High-resolution Acoustic Doppler Current Profiler Research Articles

Rain and Sun Create Slippery Layers in Eastern Pacific Fresh Pool.

An autonomous Lagrangian float equipped with a high-resolution acoustic Doppler current profiler observed the evolution of upper-ocean stratification and velocity in the Eastern Pacific Fresh Pool for over 100 days in August–November 2016. Although convective mixing homogenized the water column to 40 m depth almost every night, the combination of diurnal warming on clear days and rainfall on cloudy days routinely produced strong stratification in the upper 10 m. Whether due to thermal or freshwater effects, the initial strong stratification was mixed downward and incorporated in the bulk of the mixed layer within a few hours. Stratification cycling was associated with pronounced variability of ocean surface boundary layer turbulence and vertical shear of wind-driven (Ekman) currents. Decoupled from the bulk of the mixed layer by strong stratification, warm and fresh near-surface waters were rapidly accelerated by wind, producing the well-known “slippery layer” effect, and leading to a strong downwind near-surface distortion of the Ekman profile. A case study illustrates the ability of the new generation of Lagrangian floats to measure rapidly evolving temperature, salinity, and velocity, including turbulent and internal wave components. Quantitative interpretation of the results remains a challenge, which can be addressed with high-resolution numerical modeling, given sufficiently accurate air-sea fluxes.

Sediment transport and fluid mud layer formation in the macro-tidal Chikugo river estuary during a fortnightly tidal cycle

The erosion and deposition dynamics of fine sediment in a highly turbid estuarine channel were successfully surveyed during the period from August 29 to September 12, 2009 using an echo sounder in combination with a high-resolution acoustic Doppler current profiler. Field measurements were conducted focusing on the tide driven dynamics of suspended sediment concentration (SSC), and fluid mud at the upstream of the macrotidal Chikugo river estuary during semidiurnal and fortnightly tidal cycles. Morphological evolution was observed especially during the spring tide over a period of two weeks. The elevation of the channel bed was stable during neap tide, but it underwent fluctuations when the spring tide occurred owing to the increase in the velocity and shear stress. Two days of time lag were observed between the maximum SSC and peak tidal flow, which resulted in the asymmetry between neap-to-spring and spring-to-neap transitions. During the spring tide, a hysteresis loop was observed between shear stress and SSC, and its direction was different during flood and ebb tides. Although both fine sediments and flocs were dominant during flood tides, only fine sediments were noticed during ebb tides. Hence, the net elevation change in the bed was positive, and sedimentation took place during the semilunar tidal cycle. Finally, a bed of consolidated mud was deposited on the initial bed, and the height of the channel bed increased by 0.9 m during the two-week period. The observed hysteretic effect between shear stress and SSC during the spring tides, and the asymmetrical neap-spring-neap tidal cycle influenced the near-bed sediment dynamics of the channel, and led to the formation of a fluid mud layer at the bottom of the river.

Evaluating the role of groundwater in circulation and thermal structure within a deep inland lake

Groundwater levels in many aquifers are declining due to anthropogenic activities such as increased high-capacity pumping for agriculture or climate-related decreases in natural recharge rates or a combination of factors. At the same time, lake surface temperatures are on the rise in response to a warming climate. As a first step toward evaluating the impacts of declining groundwater levels and warming lake surface temperatures on coupled biophysical processes in lakes, we evaluate the role played by groundwater in circulation and thermal structure within Gull Lake, a deep, dimictic, inland lake in Michigan, USA. A three-dimensional, unstructured grid hydrodynamic model was developed to investigate physical processes in the lake during the summer stratified period. We used high-resolution Acoustic Doppler Current Profiler observations of currents and lake levels as well as temperature data from thermistor chains to test the numerical models. The quality of meteorological forcing fields reconstructed using data from a network of weather stations surrounding the lake were assessed using outputs from a mesoscale numerical weather forecasting model, Weather Research and Forecasting (WRF) and vice versa. Model descriptions of internal heating due to the penetration of shortwave radiation as well as turbulent mixing within the water column were improved using in situ observations. Our results indicate that meteorological forcing fields, carefully reconstructed using WRF model outputs, can provide results comparable to those obtained from a network of weather station data, an important conclusion for modeling lakes in remote parts of the world. The observed low hypolimnetic temperatures could only be explained by taking the groundwater contribution into account and water column temperatures will increase by 8 °C or more on average if the groundwater contribution is absent. These results have implications for a number of key biophysical processes that control the structure and function of lake ecosystems including the growth rates of algae, dissolved inorganic nitrogen levels in the lake as well as the depletion of oxygen in deeper layers.

Comparison of HF radar measurements with Eulerian and Lagrangian surface currents

High-frequency (HF) radar-derived ocean currents are compared with in situ measurements to conclude if the radar observations include effects of surface waves that are of second order in the wave amplitude. Eulerian current measurements from a high-resolution acoustic Doppler current profiler and Lagrangian measurements from surface drifters are used as references. Directional wave spectra are obtained from a combination of pressure sensor data and a wave model. Our analysis shows that the wave-induced Stokes drift is not included in the HF radar-derived currents, that is, HF radars measure the Eulerian current. A disputed nonlinear correction to the phase velocity of surface gravity waves, which may affect HF radar signals, has a magnitude of about half the Stokes drift at the surface. In our case, this contribution by nonlinear dispersion would be smaller than the accuracy of the HF radar currents, hence no conclusion can be made. Finally, the analysis confirms that the HF radar data represent an exponentially weighted vertical average where the decay scale is proportional to the wavelength of the transmitted signal.

The Equatorial Undercurrent and TAO Sampling Bias from a Decade at SEA

Abstract The NOAA Tropical Atmosphere Ocean (TAO) moored array has, for three decades, been a valuable resource for monitoring and forecasting El Niño–Southern Oscillation and understanding physical oceanographic as well as coupled processes in the tropical Pacific influencing global climate. Acoustic Doppler current profiler (ADCP) measurements by TAO moorings provide benchmarks for evaluating numerical simulations of subsurface circulation including the Equatorial Undercurrent (EUC). Meanwhile, the Sea Education Association (SEA) has been collecting data during repeat cruises to the central equatorial Pacific Ocean (160°–126°W) throughout the past decade that provide useful cross validation and quantitative insight into the potential for stationary observing platforms such as TAO to incur sampling biases related to the strength of the EUC. This paper describes some essential sampling characteristics of the SEA dataset, compares SEA and TAO velocity measurements in the vicinity of the EUC, shares new insight into EUC characteristics and behavior only observable in repeat cross-equatorial sections, and estimates the sampling bias incurred by equatorial TAO moorings in their estimates of the velocity and transport of the EUC. The SEA high-resolution ADCP dataset compares well with concurrent TAO measurements (RMSE = 0.05 m s−1; R2 = 0.98), suggests that the EUC core meanders sinusoidally about the equator between ±0.4° latitude, and reveals a mean sampling bias of equatorial measurements (e.g., TAO) of the EUC’s zonal velocity of −0.14 ± 0.03 m s−1 as well as a ~10% underestimation of EUC volume transport. A bias-corrected monthly record and climatology of EUC strength at 140°W for 1990–2010 is presented.

Physical structure of the Barents Sea Polar Front near Storbanken in August 2007

The Polar Front separating Atlantic Water (AW) and Arctic Water (ArW) is one of the most dominant meso- and large scale features in the Barents Sea. Here, the results of submeso-mesoscale (1–10km) variability in physical fields associated with the Barents Sea Polar Front (BSPF) are reported from a high-resolution ADCP (Acoustic Doppler Current Profiler) and CTD (Conductivity–Temperature–Depth sensors) survey near Storbanken in August 2007. A surface front separating AW and melt water with a strong salinity gradient was present, while the subsurface BSPF was characterized by a strong temperature gradient and thermohaline compensation. Isopycnal mixing leading to the formation of Polar Front Water (PFW) was observed. The dominant flow was a barotropic southeastward along-frontal jet with two cores, coinciding with the surface front and the BSPF. This gives new insights into the circulation at the BSPF. Small-scale variability in the hydrographic and dynamic structures was observed, which were rarely resolved in previous cruises. Such submeso–mesoscale physical processes can potentially have significant impacts on the biogeochemistry and biology in the area, indicating the importance of parametrizing the processes in future climate models.

Improving Estimates of the Antarctic Circumpolar Current Streamlines in Drake Passage

Abstract Accurately resolving the mean Antarctic Circumpolar Current (ACC) is essential for determining Southern Ocean eddy fluxes that are important to the global meridional overturning circulation. Previous estimates of the mean ACC have been limited by the paucity of Southern Ocean observations. A new estimate of the mean surface ACC in Drake Passage is presented that combines sea surface height anomalies measured by satellite altimetry with a recent dataset of repeat high-resolution acoustic Doppler current profiler observations. A mean streamfunction (surface height field), objectively mapped from the mean currents, is used to validate two recent dynamic height climatologies. The new streamfunction has narrower and stronger ACC fronts separated by quiescent zones of much weaker flow, thereby improving on the resolution of ACC fronts observed in the other climatologies. Distinct streamlines can be associated with particular ACC fronts and tracked in time-dependent maps of dynamic height. This analysis shows that varying degrees of topographic control are evident in the preferred paths of the ACC fronts through Drake Passage.

The role of particle aggregation/disaggregation in muddy sediment dynamics and seasonal sediment turnover in a back-barrier tidal basin, East Frisian Wadden Sea, southern North Sea

In order to better understand the processes controlling the deposition of mud in the East Frisian Wadden Sea, the seasonal response of the nearshore muddy sand facies was investigated along four shore-normal transects. For this purpose, the textural characteristics and internal sedimentary structures of the surface sediments were analysed at two-monthly intervals over two years. In addition, current velocities, in-situ particle size distributions, and the concentration of suspended sediment were measured near the dike using a high-resolution acoustic Doppler current profiler (PC-ADP) and an in-situ laser particle sizer (LISST-ST). A clear seasonal cycle emerges from the time-series data. The mud content of the surface sediment increases in summer and decreases in winter. This seasonal pattern is also reflected in the internal sedimentary structures of the upper sediment layer. In summer the surface sediment comprises a mud drape which disappears in winter when it is replaced by sand associated with wave-generated sedimentary structures. Grain-size analyses of dispersed mud samples reveal a pronounced deficiency of particles around 7 phi (8 μm). This is interpreted to reflect the transition between cohesive flocs/aggregates and non-cohesive coarser silt particles. A comparison of the particle size distributions of dispersed pump samples with those of the in-situ laser particle sizer confirms that the bulk of suspended mud consists of flocs and aggregates composed of particles finer than 8 μm. The destruction of such flocs during sample processing for size analysis in the laboratory produces strongly distorted size–frequency distributions and, as consequence, prevents a meaningful hydraulic interpretation of such data. Under calm weather conditions typical of the summer months, mean floc sizes (MFS) increase up to a critical current velocity of about 10 cm s − 1 during both the flood and ebb phase of the tidal cycle. This coincides with lower suspended sediment concentrations (SSC). Since the tidal cycle is ebb dominated under calm weather conditions, and current velocities during the flood phase do not exceed the critical value, most of the tidal cycle under typical summer conditions is dominated by large flocs and low sediment concentrations. Besides physico-chemical flocculation effects, the formation of particle aggregates is further promoted by microbial processes in summer. Such aggregates have considerably higher settling velocities than their constituent particles, especially in summer when water temperatures and, hence, kinematic viscosities are high. This explains the observed mud drape covering the intertidal area in summer. Under high-energy conditions, by contrast, the situation is reversed, the system being flood dominated, MFSs being smaller, and SSCs higher. Due to more frequent high-energy events in winter, higher current shear and turbulence cause floc break-up and resuspension of deposited muds. The sediment distribution pattern is thus clearly controlled by particle aggregation and disaggregation in response to seasonal changes in energy flux, water temperature and microbial activity.

Correction to “Development of subaqueous barchanoid-shaped dunes due to lateral grain size variability in a tidal inlet channel of the Danish Wadden Sea”

[1] High-resolution bathymetry at centimeter-scale accuracy acquired with a multibeam echo sounder system revealed the existence of barchanoid-shaped large (i.e., length 10–100 m) to very large (i.e., length >100 m) dunes in the Gradyb tidal inlet channel in the Danish Wadden Sea. The development of these dunes is due to an increase in dune celerity from 12 m/yr in the center of the channel to around 30 m/yr at the sides. This increase in dune celerity can be explained by the fact that dune heights decrease from 3.1 m in the center of the channel to 1.4 m at the sides, as a smaller sediment volume has to be moved per unit time for equal dune celerity. Water depth is uniform across the channel. Likewise, high-resolution acoustic Doppler current profiler measurements across the channel showed a uniform distribution of both ebb and flood flow. Thus no correlation between dune dimensions and water depth or flow velocity was established. Instead, high-accuracy bed sampling along the crests of the dunes showed a decrease in mean grain size from 0.63 mm in the center to 0.36 mm at the sides of the channel. The decrease in dune height is ascribed to this decrease in grain size, given that flow depth and flow velocity are uniform across the channel. The lateral decrease in grain size is suggested to result from sorting effects by secondary currents directed from the center toward the sides of the channel in the trough/lee side region of the barchanoid-shaped dunes.

HIGH-RESOLUTION ADCP MEASUREMENTS ON VERTICAL STRUCTURES OF CURRENT AND SUSPENDED SOLID IN AN URBAN RIVER

To realize more accurate measurements for pollutant loads in an urban river, in the present study, a high-resolution, acoustic Doppler current profiler (HR-ADCP) was used to measure the vertical distributions of current and SS in an urban river under rainy conditions. The field measurements with the bottom-mounted HR-ADCP were carried out at the downstream point of the Oohori River, one of main inflow rivers in Lake Teganuma. The observed results show that characteristic vertical structures of the current and SS appear in hydrologic events and then the vertical distribution of SS flux in the streamwise direction vary considerably in rising and falling stages. With the HR-ADCP measurements, we grasp the temporal variation for the dependence of the erosion rate of sediments on flow conditions.

Observations of Current Flow Through the Mouth of the Chesapeake Bay

Seven transects of the narrowly-constricted mouth of the Chesapeake Bay, U.S.A., were made using an acoustic doppler current profiler (ADCP) and conductivity-temperature-depth (CTD) sensor during the period 16–21 September 1996. Data were acquired while participating in the Chesapeake Bay Plume Outfall Experiment (COPE I). Seven CTD stations per transect were conducted across the 17km wide mouth along with continuous high-resolution ADCP coverage. Although discontinuous, the transect data covered nearly all phases of a generic tidal current cycle. The ratio of water level anomaly to average depth was O(10−3) during the transects with atmospheric pressure and wind accounting for most of the anomaly. Freshwater discharge from the Bay was above normal during this period. The resultant flow structure obtained from the ADCP is much more complete than was previously possible with point measurements. Analysis in all tidal phases revealed near-isothermal conditions with horizontal and vertical temperature differences across the mouth rarely exceeding 1·0°C in any tidal phase, higher surface salinity to the north coupled with higher subsurface salinity in the Chesapeake Channel and unidirectional flow from surface to depth. Horizontal and vertical current gradients were found to be quite strong, O(10−3) s−1and O(10−2) s−1respectively. A consistent feature found on nearly every ADCP transect was a subsurface flood current core and a surface ebb current core, both on the right side of the main channel in the direction of flow.