Bottom-mounted Acoustic Doppler Current Profiler Research Articles

High-Frequency Surface Wave Radar Current Measurement Corrections via Machine Learning and Towed Acoustic Doppler Current Profiler Integration

High-Frequency Surface Wave Radar Current Measurement Corrections via Machine Learning and Towed Acoustic Doppler Current Profiler Integration

Monitoring Tidal Currents and Macroturbulence in a High-Flow Tidal Channel Using a Kilometer-Scale Acoustic Travel-Time Instrument

A challenge in exploiting tidal energy resources is to determine the most suitable sites for development, and as the industry grows, to monitor the progressive impacts of tidal farms on the flow field and sediment transport regimes. A feasibility experiment was carried out with the fluvial acoustic tomography (FAT) system to assess the combined effects of irregular bathymetry, horizontal and vertical sound-speed gradients, and macroturbulence on the quality of FAT-estimated velocities. The experiment involved reciprocal transmissions at 30-s intervals for four days with two 7-kHz broadband transceivers. To determine the origin and potential interpretive value of the frequent multiple arrivals in the unstratified, highly-turbulent flow conditions, we performed a series of simulations with the Bellhop3D model. We demonstrate that the multiple arrivals mainly arise from out-of-plane sound propagation paths resulting from the effects of the local bathymetry. The maximum mean difference between FAT-estimated currents and the coincident moving-vessel acoustic Doppler current profiler (ADCP) transects is found to be less than 10%. Comparisons are also made between velocity spectra during the full flood and the ebb determined using the FAT velocities and the vertically-averaged velocities from a bottom-mounted ADCP deployed in a later experiment. The spectra exhibit similar shapes in the frequency and wave number overlap regions, indicating that FAT is able to detect macroturbulence. Good agreement is also obtained between the sound-speed estimates based on the travel time and those from <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">in situ</i> temperature and salinity data. Overall, the results demonstrate the potential of shallow acoustic tomography for monitoring environmental conditions in high-flow tidal channels.

Estimating Water Transport from Short-Term Vessel-Based and Long-Term Bottom-Mounted Acoustic Doppler Current Profiler Measurements in an Arctic Lagoon Connected to the Beaufort Sea.

Acoustic Doppler current profilers (ADCP) are quasi-remote sensing instruments widely used in oceanography to measure velocity profiles continuously. One of the applications is the quantification of land–ocean exchange, which plays a key role in the global cycling of water, heat, and materials. This exchange mostly occurs through estuaries, lagoons, and bays. Studies on the subject thus require that observations of total volume or mass transport can be achieved. Alternatively, numerical modeling is needed for the computation of transport, which, however, also requires that the model is validated properly. Since flows across an estuary, lagoon, or bay are usually non-uniform and point measurements will not be sufficient, continuous measurements across a transect are desired but cannot be performed in the long run due to budget constraints. In this paper, we use a combination of short-term transect-based measurements from a vessel-mounted ADCP and relatively long-term point measurements from a moored ADCP at the bottom to obtain regression coefficients between the transport from the vessel-based observations and the depth-averaged velocity from the bottom-based observations. The method is applied to an Arctic lagoon by using an ADCP mounted on a buoyant platform towed by a small inflatable vessel and another ADCP mounted on a bottom deployed metal frame. The vessel-based measurements were performed continuously for nearly 5 h, which was sufficient to derive a linear regression between the datasets with an R2-value of 0.89. The regression coefficients were in turn applied to the entire time for the moored instrument measurements, which are used in the interpretation of the subtidal transport variations.

In situ observations of turbulent ship wakes and their spatiotemporal extent

Abstract. In areas of intensive ship traffic, ships pass every 10 min. Considering the amount of ship traffic and the predicted increase in global maritime trade, there is a need to consider all types of impacts shipping has on the marine environment. While the awareness about, and efforts to reduce, chemical pollution from ships is increasing, less is known about physical disturbances, and ship-induced turbulence has so far been completely neglected. To address the potential importance of ship-induced turbulence on, e.g., gas exchange, dispersion of pollutants, and biogeochemical processes, a characterisation of the temporal and spatial scales of the turbulent wake is needed. Currently, field measurements of turbulent wakes of real-size ships are lacking. This study addresses that gap by using two different methodological approaches: in situ and ex situ observations. For the in situ observations, a bottom-mounted acoustic Doppler current profiler (ADCP) was placed at 32 m depth below the shipping lane outside Gothenburg harbour. Both the acoustic backscatter from the air bubbles in the wake and the dissipation rate of turbulent kinetic energy were used to quantify the turbulent wake depth, intensity, and temporal longevity for 38 ship passages of differently sized ships. The results from the ADCP measurements show median wake depths of 13 m and several occasions of wakes reaching depths &gt; 18 m, which is in the same depth range as the seasonal thermocline in the Baltic Sea. The temporal longevity of the observable part of the wakes had a median of around 10 min and several passages of &gt; 20 min. In the ex situ approach, sea surface temperature was used as a proxy for the water mass affected by the turbulent wake (thermal wake), as lowered temperature in the ship wake indicates vertical mixing in a thermally stratified water column. Satellite images of the thermal infrared sensor (TIRS) onboard Landsat-8 were used to measure thermal wake width and length, in the highly frequented and thus major shipping lane north of Bornholm, Baltic Sea. Automatic information system (AIS) records from both the investigated areas were used to identify the ships inducing the wakes. The satellite analysis showed a median thermal wake length of 13.7 km (n=144), and the longest wake extended over 60 km, which would correspond to a temporal longevity of 1 h 42 min (for a ship speed of 20 kn). The median thermal wake width was 157.5 m. The measurements of the spatial and temporal scales are in line with previous studies, but the maximum turbulent wake depth (30.5 m) is deeper than previously reported. The results from this study, combined with the knowledge of regional high traffic densities, show that ship-induced turbulence occurs at temporal and spatial scales large enough to imply that this process should be considered when estimating environmental impacts from shipping in areas with intense ship traffic.

Subtidal to Supertidal Variability of Reynolds Stresses in a Midlatitude Stratified Inner Shelf

AbstractWe describe the spatiotemporal variability and vertical structure of turbulent Reynolds stresses (RSs) in a stratified inner shelf with an energetic internal wave climate. The RSs are estimated from direct measurements of velocity variance derived from bottom-mounted acoustic Doppler current profilers. We link the RSs to different physical processes, namely, internal bores, midwater shear instabilities within vertical shear events related to wind-driven subtidal along-shelf currents, and nonturbulent stresses related to incoming nonlinear internal wave (NLIW) trains. The typical RS magnitudes are O(0.01) Pa for background conditions, with diurnal pulses of O(0.1–1) Pa, and O(1) Pa for the NLIW stresses. A NLIW train is observed to produce a depth-averaged vertical stress divergence sufficient to accelerate water 20 cm s−1 in 1 h, suggesting NLIWs may also be important contributors to the depth-averaged momentum budget. The subtidal stresses show significant periodic variability and are O(0.1) Pa. Conditionally averaged velocity and RS profiles for northward/southward flow provide evidence for downgradient turbulent momentum fluxes, but also indicate departures from this expected regime. Estimates of the terms in the depth-averaged momentum equation suggest that the vertical divergence of the RSs are important terms in both the cross-shelf and along-shelf directions, with geostrophy also present at leading-order in the cross-shelf momentum balance. Among other conclusions, the results highlight that internal bores and shoaling NLIWs may also be important dynamical players in other inner shelves with energetic internal waves.

Turbulence and flow field alterations inside a fish sea cage and its wake

Measurements were performed inside and in the wake of a commercial salmon sea cage. The key instrumentation included the following: sea bottom-mounted acoustic Doppler current profilers providing continuous concurrent flow velocity and turbulence information about the water columns; vessel-mounted acoustic current profilers mapping the flow pattern around the wake of the cage in a selected incoming flow; a microstructure profiler measuring the fluctuations in vertical shear in the dissipation range; an acoustic Doppler velocimeter measuring the velocity inside the sea cage; dissolved oxygen sensors and echosounders measuring the distribution of fish inside the cage. The measurements have performed with stocked and emptied sea-cage. The results showed simultaneous strong flow reductions in the wake near the cage and high turbulence in the upper part of the water column, both of which were generated by the sea cage. Measurements inside the cage showed that although the schooling fish reduced the flow, there was no evidence that they generate secondary radial and vertical flows.

Numerical investigation on the impact of wind-induced hydraulics on dissolved oxygen characteristics in a shallow stormwater pond

Abstract Stormwater ponds (SWPs) are widely utilized for flood and water quality control. Low-flow rates are common in SWPs, sometimes causing wind-driven currents to become the dominant hydrodynamic force during ice-free periods. Hence, it is essential to understand the influence of the wind-induced flow on stratification and dissolved oxygen (DO) concentrations in shallow SWPs to predict the performance and water quality of these systems. The objective of this study is to evaluate the influence of wind-driven circulation on the spatial distribution of DO in an SWP using a numerical model. A bottom-mounted acoustic Doppler current profiler (ADCP) was utilized to measure small wind-induced currents and to validate a hydrodynamic model, which suggested that a wind-dominated circulation was generated even with the moderate wind speed. Countercurrents opposite in the direction to surface wind-generated flow were also present. The DO model demonstrated that complete mixing can be produced by higher wind speed, leading to fully oxic conditions throughout the water column (7.00 mg/L DO or higher), wherein low DO water at depth was carried to the surface by upwelling circulation and was possibly replenished during the surface transportation. This sheds some light on the impact of wind-induced mixing on the water quality in shallow SWPs.

Tidal current observations through Admiralty Inlet from ferry-mounted current profilers

Admiralty Inlet is a narrow sill located at the northern end of Puget Sound (WA, USA). Circulation through Admiralty Inlet is complex, with tidal currents exceeding 3 $$\hbox {ms}^{-1}$$ , large variations in fresh water input to the system, and seasonal ocean water intrusions. Long-term observations of the currents across the entire inlet are crucial for understanding circulation through Puget Sound. In this context, the Washington State Department of Transportation (WSDOT) Ferries, which run year round through Admiralty Inlet, provide a cost-effective platform to mount instruments and obtain long time series of currents distributed across the inlet. Through the Ferry-Base Monitoring of Puget Sound Currents project, two down-looking acoustic Doppler current profilers (ADCPs) are installed on board two WSDOT ferries, providing depth profiles of velocities across the inlet since May 2014. All data are quality controlled and organized in an horizontal and vertical grid across the inlet. Data within each grid cell are analyzed to capture tidal current harmonic components. Results agree well with data from fixed bottom-mounted ADCPs, and show large spatial variability in the amplitude of harmonic components, probably related to the bathymetric features of the inlet. Further analysis provides estimates of tidal asymmetry and residual currents through the inlet, which are relevant to water quality within the Puget Sound.

Applications of ocean currents data from high-frequency radars and current profilers to search and rescue missions around Taiwan

ABSTRACTTo enhance the immediacy and accuracy of search and rescue missions conducted by Taiwan Coast Guard Administration (TCGA), this study examines the characteristics of ocean currents and estimates the drifting trajectory of objects for two cases of marine incidents. Two methods are used to predict object’s drift tracks: the first one is simply calculating the advection driven by currents and the second method adopts a Monte Carlo based software, SARMAP. Ocean currents data from a high-frequency radar system comprised of 18 radar units surrounding Taiwan supplemented by bottom-mounted Acoustic Doppler Current Profilers (ADCP) data and wind data are used as the primary oceanic environment data. The estimated ending points of drifting for both cases of missing persons in water by using both methods are consistent with the actual recovery locations. Our analyses demonstrate that ocean currents data, if properly used, can be very useful for rapid response in marine search and rescue.

Airborne Remote Sensing of the Upper Ocean Turbulence during CASPER-East

This study takes on the challenge of resolving upper ocean surface currents with a suite of airborne remote sensing methodologies, simultaneously imaging the ocean surface in visible, infrared, and microwave bands. A series of flights were conducted over an air-sea interaction supersite established 63 km offshore by a large multi-platform CASPER-East experiment. The supersite was equipped with a range of in situ instruments resolving air-sea interface and underwater properties, of which a bottom-mounted acoustic Doppler current profiler was used extensively in this paper for the purposes of airborne current retrieval validation and interpretation. A series of water-tracing dye releases took place in coordination with aircraft overpasses, enabling dye plume velocimetry over 100 m to 10 km spatial scales. Similar scales were resolved by a Multichannel Synthetic Aperture Radar, which resolved a swath of instantaneous surface velocities (wave and current) with 10 m resolution and 5 cm/s accuracy. Details of the skin temperature variability imprinted by the upper ocean turbulence were revealed in 1–14,000 m range of spatial scales by a mid-wave infrared camera. Combined, these methodologies provide a unique insight into the complex spatial structure of the upper ocean turbulence on a previously under-resolved range of spatial scales from meters to kilometers. However, much attention in this paper is dedicated to quantifying and understanding uncertainties and ambiguities associated with these remote sensing methodologies, especially regarding the smallest resolvable turbulent scales and reference depths of retrieved currents.

Observed characteristics of tidal currents and mean flow in the northern Yellow Sea

Several bottom-mounted Acoustic Doppler Current Profiler (ADCP) moorings were deployed in the northern Yellow Sea (NYS) during the four seasons of 2006–2007 and also the summertime of 2009. A synthesis analysis on the time-continuous records was performed to examine the characteristics and variations of tidal currents and mean flow over the observation period at these stations. Tidal currents accounted for ~75% of the total kinetic energy, with the absolute dominance of M2 constituent. Visible vertical variations of tidal flow were found on all sites, featured by the decrease of amplitude, increase of rotation rate as well as a decreasing trend of the phase for M2 component with depth. A notable exception was in the central NYS, where the maximum tidal currents occurred in the upper or middle layers (~20–40 m) instead of near the surface (<10 m). The observed mean flow was relatively weak, smaller than 15 cm/s. Velocity on the northern end of Yellow Sea Trough (YST) was characterized by low magnitude and an obvious layered structure vertically. In the Bohai Strait (BS) and the northern slope area, the currents weakened and the flow direction presented a major trend to deflect counterclockwise with depth in most observations. Summertime cyclonic circulation around the Yellow Sea Cold Water Mass (YSCWM), its intensification on the frontal zone and the Yellow Sea Warm Current (YSWC) for the winter season were all evident by our direct current measurements. However, the details of water exchange through the BS appeared partly different from the traditionally-accepted pattern. The vertical differences of tidal and mean flow were larger in summer than that in winter, implying the influence of thermal structure to the local currents. Affected by the water stratification, mean flow usually reached its maximum near the thermocline in spring and summer, while showing a nearly uniform vertical distribution during winter.

A NEW STRATEGIC WAVE MEASUREMENT STATION OFF NAPLES PORT MAIN BREAKWATER

The accuracy of directional wave spectra sensors is crucial for obtaining accurate forecasts of ocean and coastal wave conditions for scientific and engineering applications. In this paper, a newly designed, low-cost GPS-based wave buoy, called the Directional Wave Spectra Drifter (DWSD), is presented. A field test campaign was conducted at the Gulf of Naples, Italy with the goal of comparing the directional wave properties obtained with the DWSD and with a nearly co-located bottom-mounted Acoustic Doppler Current Profiler (ADCP) from Teledyne RD-Instruments. The comparison shows a very good agreement between the two methodologies. The reliability of this innovative instrument and its low costs allow a large variety of applications, including the implementation of a global, satellite-linked, real-time open-ocean network of drifting directional wave spectra sensors and monitoring the sea-state in harbors to aid ship transit and for planning coastal and offshore constructions. The DWSD is currently in use to better constrain the wave energy climatology with the goal of optimizing the design of a full-scale prototype Wave Energy Converter (WEC) in the port of Naples, Italy.

Observations of frazil ice formation and upward sediment transport in the Sea of Okhotsk: A possible mechanism of iron supply to sea ice

In the Sea of Okhotsk, sediment incorporation, transport and release by sea ice potentially plays important roles in the bio-related material (such as iron) cycle and ecosystem. The backscatter strength data of bottom-mounted Acoustic Doppler Current Profilers have suggested signals of frazil ice down to 30 m depth, and signals of upward sediment transport throughout the water column simultaneously in the region northeast of Sakhalin, with a water depth of ∼100 m. Such events occurred under turbulent conditions with strong winds of 10–20 m s−1. During such events, newly formed ice was present near the observational sites, shown by satellite microwave imagery. Sediment dispersion from the bottom occurred in association with strong currents of 1.0–1.5 m s−1. During these events, the mixed layer reaches near the bottom due to wind-induced stirring, inferred from the high frequency component of vertical velocity. Thus the winter time turbulent mixing brings re-suspended sediment up to near the ocean surface. This study provides the first observational evidence of a series of processes on the incorporation of sedimentary materials into sea ice: sedimentary particles are dispersed by the strong bottom current, subsequently brought up to near the surface by winter time mixing, and finally incorporated into sea ice through underwater interaction with frazil ice and/or flooding of sea ice floes. This wintertime incorporation of bottom sediment into sea ice is a possible mechanism of iron supply to sea ice which melts in spring, and releases bio-reactive iron into the ocean.

Observations of pockmark flow structure in Belfast Bay, Maine, Part 1: current-induced mixing

Field observations of current profiles and temperature, salinity, and density structure were used to examine vertical mixing within two pockmarks in Belfast Bay, Maine. The first is located in 21 m water depth (sea level to rim), nearly circular in shape with a 45 m rim diameter and 12 m rim-to-bottom relief. The second is located in 25 m water depth, more elongated in shape with an approximately 80 m (36 m) major (minor) axis length at the rim, and 17 m relief. Hourly averaged current profiles were acquired from bottom-mounted acoustic Doppler current profilers deployed on the rim and center of each pockmark over successive 42 h periods in July 2011. Conductivity–temperature–depth casts at the rim and center of each pockmark show warmer, fresher water in the upper water column, evidence of both active and fossil thermocline structure 5–8 m above the rim, and well-mixed water below the rim to the bottom. Vertical velocities show up- and down-welling events that extend into the depths of each pockmark. An observed temperature change at both the rim and center occurs coincident with an overturning event below the rim, and suggests active mixing of the water column into the depths of each pockmark. Vertical profiles of horizontal velocities show depth variation at both the center and rim consistent with turbulent logarithmic current boundary layers, and suggest that form drag may possibly be influencing the local flow regime. While resource limitations prevented observation of the current structure and water properties at a control site, the acquired data suggest that active mixing and overturning within the sampled pockmarks occur under typical benign conditions, and that current flows are influenced by upstream bathymetric irregularities induced by distant pockmarks.

Moving ship tomography experiments in a coastal sea

This study considers measuring differential travel times (DTT) between moored and ship-towed transceivers in shallow waters as a feasibility study to extend the concept of the moving ship tomography (Cornuelle et al., 1989) to current field reconstructions. The delay-Doppler ambiguity function (AF) method is used to estimate and compensate the waveform distortions induced by the relative motion between transceivers. The Doppler shift (the relative speed between transceivers) is estimated by the peak of the AF, and the delay time series associated with the peak is the Doppler compensated arrival pattern. The DTTs are determined using the compensated arrival patterns in reciprocal directions. For the current estimation the estimated relative speed is used for removing the effect of the transceiver motion from the DTTs. A reciprocal sound transmission experiment was conducted using a ship-towed transceiver and two moored transceivers in the Sizihwan coastal near Kaohsiung, Taiwan. Due to the complicated arrival patterns observed, the DTTs were determined by tracking the peaks of the cross-correlation functions between the compensated reciprocal arrival patterns over the transmissions. The path-averaged currents derived from DTT agreed well with the direct measurement using a bottom-mounted acoustic Doppler current profiler.

Temporal variations in the current structure and volume transport of the Coastal Oyashio revealed by direct current measurement

Direct current measurements by a shipboard and bottom-mounted acoustic Doppler current profiler and concurrent hydrographic observations with a CTD were conducted off southeastern Hokkaido, Japan, between January and May 2005 to reveal temporal variations in the current structure and volume transport of the Coastal Oyashio (CO). The CO, which has a baroclinic jet structure with southwestward speeds exceeding 90 cm s−1 and a width of 7–8 km, was associated with a surface-to-bottom density front and was formed on the offshore side of the shelf break. The volume transport of CO (TCO) was estimated by integrating the fluxes of lower-density water that was trapped against the coast along the density front represented by the 26.2 σθ isopycnal line. This transport decreased monotonously from 0.79 Sv (1 Sv = 106 m3 s−1) in January to 0.21 Sv in March and subsequently to 0.12 Sv in May, possibly due to the decay of the East Sakhalin Current Water in the Okhotsk Sea. Accompanied by a decrease in TCO, the location of the jet structure associated with the density front moved toward the coast while the maximum speed of the jet decreased and the tilt of the front became more horizontal. Consequently, more saline offshore Oyashio water flowed into the deep part of the shelf area, and the current structure altered from relatively barotropic in winter to baroclinic in spring. This study is the first to estimate the observed volume transport of the CO from direct current measurements.

Boundary Shear Stress in an Ice-Covered River during Breakup

River ice complicates river hydraulics and morphodynamics by adding a new boundary layer to the top of the flow. This boundary layer affects the velocity distribution throughout the depth due to increased flow resistance, and varies the local boundary shear stress on the bed (lower boundary) by adding new shear stress on the upper boundary (under surface of the ice). Variation of shear stress plays an important role in incipient motion of upper and lower boundary materials: sediment motion and transport are directly affected by local boundary shear stress, as are ice cover thickness, condition, and progression. This paper provides estimates of upper and lower boundary shear stress during stable ice cover and the important stage of ice cover breakup using available methods based on continuous field measurements of velocity profiles obtained with a bottom-mounted acoustic Doppler current profiler in the Nelson River, Canada. Boundary shear stresses varied dynamically with transformation of the ice cover, including the presence and removal of slush ice and formation of the ice jams. Estimated upper boundary shear stress varied between 0.52 and 23.87 N/m2, whereas lower boundary shear stress varied between 1.51 and 2.40 N/m2, with the highest values observed during breakup. These values are compared with estimates from previous laboratory experiments.

Monitoring tidal currents with a towed ADCP system

The tidal circulation in the semi-enclosed Boulogne harbour (eastern English Channel) is measured during the various stages of the tidal cycle with a low-cost towed Acoustic Doppler Current Profiler (ADCP) system for the first time. The system is equipped with an interpolation algorithm which allows reconstructing space-time evolution of the velocity field for surveys whose duration is comparable or larger than the typical time of tidal variation (1–2 h). The method employs space-time velocity covariances derived from a numerical simulation of the surveyed area by a high-resolution relocatable model “Model for Applications on Regional Scale” (MARS). The covariances are utilized by the optimal interpolation algorithm to obtain the most likely evolution of the velocity field under the constraints provided by the ADCP observations and their error statistics. Technically, the MARS model run provides the first guess (background) evolution of the velocity field in the surveyed area which is then corrected by the data in a statistically consistent manner as it explicitly takes into the account both observational and modeling errors. The quality of the velocity reconstruction was validated against independent bottom-mounted ADCP data, the background model evolution, and against the results of spatial interpolation by Kriging technique. All tests demonstrated significant (30 to 60 %) reduction of the model-data misfit for the velocity field obtained as a result of space-time optimal interpolation. Although the method was applied to recover surface circulation, it can be extended for assessment of the full 4D tidal flow dynamics using the data recorded throughout the entire water column.

Biomixing due to diel vertical migrations of zooplankton: Comparison of computational fluid dynamics model with observations

Recent studies (Dewar et al., 2006; Wilhelmus and Dabiri, 2014) suggest that diel vertical migrations (DVM) of zooplankton (or other migrating organisms) may have an impact on ocean mixing, though details are not completely clear. Zooplankton that undergo DVM can have an impact on oil transport through the water column, and oil and dispersants can have a negative or even lethal effect on the organisms. Kunze et al. (2006) reported an increase of dissipation rate of turbulent kinetic energy, ε, by four to five orders of magnitude during DVM of zooplankton over background turbulence in Saanich Inlet, British Columbia, Canada. However, the effect was not observed in the same area by Rousseau et al. (2010) and was later reassessed by Kunze (2011). In our work, an 11-month data set obtained in the Straits of Florida with a bottom-mounted acoustic Doppler current profiler revealed strong sound scattering layers undergoing DVM. We used a 3-D non-hydrostatic computational fluid dynamics model with Lagrangian particle injections (a proxy for migrating organisms) via a discrete phase model to simulate the effect of turbulence generation by DVM. We tested a range of organism concentrations from 1000 to 10,000 organisms/m3 based on measurements by Greenlaw (1979) and Mackie and Mills (1983) in Saanich Inlet. At a concentration close to the upper limit, the simulation showed an increase in ε by two to three orders of magnitude during DVM over background turbulence, 10−9 W kg−1. At a concentration of 1000 organisms/m3, almost no turbulence above the background level was produced in the model. These results suggest that the Kunze et al. (2006) observations could have been performed at a larger concentration of migrating zooplankton than those reported by Rousseau et al. (2010). No exact zooplankton concentrations data were provided in either work. The difference between observations and the model can, in part, be explained by the fact that Kunze et al. (2006) measured instantaneous profiles of ε, while the model results on ε were averaged horizontally over the 50 m by 50 m domain. In the Straits of Florida, we observed a small decrease in northward current velocity profiles during migration times after averaging over 11 months of observations. The computational fluid dynamics model reproduced this decrease of current velocity due to turbulence generated by DVM in the Straits of Florida model case. The deviations in the velocity profiles can be explained by the increase in turbulent mixing during vertical migration periods. Comparison of observational data to the model results was complicated by physical factors such as tides, Florida Current meandering, etc., which may have a stronger effect on current velocity profiles than DVM.

Estimation of wind drift current in the Soya Strait

Three high-frequency (HF) ocean radars were deployed around the Soya (La Perouse) Strait in 2003 to monitor the Soya Warm Current (SWC). Surface current observed by the HF radars contains a wind drift component, which must be removed in order to estimate the interior SWC. The wind drift parameters, speed factor and turning angle were derived from the surface current measured by the HF radars, the vertical current profile measured by a bottom-mounted acoustic Doppler current profiler (ADCP), and wind data from the numerical weather analysis system operated by the Japan Meteorological Agency (JMA) from October 1, 2006 to July 24, 2008. The ensemble-mean turning angle and speed factor from the entire dataset (excluding August 2007) were estimated to be 28° and 0.66 %, respectively. No significant seasonal variations were discernible in the wind drift parameters. After removal of the wind drift current estimated from the wind with the ensemble-mean drift parameters, the correlation coefficient between the along-shore current speed and sea level difference between the Sea of Japan and Sea of Okhotsk improved from 0.791 to 0.825. It was revealed that the magnitude of wind drift current reaches 45 % of that of the interior current in winter and approximately 20 % in summer, indicating the importance of wind drift current estimation in this region.