Narrowband Acoustic Doppler Current Profiler Research Articles

Time-frequency analysis of migrating zooplankton in the Terra Nova Bay polynya (Ross Sea, Antarctica)

An upward-looking 150kHz narrow-band Acoustic Doppler Current profiler was operated in Terra Nova Bay (Ross Sea, Antarctica) from 5 February 2000 to 16 January 2001 to monitor marine currents. The instrument sampled the upper 160m of the water column with a time resolution of 1h. Although the experimental setup was not specifically designed to assess zooplankton and fish distributions and behaviour, the Acoustic Doppler Current Profiler ancillary data provided useful information regarding the diel vertical migration of these acoustic targets. A time frequency analysis of the mean backscatter strength time series was conducted using a 240h-wide window with a 1day step. Assuming that the 24h period peak is associated with zooplankton diel vertical migration, the amplitude of the power spectral energy on this band was extracted from each spectrum and the time series of amplitudes was analysed. The migration signal was very weak during summer, December to January, but was evident at the beginning and end of the polar night. Interestingly, the results indicated four “migratory blooms,” the first at the end of August and the others approximately every three weeks subsequently, ending at the end of October. The daily migration was found to have a good relation with the solar cycle, while it was apparently uncorrelated with the moon phase. Migration patterns in the upper and the lower ocean layers displayed significant differences. Due to the lack of contemporary in-situ net samples, the results are more qualitative than quantitative; nonetheless, they demonstrate the validity of the method to extract relevant information even when applied to data obtained from a non-devoted low-resolution system. This may be of particular interest in polar areas where it is difficult to perform continuous biological monitoring but where a long time series of Acoustic Doppler Current profiler data is available.

Using an ADCP to Estimate Turbulent Kinetic Energy Dissipation Rate in Sheltered Coastal Waters

AbstractTurbulent microstructure and acoustic Doppler current profiler (ADCP) data were collected near Tacoma Narrows in Puget Sound, Washington. Over 100 coincident microstructure profiles have been compared to ADCP estimates of turbulent kinetic energy dissipation rate (ϵ). ADCP dissipation rates were calculated using the large-eddy method with theoretically determined corrections for sensor noise on rms velocity and integral-scale calculations. This work is an extension of Ann Gargett’s approach, which used a narrowband ADCP in regions with intense turbulence and strong vertical velocities. Here, a broadband ADCP is used to measure weaker turbulence and achieve greater horizontal and vertical resolution relative to the narrowband ADCP. Estimates of ϵ from the Modular Microstructure Profiler (MMP) and broadband ADCP show good quantitative agreement over nearly three decades of dissipation rate, 3 × 10−8–10−5 m2 s−3. This technique is most readily applied when the turbulent velocity is greater than the ADCP velocity uncertainty (σ) and the ADCP cell size is within a factor of 2 of the Thorpe scale. The 600-kHz broadband ADCP used in this experiment yielded a noise floor of 3 mm s−1 for 3-m vertical bins and 2-m along-track average (≈four pings), which resulted in turbulence levels measureable with the ADCP as weak as 3 × 10−8 m2 s−3. The value and trade-off of changing the ADCP cell size, which reduces noise but also changes the ratio of the Thorpe scale to the cell size, are discussed as well.

Circulation in the Gulf of Aqaba (Red Sea) during winter—spring

This study is devoted to oceanographic features of the semi-enclosed Gulf of Aqaba, Red Sea. The data were recorded in winter-spring 1999 on the R/V Meteor cruise leg 44/2. Temperature and salinity profiles were measured at six positions (I-VI). The shipboard NarrowBand Acoustic Doppler Current Profiler (NB ADCP) 150 kHz con- tinuously recorded current profiles down to 350 m en route. The research revealed that the current near the Strait of Tiran front (position VI) represents a semidiurnal signal of an internal tide wave (~12 h period; 0.2 ms -1 amplitude) that might be gen- erated by the barotropic tide at the sill of the Strait. A sequence of cyclonic and anti- cyclonic eddy pairs is found along the axis of the Gulf of Aqaba during winter-spring seasons. These sub-mesoscale signals are dominant above the main thermocline and might be caused by wind forcing and the narrowness of the Gulf; it might remain in other seasons with different dimensions in relation to the depth of thermocline. The total diameter of each pair was twice the baroclinic Rossby radius ( R ≈ 10 km). A single anti-cyclonic eddy was observed in the upper 300 m in the northern tip of the Gulf with a diameter of about 5-8 km.

The biological validation of ADCP acoustic backscatter through direct comparison with net samples and model predictions based on acoustic-scattering models

Abstract Mean volume-backscattering strength (MVBS) data collected using a 153-kHz, narrowband Acoustic Doppler Current Profiler (ADCP) were compared with zooplankton abundance and biovolume data collected using a Longhurst–Hardy Plankton Recorder (LHPR). A direct comparison showed that there was a linear relationship between MVBS and log-transformed zooplankton dry weight. This linear relationship, determined from a mixed zooplankton-species population, was then compared with that reported in previous work from a region dominated by a single species of copepod and found to be significantly different. The scatter around the linear relationships determined between MVBS and log-transformed dry weights in regions of complex, mixed zooplankton populations results in our inability to distinguish different relationships that could be expected from different populations in varying oceanic regions. It is suggested that, without further manipulation of the data, ADCP MVBS cannot be used to determine quantitative estimates of zooplankton abundance and biomass in mixed populations. “Observed MVBS” was compared with model-predicted backscattering, calculated using acoustic models and abundance and size measurements of zooplankton from net samples. The results show that at high backscattering intensities (>−80 dB) the observed MVBS from an ADCP was generally consistent with the model predictions. Abundance, biovolume, and model-predicted backscattering contributions of six “significant acoustic-scattering” groups (amphipods, chaetognaths, copepods, euphausiids, fish, and pteropods) are shown to vary disproportionately. In particular, a rare and small but strong acoustic scatterer such as a pteropod can contribute as little as 0.1% to the total sample abundance and 0.1% to the biovolume but represent 69.5% of the total model-predicted backscattering. Model, instrumental, and methodological artefacts are identified as potential sources of inconsistencies between the observed and model-predicted backscattering. These include the effect of the orientation of zooplankton, inadequate knowledge of model parameters such as the sound-speed and density contrasts, the mismatch between sampling volumes of the net and acoustic instrument, and net avoidance by the more mobile scatterers.

Acoustic backscatter measurements with a 153 kHz ADCP in the northeastern Gulf of Mexico: determination of dominant zooplankton and micronekton scatterers

A 153 kHz narrowband acoustic Doppler current profiler (ADCP) was used to measure volume backscattering strength ( S v) during a deepwater oceanographic survey of cetacean and seabird habitat in the northeastern Gulf of Mexico. S v was positively related to zooplankton and micronekton biomass (wet displacement volume) in ‘sea-truth’ net hauls made with a 1 m 2 Multiple Opening-Closing Net Environmental Sensing System (MOCNESS). A subset of these MOCNESS tows was used to explore the relationship between the numerical densities of various taxonomic categories of zooplankton and the ADCP backscatter signal. Crustaceans, small fish, and fragments of non-gas-bearing siphonophores in the net samples all showed significant, positive correlations with the acoustic signal, while other types of gelatinous zooplankton, pteropod and atlantid molluscs, and gas-filled siphonophore floats showed no significant correlation with S v. Previously published acoustic scattering models for zooplankton were used to calculate expected scattering for several general zooplankton types and sizes for comparison with the field data. Even though gelatinous material often made up a large fraction of the total biomass, crustaceans, small fish, and pteropods were most likely the important scatterers. Since only crustacean and small fish densities were significantly correlated with S v, it is suggested that S v at 153 kHz can be used as a relative proxy for the abundance of these organisms in the Gulf of Mexico.

Side-looking ADCP and Doppler radar measurements across a coastal front

Measurements are reported from two side-looking Doppler systems, which were used to study the discharge front located off the mouth of Chesapeake Bay. One system was a commercial 300-kHz narrow-band acoustic Doppler current profiler (ADCP), which was mounted at a depth of 0.6 m on the port side of a research ship. The other was a prototype X-band, vertically polarized, Doppler radar mounted at a height of about 4 m on the starboard side. Both velocity and backscatter intensity were measured along two beams to ranges of 120 m (ADCP) and 200 m (radar), so that by sailing alternately on each side of the front it was possible to make nearly simultaneous across-front measurements with both systems. Despite the differences in acoustic and radar scattering mechanisms, a combined backscatter intensity surface map could be made showing a continuous frontal signature about 10-m wide and 20 dB above background levels. Each system was also able to measure the same large-scale velocity change across the front, which was dominated by the discharging buoyant bay water flowing at about 50 cm/s relative to the ambient continental shelf water. However, within a 60-m wide zone, the radar system measured velocities up to 75 cm/s larger than the ADCP. Such large velocity differences arose from the radar's sensitivity to motions associated with waves reflecting from the region of strongest across-front current convergence. This frontal convergence was resolved only by the ADCP, which showed a horizontal current change of about 25 cm/s over 10 m and appeared to extend over the upper 2 m or so of the water column. These results show that the combined information from the acoustic and radar systems provide a more complete picture of the frontal currents and wave-current interactions than either system could provide alone.

Patterns of distribution of sound‐scattering zooplankton in warm‐ and cold‐core eddies in the Gulf of Mexico, from a narrowband acoustic Doppler current profiler survey

The acoustic backscatter intensity (ABI) reflected from epipelagic zooplankton communities in the central Gulf of Mexico was measured during June 1995 with a vessel‐mounted, narrowband‐153‐kHz acoustic Doppler current profiler (ADCP). Horizontal and vertical variations in ABI were documented in three kinds of mesoscale hydrographic features commonly found in the Gulf of Mexico: the warm‐core Loop Current (LC), a warm‐core Loop Current eddy (LCE), and a cold‐core region that separated the two warm‐core features. Since new nitrogen domes close to surface waters in cold‐core features whereas surface waters of warm‐core features are nutrient depleted, the cold‐core region was expected to have higher biological stocks as a result of locally higher primary production. Both ABI and net tow data confirmed that the cold‐core region was in fact a zone of local aggregation of zooplankton and micronekton. During both day and night, ABI when integrated for the upper 50 and 100 m in the cold‐core region was significantly greater than in the LC or in the LCE, and ABI was positively correlated with standing stock biomass taken by the net tows. Further investigations into the biological differences between Gulf of Mexico divergence and convergence regimes are warranted, and the ADCP will be a useful tool for examination of the distribution of sound scatterers in such features.

Effects of low‐frequency current variability on near‐inertial submesoscale vortices

From September 30 to October 14, 1993, and from May 18 to June 13, 1994, ocean surface currents along the inshore boundary of the Florida Current were measured using a land‐based ocean surface current radar (OSCR) deployed along the Florida Keys. A dual‐station OSCR mapped the current fields at 20‐min intervals with a horizontal resolution of 1.2 km over a 30×44 km domain using the high‐frequency (HF, 25.4 MHz) mode. An upward looking, narrowband acoustic Doppler current profiler (ADCP) was moored in 150 m of water seaward of the shelf break concurrently sampling the three‐dimensional current vector between 15 and 130 m at 30‐min intervals. Regression analyses between surface and subsurface currents at 15 m depth from the ADCP indicated biases of 3–12 cm s−1 and slopes of O(0.8–1). Root‐mean‐square differences were about 18 cm s−1 because of a 15‐m‐depth separation between the two measurements, and bulk current shears were O(10−3 s−1) in the Florida Current where maximum velocities exceeded 150 cm s−1 between 40 and 70 m. Surface tidal currents were 2–10 cm s−1 in the diurnal and semidiurnal bands and revealed fine‐scale variability in the tidal ellipses due to topographical variations. Differences in the tidal current amplitudes between the surface and 15 m ranged from 1 to 3 cm s−1 in these constituents and explained more of the current variance with depth. During the second experiment a submesoscale surface feature was detected along the shelf break and progressed eastward at a rate of about 30 cm s−1. This feature was embedded in the near‐inertial wave band where the local inertial period was ≈29 hours that may have been forced by an abrupt change in the wind direction. This transient feature was trapped and advected along the inshore edge of the Florida Current where the anticyclonic vorticity of the subinertial flow shifted the frequency of the nearinertial motions by a few percent of f. These energetic near‐inertial currents of 15–20 cm s−1 had horizontal and vertical wavelengths of 40 km and 50–100 m, respectively.

TheOleanderProject: Monitoring the Variability of the Gulf Stream and Adjacent Waters between New Jersey and Bermuda

Abstract An overview of the first 4.5 years of operation of a program to monitor the structure and variability of the Gulf Stream (GS) is presented. A container vessel that operates on a weekly schedule between Port Elizabeth, New Jersey, and Hamilton, Bermuda, is equipped with a 150-kHz narrowband acoustic Doppler current profiler to measure currents from the surface to ∼300 m depth. A major objective of the multiyear program is to study the annual cycle and interannual variations in velocity structure and transport by the GS. In this survey the focus is on the transport and lateral structure of the current at 52-m depth. The velocity maximum is constant at 2.07 ± 0.24 m s−1 (4 kt) with a seasonal range of ∼0.1 m s−1. Seasonal and interannual variations in total transport are observed but appear to be limited to the edges of the current, apparently reflecting low-frequency variations in the intensity of the recirculating waters adjacent to the stream. The transport by the central core of the current, def...