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Matthew H Alford - One of the best experts on this subject based on the ideXlab platform.
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the formation and fate of Internal Waves in the south china sea
Nature, 2015Co-Authors: Jonathan D Nash, Matthew H Alford, Thomas Peacock, Jennifer A Mackinnon, Maarten C Buijsman, Luca Centurioni, Shennyu Chao, Minghuei Chang, David M FarmerAbstract:Internal oceanic Waves are subsurface gravity Waves that can be enormous and travel thousands of kilometres before breaking but they are difficult to study; here observations of such Waves in the South China Sea reveal their formation mechanism, extreme turbulence, relationship to the Kuroshio Current and energy budget. Internal Waves are the underwater version of more familiar surface Waves. They can be enormous and travel thousands of kilometres before breaking. The South China Sea is known to be home to the largest Internal Waves in the world's oceans, but their size, generation mechanisms and role in the regional energy budget are unknown. Matthew Alford and colleagues now present the results from the IWISE observational campaign and reveal that Internal Waves more than 200 metres high break in the South China Sea and create turbulence that is orders of magnitude larger than in the open ocean, and that wave formation is influenced by the Kuroshio current. These results now allow for a complete energy budget of the South China Sea, and for a more accurate incorporation of Internal Waves into climate models. Internal gravity Waves, the subsurface analogue of the familiar surface gravity Waves that break on beaches, are ubiquitous in the ocean. Because of their strong vertical and horizontal currents, and the turbulent mixing caused by their breaking, they affect a panoply of ocean processes, such as the supply of nutrients for photosynthesis1, sediment and pollutant transport2 and acoustic transmission3; they also pose hazards for man-made structures in the ocean4. Generated primarily by the wind and the tides, Internal Waves can travel thousands of kilometres from their sources before breaking5, making it challenging to observe them and to include them in numerical climate models, which are sensitive to their effects6,7. For over a decade, studies8,9,10,11 have targeted the South China Sea, where the oceans’ most powerful known Internal Waves are generated in the Luzon Strait and steepen dramatically as they propagate west. Confusion has persisted regarding their mechanism of generation, variability and energy budget, however, owing to the lack of in situ data from the Luzon Strait, where extreme flow conditions make measurements difficult. Here we use new observations and numerical models to (1) show that the Waves begin as sinusoidal disturbances rather than arising from sharp hydraulic phenomena, (2) reveal the existence of >200-metre-high breaking Internal Waves in the region of generation that give rise to turbulence levels >10,000 times that in the open ocean, (3) determine that the Kuroshio western boundary current noticeably refracts the Internal wave field emanating from the Luzon Strait, and (4) demonstrate a factor-of-two agreement between modelled and observed energy fluxes, which allows us to produce an observationally supported energy budget of the region. Together, these findings give a cradle-to-grave picture of Internal Waves on a basin scale, which will support further improvements of their representation in numerical climate predictions.
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speed and evolution of nonlinear Internal Waves transiting the south china sea
Journal of Physical Oceanography, 2010Co-Authors: Matthew H Alford, Jody M Klymak, Renchieh Lien, Harper L Simmons, S R Ramp, Yiing Jang Yang, David Tang, Minghuei ChangAbstract:Abstract In the South China Sea (SCS), 14 nonlinear Internal Waves are detected as they transit a synchronous array of 10 moorings spanning the Waves’ generation site at Luzon Strait, through the deep basin, and onto the upper continental slope 560 km to the west. Their arrival time, speed, width, energy, amplitude, and number of trailing Waves are monitored. Waves occur twice daily in a particular pattern where larger, narrower “A” Waves alternate with wider, smaller “B” Waves. Waves begin as broad Internal tides close to Luzon Strait’s two ridges, steepening to O(3–10 km) wide in the deep basin and O(200–300 m) on the upper slope. Nearly all Waves eventually develop wave trains, with larger–steeper Waves developing them earlier and in greater numbers. The B Waves in the deep basin begin at a mean speed of ≈5% greater than the linear mode-1 phase speed for semidiurnal Internal Waves (computed using climatological and in situ stratification). The A Waves travel ≈5%–10% faster than B Waves until they reach...
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redistribution of energy available for ocean mixing by long range propagation of Internal Waves
Nature, 2003Co-Authors: Matthew H AlfordAbstract:Ocean mixing, which affects pollutant dispersal, marine productivity and global climate1, largely results from the breaking of Internal gravity Waves—disturbances propagating along the ocean's Internal stratification. A global map of Internal-wave dissipation would be useful in improving climate models, but would require knowledge of the sources of Internal gravity Waves and their propagation. Towards this goal, I present here computations of horizontal Internal-wave propagation from 60 historical moorings and relate them to the source terms of Internal Waves as computed previously2,3. Analysis of the two most energetic frequency ranges—near-inertial frequencies and semidiurnal tidal frequencies—reveals that the fluxes in both frequency bands are of the order of 1 kW m-1 (that is, 15–50% of the energy input) and are directed away from their respective source regions. However, the energy flux due to near-inertial Waves is stronger in winter, whereas the tidal fluxes are uniform throughout the year. Both varieties of Internal Waves can thus significantly affect the space-time distribution of energy available for global mixing.
Jorg Imberger - One of the best experts on this subject based on the ideXlab platform.
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energetics and damping of basin scale Internal Waves in a strongly stratified lake
Limnology and Oceanography, 2008Co-Authors: Kenji Shimizu, Jorg ImbergerAbstract:Energetics and damping of basin-scale Internal Waves and subsequent near-bottom transport processes in Lake Kinneret were investigated using the modal analysis in a layer-stratified irregular basin. The theory was extended to include small linear damping, and energy budgets and damping rates of five dominant Internal Waves were extracted by fitting numerically calculated Internal Waves to isotherm displacements measured by six thermistor chains distributed throughout the lake. Energy contained in the dominant Internal Waves (,3 GJ) resulted from a balance between energy input from diurnal winds and dissipation within a day, both of which were estimated to be 3, 4G J d 21. Damping was caused primarily by bottom friction, and the damping rates (e-folding time) varied from 1 to 3 d, depending on the velocity structure. Currents induced by the Internal Waves caused considerable spatial variability of the bottom shear stress and near-bottom transport processes, such as entrainment rate at the top of the benthic boundary layer and mass transfer at the sediment–water interface.
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physical properties of turbulent benthic boundary layers generated by Internal Waves
Journal of Hydraulic Engineering, 2004Co-Authors: Charles James Lemckert, Angelo Saggio, Jason P Antenucci, Jorg ImbergerAbstract:Physical properties of active turbulent benthic boundary layers (TBBL) generated by basin scale Internal Waves were studied within a Northern hemisphere thermally stratified lake. A microstructure profiler was used to measure the nature of the turbulence within the TBBLs while a series of thermistor chains were used to monitor the thermal structure of the lake. It was observed that a wind-driven anticlockwise diurnal-period vertical mode one Kelvin wave generated large scale motions within the water column, and that the interactions between this wave and the sloping lakebed induced TBBLs. A simple model, based on potential energy change and boundary shearing, was shown to describe the mean TBBL thickness.
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modeling basin scale Internal Waves in a stratified lake
Limnology and Oceanography, 2000Co-Authors: Ben R Hodges, Jorg Imberger, Angelo Saggio, Kraig B WintersAbstract:Basin-scale Internal Waves provide the driving forces for vertical and horizontal fluxes in a stratified lake below the wind-mixed layer. Thus, correct modeling of lake mixing and transport requires accurate modeling of basin- scale Internal Waves: examining this capability with a hydrostatic, z-coordinate three-dimensional (3D) numerical model at coarse grid resolutions is the focus of this paper. It is demonstrated that capturing the correct thermocline forcing with a 3D mixed-layer model for surface dynamics results in a good representation of low-frequency Internal wave dynamics. The 3D estuary and lake computer model ELCOM is applied to modeling Lake Kinneret, Israel, and is compared with field data under summer stratification conditions to identify and illustrate the spatial structure of the lowest-mode basin-scale Kelvin and PoincareWaves that provide the largest two peaks in the Internal wave energy spectra. The model solves the unsteady Reynolds-averaged Navier-Stokes equations using a semi-implicit method similar to the momentum solution in the TRIM code with the addition of quadratic Euler-Lagrange dis- cretization, scalar (e.g., temperature) transport using a conservative flux-limited approach, and elimination of vertical diffusion terms in the governing equations. A detailed description is provided of turbulence closure for the vertical Reynolds stress terms and vertical turbulent transport using a 3D mixed-layer model parameterized on wind and shear energy fluxes instead of the convential eddy viscosity/diffusivity assumption. This approach gives a good representation of the depth of the mixed-layer at coarse vertical grid resolutions that allows the Internal Waves to be energized correctly at the basin scale. Wind stresses, surface heating, and density currents form the driving energy fluxes of a stratified lake. The basin-scale energy flux from the wind is of particular interest because of its dominant role in setting the thermocline in motion, which, in the absence of inflows and outflows, is the primary energy store for transport and mixing below the wind-mixed layer. Thus, modeling the basin-scale Internal wave behavior is an a priori requirement to modeling and quantifying the flux paths of nutrients in a stratified lake (Imberger 1994). This paper takes a first step in this direction by analyzing our ability to model basin-scale Internal Waves that are seen in Lake Kinneret, Israel. Energy flux path in a stratified lake —Energy flux through a stratified lake has a fundamental dependence on forced and free baroclinic motions. The wind imparts both momentum and turbulent kinetic energy (TKE) to the water in the sur- face layer. The TKE distributes momentum vertically in the
David M Farmer - One of the best experts on this subject based on the ideXlab platform.
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the formation and fate of Internal Waves in the south china sea
Nature, 2015Co-Authors: Jonathan D Nash, Matthew H Alford, Thomas Peacock, Jennifer A Mackinnon, Maarten C Buijsman, Luca Centurioni, Shennyu Chao, Minghuei Chang, David M FarmerAbstract:Internal oceanic Waves are subsurface gravity Waves that can be enormous and travel thousands of kilometres before breaking but they are difficult to study; here observations of such Waves in the South China Sea reveal their formation mechanism, extreme turbulence, relationship to the Kuroshio Current and energy budget. Internal Waves are the underwater version of more familiar surface Waves. They can be enormous and travel thousands of kilometres before breaking. The South China Sea is known to be home to the largest Internal Waves in the world's oceans, but their size, generation mechanisms and role in the regional energy budget are unknown. Matthew Alford and colleagues now present the results from the IWISE observational campaign and reveal that Internal Waves more than 200 metres high break in the South China Sea and create turbulence that is orders of magnitude larger than in the open ocean, and that wave formation is influenced by the Kuroshio current. These results now allow for a complete energy budget of the South China Sea, and for a more accurate incorporation of Internal Waves into climate models. Internal gravity Waves, the subsurface analogue of the familiar surface gravity Waves that break on beaches, are ubiquitous in the ocean. Because of their strong vertical and horizontal currents, and the turbulent mixing caused by their breaking, they affect a panoply of ocean processes, such as the supply of nutrients for photosynthesis1, sediment and pollutant transport2 and acoustic transmission3; they also pose hazards for man-made structures in the ocean4. Generated primarily by the wind and the tides, Internal Waves can travel thousands of kilometres from their sources before breaking5, making it challenging to observe them and to include them in numerical climate models, which are sensitive to their effects6,7. For over a decade, studies8,9,10,11 have targeted the South China Sea, where the oceans’ most powerful known Internal Waves are generated in the Luzon Strait and steepen dramatically as they propagate west. Confusion has persisted regarding their mechanism of generation, variability and energy budget, however, owing to the lack of in situ data from the Luzon Strait, where extreme flow conditions make measurements difficult. Here we use new observations and numerical models to (1) show that the Waves begin as sinusoidal disturbances rather than arising from sharp hydraulic phenomena, (2) reveal the existence of >200-metre-high breaking Internal Waves in the region of generation that give rise to turbulence levels >10,000 times that in the open ocean, (3) determine that the Kuroshio western boundary current noticeably refracts the Internal wave field emanating from the Luzon Strait, and (4) demonstrate a factor-of-two agreement between modelled and observed energy fluxes, which allows us to produce an observationally supported energy budget of the region. Together, these findings give a cradle-to-grave picture of Internal Waves on a basin scale, which will support further improvements of their representation in numerical climate predictions.
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the generation and evolution of nonlinear Internal Waves in the deep basin of the south china sea
Journal of Physical Oceanography, 2011Co-Authors: Qiang Li, David M FarmerAbstract:AbstractTime series observations of nonlinear Internal Waves in the deep basin of the South China Sea are used to evaluate mechanisms for their generation and evolution. Internal tides are generated by tidal currents over ridges in Luzon Strait and steepen as they travel west, subsequently generating high-frequency nonlinear Waves. Although nonlinear Internal Waves appear repeatedly on the western slopes of the South China Sea, their appearance in the deep basin is intermittent and more closely related to the amplitude of the semidiurnal than the predominant diurnal tidal current in Luzon Strait. As the Internal tide propagates westward, it evolves under the influence of nonlinearity, rotation, and nonhydrostatic dispersion. The interaction between nonlinearity and rotation transforms the Internal tide into a parabolic or corner shape. A fully nonlinear two-layer Internal wave model explains the observed characteristics of Internal tide evolution in the deep basin for different representative forcing cond...
Jonathan D Nash - One of the best experts on this subject based on the ideXlab platform.
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the formation and fate of Internal Waves in the south china sea
Nature, 2015Co-Authors: Jonathan D Nash, Matthew H Alford, Thomas Peacock, Jennifer A Mackinnon, Maarten C Buijsman, Luca Centurioni, Shennyu Chao, Minghuei Chang, David M FarmerAbstract:Internal oceanic Waves are subsurface gravity Waves that can be enormous and travel thousands of kilometres before breaking but they are difficult to study; here observations of such Waves in the South China Sea reveal their formation mechanism, extreme turbulence, relationship to the Kuroshio Current and energy budget. Internal Waves are the underwater version of more familiar surface Waves. They can be enormous and travel thousands of kilometres before breaking. The South China Sea is known to be home to the largest Internal Waves in the world's oceans, but their size, generation mechanisms and role in the regional energy budget are unknown. Matthew Alford and colleagues now present the results from the IWISE observational campaign and reveal that Internal Waves more than 200 metres high break in the South China Sea and create turbulence that is orders of magnitude larger than in the open ocean, and that wave formation is influenced by the Kuroshio current. These results now allow for a complete energy budget of the South China Sea, and for a more accurate incorporation of Internal Waves into climate models. Internal gravity Waves, the subsurface analogue of the familiar surface gravity Waves that break on beaches, are ubiquitous in the ocean. Because of their strong vertical and horizontal currents, and the turbulent mixing caused by their breaking, they affect a panoply of ocean processes, such as the supply of nutrients for photosynthesis1, sediment and pollutant transport2 and acoustic transmission3; they also pose hazards for man-made structures in the ocean4. Generated primarily by the wind and the tides, Internal Waves can travel thousands of kilometres from their sources before breaking5, making it challenging to observe them and to include them in numerical climate models, which are sensitive to their effects6,7. For over a decade, studies8,9,10,11 have targeted the South China Sea, where the oceans’ most powerful known Internal Waves are generated in the Luzon Strait and steepen dramatically as they propagate west. Confusion has persisted regarding their mechanism of generation, variability and energy budget, however, owing to the lack of in situ data from the Luzon Strait, where extreme flow conditions make measurements difficult. Here we use new observations and numerical models to (1) show that the Waves begin as sinusoidal disturbances rather than arising from sharp hydraulic phenomena, (2) reveal the existence of >200-metre-high breaking Internal Waves in the region of generation that give rise to turbulence levels >10,000 times that in the open ocean, (3) determine that the Kuroshio western boundary current noticeably refracts the Internal wave field emanating from the Luzon Strait, and (4) demonstrate a factor-of-two agreement between modelled and observed energy fluxes, which allows us to produce an observationally supported energy budget of the region. Together, these findings give a cradle-to-grave picture of Internal Waves on a basin scale, which will support further improvements of their representation in numerical climate predictions.
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observations of polarity reversal in shoaling nonlinear Internal Waves
Journal of Physical Oceanography, 2009Co-Authors: Emily L Shroyer, James N Moum, Jonathan D NashAbstract:Observations off the New Jersey coast document the shoaling of three groups of nonlinear Internal Waves of depression over 35 km across the shelf. Each wave group experienced changing background conditions along its shoreward transit. Despite different wave environments, a clear pattern emerges. Nearly symmetric Waves propagating into shallow water develop an asymmetric shape; in the wave reference frame, the leading edge accelerates causing the front face to broaden while the trailing face remains steep. This trend continues until the front edge and face of the leading depression wave become unidentifiable and a near-bottom elevation wave emerges, formed from the trailing face of the initial depression wave and the leading face of the following wave. The transition from depression to elevation Waves is diagnosed by the integrated wave vorticity, which changes sign as the wave’s polarity changes sign. This transition is predicted by the sign change of the coefficient of the nonlinear term in the KdV equation, when evaluated using observed profiles of stratification and velocity.
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energy transport by nonlinear Internal Waves
Journal of Physical Oceanography, 2007Co-Authors: James N Moum, Jonathan D Nash, Jody M Klymak, A Perlin, W D SmythAbstract:Wintertime stratification on Oregon’s continental shelf often produces a near-bottom layer of densefluid that acts as an Internal waveguide on which nonlinear Internal Waves propagate. Shipboard profiling and bottom lander observations capture disturbances that exhibit properties of Internal solitary Waves, bores and gravity currents. Wave-like pulses are highly turbulent (instantaneous bed stresses are 1 N m 2 ), resuspending bottom sediments into the water column and raising them 30 + m above the seafloor. The Waves’ cross-shelf transport of fluid counters the time-averaged Ekman transport in the bottom boundary layer. In the nonlinear Internal Waves we have observed, the kinetic energy is roughly equal to the available potential energy and is O(0.1) MJ per m of coastline. The energy transported by these Waves includes a nonlinear advection term huEi that is negligible in linear Internal Waves. Unlike linear Internal Waves, the pressure-velocity energy flux hupi includes important contributions from nonhydrostatic effects and surface displacement. It is found that, statistically, huEi ’ 2hupi. Vertical profiles indicate that up(z) is more important in transporting energy near the seafloor while uE(z) dominates farther from the bottom. With the wave speed, c, estimated from weakly nonlinear wave theory it is verified experimentally that the total energy transported by the Waves, hupi + huEi ’ chEi. The high but intermittent energyflux by the Waves is, in an averaged sense, O(100) W per m of coastline. This is similar to independent estimates of the shoreward energy flux in the semidiurnal Internal tide at the shelfbreak.
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river plumes as a source of large amplitude Internal Waves in the coastal ocean
Nature, 2005Co-Authors: Jonathan D Nash, James N MoumAbstract:Satellite images have long revealed the surface expression of large amplitude Internal Waves that propagate along density interfaces beneath the sea surface. Internal Waves are typically the most energetic high-frequency events in the coastal ocean, displacing water parcels by up to 100 m and generating strong currents and turbulence that mix nutrients into near-surface waters for biological utilization. While Internal Waves are known to be generated by tidal currents over ocean-bottom topography, they have also been observed frequently in the absence of any apparent tide-topography interactions. Here we present repeated measurements of velocity, density and acoustic backscatter across the Columbia River plume front. These show how Internal Waves can be generated from a river plume that flows as a gravity current into the coastal ocean. We find that the convergence of horizontal velocities at the plume front causes frontal growth and subsequent displacement downward of near-surface waters. Individual freely propagating Waves are released from the river plume front when the front's propagation speed decreases below the wave speed in the water ahead of it. This mechanism generates Internal Waves of similar amplitude and steepness as Internal Waves from tide-topography interactions observed elsewhere, and is therefore important to the understanding of coastal ocean mixing.
James N Moum - One of the best experts on this subject based on the ideXlab platform.
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observations of polarity reversal in shoaling nonlinear Internal Waves
Journal of Physical Oceanography, 2009Co-Authors: Emily L Shroyer, James N Moum, Jonathan D NashAbstract:Observations off the New Jersey coast document the shoaling of three groups of nonlinear Internal Waves of depression over 35 km across the shelf. Each wave group experienced changing background conditions along its shoreward transit. Despite different wave environments, a clear pattern emerges. Nearly symmetric Waves propagating into shallow water develop an asymmetric shape; in the wave reference frame, the leading edge accelerates causing the front face to broaden while the trailing face remains steep. This trend continues until the front edge and face of the leading depression wave become unidentifiable and a near-bottom elevation wave emerges, formed from the trailing face of the initial depression wave and the leading face of the following wave. The transition from depression to elevation Waves is diagnosed by the integrated wave vorticity, which changes sign as the wave’s polarity changes sign. This transition is predicted by the sign change of the coefficient of the nonlinear term in the KdV equation, when evaluated using observed profiles of stratification and velocity.
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energy transport by nonlinear Internal Waves
Journal of Physical Oceanography, 2007Co-Authors: James N Moum, Jonathan D Nash, Jody M Klymak, A Perlin, W D SmythAbstract:Wintertime stratification on Oregon’s continental shelf often produces a near-bottom layer of densefluid that acts as an Internal waveguide on which nonlinear Internal Waves propagate. Shipboard profiling and bottom lander observations capture disturbances that exhibit properties of Internal solitary Waves, bores and gravity currents. Wave-like pulses are highly turbulent (instantaneous bed stresses are 1 N m 2 ), resuspending bottom sediments into the water column and raising them 30 + m above the seafloor. The Waves’ cross-shelf transport of fluid counters the time-averaged Ekman transport in the bottom boundary layer. In the nonlinear Internal Waves we have observed, the kinetic energy is roughly equal to the available potential energy and is O(0.1) MJ per m of coastline. The energy transported by these Waves includes a nonlinear advection term huEi that is negligible in linear Internal Waves. Unlike linear Internal Waves, the pressure-velocity energy flux hupi includes important contributions from nonhydrostatic effects and surface displacement. It is found that, statistically, huEi ’ 2hupi. Vertical profiles indicate that up(z) is more important in transporting energy near the seafloor while uE(z) dominates farther from the bottom. With the wave speed, c, estimated from weakly nonlinear wave theory it is verified experimentally that the total energy transported by the Waves, hupi + huEi ’ chEi. The high but intermittent energyflux by the Waves is, in an averaged sense, O(100) W per m of coastline. This is similar to independent estimates of the shoreward energy flux in the semidiurnal Internal tide at the shelfbreak.
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river plumes as a source of large amplitude Internal Waves in the coastal ocean
Nature, 2005Co-Authors: Jonathan D Nash, James N MoumAbstract:Satellite images have long revealed the surface expression of large amplitude Internal Waves that propagate along density interfaces beneath the sea surface. Internal Waves are typically the most energetic high-frequency events in the coastal ocean, displacing water parcels by up to 100 m and generating strong currents and turbulence that mix nutrients into near-surface waters for biological utilization. While Internal Waves are known to be generated by tidal currents over ocean-bottom topography, they have also been observed frequently in the absence of any apparent tide-topography interactions. Here we present repeated measurements of velocity, density and acoustic backscatter across the Columbia River plume front. These show how Internal Waves can be generated from a river plume that flows as a gravity current into the coastal ocean. We find that the convergence of horizontal velocities at the plume front causes frontal growth and subsequent displacement downward of near-surface waters. Individual freely propagating Waves are released from the river plume front when the front's propagation speed decreases below the wave speed in the water ahead of it. This mechanism generates Internal Waves of similar amplitude and steepness as Internal Waves from tide-topography interactions observed elsewhere, and is therefore important to the understanding of coastal ocean mixing.