Louis Gostiaux

Laboratory experiments on the generation of internal tidal beams over steep slopes

We designed a simple laboratory experiment to study internal tides generation. We consider a steep continental shelf, for which the internal tide is shown to be emitted from the critical point, which is clearly amphidromic. We also discuss the dependence of the width of the emitted beam on the local curvature of topography and on viscosity. Finally, we derive the form of the resulting internal tidal beam by drawing an analogy with an oscillating cylinder in a static fluid.

A novel internal waves generator

We present a new kind of generator of internal waves which has been designed for three purposes. First, the oscillating boundary conditions force the fluid particles to travel in the preferred direction of the wave ray, hence reducing the mixing due to forcing. Second, only one ray tube is produced so that all of the energy is in the beam of interest. Third, temporal and spatial frequency studies emphasize the high quality for temporal and spatial monochromaticity of the emitted beam. The greatest strength of this technique is therefore the ability to produce a large monochromatic and unidirectional beam.

NIOZ3: Independent Temperature Sensors Sampling Yearlong Data at a Rate of 1 Hz

Some 110 independent sensors form the NIOZ3-thermistor ldquostringrdquo to study waves in the ocean interior sampling at a rate of 1 Hz during at least one year. The string operates without connecting cables between the newly designed sensors, which are programmed and synchronized via induction. The accuracy of previous custom-made high-sampling rate thermistor strings is maintained, being better than 1 mK. This is demonstrated here using data from three recent field trials, two above seamounts and one in the ocean interior that occasionally show vigorous (nonlinear) internal wave motions.

Extracting Meaningful Information from Uncalibrated Backscattered Echo Intensity Data

The authors present an original method for the analysis of acoustic Doppler current profiler (ADCP) echo intensity profiles measured in the ocean, especially when no calibration has been performed. This study is based on data from Teledyne RD Instrument acoustic profilers but provides a methodology that can be extended to other kinds of hardware. To correctly interpret data for which the signal-to-noise ratio is below a factor of 10, the authors propose isolating the backscattered signal from noise in arithmetic space before resolving the sonar equation and compensating for transmission loss in logarithmic space. The robustness of the method is shown for several independent datasets from the Atlantic Ocean, the North Sea, and the Mediterranean Sea. Estimation of sediment concentration, planktonic migrations, or air bubbles is now possible at less than 10 dB above noise level, which can concern half of the ADCP’s range under common circumstances.

Quantitative laboratory observations of internal wave reflection on ascending slopes

Internal waves propagate obliquely through a stratified fluid with an angle that is fixed with respect to gravity. Upon reflection on a sloping bed, striking phenomena are expected to occur close to the slope. We present here laboratory observations at moderately large Reynolds number. A particle image velocimetry technique is used to provide time-resolved velocity fields in large volumes. Generation of the second and third harmonic frequencies is clearly demonstrated in the impact zone. The mechanism for nonlinear wavelength selection is also discussed. Evanescent waves with frequency larger than the Brunt-Vaisala frequency are detected and experimental results agree very well with theoretical predictions. The amplitude of the different harmonics after reflection is also obtained.

New wave generation

We present the results of a combined experimental and numerical study of the generation of internal waves using the novel internal wave generator design of Gostiaux et al. (2007). This mechanism, which involves a tunable source comprised of oscillating plates, has so far been used for a few fundamental studies of internal waves, but its full potential has yet to be realized. Our studies reveal that this approach is capable of producing a wide variety of two-dimensional wave fields, including plane waves, wave beams and discrete vertical modes in finite-depth stratifications. The effects of discretization by a finite number of plates, forcing amplitude and angle of propagation are investigated, and it is found that the method is remarkably efficient at generating a complete wave field despite forcing only one velocity component in a controllable manner. We furthermore find that the nature of the radiated wave field is well predicted using Fourier transforms of the spatial structure of the wave generator.

Where large deep-ocean waves break

Underwater topography like seamounts causes the breaking of large “internal waves” with associated turbulent mixing strongly affecting the redistribution of sediment. Here ocean turbulence is characterized and quantified in the lowest 100 m of the water column at three nearby sites above the slope of a deep-ocean seamount. Moored high-resolution temperature sensors show very different turbulence generation mechanisms over 3 and 5 km horizontal separation distances. At the steepest slope, turbulence was 100 times more energetic than at the shallowest slope where turbulence was still 10 times more energetic than found in the open ocean, away from topography. The turbulence on this extensive slope is caused by slope steepness and nonlinear wave evolution, but not by bottom-friction, “critical” internal tide reflection or lee wave generation.

High‐resolution open‐ocean temperature spectra

[1] Accurate (<1 mK) temperature sensors have been stiffly moored at � 1450 m in the open Canary Basin for 1.5 years while sampling at 1 Hz. The sensors were in an area where regular density steps occur. In this article, we investigate the variability of internal waves in such ‘‘steppy’’ environment. The waves vertically move layers of persistent temperature gradients for particular temperatures. The frequency (s) spectra of temperature, and more clearly those of inferred vertical currents w, show an internal wave band IWB that extends from 0.97f < s < Nt, where f denotes the inertial frequency or vertical Coriolis parameter. Transition frequency Nt is higher than buoyancy frequency N, computed over large vertical scales Dz = O(100) m. The extension of IWB beyond the traditional bounds [f,N] is probably due to small-vertical–scale layering (Dz � 1 m). The associated weak stratification between such thin layers provides small-scale Ns � 4fh, where fh denotes the horizontal Coriolis parameter. This minimum stable stratification Ns is associated with tilting of vorticity away from gravity, which causes the subinertial spectral extent to 0.97f. Isothermal smoothing reveals details of coherent vertical internal wave and incoherent motions. The present w spectrum continuum of coherent IWB motions is not flat, as in previous near-surface observations, but linearly increases from s = f to a peak at s � 0.8N. The coherence spectrum shows a weak, significant peak at approximately twice the local buoyancy frequency for 2.5 � Dz � 100 m. Instead of dominant mode 1, zero phase difference, observed in IWB across the 130 m of observations, these super-buoyancy motions show mode 2 dominance, p phase difference.

Large internal waves advection in very weakly stratified deep Mediterranean waters

The Eastern Mediterranean Sea contains relatively small trenches O(1-10 km) horizontal width that go deeper than 4000 m. At a first glance, these deep waters are homogeneous, with weak currents <0.1 m s(-1). This viewpoint is modified after evaluation of new detailed yearlong temperature observations using 103 high-precision sensors that reveal intense variability of internal waves. Even though temperature variations are within the range of a few mK only, requiring precise correction for the adiabatic lapse rate during post-processing, the images are permanently dynamic. The weak density stratification of buoyancy close to the inertial frequency supports large turbulent overturns indirectly governed or advected by large internal waves. In strongly stratified (near-surface) waters low-frequency inertial internal motions are horizontal, but here they attain a vertical current amplitude sometimes comparable to horizontal currents. This results in occasionally very large internal wave amplitudes (250 m peak-trough), which are generated via geostrophic adjustment presumably from local collapse of fronts. Citation: van Haren, H., and L. Gostiaux (2011), Large internal waves advection in very weakly stratified deep Mediterranean waters, Geophys. Res. Lett., 38, L22603, doi:10.1029/2011GL049707.

Internal Wave Turbulence Near a Texel Beach

A summer bather entering a calm sea from the beach may sense alternating warm and cold water. This can be felt when moving forward into the sea (‘vertically homogeneous’ and ‘horizontally different’), but also when standing still between one’s feet and body (‘vertically different’). On a calm summer-day, an array of high-precision sensors has measured fast temperature-changes up to 1°C near a Texel-island (NL) beach. The measurements show that sensed variations are in fact internal waves, fronts and turbulence, supported in part by vertical stable stratification in density (temperature). Such motions are common in the deep ocean, but generally not in shallow seas where turbulent mixing is expected strong enough to homogenize. The internal beach-waves have amplitudes ten-times larger than those of the small surface wind waves. Quantifying their turbulent mixing gives diffusivity estimates of 10−4–10−3 m2 s−1, which are larger than found in open-ocean but smaller than wave breaking above deep sloping topography.