scholarly journals Internal tide energy flux over a ridge measured by a co-located ocean glider and moored acoustic Doppler current profiler

Ocean Science ◽  
2019 ◽  
Vol 15 (6) ◽  
pp. 1439-1453 ◽  
Author(s):  
Rob A. Hall ◽  
Barbara Berx ◽  
Gillian M. Damerell
Keyword(s):  
Energy Flux ◽  
Internal Tide ◽  
Acoustic Doppler Current Profiler ◽  
Small Scale ◽  
Negative Bias ◽  
Potential Density ◽  
Shelf Slope ◽  
Current Profiler ◽  
Circle Diameter ◽  
Horizontal Wavelength

Abstract. Internal tide energy flux is an important diagnostic for the study of energy pathways in the ocean, from large-scale input by the surface tide to small-scale dissipation by turbulent mixing. Accurate calculation of energy flux requires repeated full-depth measurements of both potential density (ρ) and horizontal current velocity (u) over at least a tidal cycle and over several weeks to resolve the internal spring–neap cycle. Typically, these observations are made using full-depth oceanographic moorings that are vulnerable to being “fished out” by commercial trawlers when deployed on continental shelves and slopes. Here we test an alternative approach to minimize these risks, with u measured by a low-frequency acoustic Doppler current profiler (ADCP) moored near the seabed and ρ measured by an autonomous ocean glider holding station by the ADCP. The method is used to measure the semidiurnal internal tide radiating from the Wyville Thomson Ridge in the North Atlantic. The observed energy flux (4.2±0.2 kW m−1) compares favourably with historic observations and a previous numerical model study. Error in the energy flux calculation due to imperfect co-location of the glider and ADCP is estimated by subsampling potential density in an idealized internal tide field along pseudorandomly distributed glider paths. The error is considered acceptable (<10 %) if all the glider data are contained within a “watch circle” with a diameter smaller than 1∕8 the mode-1 horizontal wavelength of the internal tide. Energy flux is biased low because the glider samples density with a broad range of phase shifts, resulting in underestimation of vertical isopycnal displacement and available potential energy. The negative bias increases with increasing watch circle diameter. If watch circle diameter is larger than 1∕8 the mode-1 horizontal wavelength, the negative bias is more than 3 % and all realizations within the 95 % confidence interval are underestimates. Over the Wyville Thomson Ridge, where the semidiurnal mode-1 horizontal wavelength is ≈100 km and all the glider dives are within a 5 km diameter watch circle, the observed energy flux is estimated to have a negative bias of only 0.4 % and an error of less than 3 % at the 95 % confidence limit. With typical glider performance, we expect energy flux error due to imperfect co-location to be <10 % in most mid-latitude shelf slope regions.

scholarly journals Internal tide energy flux over a ridge measured by a co-located ocean glider and moored ADCP

2019 ◽  
Author(s):  
Rob Hall ◽  
Barbara Berx ◽  
Gillian Damerell
Keyword(s):  
Energy Flux ◽  
Large Scale ◽  
Low Frequency ◽  
Internal Tide ◽  
Small Scale ◽  
Potential Density ◽  
The North ◽  
Model Study ◽  
Continental Shelves ◽  
Alternative Approach

Abstract. Internal tide energy flux is an important diagnostic for the study of energy pathways in the ocean, from large-scale input by the surface tide, to small-scale dissipation by turbulent mixing. Accurate calculation of energy flux requires repeated full-depth measurements of both potential density (ρ) and horizontal current velocity (u) over at least a tidal cycle and over several weeks to resolve the internal spring-neap cycle. Typically, these observations are made using full-depth oceanographic moorings that are vulnerable to being fished-out by commercial trawlers when deployed on continental shelves and slopes. Here we test an alternative approach to minimise these risks, with u measured by a low-frequency ADCP moored near the seabed and ρ measured by an autonomous ocean glider holding station by the ADCP. The method is used to measure the M2 internal tide radiating from the Wyville Thompson Ridge in the North Atlantic. The observed energy flux (4.2 ± 0.2 kW m−1) compares favourably with historic observations and a previous numerical model study. Error in the energy flux calculation due to imperfect co-location of the glider and ADCP is estimated by sub-sampling potential density in an idealised internal tide field along pseudorandomly distributed glider paths. The error is considered acceptable (

scholarly journals Internal Wave Reflection on Shelf Slopes with Depth-Varying Stratification

2013 ◽  
Vol 43 (2) ◽  
pp. 248-258 ◽  
Author(s):  
Rob A. Hall ◽  
John M. Huthnance ◽  
Richard G. Williams
Keyword(s):  
Internal Wave ◽  
Internal Waves ◽  
Energy Flux ◽  
Internal Tide ◽  
Shelf Break ◽  
Maximum Slope ◽  
Near Surface ◽  
Shelf Slope ◽  
Linear Slope ◽  
Horizontal Wavelength

Abstract Reflection of internal waves from sloping topography is simple to predict for uniform stratification and linear slope gradients. However, depth-varying stratification presents the complication that regions of the slope may be subcritical and other regions supercritical. Here, a numerical model is used to simulate a mode-1, M2 internal tide approaching a shelf slope with both uniform and depth-varying stratifications. The fractions of incident internal wave energy reflected back offshore and transmitted onto the shelf are diagnosed by calculating the energy flux at the base of slope (with and without topography) and at the shelf break. For the stratifications/topographies considered in this study, the fraction of energy reflected for a given slope criticality is similar for both uniform and depth-varying stratifications. This suggests the fraction reflected is dependent only on maximum slope criticality and independent of the depth of the pycnocline. The majority of the reflected energy flux is in mode 1, with only minor contributions from higher modes due to topographic scattering. The fraction of energy transmitted is dependent on the depth-structure of the stratification and cannot be predicted from maximum slope criticality. If near-surface stratification is weak, transmitted internal waves may not reach the shelf break because of decreased horizontal wavelength and group velocity.

scholarly journals Velocity Structure of Internal Tide Beams Emanating from Kaena Ridge, Hawaii

2012 ◽  
Vol 42 (6) ◽  
pp. 1039-1044 ◽  
Author(s):  
Andy Pickering ◽  
Matthew H. Alford
Keyword(s):  
Numerical Simulations ◽  
Velocity Structure ◽  
Internal Tide ◽  
Acoustic Doppler Current Profiler ◽  
Ocean Model ◽  
Surface Reflection ◽  
Model Predictions ◽  
Current Profiler ◽  
North And South ◽  
Ray Paths

Abstract Observations are reported of the semidiurnal (M2) internal tide across Kaena Ridge, Hawaii. Horizontal velocity in the upper 1000–1500 m was measured during eleven ~240-km-long shipboard acoustic Doppler current profiler (ADCP) transects across the ridge, made over the course of several months. The M2 motions are isolated by means of harmonic analysis and compared to numerical simulations using the Princeton Ocean Model (POM). The depth coverage of the measurements is about 3 times greater than similar past studies, offering a substantially richer view of the internal tide beams. Sloping features are seen extending upward north and south from the ridge and then downward from the surface reflection about ±40 km from the ridge crest, closely matching theoretical M2 ray paths and the model predictions.

scholarly journals Energy Fluxes due to the Surface and Internal Tides in Knight Inlet, British Columbia

2005 ◽  
Vol 35 (11) ◽  
pp. 2219-2227 ◽  
Author(s):  
Michael W. Stacey ◽  
S. Pond
Keyword(s):  
Energy Flux ◽  
Linear Models ◽  
Internal Tide ◽  
Acoustic Doppler Current Profiler ◽  
Tidal Energy ◽  
Tidal Height ◽  
Internal Tides ◽  
Energy Fluxes ◽  
Net Flux ◽  
Adcp Observations

Abstract A laterally integrated (two dimensional) nonlinear numerical model is used to examine the flux of M2 tidal energy in Knight Inlet. The simulated flux of tidal energy into the inlet is somewhat smaller than that estimated using the change in phase of the M2 tidal height along the inlet, a method that does not account for the effect of the internal tide on the surface elevation. The simulated energy flux into the inlet is close to the energy flux of the internal tide away from the sill determined from observations using an acoustic Doppler current profiler (ADCP). The net flux due to the internal tide is significantly less than (&lt;1/2 of) the rate at which energy is removed from the surface tide. Earlier linear models of the internal tide produced energy fluxes that agreed with those estimated from the phase change of the tidal height but were larger than the fluxes that could be found in the observations. The reason for this discrepancy is not that these simple models neglected nonlinear effects, but rather that they did not take reflections of the internal tide into account. Also, the simulated flux of energy into the inlet less the net flux of internal tidal energy away from the sill is about equal to the simulated dissipation within 2 km on either side of the sill. The simulated net flux of internal tidal energy away from the sill is in agreement with the flux determined from the ADCP observations on the downinlet side of the sill, but not on the upinlet side of the sill. A possible explanation is that only the first internal mode (which is surface intensified) was important on the downinlet side but the first three internal modes were important on the upinlet side. The flux calculation using the ADCP observations took variations in the inlet width into account but did not take depth variations into account; thus, the reflection coefficients of the second and third modes may have been underestimated.

scholarly journals Spectral and Structure Function Estimates of Turbulence Dissipation Rates in a High-Flow Tidal Channel Using Broadband ADCPs

2017 ◽  
Vol 34 (1) ◽  
pp. 5-20 ◽  
Author(s):  
Justine M. McMillan ◽  
Alex E. Hay
Keyword(s):  
Structure Function ◽  
Dissipation Rate ◽  
Acoustic Doppler Current Profiler ◽  
Universal Constant ◽  
Flow Speed ◽  
High Flow ◽  
Small Scale ◽  
Order Structure ◽  
Tidal Channel ◽  
Current Profiler

AbstractSpectral and structure function methods are implemented to compute the dissipation rate ε from broadband, diverging-beam acoustic Doppler current profiler (ADCP) data collected at four sites in a high-flow tidal channel. This paper shows that middepth estimates of ε obtained from spectral and second-order structure function (SF2) methods are both lognormally distributed with comparable means and variances. Speed bin–averaged ε values agree to within 16%, depending on the site and tidal phase (ebb/flood). The close agreement between the two independent methods provides further support for the argument put forward by McMillan et al.: that is, that the factor-of-2 difference between shear probe and (spectral) ADCP estimates of ε was likely caused by spatial differences in turbulence levels. The agreement between the spectral and both second- and third-order structure function methods also supports the use of for the SF2 universal constant. Notably, however, the SF3 method was less robust for these data. Two additional aspects of the SF2 approach are examined in some detail: 1) the differences from upstream- and downstream-facing beams are shown to arise from the Reynolds stress and 2) the inability of the ADCP to resolve small-scale motions does not affect the estimates of ε but yields apparent Doppler noise levels that—counterintuitively—decrease with increasing flow speed and increasing dissipation rate. A modified SF2 method that accounts for the variance associated with the unresolved scales removes the flow speed dependence and yields noise level estimates that agree with the spectral values.

scholarly journals Storm-induced water dynamics and thermohaline structure at the tidewater Flade Isblink Glacier outlet to the Wandel Sea (NE Greenland)

2017 ◽  
Author(s):  
Sergei Kirillov ◽  
Igor Dmitrenko ◽  
Søren Rysgaard ◽  
David Babb ◽  
Leif Toudal Pedersen ◽  
...  
Keyword(s):  
Tidal Flow ◽  
Internal Tide ◽  
Acoustic Doppler Current Profiler ◽  
Water Dynamics ◽  
Storm Event ◽  
Thermohaline Structure ◽  
Relaxation Phase ◽  
Ice Cap ◽  
Glacier Terminus ◽  
Current Profiler

Abstract. In April 2015, an ice-tethered conductivity-temperature-depth (CTD) profiler and a down-looking Acoustic Doppler Current Profiler (ADCP) were deployed from the landfast ice near the tidewater glacier terminus of the Flade Isblink Glacier in the Wandel Sea, NE Greenland. The three week timeseries showed that water dynamics and the thermohaline structure were modified considerably during a storm event on 22–24 April when northerly winds exceeded 15 m/s. The storm initiated downwelling-like water dynamics characterized by on-shore water transport in the surface (0–40 m) layer and compensating off-shore flow at intermediate depths. After the storm, currents reversed in both layers, and the relaxation phase of downwelling lasted ~4 days. Although current velocities did not exceed 5 cm/s, the enhanced circulation during the storm caused cold turbid intrusions at 75–95 m depth that are likely attributed to sub-glacial water from the Flade Isblink Ice Cap. It was also found that the semidiurnal periodicities in the temperature and salinity time series were associated with the lunar semidiurnal tidal flow. The vertical structure of tidal currents corresponded to the first baroclinic mode of the internal tide with a velocity minimum at ~40 m. The tidal ellipses rotate in opposite directions above and below this depth and cause a divergence of tidal flow which was observed to induce semidiurnal internal waves of about 3 m height at the front of the glacier terminus. Our findings provide evidence that shelf-basin interaction and tidal forcing can potentialy modify coastal Wandel Sea waters even though they are isolated from the atmosphere by landfast sea ice almost year round. The northerly storms over the continental slope cause an enhanced circulation facilitating a release of cold and turbid sub-glacial water to the shelf. The tidal flow may contribute to the removal of such water from the glacial terminus.

scholarly journals Restratification of the Surface Mixed Layer with Submesoscale Lateral Density Gradients: Diagnosing the Importance of the Horizontal Dimension

2008 ◽  
Vol 38 (11) ◽  
pp. 2438-2460 ◽  
Author(s):  
P. J. Hosegood ◽  
M. C. Gregg ◽  
M. H. Alford
Keyword(s):  
Density Gradient ◽  
Mixed Layer ◽  
Large Scale ◽  
Acoustic Doppler Current Profiler ◽  
Small Scale ◽  
Density Gradients ◽  
Surface Mixed Layer ◽  
Vertical Scale ◽  
Current Profiler ◽  
Dimensional Measurements

Abstract A depth-cycling towed conductivity–temperature–depth (CTD) and vessel-mounted acoustic Doppler current profiler (ADCP) were used to obtain four-dimensional measurements of the restratification of the surface mixed layer (SML) at a submesoscale lateral density gradient near the subtropical front. With the objective of studying the role of horizontal processes in restratification, the thermohaline and velocity fields were monitored for 33 h by 16 small-scale (≤15 km2) surveys centered on a drogued float. Daytime warming by insolation caused a unidirectional displacement of the initially vertical isopycnals toward increasing density. Across the entire SML (50-m vertical scale), solar insolation accounted for 60% of observed restratification, but over 10-m scales, the percentage decreased with depth from 80% at 25–35 m to ≤25% at 55–65 m. Below 35 m, stratification was enhanced by the vertically sheared horizontal advection of the lateral density gradient due to a near-inertial wave of ∼100-m vertical wavelength that rotated anticyclonically at the inertial frequency. The phase and similar period (25.4 h) of the local inertial period to the diurnal cycle ensured constructive interference with isopycnal displacements due to insolation. Restratification by sheared advection matched that predicted due to vertically sheared inertial oscillations generated during the geostrophic adjustment of a density front, but direct wind forcing may also have generated the wave that was subsequently modified by interaction with mesoscale vorticity associated with a nearby large-scale front. By further including the effects of lateral uncompensated thermohaline inhomogeneity, the authors can account for 100% ± 20% of the observed N 2 during daytime restratification. No detectable restratification due to the slumping of horizontal density gradients under gravity alone was found.

scholarly journals Direct Observations of Along-Isopycnal Upwelling and Diapycnal Velocity at a Shelfbreak Front*

2004 ◽  
Vol 34 (3) ◽  
pp. 543-565 ◽  
Author(s):  
John A. Barth ◽  
Dave Hebert ◽  
Andrew C. Dale ◽  
David S. Ullman
Keyword(s):  
Three Dimensional ◽  
Internal Tide ◽  
Acoustic Doppler Current Profiler ◽  
Density Field ◽  
Strong Mixing ◽  
Optical Fields ◽  
Direct Observations ◽  
Current Profiler ◽  
Southern Flank ◽  
Coastal Front

Abstract By mapping the three-dimensional density field while simultaneously tracking a subsurface, isopycnal float, direct observations of upwelling along a shelfbreak front were made on the southern flank of Georges Bank. The thermohaline and bio-optical fields were mapped using a towed undulating vehicle, and horizontal velocity was measured with a shipboard acoustic Doppler current profiler. A subsurface isopycnal float capable of measuring diapycnal flow past the float was acoustically tracked from the ship. The float was released near the foot of the shelfbreak front (95–100-m isobath) and moved 15 km seaward as it rose from 80 to 50 m along the sloping frontal isopycnals over a 2-day deployment. The float's average westward velocity was 0.09 m s−1, while a drifter drogued at 15 m released at the same location moved westward essentially alongfront at 0.18 m s−1. The float measured strong downward vertical velocities (in excess of 0.02 m s−1) associated with propagation of internal tidal solibores in the onbank direction from their formation near the shelf break. The float measured large upward vertical velocities (in excess of 0.001 m s−1 ≃ 100 m day−1) as the pycnocline rebounded adiabatically after the passage of the internal tide solibore. The directly measured mean along-isopycnal vertical velocity was 17.5 m day−1. Intense mixing events lasting up to 2 hours were observed in the shelfbreak front at the boundary between cold, fresh shelf water and warm, salty slope water. Diapycnal velocities of up to 3 × 10−3 m s−1 were measured, implying a diapycnal thermal diffusivity as large as 10−2 m2 s−1, indicative of strong mixing events in this coastal front.

scholarly journals Interoperability of SeaSondes and Wellen Radars in Mapping Radial Surface Currents

2013 ◽  
Vol 30 (11) ◽  
pp. 2662-2675 ◽  
Author(s):  
J. Martinez-Pedraja ◽  
L. K. Shay ◽  
B. K. Haus ◽  
C. Whelan
Keyword(s):  
Acoustic Doppler Current Profiler ◽  
Hf Radar ◽  
Small Scale ◽  
Surface Currents ◽  
Florida Current ◽  
Current Profiler ◽  
Florida Straits ◽  
Subsurface Current ◽  
Geometric Dilution Of Precision ◽  
Good Agreement

Abstract A dual-station high-frequency (HF) Wellen Radar (WERA) transmitting at 16 MHz has observed near-real-time surface currents over an approximate range of 100 km across the Florida Straits since July 2004. During a 10-day period in April 2005 (15–25 April), a pair of 12.6-MHz SeaSondes (SS) were deployed south of the WERAs sites by NOAA's Center for Operational Oceanographic Products and Services (CO-OPS). The resulting SS grid overlapped the southern portion of the WERA domain. During the same period of time, a bottom-mounted acoustic Doppler current profiler (ADCP) acquired subsurface current measurements within these HF radar grids starting at 14 m below the surface in water of 86-m depth. The interoperability of beam-forming (WERA) and direction-finding (SS) HF radar technologies was examined. Comparisons of radial and vector currents for an 8-day concurrent time series suggested good agreement in current direction over both domains, where the surface currents' magnitudes were a maximum of 1.2 m s−1. In the core of the radar domains consisting of 108 cells, hourly vector currents were obtained by combining WERA and SS radials. Generally, this can be done in a relatively straightforward manner, considering the geometric dilution of precision (GDOP). A second key issue is downscaling the SS measurements from a 3-km grid to a 1.1-km grid to match the WERA output. This enhanced grid spacing is important along the western flank of the Florida Current, where energetic, small-scale surface features have been observed.

scholarly journals Estimates of the turbulence intensity and power density of an asymmetrical tidal flow under variability of wind forcing

2019 ◽  
Vol 55 (2) ◽  
pp. 61-72
Author(s):  
K. A. Korotenko ◽  
A. V. Sentchev
Keyword(s):  
Power Density ◽  
Turbulence Intensity ◽  
Tidal Flow ◽  
Acoustic Doppler Current Profiler ◽  
Wind Forcing ◽  
Small Scale ◽  
Reynolds Stresses ◽  
Direct Measurements ◽  
Current Profiler ◽  
Scale Turbulence

A high-frequency (1.2 MHz) four-beam Acoustic Doppler Current Profiler (ADCP) moored on the seabed has been used for direct measurements of turbulence in a shallow coastal zone of the eastern English Channel. From the measurements conducted, 5 tidal cycles covering calm and storm periods were selected. Impacts of the tidal cycle asymmetry and the variability of wind forcing on the turbulence intensity, Reynolds stresses, and the power density of the flow are assessed quantitatively. A comparison of the energy characteristics of the tidal flow during calm and storm periods revealed that the power density of the stream during the storm was about half of that during the calm period. Wave bias correction of Reynolds stresses allows estimating a contribution of small-scale turbulence to its total intensity.

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