Acoustic Doppler Current Profiler Observations Research Articles

Comparison of ADCP and ECCOv4r4 Currents in the Pacific Equatorial Undercurrent

Abstract The Pacific Equatorial Undercurrent (EUC) flows eastward across the Pacific at the equator in the thermocline. Its variability is related to El Niño. Moored acoustic Doppler current profiler (ADCP) measurements recorded at four widely separated sites along the equator in the EUC were compared to currents generated by version 4 release 4 of the Estimating the Circulation and Climate of the Ocean (ECCOv4r4) global model–data synthesis product. We are interested to learn how well ECCOv4r4 currents could complement sparse in situ current measurements. ADCP measurements were not assimilated in ECCOv4r4. Comparisons occurred at 5-m depth intervals at 165°E, 170°W, 140°W, and 110°W over time intervals of 10–14 years from 1995 to 2010. Hourly values of ECCOv4r4 and ADCP EUC core speeds were strongly correlated, similar for the EUC transport per unit width (TPUW). Correlations were substantially weaker at 110°W. Although we expected means and standard deviations of ECCOv4r4 currents to be smaller than ADCP values because of ECCOv4r4’s grid representation error, the large differences were unforeseen. The appearance of ECCOv4r4 diurnal-period current oscillations was surprising. As the EUC moved eastward from 170° to 140°W, the ECCOv4r4 TPUW exhibited a much smaller increase compared to the ADCP TPUW. A consequence of smaller ECCOv4r4 EUC core speeds was significantly fewer instances of gradient Richardson number (Ri) less than 1/4 above and below the depth of the core speed compared to Ri computed with ADCP observations. We present linear regression analyses to use monthly-mean ECCOv4r4 EUC core speeds and TPUWs as proxies for ADCP measurements. Significance Statement Hundreds of scientific papers have used ECCO data products generated with a continually evolving state-of-the-art ocean-model–data synthesis system. We ask, How representative is the latest version of ECCO equatorial ocean currents? We use independent in situ current measurements as the reference dataset to establish the accuracy of ECCO currents in the tropical Pacific. Attention is focused on the Pacific Equatorial Undercurrent (EUC) because it contributes to the formation of El Niño and La Niña events. ECCO EUC core speeds were smaller in magnitude and less variable in time compared to observations. As a consequence, ECCO currents generated smaller vertical mixing in the EUC compared to that inferred from current measurements. We developed a linear regression model to improve representation of monthly-mean ECCO currents.

A Circulation Model for Inlets Along the Central West Coast of Vancouver Island

ABSTRACT An ocean circulation model has been developed for the central west coast of Vancouver Island, with particular attention to the numerous inlets where salmon farms are located. The ultimate goal is to provide a tool to assist in informing decisions on the siting and management of fish farms and to further understanding of the regional coastal oceanography. The complicated coastline and bathymetry are approximated with an unstructured triangular grid in the horizontal direction and terrain-following coordinates in the vertical. The period of 1 March to 31 July 2016, is simulated using the Finite Volume Community Ocean Model and results are evaluated through comparisons with a combination of acoustic Doppler current profiler, tide gauge, and temperature and salinity observations. The model was found to be accurate in representing (i) tidal elevations and currents everywhere throughout the model domain and (ii) subtidal currents and temperature and salinity along the shelf. However, it fared poorly in inlets where (i) the tidal currents were relatively weak and not the dominant source of mixing and/or (ii) steep bathymetry necessitated smoothing. Sources of these and other inaccuracies, and the changes needed to overcome them, are discussed. Overall, this work highlights the complex and diverse dynamics at play in a glacially-carved coastal ocean, providing an extensive discussion of modelling challenges and potential solutions.

Comparison of Actuator Line Modelling of Tidal Power Kites with ADCP Measurements

The potential for tidal energy to be a part of future renewable energy systems is expanding. Tidal turbines deployed in open waters or channels are effective methods to harness energy from tidal currents. Sharing similar functionality with wind turbines, horizontally mounted tidal turbines require a minimum tidal current velocity to operate effectively. The Deep Green (DG) power plant which is based on a tethered kite model aims, however, to operate efficiently in tidal current velocities as low as 1.2 m/s. The kite wing is steered in a lemniscate trajectory ( ) almost perpendicular to the tidal current. In the trajectory, the relative flow velocity through the turbine attached to the wing reaches several times the tidal current velocity, enabling efficient operation of the turbine in relatively low-velocity tidal currents. This could reduce geographical limitations in installing large-scale tidal power arrays.
 In a previous project, numerical modelling of the DG was carried out in a tidal flow using Large Eddy Simulations (LES) and Actuator Line Modelling (ALM) implemented in OpenFOAM solver. ALM using momentum sources has been used in modelling wind turbines and has been validated against experimental data and observations. The ALM has been further developed in order to be able to model wings (here the kite) that move in arbitrary paths compared to horizontally mounted turbines with rotational paths. This numerical model for the DG kite in a tidal flow has, however, up to now not been validated against observations, which is beneficial before further analyses are made, e.g., optimization studies.
 In this study, Acoustic Doppler Current Profiler (ADCP) observations taken in the wake of a DG are compared to the results from the numerical model under similar conditions. Comparing the numerical results directly with the observations leads to discrepancies, hence the model data is resampled in a similar way that the ADCP would measure and process data using a virtual ADCP for the model. Flow properties such as the instantaneous and time-averaged stream velocities are in turn compared for both the tidal flow without the DG and with a DG. The effect of the DG on the tidal flow is analysed using the model and the observations. After resampling the model data, the model and observations show good agreement. This suggests 1) that the DG model using ALM can be used for further analysis and 2) that whilst comparing model data with ADCP observations for studying the small-scale effects of tidal turbine wakes, care should be taken to sample the model data consistently with observations.

Validation of Actuator Line Modeling and Large Eddy Simulations of Kite-Borne Tidal Stream Turbines against ADCP Observations

The representation of tidal energy in future renewable energy systems is growing. Most of the current tidal turbine designs are limited by the minimum current velocity required for efficient operation. The Deep Green (DG) is a kite-borne tidal power plant designed to sustain efficient operation in tidal current velocities as low as 1.2 ms−1. This could increase the geographical areas suitable for large-scale tidal power arrays. Numerical modeling of the Deep Green was carried out in a previous study using large eddy simulations and the actuator line method. This numerical model is compared with acoustic Doppler current profiler (ADCP) measurements taken in the wake of a DG operating in a tidal flow under similar conditions. To be comparable, and since the ADCP measures current velocities using averages of beam components, the numerical model data were resampled using a virtual ADCP in the domain. The sensitivity of the wake observations to ADCP parameters such as pulse length, bin length, and orientation of the beams is studied using this virtual ADCP. After resampling with this virtual ADCP, the numerical model showed good agreement with the observations. Overall, the LES/ALM model predicted the flow features well compared to the observations, although the turbulence levels were underpredicted for an undisturbed tidal flow and overestimated in the DG wake 70 m downstream. The velocity deficit in the DG wake was weaker in the observations compared to the LES. The ALM/LES modeling of kite-borne tidal stream turbines is suitable for further studies of array optimization and wake propagation, etc.

Imminent reversal of the residual flow through the Marsdiep tidal inlet into the Dutch Wadden Sea based on multiyear ferry-borne acoustic Doppler current profiler (ADCP) observations

Abstract. The Dutch Wadden Sea is a UN World Heritage Site connected to the North Sea by multiple tidal inlets. Although there are strong tidal currents flowing through these inlets, the magnitude and direction of the residual circulation in the western Dutch Wadden Sea is important for sediment, salinity and nutrient balances. We found that the direction of this residual flow is reversing. This residual circulation has been the subject of various studies since the 1970s, in which substantially different net volume fluxes were presented. Differences in tidal conditions in the main inlets, tidal rectification and meteorology were identified as driving mechanisms. Here we analysed almost 13 years of acoustic Doppler current profiler (ADCP) observations collected on the ferry crossing the Marsdiep tidal inlet in the Dutch Wadden Sea since 2009. The results are combined with earlier investigations covering the period 1998–2009. We find a significant trend in the magnitude of the residual volume flux, with decreasing export to the North Sea and with occasional imports observed in recent years. We hypothesise that this trend is related predominantly to changes in tides in the North Sea, which are caused by increased strength and duration of stratification in response to global warming. With warming projected to continue, we expect the residual flow in the Marsdiep to continue to reverse to full inflow within the current decade, with potential knock-on effects for the sediment budget and ecosystem of the western Wadden Sea.

Observed near-inertial waves and shears east of Luzon during Typhoons Mangkhut and Yutu in 2018

Based on acoustic Doppler current profiler observations from three moorings along the 18° N latitude line east of Luzon (at 122.7° E, 123° E, and 123.3° E), the characteristics, vertical propagation, modes, and shear properties of the near-inertial waves (NIWs) induced by Typhoon Mangkhut and Typhoon Yutu in 2018 were investigated. Energetic NIW events were generated that dominated ocean currents after the passage of the typhoons. The wavenumber–frequency spectra and downward/upward current decomposition demonstrated that the near-inertial currents rotated clockwise and mainly propagated downward energy in the Northern Hemisphere. The estimated phase velocities and vertical wavelengths were 4.9–9.4 m/h and 155–376 m, respectively. The NIWs at the three moorings showed red-shifted frequencies in the upper ocean but blue-shifted frequencies in the deep ocean, which were affected by background vorticity. Vertical mode analysis showed that the near-inertial kinetic energy (NIKE) was dominated by the third, fourth, and fifth higher modes above 400 m and by the first, second, and third lower modes below 400 m. The translation speeds of Typhoons Mangkhut (29 km/h) and Yutu (24 km/h) exceeded the critical value (14.4 km/h). Since the intensity of NIWs decreases with an increase in typhoon translation speed, the maximum NIKE caused by Typhoon Yutu was higher than that caused by Typhoon Mangkhut because of the lower translation speed of Typhoon Yutu. However, the NIKE caused by Typhoon Mangkhut propagated vertically into a deeper layer than that caused by Typhoon Yutu. The strong anticyclonic eddy near the moorings during the period of Typhoon Mangkhut played a more important role in the vertical propagation of NIKE than the weaker anticyclonic eddy during the period of Typhoon Yutu. This study shows the significant contribution of NIWs to shear. NIW-induced near-inertial shear was significantly enhanced after the two typhoons passed, with contributions of more than 50%. Although NIW-induced shear appeared to be stronger during the typhoons, no strong shear instabilities or turbulent mixing were observed.

ADCP Observations of Currents and Suspended Sediment in the Macrotidal Gulf of Martaban, Myanmar

The Ayeyarwady and Thanlwin Rivers, which drain Myanmar, together form one of the largest point sources of freshwater and sediment to the global ocean. Combined, these rivers annually deliver an estimated 485 Mt of sediment to the northern Andaman Sea. This sediment contributes to a perennially muddy zone within the macro-tidal Gulf of Martaban, but little is known about the processes that dominate dispersal and trapping of sediment there, as very few water column observations are available. A research cruise in December 2017 provided a rare opportunity to obtain Acoustic Doppler Current Profiler (ADCP) data along transects from the Gulf of Martaban and adjacent continental shelf. Two transects were obtained from the outer portion of the Gulf of Martaban in water depths that ranged from about 20–35 m. These showed very fast currents, especially during flood tide conditions, exceeding 1.5 m/s. The backscatter record from the ADCP indicated asymmetries in distribution of suspended sediment during the ebb versus flood phase of the tide. During ebb tidal conditions, the backscatter record indicated that sediment was transported in either a surface advected layer, or fairly well-mixed throughout the water column. In contrast, during flood tidal conditions, sediment was confined to the bottom boundary layer, even though the velocities were faster during flood than the ebb conditions. The vertical structure of the currents during flood tide conditions indicated the presence of sediment–induced stratification because currents within the near-bed turbid layers were relatively slow, but speeds increased markedly above these layers. This albeit limited dataset provides an exciting glimpse into the dynamics of sediment transport within the muddy, macrotidal Gulf of Martaban, and implies the importance of tidal straining and bottom nepheloid layer formation there.

Application of an Entropic Method Coupled with STIV for Discharge Measurement in Actual Rivers

In recent years, due to the frequent occurrence of floods that exceed the facility maintenance level due to climate change, non-contact flood flow measurement techniques have been paid attention and actually some measurements have been conducted by applying them instead of the conventional float method. The space-time image velocimetry (STIV) which can measure the surface flow velocity distribution from video images is one of such techniques. In order to calculate the river flow from the surface velocity distribution, it is necessary to determine an appropriate surface velocity coefficient, which is the ratio of the average depth velocity to the surface velocity. However, at present, empirical default value has been still used in practice. In this study, the cross-sectional velocity distribution was calculated using an entropic method by utilizing the surface velocity distribution measured by STIV and compared with Acoustic Doppler Current Profiler (ADCP) observation. It was confirmed that the introduction of the velocity dip system express the flow velocity distribution in the vertical direction, where the velocity dip occurs due to the influence of vegetation.

Optimisation of Parameters in a German Bight Circulation Model by 4DVAR Assimilation of Current and Water Level Observations

Uncertain parameters in a 3D barotropic circulation model of the German Bight are estimated with a variational optimisation approach. Surface current measurements from a high frequency (HF) radar are used in combination with acoustic Doppler current profiler (ADCP) and tide gauge observations as input for a 4DVAR assimilation scheme. The required cost function gradients are estimated using an adjoint model code. The focus of the study is on systematic errors of the model with the control vector including parameters of the bathymetry, bottom roughness, open boundary forcing, meteorological forcing as well as the turbulence model. The model uses the same bathymetry, open boundary forcing, and metereological forcing as the operational model run at the Federal Maritime and Hydrographic Agency (BSH). The baroclinic BSH model is used as a reference to put the performance of the optimised model into perspective. It is shown that the optimised model has better agreement with HF radar data and tide gauge observations both within the fortnight training period and the test period 1 month later. Current profile measurements taken at two platforms indicate that both models have comparable error magnitudes at those locations. The optimised model was also compared with independent drifter data. In this case, drifter simulations based on the BSH model and the respective operational drift model including some surface wave effects were used as a reference. Again, these comparison showed very similar results overall, with some larger errors of the tuned model in very shallow areas, where no observations were used for the tuning and surface wave effects, which are only explicitly considered in the BSH model, play a more important role. The tuned model seems to be slightly more dissipative than the BSH model with more energy entering through the western boundary and less energy leaving toward the north. It also became evident that the 4DVAR cost function minimisation process is complicated by momentum advection, which leads to non-differentiable dependencies of the model with respect to the control vector. It turned out that the omission of momentum advection in the adjoint code still leads to robust estimates of descent directions.

Comparison of ADCP measurements to Kuroshio current flow simulations for ocean current turbines

Numerical simulations are compared to field measurements of ocean currents in the Tokara Strait in this work. The Kuroshio Current flows through this strait and an ongoing ocean current turbine device is under development to harness its energy. The usefulness for engineering design input of modeling flow conditions for a large domain in space and time motivates the use of the numerical simulations. Confidence in the accuracy of the simulations can be provided from this comparison to Acoustic Doppler Current Profiler (ADCP) observations during the summer of 2018 at four locations. Numerical simulations of ocean currents which overlap in time with the field observations are presented in an attempt to compare the data on a time-domain basis. The simulations were produced using a Princeton Ocean Model based JCOPE-T-Tokara500 model. The analysis describes the capability of the numerical model to match the flow profile throughout the water column in the time domain and on a statistical basis using histograms and rose diagrams. While instances of speed peaks in the measurement data possibly representing internal waves did not readily appear in the simulation, overall the analysis supports the continued use of simulation current flow for project design input.

Three‐Dimensional Numerical Simulations of Internal Tides in the Angolan Upwelling Region

AbstractIn austral winter, biological productivity at the Angolan shelf reaches its maximum. The alongshore winds, however, reach their seasonal minimum suggesting that processes other than local wind‐driven upwelling contribute to near‐coastal cooling and upward nutrient supply, one possibility being mixing induced by internal tides (ITs). Here, we apply a three‐dimensional ocean model to simulate the generation, propagation, and dissipation of ITs at the Angolan continental slope and shelf. Model results are validated against moored acoustic Doppler current profiler and other observations. Simulated ITs are mainly generated in regions with a critical/supercritical slope typically between the 200‐ and 500‐m isobaths. Mixing induced by ITs is found to be strongest close to the coast and gradually decreases offshore thereby contributing to the establishment of cross‐shore temperature gradients. The available seasonal coverage of hydrographic data is used to design simulations to investigate the influence of seasonally varying stratification characterized by low stratification in austral winter and high stratification in austral summer. The results show that IT characteristics, such as their wavelengths, sea surface convergence patterns, and baroclinic structure, have substantial seasonal variations and additionally strong spatial inhomogeneities. However, seasonal variations in the spatially averaged generation, onshore flux, and dissipation of IT energy are weak. By evaluating the change of potential energy, it is shown, nevertheless, that mixing due to ITs is more effective during austral winter. We argue that this is because the weaker background stratification in austral winter than in austral summer acts as a preconditioning for IT mixing.

Modelling an energetic tidal strait: investigating implications of common numerical configuration choices

Representation of the marine environment is key for reliable coastal hydrodynamic models. This study investigates the implications of common depth-averaged model configuration choices in sufficiently characterising seabed geometry and roughness. In particular, applications requiring a high level of accuracy and/or exhibiting complex flow conditions may call for greater detail in marine environment representation than typically adopted in coastal models. Ramsey Sound, a macrotidal strait in Pembrokeshire, Wales, UK is considered as a case study. The site contains various steeply inclined bathymetric features, including a submerged pinnacle named Horse Rock and a rocky reef called “The Bitches”. The available energy in Ramsey Sound’s tidal currents has attracted attention from tidal energy developers. There is interest in accurately modelling the energetic hydrodynamics surrounding its pronounced bathymetry. The coastal flow solver Thetis is applied to simulate the flow conditions in Ramsey Sound. It is shown that notable prominent bathymetric features in the strait influence localised and, most importantly, regional hydrodynamic characteristics. “The Bitches” consistently accelerate flow in the strait while Horse Rock induces a notable wake structure and flow reversals. The model is calibrated against bed- and vessel-mounted Acoustic Doppler Current Profiler (ADCP) observations, by altering seabed roughness parameterisations. A spatially variable and locally scaled Manning coefficient based on diverse seabed classification observations is found to improve model performance in comparison to uniformly applied constants, the latter a more common approach. The local impact of altering the Manning coefficient configuration is found to be greatest during spring flood periods of high velocity currents. Meanwhile, the effect of coarsening the computational mesh around bathymetric features towards values more typically applied in coastal models is investigated. Results indicate severe misrepresentation of seabed geometry and subsequent wake hydrodynamics unless refined to a mesh element size that adequately represents Horse Rock and “The Bitches”.

A reciprocal acoustic transmission experiment for precise observations of tidal currents in a shallow sea

Observations of ocean currents are important information in the fields of ocean science and engineering. Reciprocal acoustic transmissions have the potential to operationally measure tidal currents in coastal seas. This study investigates the capability of the reciprocal transmissions to measure tidal currents, particularly in relation to high-frequency variations. We conducted a reciprocal acoustic transmission experiment in an area of complex coastlines in the Seto Inland Sea, Japan, in October 2019. During this experiment, a burst transmission was used, and the system performed six transmissions with 1-min intervals between them every 10 min. The estimated path-averaged currents agreed with the currents obtained from a shipboard acoustic Doppler current profiler (ADCP) on an hourly basis and had captured high-frequency variations that were not captured by the hourly ADCP observations. The measurement noise of the estimated currents, which was determined as the variation within the six transmissions, was 2.5 cm s−1 and was small enough to trace the variations in the currents over the time series even for the variations with periods considerably shorter than M2 tide. The reciprocal transmission’s capability to capture high-frequency variations will improve the capability to reproduce current velocity fields when inversion or data assimilation methods are applied.

Merging velocity measurements and modeling to improve understanding of tidal stream resource in Alderney Race

Tidal circulation and tidal stream resource in Alderney Race (Raz Blanchard) were assessed by using a towed acoustic Doppler current profiler (ADCP) system and tidal modeling. Optimal Interpolation (OI) was applied to process the underway velocity measurements recorded at neap tide flood and ebb flow. The interpolation technique allows reconstructing space-time evolution of the velocity field within the domain during surveying periods. The method employs velocity covariances derived from numerical simulations by a 2D hydrodynamic model MARS. Model covariances are utilized by the OI algorithm to obtain the most likely evolution of the velocity field under the constraints provided by the ADCP observations and their error statistics. The resulting velocity fields were used for assessing the tidal stream resource at site. The largest overall difference between the kinetic power density derived from simulated and interpolated velocity fields was found for ebb tide. Model simulations constrained by velocity measurements demonstrated a significant (up to 30%) decrease of power available in the flow. A significant change in spatial pattern of power density distribution was also identified. It is demonstrated that by merging high resolution velocity measurements at tidal energy site with modeling the tidal stream potential estimation becomes more accurate.

Validation of HF radar sea surface currents in the Malta-Sicily Channel

A network of High-Frequency radar (HFR) stations runs operationally in the Malta-Sicily Channel (MSC), Central Mediterranean Sea, providing sea surface current maps with high temporal (1 h) and spatial (3 × 3 km) resolutions since August 2012. Comparisons with surface drifter data and near-surface Acoustic Doppler Current Profiler (ADCP) observations, as well as radar site-to-site baseline analyses, provide quantitative assessments of HFR velocities accuracy. Twenty-two drifters were deployed within the HFR domain of coverage between December 2012 and October 2013. Additionally, six ADCP vertical current profiles were collected at selected positions during a dedicated field survey. External radio-frequency interferences lead to significant gaps in the HFR coverage, which were minimized by an interpolation technique applied to the surface current fields. The validation of HFR velocities was first performed for each individual station in the network, using data generated with both the ideal and the measured antenna patterns. The validation was then repeated with pairs of HFR stations using the site-to-site baseline error analysis. The analysis confirms that the baseline error estimations and the full validation metrics for this HFR frequency band are consistent, and in some aspects superior, when compared to results from similar studies in other regions. The validation with drifter tracks suggests that the adopted interpolation procedure does not in general impact on the comparison metrics, but may introduce biases when used to extrapolate currents in regions where radar geometrical constraints are suboptimal. The analysis of the percentage of contribution of individual HFR stations shown that the quality of HFR data is influenced by the network geometry.

Evaluation of GlobCurrent surface ocean current products: A case study in Australia

Australia's marginal seas include a wide range of ocean current regimes ranging from tide-dominated in the North-West where the continental shelf is wide, to boundary current- and eddy-dominated in the South-East, where the shelf is narrow. Here, we take the opportunity to test the GlobCurrent surface ocean current products against in-situ observations in these two contrasting regimes. Observations by Acoustic Doppler Current Profiler (ADCP) instruments of Australia's Integrated Marine Observing System (IMOS) and drifting buoys of the Global Drifting Programme (GDP) are used.The monthly-timescale variability of the GlobCurrent alongshore current component is in moderately good agreement with the observations on the continental shelf in the South-East but neither the shorter period variability nor the long-term mean are representative of the ADCP observations. While the observed tidal currents are negligibly small, the wind-driven signals are not. But these are evidently too transient to be adequately sampled by altimetry. The inclusion of an Ekman component does not represent these signals because the interaction of the Ekman transport with the coastal boundary condition is not included. Similarly, the error of the time-mean velocity, which is the dominant error, is because that product is not designed to represent the highly anisotropic nature of the sea level gradients over the continental shelf nor the constraints on the flow field that are imposed by the topography. We thus conclude that the GlobCurrent product needs improvements before it can be described as very suitable for applications on this, and probably other, narrow continental shelf. Off the continental shelf, in contrast, the GlobCurrent products compare quite well with the trajectories of drifting buoys, confirming that the products are quite suitable for blue-water applications.In contrast to the South-East, the tides are very strong in the North-West region of Australia. The sub-tidal variability is weak, in both relative and absolute senses. Consequently, the removal of the tidal signal from the sea level observations needs to be very complete for the residual error to be smaller than the true sub-tidal signal. Transient wind forced signals are also occasionally large so this step of the de-aliasing also needs to be very accurate. Unfortunately, it appears that more work is required before accurate estimates of sub-tidal variability are available from GlobCurrent: the magnitude of the GlobCurrent estimates of sub-tidal current variability far exceed the magnitude of, and are uncorrelated with, the detided ADCP data.

Rock Control of River Geometry: The Fraser Canyons

AbstractA 524‐km reach of Fraser River, British Columbia, was traversed, where the channel alternates between gravel‐bedded reaches that are incised into semiconsolidated glacial deposits and bedrock‐bound reaches, including Fraser Canyon. Continuous centerline acoustic Doppler current profiler survey and observations of bedrock confinement, supplemented with measurements on air photos, reveal the influence of bedrock on channel properties. The surveyed reach was subdivided into 10 morphological reaches, ranging from dominance by alluvial gravel‐bed channels to dominantly rock‐walled canyons. Measured data were grouped within each reach based on the presence of bedrock on both banks, one bank, or neither bank. From alluvial to mixed to canyon‐dominated reaches, river widths decreased while water depths, flow velocities, water surface slopes, and bed shear stresses increased. Within reaches, subreaches with bedrock on both banks were narrower and deeper with higher bed shear stresses but, at the observed stage, had lower depth‐averaged velocity and water surface slope than partly confined and alluvial (unconfined) subreaches. A conceptual model is presented representing a bedrock river as an alternating series of relatively low gradient, deep, rock‐walled pools and steep semialluvial or alluvial sections. Downstream hydraulic geometry relations have scaling exponents expected for alluvial channels provided that the alluvial and rock‐influenced sections are analyzed separately. In Fraser River, canyons become relatively narrower downstream, apparently in response to increasing rock uplift during the Quaternary period. The observations are compared against prior semitheoretical frameworks established to explain variations in width, slope, and channel shape along rockbound channels.

Observations of a Submesoscale Cyclonic Filament in the Marginal Ice Zone

AbstractSubmesoscale flows are energetic motions on scales of several kilometers that may lead to substantial vertical motions. Here we present satellite and ship radar as well as underway conductivity‐temperature‐depth and Acoustic Doppler Current Profiler observations of a cyclonic submesoscale filament in the marginal ice zone of Fram Strait. The filament created a 500‐m thin and 50‐km long sea ice streak and extends to >250‐m depth with a negative/positive density anomaly within/below the halocline. The frontal jets of 0.5 m/s are in turbulent thermal wind balance while the ageostrophic secondary circulation in places appears to subduct Atlantic Water at >50 m/day. Our study reveals the submesoscale dynamics related to sea ice shapes that can be sensed remotely and shows how submesoscale dynamics contribute to shaping the marginal ice zone. It also demonstrates the co‐occurrence and mixing of water masses over short horizontal scales, which has implications for ocean and sea ice models and understanding of patch formation of planktonic organisms.

Comparing ADCP data collected during a seismic survey off the coast of Newfoundland with analysis data from the CONCEPTS operational ocean model

ABSTRACTOceanographic data collected during seismic surveys have potential to provide a significant resource for oceanographic research. In this paper, acoustic Doppler current profiler (ADCP) data collected during the Chevron 2011 North East Grand Banks survey are analysed. The survey took place in a 32,000-km2 area in the North East Grand banks, Orphan Basin region in water depths ranging from 100 to 1500 m and notably, the survey area is crossed by the offshore branch of the Labrador current. Data quality is evaluated by comparing velocities before and after ship turns where we find no significant velocity bias. The ADCP data are compared to data products from the CONCEPTS global ice ocean prediction system operational ocean model. ADCP data and model output agree qualitatively with the model reproducing the long-term fluctuations seen in the data. At higher frequencies between 0.3 and 2.0 cycles/day, the model tends to under-represent the current velocities by about 20%, but the spectral peak associated with inertial oscillations is well resolved. Comparisons of drift track predictions based on the ADCP observations and model output show rms displacement differences of 8 km after 24 h and ∼10 km separations for drift tracks with length under 50 km.

Seasonal and Interannual Variability in Coastal Circulations in the Northern South China Sea

Seasonal cycle and interannual variability in coastal circulations in the northern South China Sea (NSCS) are investigated using satellite altimeter data from March 1993 to September 2016. Altimeter-derived velocity anomalies are in good agreement with acoustic Doppler current profilers (ADCP) observations at an adjacent location. Along-shelf volume transport anomalies in the NSCS indicate northeastward transports from mid-spring to summer and southwestward transports from mid-autumn to winter, which are consistent with previous studies in this region. According to convergence and divergence in the target control volumes, cross-shelf volume transports are estimated as the differences between two neighboring along-shelf volume transport anomalies, with the assumption that long-term mean along-shelf volume transports at each cross-sections are identical. The results show onshore transports in mid-autumn and offshore transports in early summer. The comparison between altimeter-derived and ADCP-estimated cross-shelf volume transports is encouraging, especially when the region has relatively low mesoscale activities and a low freshwater input. Reconstructed cross-shelf volume transports through multiple linear regression reveal that seasonal harmonics is the primary force in driving cross-shelf volume transports in the NSCS, while wind and El Nino have secondary effects on controlling cross-shelf volume transports in different regions. The present study helps to quantify the long-term coastal circulations, especially cross-shelf volume transports, based on altimeter data, which has important implications on the dynamics in coastal regions where observational data is limited.