Shipborne Acoustic Doppler Current Profiler Research Articles

Dynamic Structure of Eddies of the Brazil‐Malvinas Confluence Zone Revealed by Direct Measurements and Satellite Altimetry

AbstractThe goal of this work is to study the dynamical structure of eddies of the Brazil‐Malvinas Confluence zone (BMC eddies) using direct velocity measurements carried out by Shipborne Acoustic Doppler Current Profiler during five oceanographic cruises performed in 2016–2022. In total, in situ data of 13 BMC eddies, including nine anticyclones and four cyclones are available. These data show that the orbital velocity in such eddies can reach 189 cm/s and their vertical structure is highly barotropic. In several eddies, the velocities exceeding 100 cm/s are observed down to a depth of 560 m and at a depth of 800 m they are still higher than 80 cm/s. The spatial structure of velocity and horizontal shear in the eddies is strongly asymmetric, with higher velocities in the southern part near the intense thermohaline BMC front. Altimetry data show qualitative agreement with in situ data, but underestimate the horizontal velocity shear and the maximum velocities at the periphery of the BMC eddies. We also use satellite altimetry and Argo float measurements to study these eddies, and estimate their impact on the thermohaline structure. The analysis shows that the eddies with orbital velocities exceeding 100 cm/s cause intense temperature and salinity anomalies reaching 7–9°C and 1 psu in anticyclones and −4°C and 0.8 psu in cyclones at 100–300 m depth.

A lens-shaped, cold-core anticyclonic surface eddy in the northern South China Sea

Typically, compared with the normal ocean, an anticyclonic eddy has higher sea surface temperature (SST), greater surface mixed-layer depth (MLD), and a bowl-shaped structure under the action of geostrophy. This study is the first to report an abnormal anticyclonic eddy characterized by a lens-shaped structure, cold core, and shallower MLD, which were observed in situ in the northern South China Sea (SCS) in September 2021. The SST at core of the anticyclonic eddy was 0.4°C lower than that in its peripheral region. The MLD at the center of the eddy was about half of that outside the eddy. Below the surface mixed layer, a lens-shaped structure containing relatively well-mixed water was observed between the two high-gradient layers. Within this lens-shaped structure, the isothermal layers were stretched, but accompanied by water mixing that was about one third of that at the upper and lower bounds of the structure. This eddy originated from a Kuroshio Current intrusion in late October 2020 and died in November 2021, such that its lifespan exceeded 1 year. The shedding of the Kuroshio Loop into the SCS under strong air–sea interactions under continuous sea surface cooling in winter was considered a key mechanism for the generation of the cold core of the eddy. The lens-shaped structure formed below the surface mixed layer and was maintained through geostrophic balance by the subsurface maximum speeds of the Kuroshio Current intrusion (50–100 m), thereby forming a shallower MLD in the eddy center. The subsurface speed maximum within the eddy was also observed by a shipborne acoustic Doppler current profiler at 10 months after its formation, confirming the hypothesized mechanism. This type of abnormal anticyclonic may be a common phenomenon in the northern SCS and has potential implications for the local biogeochemistry, local heat budget, and regional oceanic models.

Physical Characteristics and Evolution of a Long-Lasting Mesoscale Cyclonic Eddy in the Straits of Florida

Ocean eddies along the Loop Current (LC)/Florida Current (FC) front have been studied for decades, yet studies of the entire evolution of individual eddies are rare. Here, satellite altimetry and ocean color observations, Argo profiling float records and shipborne acoustic Doppler current profiler (ADCP) measurements, together with high-resolution simulations from the global Hybrid Coordinate Ocean Model (HYCOM) are used to investigate the physical and biochemical properties, 3-dimensional (3-D) structure, and evolution of a long-lasting cyclonic eddy (CE) in the Straits of Florida (SoF) along the LC/FC front during April–August 2017. An Angular Momentum Eddy Detection Algorithm (AMEDA) is used to detect and track the CE during its evolution process. The long-lasting CE is found to form along the eastern edge of the LC on April 9th, and remained quasi-stationary for about 3 months (April 23 to July 15) off the Dry Tortugas (DT) until becoming much smaller due to its interaction with the FC and topography. This frontal eddy is named a Tortugas Eddy (TE) and is characterized with higher Chlorophyll (Chl) and lower temperature than surrounding waters, with a mean diameter of ∼100 km and a penetrating depth of ∼800 m. The mechanisms that contributed to the growth and evolution of this long-lasting TE are also explored, which reveal the significant role of oceanic internal instability.

Remote impacts of 2009 and 2015 El Ni\xf1o on oceanic and biological processes in a marginal sea of the Northwestern Pacific

The significance of long-term teleconnections derived from the anomalous climatic conditions of El Niño has been a highly debated topic, where the remote response of coastal hydrodynamics and marine ecosystems to El Niño conditions is not completely understood. The 14-year long data from a ship-borne acoustic Doppler current profiler was used to examine the El Niño’s impact, in particular, 2009 and 2015 El Niño events, on oceanic and biological processes in coastal regions across the Korea/Tsushima Strait. Here, it was revealed that the summer volume transport could be decreased by 8.7% (from 2.46 ± 0.39 to 2.24 ± 0.26 Sv) due to the anomalous northerly winds in the developing year of El Niño. Furthermore, the fall mean volume backscattering strength could be decreased by 1.8% (from − 97.09 ± 2.14 to − 98.84 ± 2.10 dB) due to the decreased surface solar radiation after the El Niño events. Overall, 2009 and 2015 El Niño events remotely affected volume transport and zooplankton abundance across the Korea/Tsushima Strait through climatic teleconnections.

СТРУКТУРА ПОЛЯ СКОРОСТИ ЭКВАТОРИАЛЬНОГО ТЕЧЕНИЯ ЛОМОНОСОВА ПО ДАННЫМ ИЗМЕРЕНИЙ ВЕСНОЙ 2014 И ОСЕНЬЮ 2015 гг.

Two examples of measurements of oceanic equatorial currents in the Atlantic using an shipborne acoustic Doppler current profiler SADCP on meridional sections across the equator in spring 2014 at 23°W and autumn 2015 at 34°W are considered. The relevant data of some previous observations in this region are also used for comparison. The methods for studying the Lomonosov Current using ship profiling and moorings are discussed. According to the results of profiling with a two-minute averaging of the source data of the section of September 27, 2015, a structure of the two-dimensional section of the high-resolution velocity field is presented, which is characterized by small-scale fragmentary stratification, which was previously detected only from one-dimensional data of vertical soundings of velocity and other parameters of the ocean environment. Using the example of comparing this detailed section with the versions for other time averages (15 and 60 min) differences in the estimates of parameters of the velocity field due to the averaging are specified. Using the example of vertical profiles through the core of the Lomonosov Undercurrent (EUC) in the springs of 2002 and 2014 and winters of 2002 and 1971 at the locations close to 23°W, the stability of the annual cycle of fluctuations of the intensity of currents in the core and the position of the core in a multiyear aspect is illustrated. The agreement of our results with other previously published data is shown. The main parameters of the velocity field over the equatorial section on April 17, 2014 indicate a high probability of the outcropping of the easterly flow on the surface. Similar episodes of outcropping in 2002 within the PIRATA program together with corresponding intervals of changing the direction of the zonal component of wind speed derived from the Era Interim reanalysis from westward to eastward are demonstrated. A similar correlation with the wind is assumed to occur on April 17, 2014.

Analysis of tidal currents in the North Sea from shipboard acoustic Doppler current profiler data

North Sea tidal currents are determined by applying harmonic analysis to ship-borne acoustic Doppler current profiler data recorded from 1999 to 2016, covering large areas of the northern North Sea. Direct current measurement data sets of this magnitude are rare in the otherwise well investigated North Sea, and thus it is a valuable asset in studying and expanding our understanding of its tidal currents and circulation in general. The harmonic analysis is applied to a least squares fit of the current observations at a set of knot points. Results from the harmonic analysis compare favorably to tidal parameters estimated from observations from moored instruments. The analysis shows that the tides are characterized by strong semi-diurnal component, with amplitudes of the principal Lunar constituent ranging from 1.6 cm/s in the Skagerrak to 67 cm/s in the Fair Isle Channel. Diurnal tides are found to be approximately one fifth the strength of the predominant semi-diurnal constituent. Output from a regional barotropic tide model compares well to tidal current determined from the harmonic analysis of the Acoustic Doppler Current Profiler data.

Spatial and temporal variability of zooplankton off New Caledonia (Southwestern Pacific) from acoustics and net measurements

AbstractSpatial and temporal distribution of zooplankton off New Caledonia in the eastern Coral Sea was studied during two multidisciplinary cruises in 2011, during the cool and the hot seasons. Acoustic measurements of zooplankton were made using a shipborne acoustic Doppler current profiler (S‐ADCP), a scientific echosounder and a Tracor acoustic profiling system (TAPS). Relative backscatter from ADCP was converted to biomass estimates using zooplankton weights from net‐samples collected during the cruises. Zooplankton biomass was estimated using four methods: weighing, digital imaging (ZooScan), ADCP and TAPS. Significant correlations were found between the different biomass estimators and between the backscatters of the ADCP and the echosounder. There was a consistent diel pattern in ADCP derived biomass and echosounder backscatter resulting from the diel vertical migration (DVM) of zooplankton. Higher DVM amplitudes were associated with higher abundance of small zooplankton and cold waters to the south of the study area, while lower DVM amplitudes in the north were associated with warmer waters and higher abundance of large organisms. Zooplankton was largely dominated by copepods (71–73%) among which calanoids prevailed (40–42%), with Paracalanus spp. as the dominant species (16–17%). Overall, zooplankton exhibited low abundance and biomass (mean night dry biomass of 4.7 ± 2.2 mg m3 during the cool season and 2.4 ± 0.4 mg m3 during the hot season) but high richness and diversity (Shannon index ∼4). Substantially enhanced biomass and abundance appeared to be episodically associated with mesoscale features contributing to shape a rather patchy zooplankton distribution.

On the use of horizontal acoustic Doppler profilers for continuous bed shear stress monitoring

Continuous monitoring of bed shear stress in large river systems may serve to better estimate alluvial sediment transport to the coastal ocean. Here we explore the possibility of using a horizontally deployed acoustic Doppler current profiler (ADCP) to monitor bed shear stress, applying a prescribed boundary layer model, previously used for discharge estimation. The model parameters include the local roughness length and a dip correction factor to account for sidewall effects. Both these parameters depend on river stage and on the position in the cross-section, and were estimated from shipborne ADCP data. We applied the calibrated boundary layer model to obtain bed shear stress estimates over the measuring range of the HADCP. To validate the results, co-located coupled ADCPs were used to infer bed shear stress, both from Reynolds stress profiles and from mean velocity profiles.From HADCP data collected over a period of 1.5 years, a time series of width profiles of bed shear stress was obtained for a tidal reach of the Mahakam River, East Kalimantan, Indonesia. A smaller dataset covering 25 hours was used for comparison with results from the coupled ADCPs. The bed shear stress estimates derived from Reynolds stress profiles appeared to be strongly affected by local effects causing upflow and downflow, which are not included in the boundary layer model used to derive bed shear stress with the horizontal ADCP. Bed shear stresses from the coupled ADCP are representative of a much more localized flow, while those derived with the horizontal ADCP resemble the net effect of the flow over larger scales. Bed shear stresses obtained from mean velocity profiles from the coupled ADCPs show a good agreement between the two methods, and highlight the robustness of the method to uncertainty in the estimates of the roughness length.

Shelf currents, lee-trapped and transient eddies on the inshore boundary of the Agulhas Current, South Africa: their relevance to the KwaZulu-Natal sardine run

The existence and strength of the annual KwaZulu-Natal (KZN) sardine run has long been a conundrum to fishers and scientists alike ― particularly that the sardine Sardinops sagax migrate along the narrow Transkei shelf against the powerful, warm Agulhas Current. However, examination of ship-borne acoustic Doppler current profiler (S–ADCP) data collected during two research surveys in 2005 indicated that northward-flowing coastal countercurrents exist at times between the Agulhas Bank and the KZN Bight, near Port Alfred, East London, Port St Johns and Durban. The countercurrent near Port Alfred extended as far east as the Keiskamma River, within an upwelling zone known to exist there. An ADCP mooring at a depth of 32 m off Port Alfred indicated that the countercurrent typically lasted a few days, but at times remained in the same direction for as long as 10 days. Velocities ranged between 20 and 60 cm s−1 with maximum values of ∼80 cm s−1. The S–ADCP data also highlighted the existence of cyclonic flow in the Port St Johns–Waterfall Bluff coastal inset, with a northward coastal current similarly ranging in velocity between 20 and 60 cm s−1. CTD data indicated that this was associated with shelf-edge upwelling, with surface temperatures 2–4 °C cooler than the adjacent core temperature (24–26 °C) of the Agulhas Current. Vertical profiles of the S–ADCP data showed that the countercurrent, about 7 km wide, extends down the slope to at least 600 m, where it appeared to link with the deep Agulhas Undercurrent at 800 m. S–ADCP and sea surface temperature (SST) satellite data confirmed the existence of the semi-permanent, lee-trapped, cyclonic eddy off Durban, associated with a well-defined northward coastal current between Park Rynie and Balito Bay. Analysis of three months (May–July 2005) of satellite SST and ocean colour data showed the shoreward core-boundary of the Agulhas Current (24 °C isotherm) to commonly be close to the coast along the KZN south coast, as well as between the Kei and Mbhashe rivers on the Transkei shelf. The Port St Johns–Waterfall Bluff cyclonic eddy was also frequently visible in these satellite data. Transient cyclonic eddies, which spanned 150–200 km of shelf, appeared to move downstream in the shoreward boundary of the Agulhas Current at a frequency of about once a month. These seemed to be break-away Durban eddies. Data collected by ADCP moorings deployed off Port Edward in 2005 showed that these break-away eddies and the well-known Natal Pulse are associated with temporary northward countercurrents on the shelf, which can last up to six days. It is proposed that these countercurrents off Port Alfred, East London and Port St Johns assist sardine to swim northwards along the Transkei shelf against the Agulhas Current, but that their progress north of Waterfall Bluff is dependent on the arrival of a transient, southward-moving, break-away Durban cyclonic eddy, which apparently sheds every 4–6 weeks, or on the generation of a Natal Pulse. This passage control mechanism has been coined the ‘Waterfall Bluff gateway’ hypothesis. The sardine run survey in June–July 2005 was undertaken in the absence of a cyclonic eddy on the KZN south coast, i.e. when the ‘gate’ was closed.

Variability of the volume transport through the Korea/Tsushima Strait as inferred from the shipborne acoustic Doppler current profiler observations in 1997–2007

The 10-year series of observations of currents directed along the Korea/Tsushima Strait, which were measured with an acoustic Doppler current profiler aboard a ferry boat that cruised several times a week between the Hakata (Japan) and Pusan (South Korea) ports, is analyzed. Robust estimation methods are used to separate the tidal signal from the inhomogeneous series of the current data in the problem of the harmonic analysis. The MU2, NO1, PHI1, and J1 constituents have been estimated in addition to the MSF, MF, Q1, O1, P1, K1, N2, M2, S2, and K2 tidal harmonics detected previously. The annual variations in the amplitude of the M2 fundamental harmonic have also been taken into account. The current series cleared from the tidal signal has been processed in order to analyze the spatio-temporal variability of the volume transport through the Korea Strait. The normal annual velocity of the water inflow into the Japan Sea through the Korea Strait was 2.77 × 10 6 m 3 s -1 . The ratio of the flow rates in the eastern and western zones of the strait separated by the Tsushima Islands was 2/3. Considerable seasonal variations in the dis- charge are observed in the western strait zone: the flow rate annual maximum in October is 1.75 times as high as the minimum in February. An insignificant (not more than 0.1 × 10 6 m 3 s -1 on average) southward flow can cross the eastern channel. Mesoscale vortices are generated in the lee of the Tsushima Islands when the northeastern current flows around them. The energy spectrum of the total nonseasonal flow rate through the Korea Strait has been con- structed in the frequency range of 8-500 days. The spectrum has three significant maximums near periods of 10, 19, and 64 days. It has been indicated that this spectrum flattens at low frequencies (<0.1 day -1 ) in the vicinity of the for- mation of mesoscale vortices behind the Tsushima Islands.

A four-dimensional inversion of the acoustic tomography, satellite altimetry and in situ data using Quasigeostrophic constraints

A least square dynamically constrained inversion algorithm (DCIA) is developed for monitoring open ocean eddies in the regions, where open boundaries are the major sources of model forecast uncertainties on time scales larger than one week. We employ a finite-difference quasigeostrophic numerical model to constrain evolution of the ocean state. The inversion algorithm is based upon minimization of a quadratic cost function in the space of initial and boundary conditions for quasigeostrophic equations. The problem is regularized by enforcing smoothness in the fields of vorticity, buoyancy, and vertical velocity. We demonstrate that quasigeostrophic currents in an open ocean 1000 × 1000 km region can be effectively reconstructed using acoustic tomography and satellite altimetry data supported by additional sources of information. Performance of the algorithm is tested in the framework of twin-data experiments and then applied to the real observations. The analyzed observations span over the period of two months and include (i) direct and differential acoustic travel times measured at the array of five acoustic transceivers; (ii) satellite sea surface height measurements; (iii) vertical density profiles, and (iv) current velocities observed by the ship-borne acoustic Doppler current profiler (ADCP). We discuss interpolation properties of the travel time operators, study the impact of various data types on the quality of inversion, and compare performance of the proposed algorithm with the standard least square inversion of the acoustic travel times.

Comparison of Remotely Measured and Modelled Currents in Coastal Areas of Norway and Spain

Data from two six-week current measurement campaigns at the coasts of Norway and Spain are presented. Spatial coverage of surface currents was obtained by an HF (High Frequency) radar. Subsurface currents were measured by bottom-mounted and ship-borne ADCPs (Acoustic Doppler Current Profiler). The HF data were assimilated by a fine gridded model with the aim of predicting currents for about 6 h. The objective of this article is twofold, to investigate the performance of the instruments and the model, and to show the high temporal and spatial variability of currents in the coastal zones under investigation. Several strong storms occurred during the experiments with significant waveheights exceeding 11 m (Norway) and 8 m (Spain), respectively. High waves affected both the WEllen RAdar (WERA) (reduced ranges) and the ADCP (unreliable near-surface current velocities). The assimilation algorithm of the model worked well. The comparisons of measured and modelled maps of the surface-current velocity, time series...

Seasonal transformation of water masses, circulation and brine formation observed in Storfjorden, Svalbard

AbstractThe transformation of ocean water masses at high latitudes is closely related to the freezing and melting processes during the year. Downward salt fluxes from brine rejection during freezing increase the salinity and density of the water column underneath. Fresh-water input from river run-off and melting of sea ice reduces the density, mainly of the surface layer. Hydrographic profiles collected in Storfjorden, Svalbard, in spring and summer, show the strong seasonal and interannual variability of the water masses. Using, in addition, data from moorings, a ship-borne acoustic Doppler current profiler and drifting buoys, and results from models of ice drift, polynya evolution, ice formation and convection processes during freezing, we document the seasonal water-mass transformation and try to explain its interannual variability. The advection of ice and water through the two northern sounds and over the sill in the south of the fjord is examined. The release of brine-enriched bottom water over the sill must be balanced by advection from the Barents Sea in the upper layers. The interannual variability of the brine-enriched bottom layer is very high, and higher salinities are observed in a milder winter. The density anomaly resulting from freezing might depend more on the ice cover and geographical position of the polynya than on the total atmospheric forcing during winter.

Upper-ocean water mass characteristics of the California current, Summer 1993

Two large, high-resolution upper-ocean surveys of the California Current region were conducted in the summer of 1993. Temperature and salinity were measured from a Seasoar vehicle, and velocity was measured by a ship-borne acoustic Doppler current profiler. Both surveys extended from the continental margin to 128°W, and consisted of zonal sections 28 km apart. The first survey (7–28 June) extended from 39.5°N to 36.25°N, and the second (16 August to 1 September) extended from 39.0°N to 37.0°N. The current field was fairly simple in June, showing an equatorward surface jet with only one gentle meander, a nearly continuous poleward California Undercurrent near the continental margin, and two anticyclonic subsurface eddies offshore of the jet. By August meanders in the surface jet had strengthened, and eddies dominated the flow field. Large-scale trends in the water-mass characteristics (“spiciness”) along three isopycnal surfaces (26.4, 26.0 and 25.6 kg m -3) show that temperature and salinity tend to increase towards the equator and towards shore, consistent with poleward advection along the continental margin. Local minima in spiciness tend to coincide with the equatorward surface jet. Local maxima in spiciness are associated with subsurface anticyclonic eddies which have a core depth of about 150 m, and seem to originate in the California Undercurrent over the continental slope at local latitudes. We identified several different anticyclonic eddies, each with different water-mass characteristics; all of the eddies we observed had diameters larger than the baroclinic radius of deformation. Water-mass characteristics and migration rates are consistent with formation over the continental margin within the preceding 6–8 months.

Separation of a coastal upwelling jet at Cape Blanco, Oregon, USA

The coastal upwelling region near Cape Blanco (43°N), Oregon, off the west coast of the United States, was studied using a towed conductivity-temperature-depth instrument on SeaSoar, a shipborne Acoustic Doppler Current Profiler, satellite sea surface temperature maps and satellite-tracked surface drifters during three cruises: August 1994, May and August 1995. Results demonstrate that the baroclinic coastal upwelling jet (and associated front), which was over the shelf poleward of Cape Blanco in all three cruises, separates from the continental shelf, providing an important mechanism for transporting material across the continental margin to the deep ocean. This flow-topography interaction mechanism is a universal phenomenon, and is likely to be important in other eastern boundary current regions of the world. The observations from the two August cruises show two different phenomena. In 1994, cyclogenesis was observed, during which the coastal jet was connected with a cyclonic eddy offshore before the connection was severed and the jet again flowed continuously around Cape Blanco but shifted eastwards. In 1995, the coastal jet meandered in the vicinity of Cape Blanco and then continued equatorward as an oceanic jet in deep water, but it was stronger and displaced farther seawards than the previous year. Drifters released early in the upwelling season (May 1995), when the strengthening longshore upwelling jet was only minimally perturbed by the Cape, were transported rapidly equatorwards and were swept through a large portion of the eastern boundary current region. Drifters released later in the upwelling season (August) were initially swept offshore and equatorwards near the Cape, but after interaction with the spatially complex mesoscale circulation, eventually returned to the continental margin with the seasonal reversal in winds and near-surface currents. These differing flow trajectories are likely to have a significant impact on the biology of eastern boundary currents.

Calculation of ADCP Alignment Offsets Using Single-Beam Velocity and Depth Data

Abstract To extract the maximum quality results from shipborne acoustic Doppler current profiler data, it is necessary to know how the beams are aligned with respect to the earth. Individual beam velocities, relative to the water or the bottom, have been used to estimate the rotation of the transducer relative to the ship and the pitch offset of the transducer. Individual beam bottom-track ranges can be used to make an independent estimate of pitch offset and an estimate of the roll offset.

The Kuroshio Structure and Transport Estimated by the Inverse Method

Abstract Structure and transport of the Kuroshio are examined by the inverse method, based on a dataset around the Tokara Strait. Subsurface velocities measured by a shipborne acoustic Doppler current profiler are used together with CTD sections across the Kuroshio to estimate the absolute flow fields. The dataset was obtained along a closed line south off the Kyushu by the R/V Chofu Maru in September 1987. The Kuroshio transport through the Tokara Strait was 28 × 109 kg s−1 at the time of observations. The transport of warm water eddy south of the Kuroshio is estimated at larger than 39 × 109 kg s−1, but its precise value is difficult to determine because of strong transport dependency on the solution rank. The result also suggests a southward flow below the Kuroshio off Cape Toimisaki and in the Tokara Strait. In the section east of Amamioshima Island, a broad northward flow is suggested on the continental slope.

Combining hydrographic and shipborne ADCP measurements

Observations with a shipborne Acoustic Doppler Current Profiler (ADCP) were made in the Faeroe Bank Channel. When underway in strong winds and rough seas, spurious currents (>50 cm s −1) were observed which vanished when the ship was hove to. On the station strong tidal/inertial currents were detected ( ca 20 cm s −1) at two locations, over periods of 12 and 27 h. Transient current estimates were extracted by a least-squares model fit and the residuals filtered by optimum interpolation. The resulting non-divergent currents were combined with geostrophic shear derived from simultaneous CTD and XBT observations. Transport of, and mixing within, the Norwegian Sea overflow confirm previous determinations. The implication of this research for the conduct of WOCE transoceanic sections is described.

Cold Outflow from the Faroe Bank Channel

Abstract The exchange of water between the Norwegian Sea and the North Atlantic around the Faroe Islands was investigated in 1987 and 1988 combining CTD data with shipborne Acoustic Doppler Current Profiler (ADCP)measurements. Tests were devised to assess the accuracy of the ADCP measurements and the results are described; it is found that estimating a reference velocity for geostrophic measurements is limited by the presence of tides of order 10 cm s−1. In the outflow of cold water from the Faroe Bank Channel, which reaches speeds of nearly 1 m s−1 the error is acceptably small but strong time dependent components are revealed. The discharge of cold water is traced 75 km beyond the sill of this channel and changes in potential temperature, salinity and potential vorticity on an isopycnal surface are attributed to intense mixing. An effective diffusivity near 100 cm2 s−1 is derived and K-H instability demonstrated as a plausible source of the turbulent energy. Six current meter moorings were deployed from...