Observations of turbulence in a tidal beam and across a coastal ridge (original) (raw)
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Tidal mixing processes amid small-scale, deep-ocean topography
Geophysical Research Letters, 2015
The nature of tide-topography interaction reflects the topographic scales experienced by water parcels during their tidal excursions. In the deep ocean these scales are typically subkilometer, yet direct observations of tidal processes on such scales are lacking. At one site, a saddle amid steep and complex Mid-Atlantic Ridge topography, observations reveal tidally pulsed, bottom-trapped fronts, overflows, and lee waves in response to a tide combined with a mean flow of similar amplitude. The tidal pulsing of the fronts and overflows was only evident locally, and their phase became unpredictable over scales of hundreds of meters. Enhanced turbulence in a 100-200 m thick bottom boundary layer had an estimated dissipation rate of 2.6 × 10 À2 W m À2 , exceeding the large-scale average of tidal dissipation in mid-ocean ridge environments but by less than an order of magnitude. This site was not a dissipation "hotspot," and the processes observed could provide widely distributed mixing to the meridional overturning circulation.
From Tides to Mixing Along the
The cascade from tides to turbulence has been hypothesized to serve as a major energy pathway for ocean mixing. We investigated this cascade along the Hawaiian Ridge using observations and numerical models. A divergence of internal tidal energy flux observed at the ridge agrees with the predictions of internal tide models. Large internal tidal waves with peak-to-peak amplitudes of up to 300 meters occur on the ridge. Internal-wave energy is enhanced, and turbulent dissipation in the region near the ridge is 10 times larger than open-ocean values. Given these major elements in the tides-to-turbulence cascade, an energy budget approaches closure.
Internal waves, solitary-like waves, and mixing on the Monterey Bay shelf
Continental Shelf Research, 2005
Microstructure measurements taken on the Monterey Bay continental shelf, within 4 km of the shelf break, reveal a complex mixing environment. Depth-and time-averaged dissipation rates ð ¼ 7:4255:8 Â 10 À9 W kg À1 Þ and diapycnal diffusivities ðK r ¼ 6:1237:8 Â 10 À5 m 2 s À1 Þ were elevated above observations made over other continental shelves with no significant topography, but were below those influenced by topographic features. The close proximity of the shelf break/canyon rim, locally generated internal tides, and nonlinear internal waves all contributed to the elevated turbulence. The complex bathymetry associated with Monterey Submarine Canyon allowed an internal tide to be generated at depths greater than 1500 m, as well as at the shelf break. The observed velocity field was normally dominated by upward energy propagation from the local shelf break generated internal tide, but near low tide downward energy propagation from a surface reflection of the internal tide generated below 1500 m was observed. Turbulent dissipation rates were not well parameterized by either the open-ocean Gregg-Henyey model or the recently developed MacKinnon-Gregg shelf model. Like its application on the New England shelf, the MacKinnon-Gregg model had the correct functional dependence on shear and stratification (dissipation increasing with increasing shear and increasing stratification), however, the magnitude and range of values were too small. The most surprising finding was the presence of what we believe to be large, high-aspect-ratio, downslope-propagating nonlinear internal solitarylike waves of elevation. Upon reaching the canyon rim, these waves propagated into deep water and transformed into waves of depression. On the shelf south of the canyon, the waves of elevation accounted for 20% of the observed turbulent kinetic energy dissipation. Off the shelf, where the solitary-like waves changed to downward displacement, 0278-4343/$ -see front matter r (R.-C. Lien). their average dissipation increased 10-fold to ¼ 2:6 Â 10 À6 W kg À1 , and accounted for nearly half the dissipation in the upper 150 m. r
Along-slope generation as an explanation for some unusually large internal tides
Deep Sea Research Part I: Oceanographic Research Papers, 2002
Why are the internal waves observed on the Portuguese shelf at 411N (which have thermocline displacements of up to 45 m) many times larger than expected from 2D shelf edge internal tide generation theory? Barotropic tidal forcing is too small to create them, either at the local shelf edge or from the nearby Oporto seamount. Using wave refraction techniques it is demonstrated that they must be created by the interaction between tidal currents and a major westward projection of the shelf edge about 50 km to the south. The off-shelf propagating internal tidal energy thus generated is subsequently refracted back onto the shelf in the form of non-linear internal wave packets. Refraction explains not only how the energy reaches the shelf, but also the orientation of the waves relative to the shelf edge and details of their appearance in a synthetic aperture radar image. The result demonstrates that shelf edge internal tide generation can be more complex than is suggested by the 2D approach, and that global shelf edge internal tide energy must be larger than previously thought. r
Continental Shelf Research, 2001
A simple parameterization of eddy diffusivity is used to simulate the shear mixing from tidally induced internal waves generated in the continental slope region southwest of Brittany. Near the edge of the shelf, the seasonal thermocline oscillates under the forcing of the barotropic tide which propagates over the shelf break. A composite model is constructed to simulate the mixing of the upper ocean from both external (wind stress) and internal (internal waves) sources. A simple one-dimensional eddy kinetic energy model, which predicts the temperature profile from heat flux and wind stress inputs, is validated with respect to regional hydroelimatic conditions, then coupled with a two-layer model of non-linear internal waves, to simulate the mixing encountered in shelf break fronts submitted to tidal forcing. Numerical runs on a transect perpendicular to the shelf break show the formation of a spot of cool water over the edge of the continental margin. The one-dimensional eddy kinetic energy model has been validated over a decade with temperature profiles over the abyssal plain adjacent to the continental slope. An annual validation experiment has also been conducted for the combined models, beginning on 1st January 1985, as well as a short-term validation experiment, using a set of highfrequency temperature measurements at two stations near the shelf edge in September and early October 1985. The simulation has also been spatially validated against three sets of infrared satellite images. The one-dimensional model is calibrated for the minimum turbulent kinetic energy, whereas the best fit to the high-frequency measurements in autumn 1985 above the slope provides the optimum values for the initial thermal content and for the parameterization constant of internal wave diffusivity. The combined model reproduces successfully the seasonal and the high-frequency (neapspring tidal cycle) variation of the temperature field in the upper ocean along a transect perpendicular to the shelf break. Horizontal advection and mesoscale turbulence somewhat limit the performance of the model at low-amplitude tides and over the shallower part of the shelf, but the satisfactory overall agreement between the results and the measurements is consistent with the formation of a shelf break front in the northern Bay of Biscay and southern Celtic Sea, mainly as a result of mixing enhanced by tidally induced internal waves.
Internal Tide Reflection and Turbulent Mixing on the Continental Slope
Journal of Physical Oceanography, 2004
Observations of turbulence, internal waves, and subinertial flow were made over a steep, corrugated continental slope off Virginia during May-June 1998. At semidiurnal frequencies, a convergence of low-mode, onshore energy flux is approximately balanced by a divergence of high-wavenumber offshore energy flux. This conversion occurs in a region where the continental slope is nearly critical with respect to the semidiurnal tide. It is suggested that elevated near-bottom mixing (K ϳ 10 Ϫ3 m 2 s Ϫ1 ) observed offshore of the supercritical continental slope arises from the reflection of a remotely generated, low-mode, M 2 internal tide. Based on the observed turbulent kinetic energy dissipation rate ⑀, the high-wavenumber internal tide decays on time scales O(1 day). No evidence for internal lee wave generation by flow over the slope's corrugations or internal tide generation at the shelf break was found at this site.
Toward global maps of internal tide energy sinks
Ocean Modelling, 2019
Internal tides power much of the observed small-scale turbulence in the ocean interior. To represent mixing induced by this turbulence in ocean climate models, the cascade of internal tide energy to dissipation scales must be understood and mapped. Here, we present a framework for estimating the geography of internal tide energy sinks. The mapping relies on the following ingredients: (i) a global observational climatology of stratification; (ii) maps of the generation of M 2 , S 2 and K 1 internal tides decomposed into vertical normal modes; (iii) simplified representations of the dissipation of low-mode internal tides due to wave-wave interactions, scattering by smallscale topography, interaction with critical slopes and shoaling; (iv) Lagrangian tracking of low-mode energy beams through observed stratification, including refraction and reflection. We thus obtain a global map of the column-integrated energy dissipation for each of the four considered dissipative processes, each of the three tidal constituents and each of the first five modes. Modes ≥6 are inferred to dissipate within the local water column at the employed half-degree horizontal resolution. Combining all processes, modes and constituents, we construct a map of the total internal tide energy dissipation, which compares well with observational inferences of internal wave energy dissipation. This result suggests that tides largely shape observed spatial contrasts of dissipation, and that the framework has potential in improving understanding and modelling of ocean mixing. However, sensitivity to poorly constrained parameters and simplifying assumptions entering the parameterized energy sinks calls for additional investigation. The attenuation of low-mode internal tides by wave-wave interactions needs particular attention.
Eddy-Modulated Internal Waves and Mixing on a Midocean Ridge
Journal of Physical Oceanography, 2012
Mesoscale eddies are ubiquitous in the World Ocean and dominate the energy content on subinertial time scales. Recent theoretical and numerical studies suggest a connection between mesoscale eddies and diapycnal mixing in the deep ocean, especially near rough topography in regions of strong geostrophic flow. However, unambiguous observational evidence for such a connection has not yet been found, and it is still unclear what physical processes are responsible for transferring energy from mesoscale to small-scale processes. Here, the authors present observations demonstrating that finescale variability near the crest of the East Pacific Rise is strongly modulated by low-frequency geostrophic flows, including those due to mesoscale eddies. During times of strong subinertial flows, the authors observed elevated kinetic energy on vertical scales ,50 m and in the near-inertial band, predominantly upward-propagating near-inertial waves, and increased incidence of layers with Richardson number (Ri) , 1 /4. In contrast, during times of weak subinertial flows, kinetic energy in the finescale and near-inertial bands is lower, Ri values are higher, and near-inertial waves propagate predominantly downward through the water column. Diapycnal diffusivities estimated indirectly from a simple Ri-based parameterization are consistent with results from a tracer-release experiment and a microstructure survey bracketing the mooring measurements. These observations are consistent with energy transfer (a ''cascade'') from subinertial flows, including mesoscale eddies, to nearinertial oscillations, turbulence, and mixing. This interpretation suggests that, in addition to topographic roughness and tidal forcing, parameterization of deep-ocean mixing should also take subinertial flows into account. The findings presented here are expected to be useful for validating and improving numerical-model parameterizations of turbulence and mixing in the ocean.
Numerical and experimental modelling of the internal tide near a continental shelf
Les processus de mélange sont essentiels au fond de l'océan car ils permettent la remontée des eaux froides abyssales vers la surface. Une grande question de la communauté océanographique concerne la contribution des ondes internes à ces processus car ces ondes, bien que peu énergétiques en regard des courants marins par exemple, sont présentes partout dans l'océan et y déferlent. Les principales sources d'énergie des ondes internes sont le vent et l'interaction de la marée avec la topographie sous-marine. C'est cette dernière configuration que nous considérons ici, au travers d'expériences de laboratoire et numériques, dans le contexte académique d'un talus continental bidimensionnel dans un océan uniformément stratifié. Nous examinons plus particulièrement le processus de génération du champ d'ondes internes et la structure cinématique de ce champ. Nous discutons également de la manifestation des effets non linéaires lorsque le champ d'ondes se ...
Observations of Breaking Internal Tides on the Australian North West Shelf Edge
Frontiers in Marine Science, 2021
A comprehensive observational data set was used to examine shoreward propagating semidiurnal internal tides as they shoal, break and run-up as turbulent boluses across the edge of the Australian North West Shelf (NWS), offshore Dampier, during late winter 2013. The measured waveforms and wavefields supported the grouping of events into two distinct categories: (1) pre-; and, (2) post- wave breaking. It was found that the transition from (1) to (2) was marked by the rise of nonlinear steepening (α) and reduction in dispersion (β), both coefficients that parameterize nonlinear wave effects on the Korteweg-de Vries (KdV) equation. We introduced a criterion for wave breaking from the dimensionless parameter (δ) that relates these two terms: wave breaking occurs when δ < 1. In the first group, dispersive effects were dominant to spread energy out of the semidiurnal wave to a dispersive wave packet of short-period internal solitary waves (ISWs). In the second, dispersion was considered...