Energy Release Through Internal Wave Breaking

The sun inputs huge amounts of heat to the ocean, heat that would stay near the ocean's surface if it were not mechanically mixed into the deep. Warm water is less dense than cold water, so that heated surface waters "float" on top of the cold deep waters. Only active mechanical turbulent mixing can pump the heat downward. Such mixing requires remarkably little energy, about one-thousandth of the heat stored, but it is crucial for ocean life and for nutrient and sediment transport. Several mechanisms for ocean mixing have been studied in the past. The dominant mixing mechanism seems to be breaking of internal waves above underwater topography. Here, we quantify the details of how internal waves transition to strong turbulent mixing by using high-sampling-rate temperature sensors. The sensors were moored above the sloping bottom of a large guyot (flat-topped submarine volcano) in the Canary Basin, North Atlantic Ocean. Over a tidal period, most mixing occurs in two periods of less than half an hour each. This "boundary mixing" dominates sediment resuspension and is 100 times more turbulent than open ocean mixing. Extrapolating, the mixing may be sufficiently effective to maintain the ocean's density stratification

Fine-structure contamination by internal waves in the Canary Basin

Over a range of 132.5 m, 54 temperature sensors (1 mK relative accuracy) were moored yearlong while sampling at 1 Hz around 1455 m in the open Canary Basin. Coherence between individual records shows a weak but significant peak above the local buoyancy frequency N for all vertical separations Delta z < 100 m, including at sensor interval Delta z = 2.5 m. Instead of a dominant zero-phase difference over the range of sensors, as observed for internal waves at frequencies f < sigma < N, with f denoting the inertial frequency, this superbuoyant coherence shows pi-phase difference. The transition from zero-phase difference, for internal waves, to pi-phase difference is abrupt and increases in frequency for decreasing Delta z < 10 m. For Delta z > 10 m, the transition is fixed at N-t similar or equal to 1.6N, which is also the maximum value of the small-scale buoyancy frequency, and limits the internal wave band on its high-frequency side. In the time domain it is observed that this high-frequency coherence mainly occurs when nonlinearities in the temperature gradient, such as steps in the temperature profile, are advected past the sensors. A simple kinematic model of fine-structure contamination is proposed to reproduce this observation. The canonical -2 slope of the temperature spectrum above N is not observed in the in situ data, which rather slope as -8/3. The -8/3 slope can, however, be reproduced in our model, provided the jumps in the temperature profile are not infinitely thin

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

Soliton generation by internal tidal beams impinging on a pycnocline: laboratory experiments

In this paper, we present the first laboratory experiments that show the generation of internal solitary waves by the impingement of a quasi-two-dimensional internal wave beam on a pycnocline. These experiments were inspired by observations of internal solitary waves in the deep ocean from synthetic aperture radar (SAR) imagery, where this so-called mechanism of 'local generation' was argued to be at work, here in the form of internal tidal beams hitting the thermocline. Nonlinear processes involved here are found to be of two kinds. First, we observe the generation of a mean flow and higher harmonics at the location where the principal beam reflects from the surface and pycnocline; their characteristics are examined using particle image velocimetry (PIV) measurements. Second, we observe internal solitary waves that appear in the pycnocline, detected with ultrasonic probes; they are further characterized by a bulge in the frequency spectrum, distinct from the higher harmonics. Finally, the relevance of our results for understanding ocean observations is discussed

Deep-sea deployment of the KM3NeT neutrino telescope detection units by self-unrolling

KM3NeT is a research infrastructure being installed in the deep Mediterranean Sea. It will house a neutrino telescope comprising hundreds of networked moorings — detection units or strings — equipped with optical instrumentation to detect the Cherenkov radiation generated by charged particles from neutrino-induced collisions in its vicinity. In comparison to moorings typically used for oceanography, several key features of the KM3NeT string are different: the instrumentation is contained in transparent and thus unprotected glass spheres; two thin Dyneema® ropes are used as strength members; and a thin delicate backbone tube with fibre-optics and copper wires for data and power transmission, respectively, runs along the full length of the mooring. Also, compared to other neutrino telescopes such as ANTARES in the Mediterranean Sea and GVD in Lake Baikal, the KM3NeT strings are more slender to minimise the amount of material used for support of the optical sensors. Moreover, the rate of deploying a large number of strings in a period of a few years is unprecedented. For all these reasons, for the installation of the KM3NeT strings, a custom-made, fast deployment method was designed. Despite the length of several hundreds of metres, the slim design of the string allows it to be compacted into a small, re-usable spherical launching vehicle instead of deploying the mooring weight down from a surface vessel. After being lowered to the seafloor, the string unfurls to its full length with the buoyant launching vehicle rolling along the two ropes. The design of the vehicle, the loading with a string, and its underwater self-unrolling are detailed in this paper.French National Research Agency (ANR) ANR-15-CE31-0020Centre National de la Recherche Scientifique (CNRS)European Union (EU)Institut Universitaire de France (IUF)LabEx UnivEarthS ANR-10-LABX-0023 ANR-18-IDEX-0001Paris Ile-de-France Region, FranceShota Rustaveli National Science Foundation of Georgia (SRNSFG), Georgia FR-18-1268German Research Foundation (DFG)Greek Ministry of Development-GSRTIstituto Nazionale di Fisica Nucleare (INFN), Ministero dell'Universita e della Ricerca (MUR), PRIN Italy NAT-NET 2017W4HA7SMinistry of Higher Education, Scientific Research and Professional Training, MoroccoNetherlands Organization for Scientific Research (NWO) Netherlands GovernmentNational Science Center, Poland National Science Centre, Poland 2015/18/E/ST2/00758National Authority for Scientific Research (ANCS), RomaniaMinisterio de Ciencia, Innovación, Investigación y Universidades (MCIU): Programa Estatal de Generación de Conocimiento (MCIU/FEDER) PGC2018-096663-B-C41 PGC2018-096663-B-A-C42 PGC2018-096663-B-BC43 PGC2018-096663-B-B-C44Severo Ochoa Centre of Excellence and MultiDark Consolider (MCIU), Junta de Andalucía SOMM17/6104/UGRGeneralitat Valenciana GRISOLIA/2018/119 CIDEGENT/2018/034La Caixa Foundation LCF/BQ/IN17/11620019EU: MSC program, Spain 71367

Characterization of turbulent overturns in stratified turbulence

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Detailed internal wave mixing above a deep-ocean slope

Turbulent vertical eddy diffusivity (K-z) and dissipation rate (epsilon) are estimated between 0.5 and 50 m above the sloping side of Great Meteor Seamount, Canary Basin, using 101 moored temperature sensors, 1-mK precision, sampling at 1 Hz. Effectively, detailed observed time-depth temperature images are split in two: a statically stable and a turbulence image. Tides dominate the temperature variations, but the local bottom slope is supercritical to motions at semidiurnal frequencies. Averaged over a fortnight, the observed overall time-depth mean K-z = 3 +/- 1 x 10(-3) m(2) s(-1) and epsilon = 1.5 +/- 0.7 x 10(-7) W kg(-1). Variations with time and depth are large, by up to four orders of magnitude. Although variations do occur having tidal periodicity, shorter-scale variations are more intense. A particular tidal period shows multiple vigorous overturning events, the largest found away from the bottom during the downslope phase but just prior to arrival of an upslope moving, equally vigorous bore. The strength of the bore may be controlled by the intensity of the mixing just prior to it. The bore itself is turbulent from the bottom upward, up to some 40 m above it. Its mixing is most efficient providing large fluxes in extremely thin layers. Parameterizations of turbulence estimates are inconclusive using powers of N, as they show different relationships for different depths, time-ranges and averaging

Distinguishing turbulent overturns in high-sampling-rate moored thermistor string observations

Turbulent overturns are distinguished from salinity-compensated intrusions in high-resolution moored thermistor string observations. The buoyancy frequency N is used to make the time dimensionless, "t*." This results in a primary, visual means to easily compare the duration of overturns with N, the natural frequency that separates internal waves from turbulent overturns. As a secondary means, the shapes of overturns are investigated. Above various sloping topography between 500 and 1,000 m water depth where the buoyancy period varies between ~1,300 and 2,600 s, vertical overturns of ~40 m last ?t* =0.2–0.4. This corresponds with the timescale of growth of model-stratified turbulence in the wake of a grid. Smaller-scale, weaker-turbulent, shear-induced Kelvin-Helmholtz overturns of ~5 m are observed to last approximately ?t* = 0.03, whereas the passage of their train of multiple consecutive overturns lasts up to approximately ?t* = 0.95. Although the shape of overturns can distinguish salinity-compensated intrusions from turbulent overturns, the present observations from internal wave breaking above sloping topography show complex results of mixed feature