Showing papers on "Internal wave published in 2006"

Propagation of Low-Mode Internal Waves through the Ocean

Abstract: The baroclinic tides play a significant role in the energy budget of the abyssal ocean. Although the basic principles of generation and propagation are known, a clear understanding of these phenomena in the complex ocean environment is only now emerging. To advance this effort, a ray model is developed that quantifies the effects of spatially variable topography, stratification, and planetary vorticity on the horizontal propagation of internal gravity modes. The objective is to identify “baroclinic shoals” where wave energy is spatially concentrated and enhanced dissipation might be expected. The model is then extended to investigate the propagation of internal waves through a barotropic mesoscale current field. The refraction of tidally generated internal waves at the Hawaiian Ridge is examined using an ensemble of mesoscale background realizations derived from weekly Ocean Topography Experiment (TOPEX)/Poseidon altimetric measurements. The path of mode 1 is only slightly affected by typical cur...

Prototypical solitons in the South China Sea

Abstract: [1] Surface signatures associated with non-linear internal waves are often seen in satellite images of the western South China Sea (SCS) slope and shelf. Observation in the deep sea, to the east, are rare. Here we report on the evolution of an energetic packet as it propagated through the deep central basin of the SCS, toward the western slope and shelf. The waves have amplitudes estimated at 170 m, half widths of 3 km, and phase speeds of 2.9 ± 0.1 m/s, faster than the mode-1 linear phase speed of 2.6 m/s. The shape and observed phase speed were consistent with the Korteweg-deVries (KdV) model over the 65-km path that they were tracked. The intrinsic velocity shear of the waves is small compared to pre-existing shears, and the waves exhibit weak turbulence. The KdV fit and a satellite-derived estimate of horizontal wave extent imply a westward energy flux of 4.5 GW for each crest.

Source and propagation of internal solitary waves in the northeastern South China Sea

Abstract: [1] Large-amplitude internal solitary waves (ISWs) observed near Dongsha Island in the South China Sea originate in tide-topography interactions at Luzon Strait. Their arrival times at two moorings (S7 at 117°17'E, 21°37'N, and Y at 117°13.2'E, 21°2.8'N) are investigated, with respect to model-predicted barotropic tidal currents over Lan-Yu ridge at Luzon Strait. Each ISW packet can be associated with a westward tidal current peak. The time lags between the ISWs and the barotropic tidal currents are 57.6 ± 0.9 hours at S7 and 55.1 ± 1.0 hours at Y, consistent with the mode-one internal waves propagating nondispersively through the region's bathymetry and climatological stratification. Larger ISWs usually arrive earlier than smaller ones, consistent with the theoretical relation between nonlinear wave speed and wave amplitude. The observation that the ISWs are associated with westward tidal currents, with/without the presence of earlier eastward tidal currents, suggests that they are generated by nonlinear steepening of internal tides, rather than by the lee-wave mechanism. An idealized nonlinearization distance, over which the ISWs are generated in internal tide troughs, is estimated to be 260 ± 40 km from Luzon Strait.

Internal solitons in the ocean

Abstract: Internal waves (IW) are among the important factors affecting sound propagation in the ocean. A special role may be played by solitary IWs because of their spatial localization and high magnitudes. Here, nonlinear IWs are discussed (a) from the standpoint of soliton theory and (b) from the viewpoint of experimental measurements. First, basic theoretical models for solitary IWs in the ocean are described, and various analytical solutions are treated, commencing with the well‐known Korteweg–de Vries equation and its important generalizations including effects of rotation, cylindrical divergence, eddy viscosity, shear flows and instabilities, and turbulence. Experimental evidence for the existence of solitons in the upper ocean is presented both for shallow and deep sea regions. The data include radar and optical images and in situ measurements of waveforms, propagation speeds, and dispersion characteristics. It is suggested that internal solitons in the ocean are ubiquitous and are generated primarily by ti...

A novel internal waves generator

Abstract: 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.

Tidal Conversion at a Submarine Ridge

Abstract: The radiative flux of internal wave energy (the “tidal conversion”) powered by the oscillating flow of a uniformly stratified fluid over a two-dimensional submarine ridge is computed using an integral-equation method. The problem is characterized by two nondimensional parameters, A and B. The first parameter, A, is the ridge half-width scaled by h, where h is the uniform depth of the ocean far from the ridge and is the inverse slope of internal tidal rays (horizontal run over vertical rise). The second parameter, B ,i s the ridge height scaled by h. Two topographic profiles are considered: a triangular or tent-shaped ridge and a “polynomial” ridge with continuous topographic slope. For both profiles, complete coverage of the (A, B) parameter space is obtained by reducing the problem to an integral equation, which is then discretized and solved numerically. It is shown that in the supercritical regime (ray slopes steeper than topographic slopes) the radiated power increases monotonically with B and decreases monotonically with A. In the subcritical regime the radiated power has a complicated and nonmonotonic dependence on these parameters. As A → 0 recent results are recovered for the tidal conversion produced by a knife-edge barrier. It is shown analytically that the A → 0 limit is regular: if A 1 the reduction in tidal conversion below that at A 0 is proportional to A 2 . Further, the knife-edge model is shown to be indicative of both conversion rates and the structure of the radiated wave field over a broad region of the supercritical parameter space. As A increases the topographic slopes become gentler, and at a certain value of A the ridge becomes “critical”; that is, there is a single point on the flanks at which the topographic slope is equal to the slope of an internal tidal beam. The conversion decreases continuously as A increases through this transition. Visualization of the disturbed buoyancy field shows prominent singular lines (tidal beams). In the case of a triangular ridge these beams originate at the crest of the triangle. In the case of a supercritical polynomial ridge, the beams originate at the shallowest point on the flank at which the topographic slope equals the ray slope.

Observations and models of the energy flux from the wind to mixed-layer inertial currents

Abstract: Observations of wind stress, upper-ocean currents, and stratification from a mooring at 10 ∘ N in the Pacific are used along with two different one-dimensional mixed-layer models to investigate the horizontal kinetic energy (KE) balance for inertial motions in the mixed-layer. Results from four other sites spanning 25 ∘ N to 60 ∘ N in the Atlantic also are presented. Determining the work done by the wind on the ocean during strong resonant forcing events is of particular interest. The damped-slab mixed-layer model used in previous studies is appealing for this purpose, since it is possible to estimate the wind work using only the wind stress, prescribed mixed-layer depth, and an empirical damping constant. However, it is found that this model, which does not allow for interaction between the mixed-layer and the stratified water column below, does not reproduce the observed KE balance for strong forcing events and systematically over-estimates the wind work. A slightly more sophisticated model, which includes a shear instability mechanism to facilitate momentum transfer between the mixed-layer and a transition layer below, reproduces the observed KE balance and the cumulative wind work much more accurately. Mixed-layer inertial currents unrelated to wind forcing (e.g., due to upward propagating internal waves) are found in the observations, but are not accounted for in either model. When wind forcing is relatively weak, these background currents may be of comparable amplitude to wind-forced currents, and can have a significant impact on the inertial response.

Preliminary simulations of internal waves and mixing generated by finite amplitude tidal flow over isolated topography

Abstract: Much recent observational evidence suggests that energy from the barotropic tides can be used for mixing in the deep ocean. Here the process of internal-tide generation and dissipation by tidal flow over an isolated Gaussian topography is examined, using two-dimensional numerical simulations employing the MITgcm. Four different topographies are considered, for five different amplitudes of barotropic forcing, thereby allowing a variety of combinations of key nondimensional parameters. While much recent attention has focused on the role of relative topographic steepness and height in modifying the rate of conversion of energy from barotropic to baroclinic modes, here attention is focused on parameters dependent on the flow amplitude. For narrow topography, large amplitude forcing gives rise to baroclinic responses at higher harmonics of the forcing frequency. Tall narrow topographies are found to be the most conducive to mixing. Dissipation rates in these calculations are most efficient for the narrowest topography.

A quasi-normal scale elimination model of turbulence and its application to stably stratified flows

Abstract: . Models of planetary, atmospheric and oceanic circulation involve eddy viscosity and eddy diffusivity, KM and KH, that account for unresolved turbulent mixing and diffusion. The most sophisticated turbulent closure models used today for geophysical applications belong in the family of the Reynolds stress models. These models are formulated for the physical space variables; they consider a hierarchy of turbulent correlations and employ a rational way of its truncation. In the process, unknown correlations are related to the known ones via "closure assumptions'' that are based upon physical plausibility, preservation of tensorial properties, and the principle of the invariant modeling according to which the constants in the closure relationships are universal. Although a great deal of progress has been achieved with Reynolds stress closure models over the years, there are still situations in which these models fail. The most difficult flows for the Reynolds stress modeling are those with anisotropy and waves because these processes are scale-dependent and cannot be included in the closure assumptions that pertain to ensemble-averaged quantities. Here, we develop an alternative approach of deriving expressions for KM and KH using the spectral space representation and employing a self-consistent, quasi-normal scale elimination (QNSE) algorithm. More specifically, the QNSE procedure is based upon the quasi-Gaussian mapping of the velocity and temperature fields using the Langevin equations. Turbulence and waves are treated as one entity and the effect of the internal waves is easily identifiable. This model implies partial averaging and, thus, is scale-dependent; it allows one to easily introduce into consideration such parameters as the grid resolution, the degree of the anisotropy, and spectral characteristics, among others. Applied to turbulent flows affected by anisotropy and waves, the method traces turbulence anisotropization and shows how the dispersion relationships for linear waves are modified by turbulence. In addition, one can derive the internal wave frequency shift and the threshold criterion of internal wave generation in the presence of turbulence. The spectral method enables one to derive analytically various one-dimensional and three-dimensional spectra that reflect the effects of waves and anisotropy. When averaging is extended to all scales, the method yields a Reynolds-averaged, Navier-Stokes equations based model (RANS). This RANS model shows that there exists a range of Ri, approximately between 0.1 and 1, in which turbulence undergoes remarkable anisotropization; the vertical mixing becomes suppressed while the horizontal mixing is enhanced. Although KH decreases at large Ri and tends to its molecular value, KM remains finite and larger than its molecular value. This behavior is attributable to the effect of internal waves that mix the momentum but do not mix a scalar. In the Reynolds stress models, this feature is not replicated; instead, all Reynolds stress models predict KM→0 at some value of Ri≤1 which varies from one model to another. The presented spectral model indicates that there is no a single-valued critical Richardson number Ri at which turbulence is fully suppressed by stable stratification. This result is in agreement with large volume of atmospheric, oceanic and laboratory data. The new spectral model has been implemented in the K-e format and tested in simulations of the stably stratified atmospheric boundary layers. The results of these simulations are in good agreement with the data collected in BASE, SHEBA and CASES99 campaigns. Implementation of the QNSE-derived KM and KH in the high-resolution weather prediction system HIRLAM results in significant improvement of its predictive skills.

On the realm of validity of strongly nonlinear asymptotic approximations for internal waves

Abstract: Analytical and numerical results from recently developed strongly nonlinear asymptotic models are compared and validated with experimental observations of internal gravity waves and results from the numerical integrations of Euler equations for solitary waves at the interface of two-fluid systems. The focus of this investigation is on regimes where large amplitudes are attained, where the classical weakly nonlinear theories prove inadequate. Two asymptotically different regimes are examined in detail: shallow fluids, in which the typical wavelengths of the interface displacement are long with respect to the depths of both fluids, and deep fluids, where the wavelengths are comparable to, or less than, the depth of one of the two fluids. With the aim of illustrating the breakdown of the asymptotic assumptions, the transition from a shallow to a deep regime is examined through numerical computation of Euler system's solutions and by comparisons with solution to models.

Numerical investigation of solitary internal wave-induced global instability in shallow water benthic boundary layers

Abstract: The time-dependent boundary layer induced by a weakly nonlinear solitary internal wave in shallow water is examined through direct numerical simulation. Waves of depression and elevation are both considered. The mean density field corresponds to that typical of the coastal ocean and lakes where the lower fraction of the water column is subject to the stabilizing effect of a diffuse stratification. Sufficient resolution of the “inviscid” dynamics of the boundary layer is ensured through use of a Legendre spectral multidomain discretization scheme in the vertical direction. At higher Reynolds numbers, where the simulations become underresolved, because of restrictions in available computational resources, spectral accuracy and numerical stability at the scales of physical interest are preserved through use of a penalty scheme in the vertical and explicit spectral filtering. Thus, a highly accurate description of the qualitative dynamics of the wave-induced global instability is possible and finesca...

Internal Tides and Turbulence along the 3000-m Isobath of the Hawaiian Ridge

Abstract: Full-depth velocity and density profiles taken along the 3000-m isobath characterize the semidiurnal internal tide and bottom-intensified turbulence along the Hawaiian Ridge. Observations reveal baroclinic energy fluxes of 21 5k W m 1 radiating from French Frigate Shoals, 17 2.5 kW m 1 from Kauai Channel west of Oahu, and 13 3.5 kW m 1 from west of Nihoa Island. Weaker fluxes of 1–4 2k W m 1 radiate from the region near Necker Island and east of Nihoa Island. Observed off-ridge energy fluxes generally agree to within a factor of 2 with those produced by a tidally forced numerical model. Average turbulent diapycnal diffusivity K is (0.5–1) 10 4 m 2 s –1 above 2000 m, increasing exponentially to 20 10 4 m 2 s –1 near the bottom. Microstructure values agree well with those inferred from a finescale internal wave-based parameterization. A linear relationship between the vertically integrated energy flux and vertically integrated turbulent dissipation rate implies that dissipative length scales for the radiating internal tide exceed 1000 km.

Energy flux of nonlinear internal waves in northern South China Sea

Abstract: [1] We analyze three sets of ADCP measurements taken on the Dongsha plateau, on the shallow continental shelf, and on the steep continental slope in the northern South China Sea (SCS). The data show strong divergences of energy and energy flux of nonlinear internal waves (NLIW) along and across waves' prevailing westward propagation path. The NLIW energy flux is 8.5 kW m−1 on the plateau, only 0.25 kW m−1 on the continental shelf 220 km westward along the propagation path, and only 1 kW m−1 on the continental slope 120 km northward across the propagation path. Along the wave path on the plateau, the average energy flux divergence of NLIW is ∼0.04 W m−2, which corresponds to a dissipation rate of O(10−7−10−6) W kg−1. Combining the present with previous observations and model results, a scenario of NLIW energy flux in the SCS emerges. NLIWs are generated east of the plateau, propagate predominantly westward across the plateau along a beam of ∼100 km width that is centered at ∼21°N, and dissipate nearly all their energy before reaching the continental shelf.

Persistent Near-Diurnal Internal Waves Observed above a Site of M2 Barotropic-to-Baroclinic Conversion

Abstract: Near-diurnal internal waves were observed in velocity and shear measurements from a shipboard survey along a 35-km section of the Kaena Ridge, northwest of Oahu. Individual waves with upward phase propagation could be traced for almost 4 days even though the ship transited approximately 20 km. Depth–time maps of shear were dominated by near-diurnal waves, despite the fact that Kaena Ridge is a site of considerable M2 barotropic-to-baroclinic conversion. Guided by recent numerical and observational studies, it was found that a frequency of ½M2 (i.e., 24.84-h period) was consistent with these waves. Nonlinear processes are able to transfer energy within the internal wave spectrum. Bicoherence analysis, which can distinguish between nonlinearly coupled waves and waves that have been independently excited, suggested that the ½M2 waves were nonlinearly coupled with the dominant M2 internal tide only between 525- and 595-m depth. This narrow depth range corresponded to an observed M2 characteristic ema...

Estimating Open-Ocean Barotropic Tidal Dissipation: The Hawaiian Ridge

Abstract: The generalized inverse of a regional model is used to estimate barotropic tidal dissipation along the Hawaiian Ridge. The model, based on the linear shallow-water equations, incorporates parameterizations for the dissipation of energy via friction in the bottom boundary layer and form drag due to internal waves generated at topographic slopes. Sea surface height data from 364 orbit cycles of the Ocean Topography Experiment (TOPEX)/Poseidon satellite mission are used to perform inversions at eight diurnal and semidiurnal tidal frequencies. It is estimated that the barotropic M2 tide loses energy at a rate of 19 GW, of which 88% is lost within 250 km of the ridge, presumably via conversion to the internal or baroclinic tide. Uncertainty in the assumed model error and wave drag in the forward model suggest that M2 dissipation values from 18 to 25 GW are consistent with the altimetric observations. Other barotropic tidal constituents are estimated to lose a total of 5.7 GW. The spatial distribution of barotropic dissipation along the ridge is similar to that inferred from three-dimensional primitive equation models, and it is largely insensitive to details of assumed model and data errors. Dissipation at semidiurnal frequencies is most intense at the French Frigate Shoals with lesser, but significant, contributions at other sites. Diurnal tidal dissipation is concentrated to the east of the French Frigate Shoals, at the Gardner Pinnacles. Further work with threedimensional models will be necessary to determine the fate of the energy that is removed from the barotropic tide.

Baroclinic Energy Flux at the Hawaiian Ridge: Observations from the R/P FLIP

Abstract: Estimates of baroclinic energy flux are made in the immediate “Nearfield” (September–October 2002) and 450 km offshore (“Farfield”; October–November 2001) of the Kaena Ridge, an active barotropic-to-baroclinic conversion site. The flux estimates are based on repeated profiles of velocity and density obtained from the Research Platform Floating Instrument Platform (FLIP) as an aspect of the Hawaii Ocean Mixing Experiment. Energetic beams associated with both semidiurnal and diurnal internal waves are observed in the Kauai Channel. Beam depths and orientations are consistent with generation along the upper flanks of the ridge. At the far-field site, the baroclinic energy flux is borne primarily by first-mode semidiurnal waves. The energy flux associated with the entire spectrum of internal waves is computed by cross-spectral analysis. Significant energy fluxes are found in the inertial, diurnal, semidiurnal, and twice-semidiurnal frequency bands. The semidiurnal energy flux strongly dominates the s...

Long waves in erodible channels and morphodynamic influence

Abstract: [1] Like most media, open channel flows propagate information through waves When the channel boundary is fixed, the vectors of information consist typically of surface gravity waves In the less straightforward case of channels with cohesionless bed and possibly erodible banks, other types of waves arise from the erodible nature of the boundaries and the ability of the stream to transport sediments In this paper we restrict our attention to the important case of long waves, which can be described by employing the shallow water approximation for the flow field and a quasi-equilibrium assumption for sediment transport on weakly sloping beds We focus on a major issue: In which direction is information propagated? This is a problem raised and partially solved by de Vries in the context of one-dimensional morphological modeling as early as 1965 We review some of the available knowledge on this subject, viewed in a more general context where vectors of information can be a variety of waves: purely longitudinal one-dimensional sediment waves, two-dimensional waves driven by large-scale bed forms (bars), and plan form waves carrying information related to the planform shape of the channel Both linear and nonlinear, migrating and stationary waves are considered It turns out that the role played by the Froude number in determining the direction of one-dimensional perturbations of bed topography is somewhat taken by the aspect ratio of the channel when large-scale two-dimensional bed forms as well as planform waves are considered

On the interpretation of energy and energy fluxes of nonlinear internal waves: an example from Massachusetts Bay

Abstract: Author Posting. © Cambridge University Press, 2006. This article is posted here by permission of Cambridge University Press for personal use, not for redistribution. The definitive version was published in Journal of Fluid Mechanics 561 (2006):103–112, doi:10.1017/S0022112006000991

Density variability at scales typical of gravity waves observed in Mars' thermosphere by the MGS accelerometer

Abstract: [1] Examination of the small-scale density perturbations in Mars' thermosphere inferred from Mars Global Surveyor (MGS) accelerometer data reveals wave-like structures consistent with gravity waves. The structures are interpreted to be horizontal with dominant wavelengths of 100-300 km observed along the orbital path. Density perturbations are significantly stronger in winter versus spring/summer, suggesting that the zonal mean winds play a major role in filtering these waves. There is also evidence of possible modulation by the mean winds due to local time differences. No correlation is found with the underlying topography, a source region on Earth for gravity waves. In fact, density perturbations in the northern hemisphere are observed to be greater than those in the southern hemisphere, which has a higher orographic variance. Over the Martian tropics, an observed source region for gravity waves, density perturbations are again not elevated, leaving questions on how gravity waves observed in the lower atmosphere are related to those observed in the thermosphere.

Spatial structure of the dominant basin-scale internal waves in Lake Kinneret

Abstract: Field data and numerical simulations were used to investigate the effects of basin shape, continuous stratification, and rotation on the three-dimensional structure of the dominant natural basin-scale internal-wave modes in Lake Kinneret, a stratified lake large enough so that the earth’s rotation influences the wave motion. The structure of the modes was inferred from power spectral density of measured and simulated isotherm vertical displacements and from rotary-power spectral density of isopycnal velocities obtained from numerical simulations. The shape of the lake at the level of the thermocline, in conjunction with the dispersion relationship, determines the horizontal configuration of the natural modes. The dominant response to wind forcing was an azimuthal and vertical mode 1 Kelvin wave with a natural period of 22.6 h that propagated around the entire basin; the second most dominant response was a vertical mode 1 wave with a natural period close to 10.5 h and composed of two counter-rotating

Gravity wave breaking, secondary wave generation, and mixing above deep convection in a three-dimensional cloud model

Abstract: [1] This paper documents the breakdown of gravity waves generated by deep convection in a three-dimensional cloud-resolving model. The convection generates gravity waves that propagate into the lower stratosphere, with horizontal wavelengths between 5 and 10 km. Above-cloud wind shear causes part of the spectrum of these waves to break, inducing overturning. The model grid spacing is small enough (150 m) that the gravity waves are well resolved, and the turbulent cascade induced by the breakdown is partially resolved. Previous model simulations of wave breakdown above deep convection, at this resolution, have only been achieved in two-dimensional models. The wave breakdown generates secondary waves, which have much shorter horizontal wavelengths, and different propagation characteristics compared to the primary waves. Secondary wave generation in the lower stratosphere above deep convection has not been identified in previous studies. The wave breakdown also induces irreversible mixing, which is quantified in terms of the vertical transport of water vapor.

On the generation and propagation of internal solitary waves in the southern Bay of Biscay

Abstract: Internal solitary waves (ISWs), travelling towards the South–South-West (SSW), are now well documented in the northern and central Bay of Biscay. These are formed from large-amplitude internal tides which result from the interaction of the barotropic tide with the steep shelf-break topography. In the present paper, we investigate available satellite imagery (Synthetic Aperture Radar (SAR) and ASAR data) to reveal that the southern Bay of Biscay is also a ‘‘hotspot’’ region which has a high level of ISW activity. Here, the ISWs travel towards the East–North-East from the Cape Finisterre region off North-West Spain. In fact, we reveal the presence of two wave-trains travelling in slightly different directions (0551T and 0401T). By calculating the strength of the barotropic tidal forcing in the region, and identifying the likely propagation pathways (rays) of internal tidal (IT) energy, we identify the generation sites for these wave-trains as lying on either side of the Ortegal Promontory (OP). This is an undersea ‘‘headland’’ projecting towards the North-West from the north-western coast of Spain (near 441N, 8.51W), and over which the barotropic tides are forced to flow. For each generation site, IT rays emanating from ‘‘critical’’ topography (where the ray slope is equal to the topographic slope) in regions of strong barotropic forcing, rise to the surface (for one site after a reflection from the sea-floor) and pass through the thermocline close to the earliest occurrences of the ISWs in the respective wave trains. These rays would then produce, through nonlinear processes, the ISWs through the same ‘‘local generation’’ mechanism that has been used to explain the occurrence of the ISWs in the northern and central Bay. The ‘‘local generation’’ mechanism may therefore be more widely applicable than previously thought. In addition, the methods we have used to deduce the generation sites for these waves are expected to prove equally useful for studies in other areas of the world’s oceans.

Numerical and Analytical Estimates of M2 Tidal Conversion at Steep Oceanic Ridges

Abstract: Numerical calculations of the rate at which energy is converted from the external to internal tides at steep oceanic ridges are compared with estimates from analytic theories. The numerical calculations are performed using a hydrostatic primitive equation ocean model that uses a generalized s-coordinate system as the vertical coordinate. The model [Regional Ocean Modeling System (ROMS)] estimates of conversion compare well with inviscid and nondiffusive theory in the sub- and supercritical regimes and are insensitive to the strength of viscosity and diffusivity. In the supercritical regime, the nondissipative analytic solution is singular all along the internal tide beams. Because of dissipation the ROMS solutions are nonsingular, although the density gradients are strongly enhanced along the beams. The agreement between model and theory indicates that the prominent singularities in the inviscid solution do not compromise the estimates of tidal conversion and that the linearization used in derivi...

Dynamics of a stratified shear layer with horizontal shear

Abstract: The evolution of a stratified shear layer with mean shear in the horizontal direction, orthogonal to gravity, is numerically investigated with focus on the structural organization of the vorticity and density fields. Although the Reynolds number of the flow increases with time, facilitating instabilities and turbulence, the bulk Richardson number signifying the level of stratification also increases. Remarkably rich dynamics is found: turbulence; the emergence of coherent core/braid regions from turbulence; formation of a lattice of dislocated vortex cores connected by thin horizontal sheets of collapsed density and vorticity; density-driven intrusions at the edges of the shear layer; and internal wave generation and propagation. Stratification introduces significant vertical variability although it inhibits the vertical velocity. The molecular dissipation of turbulent kinetic energy and of turbulent potential energy are both found to be substantial even in the case with highest stratification, and primarily concentrated in thin horizontal sheets. The simulation data are used to help explain how buoyancy induces the emergence of columnar vortex cores from turbulence and then dislocates these cores to eventually form a lattice of ‘pancake’ eddies connected by thin sheets with large vertical shear (horizontal vorticity) and density gradient.

Numerical simulation of internal tides and the resulting energetics within Monterey Bay and the surrounding area

Abstract: [1] We use a three-dimensional nonhydrostatic unstructured-grid code to simulate internal tides within Monterey Bay. The predicted water surface, barotropic and baroclinic velocities, and internal tides in the greater coastal region agree with observations. The agreement is due to the use of a high-resolution mesh, alongshore prescription of the barotropic M2 tidal velocities, and specification of initial conditions consistent with field data. Results of depth-integrated, M2-period-averaged energy flux and energy flux divergence are presented in order to identify locations of significant internal wave generation and dissipation. Based on the results, there is a domain-wide power surplus of +52 MW due to internal tides that is available for pelagic mixing, yet isolated bathymetric features, such as Monterey Submarine Canyon and Smooth Ridge, are net dissipative, with dissipation rates of −8.3 and −1.5 MW, respectively.

Numerical simulations of the interaction of internal waves with a shelf break

Abstract: The energetics of the interaction of internal gravity waves with a shelf break is investigated by means of high-resolution two-dimensional numerical simulations, with an emphasis on understanding the partitioning of the internal wave energy over the course of the interaction process and the subsequent dynamics of the onshore propagating internal waves. Some of the energy is dissipated as a result of the instabilities associated with breaking, while the remaining energy is either reflected back away from or transmitted onto the shelf. We employ an analysis of the distribution of the energy flux across the shelf break taking into account the contributions from nonhydrostatic as well as nonlinear effects to quantify the percentage of energy flux that is transmitted onto the shelf, as well as the percentages of reflected and dissipated energy fluxes, from an incoming wave field. For a given frequency of an incoming wave, we vary the amplitude of the wave to vary the incident energy flux, and we simulate condi...

Spatially Broad Observations of Internal Waves in the Upper Ocean at the Hawaiian Ridge

Abstract: The density and current structure at the Hawaiian Ridge was observed using SeaSoar and Doppler sonar during a survey extending from Oahu to Brooks Banks. Across- and along-ridge changes in internal wave statistics in the upper ocean within 200 km of the ridge are investigated. Internal waves with trough-to-crest amplitude as large as 60 m and horizontal wavelength of about 50 km are observed repeatedly in across-ridge sections of potential density. Within 150 km of the ridge, kinetic and potential energy density exceed open-ocean values with maxima about 10 times Garrett–Munk levels. In the Kauai Channel (KC), the kinetic energy density is largest along an M2 internal tide ray. The ray originates at the northern edge of the ridge peak at a large across-ridge change in topographic slope and terminates at the ocean surface about 30–40 km south of the ridge peak. Kinetic and potential energy density are larger on the south side of the ridge at KC, the side with larger topographic slope. Energy densi...

Tidally forced internal waves and overturns observed on a slope: Results from HOME

Abstract: Tidal mixing over a slope was explored using moored time series observations on Kaena Ridge extending northwest from Oahu, Hawaii, during the Survey component of the Hawaii Ocean Mixing Experiment (HOME). A mooring was instrumented to sample the velocity and density field of the lower 500 m of the water column to look for indirect evidence of tidally induced mixing and was deployed on a slope in 1453-m water depth for 2 months beginning in November 2000. The semidiurnal barotropic tidal currents at this site have a significant cross-ridge component, favorable for exciting an internal tidal response. A large-amplitude response is expected, given that the slope of the topography (4.5°) is nearly the same as the slope of the internal wave group velocity at semidiurnal frequency. Density overturns were inferred from temperature profiles measured every 2 min. The number and strength of the overturns are greater in the 200 m nearest the bottom, with overturns exceeding 24 m present at any depth nearly ...

Internal wave generation from rough topography

Abstract: Through laboratory experiments we examine internal wave generation above and in the lee of finite-amplitude periodic topography having various degrees of roughness. We show that internal waves are generated not only by flow over the hills but also by flow over “boundary-trapped” lee waves and by vigorous turbulence created in the lee of sharp-crested hills. For low values of the excitation frequency, linear theory well predicts the internal wave frequencies but significantly overestimates the wave amplitudes because it neglects processes associated with boundary layer separation. When the excitation frequency exceeds the buoyancy frequency, turbulence results in the excitation of internal waves with frequencies approximately 0.72±0.05 of the buoyancy frequency and vertical displacement amplitudes ranging between 1.5% and 2% of the horizontal wavelength.