Ship-mounted Acoustic Doppler Current Profiler Research Articles

Observations of Currents in Seto Inland Sea using a Ship-mounted Acoustic Doppler Current Profiler -Comparison of Onboard Observations Performed on Training Voyages and Tidal Current Estimations-

Observations of Currents in Seto Inland Sea using a Ship-mounted Acoustic Doppler Current Profiler -Comparison of Onboard Observations Performed on Training Voyages and Tidal Current Estimations-

Reverse circulation in Bahía Santa Elena, North Pacific of Costa Rica

This work describes the water circulation of Bahía Santa Elena using velocity fields and hydrographic data obtained from a ship-mounted Acoustic Doppler Current Profiler and a vertical profiler Conductivity–Temperature–Depth sonde, during two campaigns in August and October of 2019. Data were taken in a cross-bay transect in the main channel of the Bahía Santa Elena during a complete tidal cycle. The hydrographic data obtained during the two sampling campaigns suggest a reverse estuarine circulation; tidally averaged values of density and water velocities revealed a dense outflow at the bottom layer in the bay. This is the first description of an inverse estuary circulation pattern in Costa Rica and the second in Central America. Understanding the hydrographic properties of the bay is crucial for undertaking future research projects on water quality and biological communities.

Near-surface western boundary circulation off Northeast Brazil

In the tropical Atlantic Ocean, the North Brazil Undercurrent (NBUC) and the North Brazil Current (NBC) play important roles as near-surface northward paths for the Atlantic meridional overturning circulation. To investigate the meridional evolution of the near-surface western boundary circulation in that region, two surveys were carried out off Northeast Brazil during austral spring 2015 and fall 2017, periods that we show to be representative of typical conditions for the spring and fall seasons. Using conductivity temperature depth oxygen probe and ship-mounted acoustic Doppler current profiler measurements, altimetry data, and numerical reanalysis products, we examine the spatiotemporal variability of the NBUC, providing a comprehensive view of the circulation, including NBUC-NBC transition and the potential impact of mesoscale eddies. The NBUC originating south of 10°S flows equatorward over the continental slope. NBUC mean velocity, transport and vertical extent of the velocity core were higher in spring 2015 (0.81 m s−1, 15.6 Sv, and >400 m) than in fall 2017 (0.65 m s−1, 11.0 Sv, and ~300 m). The upper limit of the NBUC velocity core rose slightly from 160 m at 9°S to 105 m depth at 6°S. This pattern is associated with a northward increase in current velocity from 0.7 m s−1 at 9°S to 1 m s−1 at 5°S. The orographic effect shifts the flow from northeastward south of 7.5°S to northwestward north of 7.5°S, following the shoreline. Besides, altimetry data show that this flow can be locally influenced by mesoscale activity. Farther north, at ~4.8°S, data obtained in fall 2017 show that the central branch of the South Equatorial Current (cSEC) enters into the western boundary system where it coalesces with the NBUC to form the NBC, flowing toward the Caribbean Sea. Finally, in fall, the NBUC retroflection does not feed the South Equatorial Undercurrent (SEUC), which, instead originates from retroflections of the cSEC and the equatorial branch of the South Equatorial Current (eSEC).

The spatio-temporal variation of wintertime subtidal currents in the western Taiwan Strait

A new data set of observations by six cruises of ship-mounted acoustic doppler current profiler (SADCP) and three 40 d long bottom-mounted ADCPs (BADCPs) is employed to reveal the spatiotemporal variability of tidal and subtidal currents in the western Taiwan Strait (TWS) during winter season. The results confirm the existence of intense cotidal lines for M2 tidal current, which is located north of 25°N. In this case, no existence of an amphidromic point can be identified. It is also revealed that the counter-wind current (CWC) can extend through the whole western TWS and even occupy the entire water column during winter monsoon relaxation. However, this CWC is observed to be thoroughly overwhelmed by the downwind China coastal current (CCC) during the two big monsoon bloom events in the winter of 2007, and the CCC consequently extends southward throughout the western TWS instead. Most importantly, the variation of the spatial extent for the CWC and the CCC in the western TWS is found to be well explained by the first two modes of the vector empirical orthogonal function (VEOF) analysis, that is, it is mainly controlled by a wind-driven quasi barotropic current as the first mode and slightly modulated by a relatively weak background current with a first-order baroclinic structure as the second mode.

Observations of the East Madagascar Current system: Dynamics and volume transports

The South Equatorial Current (SEC) in the Indian Ocean bifurcates when it reaches Madagascar leading, respectively, to the North East Madagascar Current (NEMC), which contours the island flowing northwestward, and to the South East Madagascar Current (SEMC), which flows southwestward. Both branches eventually contribute to the greater Agulhas Current system and thus play a part in the global ocean circulation. In addition, these currents have important effects on the local conditions for marine life. In this study, the NEMC and the SEMC are investigated based on a comprehensive, multidisciplinary survey of the east coast of Madagascar in 2008. Results from conductivity-temperaturedepth stations, underway measurements with thermosalinograph and fluorometer, and ship-mounted acoustic Doppler current profiler are discussed along with concomitant remotely sensed data. Maximum core velocities of >150 cm s–1 were observed in both the NEMC and the SEMC. The SEMC appeared as a nearshore southward jet, which extended at its widest nearly 200 km offshore. Near the southern tip of Madagascar, the SEMC was flanked by reverse velocities: a northeastward current about 30 km wide along the coast and a northeastward offshore current greater than 140 km wide suggesting the presence of the South Indian Ocean Countercurrent. The NEMC had the structure of a narrow nearshore jet, with weak currents beyond 100 km offshore. Volume transport for the upper 1,100 m was estimated to be 22 Sverdrup (Sv) for the SEMC and 48 Sv for the NEMC. The high NEMC transport compared with earlier estimates might be because of anomalously high influx of the SEC during the measurement period. Off the south coast, an undercurrent below 900 m carried an equatorward transport of 3.1 Sv. No undercurrent was observed in the north, but a geostrophic velocity minimum and a similar vertical velocity shear between surface and subsurface currents were found at the level where such a countercurrent has been previously observed from direct measurements.

Observations of strong ocean current events offshore Pemba, Northern Mozambique

ABSTRACTThree years (2008–2011) of current measurements outside Pemba (∼13°S), Mozambique, are presented. Oil and gas reserves are found in the region, and knowledge of the ocean current climate is important for safe offshore operations. The full-depth current profile at Pemba was estimated by comparing with moorings further south. The moored record was also compared with geostrophic velocities obtained from sea surface height (SSH) fields, current profiles from ship-mounted Acoustic Doppler Current Profiler and surface currents from seven drifters passing the mooring. Velocities up to 60 cm/s were observed at 850 m depth. Altimetry-derived currents, while strong (often > 100 cm/s), are probably underestimates due to coarse resolution. Direct observations show surface currents up to 170 cm/s. Four strong current events were studied in detail. Three of the four events appeared to be associated with anticyclonic eddies nearby. The time series of SSH-derived geostrophic current always showed southward velocities, never below 20 cm/s. These findings indicate that currents off Pemba may display characteristics of a western boundary current rather than being completely dominated by eddies. Strong current events are probably caused by anticyclones interacting with the boundary current and speeds on the order of 200 cm/s can be expected.

船舶搭載型CTD・ADCPによる富山湾の海洋環境調査

Toyama Bay, a semi-enclosed water area on the northern coast of Japan with an average water depth at 600 m, is known to have three primary water masses: the coastal surface layer, Tsushima Warm Current water and Japan Sea proper water. The present study describes the oceanographic characteristics of the bay based on the oceanographic observation programs, which include temperature and salinity with a CTD (Conductivity, Temperature and Depth profiler) system and current velocities with shipmounted ADCP (Acoustic Doppler Current Profiler), onboard, Toyama National College of Technology (TNCT) training ship, Wakashio-maru, from 2004 to 2012. By the ADCP observations at 2010, 2011 and 2012, the authors observed the main path of Tsushima Warm Current and an anticlockwise eddy with its diameters of about 20-30 nautical miles in the inner parts of the bay. The authors compared these ADCP data with a numerical simulation results for ocean current and satellite remote sensing data for the Chlorophyll-a concentration.

Currents Associated with the Quasi-Stationary Jet Separated from the Kuroshio Extension

Abstract The hydrographic structure of a quasi-stationary jet separated from the Kuroshio Extension, a phenomenon that possibly leads to deepening of the winter mixed layer, is revealed via intensive observations using a conductivity–temperature–depth profiler and a ship-mounted acoustic Doppler current profiler (ADCP) carried out in September 2009. Data collected from profiling floats set to drift isopycnal layers and time series of sea surface buoys and satellite altimeters were also analyzed to understand the continuity, water characteristics, and time variation of the jet. Although the flow field was complex due to disturbances such as energetic eddies, the jet transported subtropical water to the downstream (northeastern) observation region in a layer shallower than 400 dbar and its highest velocity was concentrated in the mid- and deep layers (≤0.30 m s−1 at the sea surface and ≥0.05 m s−1 at 800-m depth). The velocity axes of the jet detected from the satellite data corresponded to those detected through analysis of the ADCP data, and the intensity of the jet at the sea surface varies over interannual-to-decadal time scales. Part of the interannual-to-decadal variation in the velocity field of the jet is controlled by the dynamic state of the Kuroshio Extension and is correlated with that of the Kuroshio Extension latitude (linear correlation coefficient r ~ 0.67). The relationship between these variations can be interpreted qualitatively as being responsible for the inertial streamers that separate from the crests of meanders of the Kuroshio Extension to the jet region due to the large steering effect.

Turbulent Wake of a Bridge Pier in a Tidal Current

The evolution of a large Reynolds number O(10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> ) wake was investigated using data from a ship-mounted acoustic Doppler current profiler (ADCP). The ADCP was a custom configured 600-kHz Teleydyne RD Instruments Workhorse augmented to have a fifth beam that was nominally vertical. The test was conducted in Narragansett Bay, RI, USA, using a large support pier of the Newport Bridge as the turbulent source in the bay's tidal flow. The turbulence was manifested as a vigorously fluctuating wake in the lee of the bridge pier. A series of wake crossings between x = 10-100 D were made during the ebb of the tidal cycle, where x and D are, respectively, the downstream distance and bridge pier diameter. The data set serves as one of a handful where wake evolution has been documented for a large Reynolds number system, and is unique in that high-horizontal resolution vertical velocity profiles were taken with the vertical fifth beam of the ADCP. A range of scales was observed in the vertical velocity field within the wake, including small-scale turbulence O(2-10 m) and large vortex structures O(20-40 m). When cast in terms of nondimensional wake age ( N τ), where N and τ are, respectively, the buoyancy frequency and wake age, the decay of root mean square vertical velocity fluctuations was observed to be proportional to (N τ) <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-0.82</sup> , faster than the decay of an unstratified 2-D plane wake.

Variability of the meridional overturning circulation at the Greenland–Portugal OVIDE section from 1993 to 2010

The meridional overturning circulation (MOC) in the North Atlantic transports heat from the subtropics to high latitudes and hence plays an important role in the Earth’s climate. A region crucial for the MOC is the northern North Atlantic and the adjacent Nordic Seas, where waters transported northward in the MOC upper limb progressively cool, gain density and eventually sink. Here we discuss the variability of the gyre circulation, the MOC and heat flux as quantified from a joint analysis of hydrographic and velocity data from six repeats of the Greenland to Portugal OVIDE section (1997–2010), satellite altimetry and Argo float measurements. For each repeat of the OVIDE section, the full-depth absolute circulation and transports were assessed using an inverse model constrained by ship-mounted Acoustic Doppler Current Profiler data and by an overall mass balance. The obtained circulation patterns revealed remarkable transport changes in the whole water column and evidenced large variations (up to 50% of the lowest value) in the magnitude of the MOC computed in density coordinates (MOCσ). The extent and time scales of the MOCσ variability in 1993–2010 were then evaluated using a monthly MOCσ index built upon altimetry and Argo. The MOCσ index, validated by the good agreement with the estimates from repeat hydrographic surveys, shows a large variability of the MOCσ at OVIDE on monthly to decadal time scales. The intra-annual variability is dominated by the seasonal component with peak-to-peak amplitude of 4.3Sv (1Sv=106m3s–1). On longer time scales, the MOCσ index varies from less than 15Sv to about 25Sv. It averages to 18.1±1.4Sv and shows an overall decline of 2.5±1.4Sv (95% confidence interval) between 1993 and 2010. The heat flux estimates from repeat hydrographic surveys, which vary between 0.29 and 0.70±0.05PW, indicate that the heat flux across the OVIDE section is linearly related to the MOCσ intensity (0.054PW/Sv).

Characteristics of nonlinear internal waves observed in the northern South China Sea

The South China Sea (SCS) is one of the most active areas of internal waves. We undertook a program of physical oceanography in the northern South China Sea from June to July of 2009, and conducted a 1-day observation from 15:40 of June 24 to 16:40 of June 25 using a chain of instruments, including temperature sensors, pressure sensors and temperature-pressure meters at a site (117.5°E, 21°N) northeast of the Dongsha Islands. We measured fluctuating tidal and subtidal properties with the thermistor-chain and a ship-mounted Acoustic Doppler Current Profiler, and observed a large-amplitude nonlinear internal wave passing the site followed by a number of small ones. To further investigate this phenomenon, we collected the tidal constituents from the TPXO7.1 dataset to evaluate the tidal characteristics at and around the recording site, from which we knew that the amplitude of the nonlinear internal wave was about 120 m and the period about 20 min. The horizontal and vertical velocities induced by the soliton were approximately 2 m/s and 0.5 m/s, respectively. This soliton occurred 2–3 days after a spring tide.

Geometric properties of river cross sections and associated hydrodynamic implications in Wuhan-Jiujiang river reach, the Yangtze River

Based on measured hydrological data by using ship-mounted Acoustic Doppler Current Profiler (ADCP) instrument, we analyzed shapes of river cross sections of the middle Yangtze River basin (mainly focusing on Makou and Tianjiazhen river reach). Hydrodynamic properties of river channels were also discussed. The research results indicate that nonlinear relationships can be identified between river-width/river-depth ratio (W/D ratio), sizes of cross section and mean flow velocity. Positive relations are detected between W/D ratio and mean flow velocity when W/D 1. Adverse relationships can be obtained between W/D ratio and cross-section area. Geomorphologic and geologic survey indicates different components of river banks in the wider and narrower river reaches respectively. These may be the main driving factors causing unique hydrological properties of river channels in the middle Yangtze River basin. Narrower river cross sections tend to raise water level in the upstream river reach near narrower river channel, giving rise to backwater effects. River knots can cause serious backwater effects, which is harmful for flood mitigation. However river knots will also stabilize river channel and this will be beneficial for river channel management. The results of this paper may be helpful for flood mitigation and river channel management in the middle Yangtze River basin.

ADCP measured flow current of the middle-lower Changjiang River channel

The water column flow velocity of 36 river sections in the river reach between Hankou (Wuhan) and Wuxue of the middle-lower Changjiang River. Their cross sectional distribution patterns in relation to the river channel morphologies were examined by using ship-mounted ADCP (Acoustic Doppler Current Profiler) instrument. The results indicate four (I-VI) types of river channel morphology associated flow patterns: I—laterally deepening riverbed topographic pattern; II—symmetrical to asymmetrical riverbed topographic pattern; III—relative flat riverbed topographic pattern, and IV—sandbar supported riverbed topographic pattern. All these correspond to the different patterns of flow velocity distribution. The maximum flow velocity is usually related to the deeper water depth, but irregular water column distribution of flow current velocity results often from the vortices’ current associated with river knots. Deeper river water depth is usually identified in the river reach located slightly downstream to the river knot, where faster flow velocity occurs. Downward change in flow velocity fits semi-log law, showing an exponential decreasing flow current with the maximum flow velocity near the water surface. However, in the river reach near the river knots, the water column distribution of flow current velocity does not fit the semi-log law, showing the irregular flow current pattern. This study, in context of river catchment management, highlights the controls of riverbed morphology to the flow current structure, which will shed light on the post study of Three Gorges damming in 2009.

Spatial continuity of measured seawater and tracer fluxes through Nares Strait, a dynamically wide channel bordering the Canadian Archipelago

Freshwater delivered as precipitation and runoff to the North Pacific and Arctic oceans returns to the Atlantic principally via the Canadian polar shelf and Fram Strait. It is conveyed as ice or freshened seawater. Here we use detailed ship-based measurements to calculate a snap-shot of volume, freshwater, and tracer fluxes through Nares Strait, a 500-km long waterway separating Greenland and Ellesmere Island. We use quasi-synoptic observations of current by ship-mounted acoustic Doppler current profiler (ADCP), of salinity and temperature by CTD probe and of dissolved nutrients by rosette bottle sampler on four cross-sections between 82 and 78N latitude. Data were collected during the first half of August 2003. We partition the fluxes into components derived from Pacific and Atlantic inflows into the Arctic Ocean. During the time of the survey, there was a net southward 0.91 0.10 Sv (10 m s) flux of volume and a net southward 31 4 10 Sv (977 127 km y) flux of freshwater relative to a salinity of 34.8. Much of the volume flux was carried within a strong (40 cm s), narrow (10 km) subsurface jet hugging the western (Ellesmere Island) side of the strait. The presence of this jet in four sections spanning the 500-km length of the strait is evidence of a buoyant boundary current through the strait. The jet was coincident with elevated concentrations of phosphate (1.0 mmol m) and silicate (11 mmol m) which both indicate a Pacific Ocean source. We interpreted the ratio of dissolved total inorganic nitrogen to phosphate in terms of fractional dilution of Atlantic by Pacific waters. About 0.43 0.10 Sv (39%) of the southward flow was of Pacific origin. These results are a snapshot during the summer of 2003 following a prolonged period of northward directed wind stress when ice cover was mobile. Although long-term mean values are likely different, we determined that the major fraction of the through-flow is carried by a jet of scale determined by the internal Rossby radius (5-10 km).

The tidal flux in the Firth of Forth

The tidal flow into an estuary controls the export of pollutants and determines the net flux and redistribution of sediments. To make an assessment of the accuracy of model simulations, a ship-mounted acoustic Doppler current profiler was used to measure the flow into the Firth of Forth near the Edinburgh road bridge during neap and spring tide conditions. A pressure gauge was used to record the tidal elevation and concurrent meteorological data were obtained from local weather sensors. The maximum tidal flux into the estuary during spring tides reached 30 000 m3/s. The observations were simulated using nested versions of the DIVAST depth-averaged tidal model. Good agreement was obtained between the observed and simulated tidal elevations, and also in the magnitude of the computed tidal fluxes. There were, however, sometimes large instantaneous differences between the observed and computed fluxes. An analysis of low-pass sea-level data, atmospheric pressure and wind forcing, suggests that the errors in the flux estimates were due to meteorologically induced shifts in the phase of the tide at the open boundary of the model. Such shifts limit the accuracy that can be obtained by a tidally forced model when the impact of meteorological forcing is not included at the open boundary.

Evaluation of AUV-based ADCP measurements

Recent development and commercialization of relatively portable and inexpensive autonomous underwater vehicles (AUVs) have led to new methods for making hydrodynamic, chemical, and biological measurements in near-coastal, lake, and estuarine systems. Equipped with Doppler velocity log (DVL) technology to allow for deadreckoning navigation, this new class of AUVs has the potential to log not only vehicle motions, but water velocities as well. In this article, we assess the performance of the DVL acoustic Doppler current profiler (ADCP) equipped on the REMUS AUV (Hydroid) in three different environments: a tidally forced lake; a wave-forced, near-shore fringing coral reef; and a gently sloping continental shelf. In all three data sets, the water velocities measured by the AUV DVL compare favorably with measurements from nearby stationary acoustic Doppler current profilers. Nevertheless, the water velocity component measured parallel to the vehicle's tracks exhibited a bias in the direction of AUV motion, consistent with a previously reported bias found for the ship-mounted ADCP application in low-scattering environments. The fringing reef data suggest that DVLs are capable of partially resolving wave motions.

Detiding ADCP Data in a Highly Variable Shelf Sea: The Celtic Sea

Abstract This paper presents a comparison of two conventional detiding techniques carried out for ship-mounted acoustic Doppler current profiler (ADCP) data collected in the European shelf area of the Celtic Sea during the summer of 1998. One technique consisted of extracting the vertically averaged tidal currents obtained from a barotropic three-dimensional numerical tidal model. The second technique consisted of fitting the spatiotemporal ADCP data using least squares and polynomial spatial functions. In the least squares technique, the incorporation of zero velocity normal to the coast appears to improve the estimation of the tidal currents near the coast. Quantitative comparisons of the results from both techniques with historical current meter observations are shown. However, both methods showed limitations in accurately representing the tidal currents in the study area. Consequently, an alternative detiding technique is proposed. This technique consists of blending the tidal currents derived from the numerical model with those fitted to the ADCP data from the least squares method. Improved results were obtained using the blending technique. ADCP-derived residual currents were comparable with contemporaneous flows measured using drifting buoys and also with estimates obtained by geostrophic calculations.

Variability of Northeastward Current Southeast of Northern Ryukyu Islands

To better understand the mechanism underlying the variation of the Kuroshio south of central Japan, we have examined the variability of current structure in its upstream region, southeast of Amami-Ohshima Island in the northern Ryukyu Islands. By combined use of ship-mounted Acoustic Doppler Current Profiler (ADCP) and the TOPEX/POSEIDON satellite altimeter data on Path 214, the sea surface absolute geostrophic currents were estimated every ten days from January 1998 to July 2002. The 4.5-year mean surface current was found to flow northeastward north of 26.8°N with a maximum speed of 14 cm s−1 over the shelf slope at 3000 m depth. The moored current-meter observations at three or four mooring stations from Dec. 1998 to Oct. 2002 suggested the existence of a northeastward undercurrent with a maximum core velocity of 23 cm s−1 at 600 m depth over the shelf slope at 1600 m depth. The mean volume transport in the top 1500 m between 27.9°N and 26.7°N is estimated to be 16 × 106 m3s−1 northeastward, including the subsurface core current related component of 4 × 106 m3s−1.

The Response of a Strongly Stratified Fjord to Energetic Tidal Forcing

The response of Upper Loch Linnhe, a strongly stratified Scottish fjord, to energetic tidal forcing is studied by new analysis of two observational data sets. Using a ship-mounted Acoustic Doppler Current Profiler (ADCP) and an undulating Conductivity-Temperature-Depth (CTD) we are able to repeatedly sample over a tidal cycle at sufficiently high spatial and temporal resolutions to clearly define the tidal variability in the vertical and horizontal. Away from the near-sill region, horizontal velocities are generally orientated along the longitudinal axis of the fjord. Analysis of the along-fjord velocities shows that at the M 2-frequency there is a strong 1st mode internal tide with maximum horizontal velocity amplitudes of 23·4 cm s −1. The observed vertical structure of the complex amplitudes of the horizontal velocities are in close agreement with normal mode theory. Along-fjord variations of the fitted amplitude and phase lags from further analysis of the two independent data sets show that the 1st mode internal tide is behaving as a standing wave. Within the near-sill region, the circulation is complicated by the formation of a horizontal gyre and a large perturbation in the density field on the flood tide. These features are a response to the flood tide currents advecting denser water into the fjord in the form of a strong jet. Due to the narrow entrance sill being off-centre to the axis of the fjord, the strong flood tide jet gives rise to eddy formation. As the incoming flood tide water is denser than the resident surface water of the fjord the tidal jet sinks and advects the resident water landwards giving rise to the observed density perturbations.

Diagnostic analysis of the 3D ageostrophic circulation from a multivariate spatial interpolation of CTD and ADCP data

A multivariate (MV) optimal statistical interpolation method is applied to conductivity–temperature–depth (CTD) and ship-mounted acoustic doppler current profiler (ADCP) data from quasi-synoptic oceanographic surveys. MV analysis aims to improve the spatial interpolation of any particular variable (e.g., dynamic height) by including in the analysis observations of other physically related variables (e.g., current). The version used in this work also provides estimates of the non-divergent and irrotational components of the flow. The method is tested in a sharp frontal region to the north of the Western Alborán gyre. After deriving the optimal analysis parameters, we first show that MV statistical dynamic height analysis errors are significantly smaller than those derived from univariate (UV) analysis. In our region, this translates in a more realistic shape for the geostrophic relative vorticity and the vertical velocity field. The latter peaks at about 45 m/day (as given by the quasi-geostrophic omega equation), with a tendency for light water to be upwelled upstream of the gyre while denser water is downwelled downstream of the gyre. For the horizontal velocity we show the existence of large (up to 40 cm/s) ageostrophic velocities. These are mainly non-divergent and can be explained by the cyclostrophic acceleration induced by the anticyclonic gyre. The irrotational velocity component is of the order of 10 cm/s towards the dense side of the front. The robustness of the method is checked by means of several tests that evaluate the sensitivity of results with respect to the synopticity of the data, the analysis parameters, the reference level and the presence of tidal or inertial currents.