region makes it highly challenging to maintain moorings; however, extended time series with better cross-slope and vertical coverage are needed to study the dynamics and variability of the slope current. The attempts to calcu-305 late (observation-based) energy conversion rates remain inconclusive. Utilization of autonomous underwater vehicles, such as gliders, can help collecting high quality observations, but will be difficult to operate in the strong boundary current. The slope current and its instability is an important player in the energetics of the Lofoten Basin and merits further studies.
Data availability. Data used in this analysis have been submitted to the Norwegian Marine Data Centre and will be promptly available for open public access upon acceptance of the paper.
coordinates that solves the primitive equations on a staggered C-grid (Shchepetkin and McWilliams, 2009;Haidvogel et al., 2008). The model fields used here have a horizontal resolution of 800 m, 60 vertical layers with increased resolution towards the surface (1-3 m at the surface, about 60 m at the bottom) and are stored as 6 hourly outputs. The model fields are used and described in detail in Dugstad et al. (2019b).
We first compute the baroclinic and barotropic conversion rates over a domain covering the slope region identified in Fig. A1.

320
The conversion rates in a 3D, right-handed coordinate system are given in Olbers et al. (2012, , pp.376-377). The baroclinic conversion rates are computed from Here u h = (u, v) denotes the horizontal velocity field with the components pointing in the x-and y-direction on the model grid, b = −gρ ρ0 is the buoyancy, N 2 = − g ρ0 ∂ρ ∂z is the buoyancy frequency, ρ the potential density referenced to surface, g the gravitational acceleration, and ρ 0 = 1027 kg m −3 is a reference density. The primes denote deviations from an average state (overbar), averaged over multiple eddy time scales, e.g. for velocity u = u − u. In this coordinate system, a positive value of BC indicates a transfer of potential energy from the mean flow to eddies.
We calculate the barotropic conversion rates from A positive value of BT indicates a transfer of kinetic energy from the mean flow to eddies.
We compute BC and BT after interpolating the model fields to uniform z-levels of 10 m vertical spacing. The time averaging and fluctuations are calculated over monthly windows to avoid any seasonal bias. We arbitrarily chose the year 1999 from the model fields (available from 1996 to the end of 1999). Monthly conversion rates are then averaged vertically between 100-1000 m depth (i.e., we exclude the near-surface variability). A global annual average is then obtained by averaging over these 12 335 months. Results are shown in Fig. A1.
The baroclinic conversion rates are typically positive and largest along the slope, indicating that potential energy is extracted from the slope current to feed eddies that are generated there. The barotropic conversion rates, on the other hand, show larger spatial variability. The magnitudes are smaller and the sign often changes. The baroclinic processes therefore appear to be the main contributor to the conversion of energy from the mean flow to eddies along the slope. Monthly conversion rates over the slope, volume averaged over the red box identified in Fig A1a and between 100 and 1000 m depth show that the baroclinic conversion rates dominate (Fig A2a), implying the baroclinic instability of the slope current extracts energy from the mean flow to eddies.
The motivation here is to assess whether the conversion rates obtained from a mooring array are representative of the volumeaveraged values. To do this we define a segment across the slope (magenta in Fig A1b), that stretches between the mooring  Observed BC is available only in the summer months (red line in Fig 8 d), and compare fairly well with the BC from the virtual moorings. However, a comparison with the volume-averaged conversion rates shows that calculations using virtual moorings overestimate BT, underestimate BC, introduce spurious changes in sign, and are not representative of the overall conversion 360 rates on the slope. The discrepancy in BT is partly due to the different depth-averaging (100-1000 m vs 200-600 m, note the latter range is constrained by available observations whereas the former covers the depth range of interest on the slope region, excluding the upper surface processes) , and partly because the volume-averaged calculations include the divergent terms (first and last term in Equation A2) in addition to the terms related to shear (second term). We notice that the divergent terms often occur with opposite signs (not shown) and therefore to some extent cancel out the contribution from the terms 365 related to shear. Also note that the highly variable spatial structure observed in BT cannot be resolved with a single segment.
The discrepancy in BC is mainly because the volume-averaged calculations are based on a depth average between 100-1000 m, whereas the mooring calculations are only taken at 400 m depth due to limited observations. We therefore conclude that the 19 https://doi.org/10.5194/os-2020-15 Preprint. Discussion started: 27 February 2020 c Author(s) 2020. CC BY 4.0 License.
mooring-derived conversion rates must be interpreted with caution and may not be representative of the real conversion rates in the region.