Upward-looking Acoustic Doppler Current Profiler Research Articles

Acoustic Sensing of Ocean Mixed Layer Depth and Temperature from Uplooking ADCPs

Abstract Properties of the surface mixed layer (ML) are critical for understanding and predicting atmosphere–sea ice–ocean interactions in the changing Arctic Ocean. Mooring measurements are typically unable to resolve the ML in the Arctic due to the need for instruments to remain below the surface to avoid contact with sea ice and icebergs. Here, we use measurements from a series of three moorings installed for one year in the Beaufort Sea to demonstrate that upward-looking acoustic Doppler current profilers (ADCPs) installed on subsurface floats can be used to estimate ML properties. A method is developed for combining measured peaks in acoustic backscatter and inertial shear from the ADCPs to estimate the ML depth. Additionally, we use an inverse sound speed model to infer the summer ML temperature based on offsets in ADCP altimeter distance during open-water periods. The ADCP estimates of ML depth and ML temperature compare favorably with measurements made from mooring temperature sensors, satellite SST, and from an autonomous Seaglider. These methods could be applied to other extant mooring records to recover additional information about ML property changes and variability.

Acoustic backscatter from bubbles as a (quasi)passive tracer of turbulent mixing in high-flow tidal channels

In high-flow tidal channels, the water column tends to be well-mixed vertically due to the high levels of turbulence. Under favorable circumstances, such as those in which wind waves incident at the channel entrance from the adjoining open ocean or large embayment are opposed by the tidal flow, the waves steepen and break as they propagate upstream, resulting in bubble injection at the sea surface. These bubbles are then mixed downward by the tidally generated turbulence, resulting in pronounced, surface-connected, downward-propagating plumes of high backscatter in records from bottom-mounted upward-looking acoustic Doppler current profilers, typically operating at 100s of kHz. Letting the backscatter amplitude represent a pseudo-concentration, C, we demonstrate that the vertical turbulent diffusivity, K, can be estimated from the vertical turbulent flux i < c′w′> along the axis of the vertical beam, and the mean vertical gradient, dC/dz. Intriguingly, and despite the fact that bubbles are positively buoyant, the resulting estimates of K are very close to the values of momentum diffusivity required to obtain good agreement between the observed tidal velocities and a data-validated numerical tidal circulation model for the particular channel in question

An Adaptive Approach to Derive Sea Ice Draft from Upward-Looking Acoustic Doppler Current Profilers (ADCPs), Validated by Upward-Looking Sonar (ULS) Data

Moored upward-looking Acoustic Doppler Current Profilers (ADCPs) can be used to observe sea ice draft. While previous studies relied on the availability of auxiliary pressure sensors to measure the instrument depth of the ADCP, we present an adaptive approach that infers instrument depth from ADCP bottom track (BT) mode measurements of error velocity and range. The ADCP-derived ice draft time series are validated with data from adjacent Upward-Looking Sonar (ULS) moorings. We demonstrate that this method can be used to obtain daily mean sea ice draft time series that, on average, are within 20% of ULS-derived draft time series. ULS and ADCP ice draft time series were observed by four moorings in the Laptev Sea and show correlations between 0.7 and 0.9. This new approach is not a substitute for high-frequency, high-precision ULS measurements of ice draft but it provides a low-cost opportunity to derive daily mean ice draft time series accessing existing ADCP data that have not been not used for that purpose to date. This method has the potential to close data gaps and extend existing ice draft time series in all ice-covered regions and supports the validation of sea ice thickness products from satellite missions such as CryoSat-2, SMOS or ENVISAT.

A Decade of Eastern Tropical Indian Ocean Observation Network (TIOON)

AbstractAs an important part of the Indo-Pacific warm pool, the Indian Ocean has great significance for research on the Asian monsoon system and global climate change. From the 1960s onward, several international and regional programs have led to important new insights into the Indian Ocean. The eastern Tropical Indian Ocean Observation Network (TIOON) was established in 2010. The TIOON consists of two parts: large-scope observations and moored measurements. Large-scope observations are performed by the eastern Tropical Indian Ocean Comprehensive Experiment Cruise (TIO-CEC). Moored measurements are executed by the TIOON mooring array and the hydrological meteorological buoy. By 2019, 10 successful TIO-CEC voyages had been accomplished, making this mission the most comprehensive scientific investigation in China. The TIO-CEC voyages have collected temperature/salinity profiles, GPS radiosonde profiles, and other observations in the Indian Ocean. To supplement the existing buoy array in the Indian Ocean, an enhanced TIOON mooring array consisting of eight subthermocline acoustic Doppler current profiler (ADCP) moorings, was established since 2013. The TIOON mooring equipped with both upward-looking and downward-looking WHLS75K ADCP provide valuable current monitoring information to depth of 1,000 m in the Indian Ocean. To improve air–sea interaction monitoring, two real-time hydrological–meteorological buoys were deployed in 2019 and 2020 in the equatorial Indian Ocean. A better understanding of the Indian Ocean requires continuous and long-term observations. The TIOON program and other aspiring field investigation programs will be promoted in the future.

Satellite-based sea ice thickness changes in the Laptev Sea from 2002 to 2017: comparison to mooring observations

Abstract. The gridded sea ice thickness (SIT) climate data record (CDR) produced by the European Space Agency (ESA) Sea Ice Climate Change Initiative Phase 2 (CCI-2) is the longest available, Arctic-wide SIT record covering the period from 2002 to 2017. SIT data are based on radar altimetry measurements of sea ice freeboard from the Environmental Satellite (ENVISAT) and CryoSat-2 (CS2). The CCI-2 SIT has previously been validated with in situ observations from drilling, airborne remote sensing, electromagnetic (EM) measurements and upward-looking sonars (ULSs) from multiple ice-covered regions of the Arctic. Here we present the Laptev Sea CCI-2 SIT record from 2002 to 2017 and use newly acquired ULS and upward-looking acoustic Doppler current profiler (ADCP) sea ice draft (VAL) data for validation of the gridded CCI-2 and additional satellite SIT products. The ULS and ADCP time series provide the first long-term satellite SIT validation data set from this important source region of sea ice in the Transpolar Drift. The comparison of VAL sea ice draft data with gridded monthly mean and orbit trajectory CCI-2 data, as well as merged CryoSat-2–SMOS (CS2SMOS) sea ice draft, shows that the agreement between the satellite and VAL draft data strongly depends on the thickness of the sampled ice. Rather than providing mean sea ice draft, the considered satellite products provide modal sea ice draft in the Laptev Sea. Ice drafts thinner than 0.7 m are overestimated, while drafts thicker than approximately 1.3 m are increasingly underestimated by all satellite products investigated for this study. The tendency of the satellite SIT products to better agree with modal sea ice draft and underestimate thicker ice needs to be considered for all past and future investigations into SIT changes in this important region. The performance of the CCI-2 SIT CDR is considered stable over time; however, observed trends in gridded CCI-2 SIT are strongly influenced by the uncertainties of ENVISAT and CS2 and the comparably short investigation period.

Near-inertial internal waves and multiple-inertial oscillations trapped by negative vorticity anomaly in the central Sea of Japan

This study aims to reveal the interaction between wind-induced internal waves and mesoscale features in a frontal region of the central Sea of Japan. A mooring system with an upward-looking acoustic Doppler current profiler was deployed over a meandering pathway of the Tsushima Warm Current. The annual record of horizontal current showed an extraordinary event of near-inertial internal waves (NIWs) that were trapped in a mesoscale warm-core eddy, characterized by negative vorticity anomaly. Near the critical depth inside the eddy, where background vorticity vanishes, the amplitude of downward-traveling NIW reached 50–70 cm s−1 in root-mean-square current magnitude. The amplified NIWs were concurrent with the emergence of superinertial oscillations, peaking at multiple inertial (MI) frequencies, i.e., double, triple, and quadruple inertial. The MI oscillations are regarded as arising through nonlinear resonance of NIW. Examining local surface winds suggest that a fast-moving cyclone passing over the site initially generated the NIW event. The trapping and amplification processes of wind-generated NIW are examined by a series of ray-tracing simulations, where background vorticity is reconstructed from in situ hydrographic observations. Based on the results above, a specific picture model of kinetic energy circulation around the Sea of Japan can be presented: the atmospheric event provides momentum energy into the ocean surface, resulting in the generation of NIWs and trapping within anticyclonic vortices. A part of the near-inertial kinetic energy is then transferred into the MI oscillations if NIW is vigorously amplified near the critical depth; then it likely dissipates into turbulence.

A comprehensive assessment of turbulence at a tidal-stream energy site influenced by wind-generated ocean waves

Velocity measurements collected by an upward-looking acoustic Doppler current profiler were used to provide the first study of ambient turbulence in Alderney Race. Turbulence metrics were estimated at mid-depth during peak flooding and ebbing tidal conditions. The dissipation rate ε and the integral lengthscale (L) were estimated using two independent methods: the spectral method and the structure function method. The spectral method provided ε and (L) estimates with standard deviations twice lower than that obtained from the structure function method. Removal of wave and Doppler noise-induced bias when estimating the dissipation rate was shown to be a crucial step in turbulence characterization. It allowed for a significant refining in (L) estimates derived from the spectral and structure function methods of 35% and 20% respectively. The integral lengthscale was found to be 2–3 times the local water depth. It is considered that these findings could be valuable for current turbine designers, helping them optimizing their designs as well as improving loading prediction through the lifetime of the machines.

Observations, modeling, and inter-comparison of waves from deep to intermediate waters in the East Mediterranean basin

Wind waves are of great importance in various fields of oceanography and engineering. Although they propagate along entire seas and oceans, due to the complexity and high costs in setting deep water measurements, they are mostly measured only in nearshore regions. The connection between deep sea wave conditions and nearshore ones is commonly done using wave propagation models. Nevertheless, due to the scarcity of deep water measurements, the validity of this process is seldomly tested. This work presents newly acquired data for investigating wave evolution from the deep sea far beyond the continental shelf to intermediate coastal waters. The deep water wave measurements were obtained using an upward-looking Acoustic Doppler Current Profiler (ADCP) mounted on a submerged buoy (35 m depth) in the East Mediterranean 50 km offshore Israel (bottom depth of 1500 m). To the best of our knowledge, it is the first time that such a deep sea measurement is carried out in the region. Another ADCP was deployed on the sea bed (26 m depth) 2 km offshore. During this campaign, a substantial storm (Hm0=6 m) was recorded simultaneously by both ADCPs. A wave evolution model accounting for triad and quartet interactions was employed for connecting the two measurements, in order to explain the differences between the two measurements with numerical estimations. A new methodology was developed for resolving spatial variability in the deep sea conditions by assimilating the deep water data into the output of an operational wave model for the Mediterranean Sea. This allowed forming appropriate boundary conditions for propagation of the wave field from deep to coastal waters. Model results show an importance of accounting for inhomogeneity of the wave field in deep water. Nevertheless, the measurements indicate that one cannot expect to fully resolve the exact wave spectra only by applying wave modeling. This emphasizes the importance of conducting such deep wave measurements.

Tidal Effects on the Bottom Thermal Front of North Yellow Sea Cold Water Mass near Zhangzi Island in Summer 2009

Three seabed-mounted TD/CTD chains and two upward-looking acoustic Doppler current profilers (ADCPs) in the southwest of Zhangzi Island are used and a simultaneous cruise observation in the northern North Yellow Sea (NYS) is conducted to study temperature variation in the bottom thermal front zone of the NYS Cold Water Mass (NYSCWM) during the summer of 2009. In the flood-ebb tidal cycles, the bottom temperature decreases (increases) during flood (ebb) tides, which are dominated by the tidal-current induced horizontal advection. The ebb tide-induced temperature increase is larger than the flood tide-induced temperature decrease due to seasonal warming. In the spring-neap tidal cycles, the temperature and the vertical temperature structure show notable fortnightly variation from 16 July to 25 August. The bottom temperature increases from neap to spring tides and decreases from spring to neap. The Richardson number demonstrates strengthened vertical mixing during spring tides but enhanced stratification during neap tides. The spring-neap variation in vertical shear caused by tidal current is the dominant factor that induces the fortnightly variation in vertical mixing and thus bottom temperature.

Diel vertical migration of zooplankton and micronekton on the northern slope of the South China Sea observed by a moored ADCP

Acoustic Doppler current profiler (ADCP) echoes can be transformed into mean volume backscattering strength (MVBS), which is positively proportional to the biomass of zooplankton and micronekton. A mooring with an upward-looking 75-kHz ADCP was deployed on the northern slope of the South China Sea to monitor diel vertical migration (DVM) and vertical distributions of zooplankton and micronekton under general conditions and extreme conditions such as typhoons. DVM occurs throughout the year, with the maximum migrating speed reaching 9.0 cm s−1. It had some cyclic variation, which was in response to marine environment and astronomical influence. Light initiates the ascents and descents of the migrators, and migration intensity may be proportional to the solar altitude. Neap-spring tidal cycle and full moon phase influences are prominent, and the peak at 112 d may be driven by variation of currents. In addition, 270 m is the demarcation between upward and downward migrations; some organisms living below this layer, especially zooplankton and micronekton, migrate to the surface to obtain nutrients and energy; other marine mesopelagic organisms also survive in this manner. Super and severe typhoons reduced vertical migration, having less influence on the deep scattering layer. As Super Typhoon Rammasun passed by the mooring station, current speed increases and temperature decreases were synchronous with changes in the deep scattering layer; the migrators swam downward to evade the influence of a higher-speed current rather than swam upward to compensate for the decrease of temperature.

Incorporating real-time acoustic ranging and glider-based Doppler measurements to aid vehicle navigation

In the summer of 2017, two Seagliders equipped with low frequency acoustic recorders and 1 MHz acoustic Doppler current profilers (ADCPs) were deployed in the Canada Basin as part of a large-scale acoustic tomography experiment. Acoustic tomography sources were moored at approximately 175 m depth within an acoustic duct enabling acoustic transmission to be received at long ranges. The sources transmitted at a center frequency of approximately 250 Hz every four hours, and ranges between the sources and Seaglider were estimated in real time using a WHOI MicroModem. These ranges can be incorporated into an extended Kalman filter for navigation. In addition, glider-mounted upward-looking ADCPs recorded ~26.5 m shear profiles every 15 seconds. These overlapping profiles can be used to estimate (in post-processing) both the local current profile on a per-dive basis and the glider's relative velocity through the water. In real time, the close-range ADCP velocity measurements can be used to estimate the glider's relative (Through The Water, TTW) velocity to improve the glider's subsea position estimate. We will describe our recent developments in using these Doppler measurements to aid glider navigation, comparing the results to previous developments in range-aided navigation.

The 2016–2017 deep-water Canada Basin Acoustic Propagation Experiment (CANAPE): A preliminary report

The Arctic Ocean is undergoing dramatic changes in both ice cover and ocean structure. The Canada Basin Acoustic Propagation Experiment (CANAPE), which includes both deep and shallow water components, was designed to understand the effects of changing Arctic conditions on low-frequency propagation and ambient noise. The deep-water component, which is reported on here, includes a yearlong experiment in the Canada Basin during 2016–2017, preceded by a short Pilot Study during July–August 2015. During 2016-2017, a Distributed Vertical Line Array (DVLA) receiver with 60 Hydrophone Modules was moored within a six-element acoustic transceiver array with a 150-km radius. Environmental measurements on the DVLA include 28 Sea-Bird MicroCATs and upward- and downward-looking Acoustic Doppler Current Profilers (ADCPs) located below the hydrophone array. The acoustic transceivers had sources at 175-m depth and 15 Hydrophone Modules located above the sources. Environmental measurements on the transceiver moorings include ice-profiling sonars, upward-looking ADCPs on the subsurface floats, and 10 temperature sensors located below the acoustic transceivers. The one-year deployment provides measurements at least partially in open water during summer, in the marginal ice zone (MIZ) as it transitions across the array during the spring and autumn, and under complete ice cover during winter.

Wave resource characterization through in-situ measurement followed by artificial neural networks' modeling

This research presents a mathematical model that uses artificial neural networks for the assessment of the wave energy potential of sites, based on data recorded by wave monitoring instrumentation. The model was implemented and validated in two different sites. The first one had a dataset from an upward-looking acoustic Doppler current profiler that recorded a hindcast during 2½ years. The second consisted in data from a buoy using motion sensors that recorded continuously during 23 years. For this second site, the performance of the neural network model was compared to that of the Nearshore Wave Prediction System (NWPS), which combines SWAN, Wavewatch III and other numerical models. For the 2½ years' hindcast, the error of the neural network was significant which suggests a better use for filling missing gaps within datasets than for resource assessment. Meanwhile the performance of the neural network trained with the 23 years' hindcast was satisfactory; better than the NWPS in terms of relative bias but worse in terms of scatter index. Therefore it is concluded that neural networks can make an optimal use of the data produced by wave monitoring instrumentation and are useful to characterize the wave energy resource of a coastal site.

Acoustic backscatter observations with implications for seasonal and vertical migrations of zooplankton and nekton in the Amundsen shelf (Antarctica)

High-temporal resolution profiles of acoustic backscatter were collected in the Dotson Trough on the Amundsen shelf in the Antarctica, using a bottom-moored, upward-looking acoustic Doppler current profiler (ADCP). This data set was used to examine the impact of seasonal variations in surface solar radiation (SSR), sea ice concentration (SIC), and Circumpolar Deep Water (CDW) thickness on acoustic backscatter in the lower water column (250−540-m depth). A recorded high acoustic backscatter (−75 to −70 dB) at depth >400 m from April to November compared to the rest of the year (−90 to −80 dB) suggests that zooplankton and nekton migrated towards the bottom during winter. The depth of maximum mean volume backscattering strength showed a significant correlation with SSR, SIC and CDW thickness. A daily cycle of vertical migration was also recorded. This varied with changing surface ice conditions. When sea ice cover was low, the acoustic backscatter descended at sunrise, and ascended at sunset. When sea ice cover was high, the daily migration was not pronounced, and the layer of high acoustic backscatter remained near the bottom. This is the first study of seasonal and vertical migration of zooplankton and nekton that has been conducted on the Amundsen Sea shelf, one of the world's most productive areas. The findings provide implications to understand the behavior of zooplankton and nekton below the euphotic zone in the Southern Ocean.

Variability of turbulent quantities in the tidal bottom boundary layer: Case study in the eastern English Channel

Tidal current structure and turbulent quantities within a tidal bottom boundary layer (BBL) have been examined using an upward-looking acoustic Doppler current profiler (ADCP). The instrument was deployed on the seafloor, off the north-eastern French coast in the eastern English Channel over 12 tidal cycles and covered the period of the transition from mean spring to neap tide. Forcing regimes varied from calm to moderate storm conditions during the deployment. For the study of turbulent quantities in the BBL, we have chosen a calm period, when an effect of surface waves on the velocity structure was negligible. Stresses were found to vary regularly with the predominantly semidiurnal tidal flow, with the along-shore stress being generally greater during the flood flow (∼3.0Pa) than during the ebb flow (∼−1.5Pa). The turbulent kinetic energy (TKE) production rate, P, TKE density, Q, and its dissipation rate, ε, followed a nearly regular cycle with close to quarter-diurnal period. Near the seabed, peak values of P, Q and ε were found to be 0.5Wm−3, 0.5m2s−2 and, 0.04Wm−3, respectively, during the flood while, during the ebb, these quantities reached lesser values: 0.1Wm−3, 0.1m2s−2 and 0.03Wm−3, respectively. Near the bottom, eddy viscosity, Az, peak ranged from about 0.1m2s−1 during the flood to 0.03m2s−1 during the ebb flow. Away from the bottom, Az increased to reach a maximum near the mid-depth. Time–depth variation of the P/ε ratio indicated that the turbulence in the BBL, most of the time, was at a non-equilibrium state (P/ε≠1). The largest deviation from the equilibrium occurred during the flood, when P/ε exceeded about one decade near the bottom. During the ebb, P/ε was close to the equilibrium state, slightly decreasing with height above the bottom. Results are found to be in a good agreement with those of the other researches working on direct measurements of turbulence in tidal flows.

Effect of variable winds on current structure and Reynolds stresses in a tidal flow: analysis of experimental data in the eastern English Channel

Abstract. Wind and wave effects on tidal current structure and turbulence throughout the water column are examined using an upward-looking acoustic Doppler current profiler (ADCP). The instrument has been deployed on the seafloor of 18-m mean depth, off the north-eastern French coast in the eastern English Channel, over 12 tidal cycles, and covered the period of the transition from mean spring to neap tide, and forcing regimes varied from calm to moderate storm conditions. During storms, we observed gusty winds with magnitudes reaching 15 m s−1 and wave heights reaching up to 1.3 m. Analysis of velocity spectra revealed a noticeable contribution of wind-induced waves to spectral structure of velocity fluctuations within the subsurface layer. Near the surface, stormy winds and waves produced a significant intensification of velocity fluctuations, particularly when the sustained wind blew against the ebb tide flow. As during wavy periods, the variance-derived Reynolds stress estimates might include a wave-induced contamination, we applied the Variance Fit method to obtain unbiased stresses and other turbulent quantities. Over calm periods, the turbulent quantities usually decreased with height above the seabed. The stresses were found to vary regularly with the predominantly semidiurnal tidal flow. The along-shore stress being generally greater during the flood flow (~2.7 Pa) than during the ebb flow (~−0.6 Pa). The turbulent kinetic energy production rate, P, and eddy viscosity, Az, followed a nearly regular cycle with close to a quarter-diurnal period. As for the stresses, near the seabed, we found the maximum values of estimated quantities of P and Az to be 0.1 Wm−3 and 0.5 m2 s−1, respectively, during the flood flow. Over the storm periods, we found the highest unbiased stress values (~−2.6 Pa) during ebb when tidal currents were opposite to the southwesterly winds while, during the flood, the surface stresses slightly exceeded those estimated for a calm period. A comparison of obtained results gives a good agreement with those of other researchers working on direct measurements of turbulence in tidal flows.

Responses of Guangdong coastal upwelling to the summertime typhoons of 2006

The effects of typhoon intrusion on the Guangdong coastal upwelling system were investigated on the basis of in situ CTD (conductivity-temperature-depth) cruise observations and especially upward-looking ADCP (Acoustic Doppler Current Profilers) measurements obtained from a comprehensive survey of the Guangdong coastal region carried out by the Chinese Offshore Investigation and Assessment Project in the summer of 2006. It was found that northeastward geostrophic advection driven by the summer monsoon has a significant near-seabed onshore component adjacent to Shantou, which in conjunction with upper-level offshore Ekman flow, constitutes the canonical Guangdong coastal upwelling system. Further analyses suggested that the Guangdong coastal upwelling system is sensitive to subtle changes in the typhoon intensity and migration pathway. On one hand, as a typhoon approaches from north of the upwelling system (e.g. Typhoon 0604 (Bilis) and Typhoon 0605 (Kaemi)) in the early phase of intrusion, the enhanced southwesterly leads to exceptional enhancement of the onshore flow; i.e., enhanced upwelling. Afterward, irrespective of the forced ocean responses resulting from the stronger local winds (Typhoon 0604) or the moderate typhoon-induced inertial oscillations (Typhoon 0605), the situation is not conducive to sustaining a stable, persistent upwelling system. On the other hand, when there is typhoon intrusion south of the upwelling system (e.g. Typhoon 0606 (Prapiroon)), the favorable southwesterly tends to be substituted by an anomalous northeasterly, which destroys the traditional coastal upwelling pattern. However, the canonical upwelling system tends to recover within 1–2 days of the typhoon passing.

Sea-ice drift characteristics revealed by measurement of acoustic Doppler current profiler and ice-profiling sonar off Hokkaido in the Sea of Okhotsk

AbstractIn the southwestern part of the Sea of Okhotsk off Hokkaido, sea-ice drift characteristics are investigated using the ice and water velocities obtained from a moored upward-looking acoustic Doppler current profiler (ADCP) during the winters of 1999–2001. Using hourly-mean values of these data along with the wind data measured at a nearby coastal station, the wind factor and turning angle of the relative velocity between the ice and water velocities with respect to the wind are calculated assuming free drift under various conditions. Since the simultaneous sea-ice draft data are also available from a moored ice-profiling sonar (IPS), we examine the dependence of drift characteristics on ice thickness for the first time. As ice thickness increases and wind decreases, the wind factor decreases and the turning angle increases, as predicted by the theory of free drift. This study clearly shows the utility of the moored ADCP measurement for studying sea-ice drift, especially with the simultaneous IPS measurement for ice thickness, which cannot be obtained by other methods.

Characterization of sea ice cover, motion and dynamics in Marguerite Bay, Antarctic Peninsula

As part of the U.S. GLOBEC Southern Ocean Program, data from two automatic weather stations (AWSs) on small islets and upward-looking acoustic Doppler current profilers (ADCPs) and an upward-looking sonar (ULS) on sub-surface moorings have been analyzed to produce time series of atmospheric forcing, sea ice thickness, sea ice and ocean velocities, and sea ice momentum balances within Marguerite Bay and at mid-shelf on the central west Antarctic Peninsula continental shelf for the austral winter–spring seasons of 2001 and 2002. Both years had roughly seven months of nearly complete sea ice cover, but ice onset was about two months earlier in 2002 than 2001 mostly due to extremely cold surface air temperatures in Marguerite Bay during May–June 2002. Sea ice draft was quite variable, but generally thickened with time, reaching ∼2–3 m by the end of September before thinning. In October, 2002, a polynya was observed at one site within Marguerite Bay that lasted for 4 days. The sea ice motion and dynamics in Marguerite Bay were analyzed when sea ice draft was greater than 2 m. Wind stress during these periods was predominantly southward and southeastward. Wind stress, sea ice, and near-surface water motion were partially correlated, with sea ice motion greater than water motion. Large wind stress and sea ice and water motions occurred during short energetic events of a few days or shorter; however, the rms wind stress, sea ice, and water velocities were small, about 0.3 N m −2, 8–16 cm s −1, and 5–7 cm s −1, respectively. Mean sea ice motion was southward, but quite small, of order 2 cm s −1, while mean near-surface water motion was not statistically different from zero. The dominant sea ice momentum balance was between wind and internal ice stresses on time scales of several days and longer, with water stress, Coriolis, and tilt terms playing secondary roles. The mean southward wind stress was balanced, within uncertainty, by the mean northward internal ice stress. The strong and variable internal ice stresses were likely due to the onshore winds, the thickness of the sea ice, and the nearby coastline.

Modelling fluxes of water and sediment between Venice Lagoon and the sea

To describe the exchange of water and sediment through the Venice Lagoon inlets a 3-D hydrodynamic and sediment transport model has been developed and applied to a domain comprising Venice Lagoon and a part of the Adriatic Sea. The model has been validated for both current velocities and suspended particle concentration against direct observations and from observations empirically derived fluxes from upward-looking acoustic Doppler current profiler probes installed inside each inlet. The model provides estimates of the suspended sediment transport in the lower 3 m of the water column that is not detected by acoustic Doppler current profiler sensors. The bedload model prediction has been validated against measured sand transport rates collected by sand traps deployed in the Lido and Chioggia inlets. Results indicate that, in the Lido inlet, 87% of the total load is in suspension, while the rest moves as bedload.