Showing papers on "Internal wave published in 2012"

Gravity Wave Characteristics in the Southern Hemisphere Revealed by a High-Resolution Middle-Atmosphere General Circulation Model

Abstract: Gravity wave characteristics in the middle- to high-latitude Southern Hemisphere are analyzed using simulation data over 3 yr from a high-resolution middle-atmosphere general circulation model without using any gravity wave parameterizations. Gravity waves have large amplitudes in winter and are mainly distributed in the region surrounding the polar vortex in the middle and upper stratosphere, while the gravity wave energy is generally weak in summer. The wave energy distribution in winter is not zonally uniform, but it is large leeward of the southern Andes and Antarctic Peninsula. Linear theory in the three-dimensional framework indicates that orographic gravity waves are advected leeward significantly by the mean wind component perpendicular to the wavenumber vector. Results of ray-tracing and cross-correlation analyses are consistent with this theoretical expectation. The leeward energy propagation extends to several thousand kilometers, which explains part of the gravity wave distribution aro...

Direct numerical simulation of stationary homogeneous stratified sheared turbulence

Abstract: Using direct numerical simulation, we investigate stationary and homogeneous shear-driven turbulence in various stratifications, ranging from neutral to very stable. To attain and maintain a stationary flow, we throttle the mean shear so that the net production stays constant for all times. This results in a flow that is characterized solely by its mean shear and its mean buoyancy gradient, independent of initial conditions. The method of throttling is validated by comparison with experimental spectra in the case of neutral stratification. With increasing stratification comes the emergence of vertically sheared large-scale horizontal motions that preclude a straightforward interpretation of flow statistics. However, once these motions are excluded, simply by subtracting the horizontal average, the underlying flow appears amenable to the standard methods of turbulence analysis. It is shown that a direct acknowledgement of the confining influence of the periodic simulation box can lead to a meaningful physical interpretation of the large scales. Once an appropriate confinement scale is identified, many features, including horizontal spectra, flux-gradient relationships and length scales, of stratified sheared turbulence can be readily understood, both qualitatively and quantitatively, in terms of Monin-Obukhov similarity theory. Finally, the similarity-theory framework is used to interpret the scaling of the vertical diapycnal diffusivity in stratified turbulence. © 2012 Cambridge University Press.

Annual Cycle and Depth Penetration of Wind-Generated Near-Inertial Internal Waves at Ocean Station Papa in the Northeast Pacific

Abstract: The downward propagation of near-inertial internal waves following winter storms is examined in the context of a 2-yr record of velocity in the upper 800 m at Ocean Station Papa. The long time series allow accurate estimation of wave frequency, whereas the continuous data in depth allow separation into upward- and downward-propagating components. Near-inertial kinetic energy (KEin) dominates the record. At all measured depths, energy in downgoing motions exceeds that of upward-propagating motions by factors of 3–7, whereas KEin is elevated by a factor of 3–5 in winter relative to summer. The two successive winters are qualitatively similar but show important differences in timing and depth penetration. Energy is seen radiating downward in a finite number of wave groups, which are tagged and catalogued to determine the vertical group velocity cgz, which has a mean of about 1.5 × 10−4 m s−1 (13 m day−1). Case studies of three of these are presented in detail.Downward energy flux is estimated as cgz ...

Advantages of Using Multichannel Analysis of Love Waves (MALW) to Estimate Near-Surface Shear-Wave Velocity

Abstract: As theory dictates, for a series of horizontal layers, a pure, plane, horizontally polarized shear (SH) wave refracts and reflects only SH waves and does not undergo wave-type conversion as do incident P or Sv waves. This is one reason the shallow SH-wave refraction method is popular. SH-wave refraction method usually works well defining near-surface shear-wave velocities. Only first arrival information is used in the SH-wave refraction method. Most SH-wave data contain a strong component of Love-wave energy. Love waves are surface waves that are formed from the constructive interference of multiple reflections of SH waves in the shallow subsurface. Unlike Rayleigh waves, the dispersive nature of Love waves is independent of P-wave velocity. Love-wave phase velocities of a layered earth model are a function of frequency and three groups of earth properties: SH-wave velocity, density, and thickness of layers. In theory, a fewer parameters make the inversion of Love waves more stable and reduce the degree of nonuniqueness. Approximating SH-wave velocity using Love-wave inversion for near-surface applications may become more appealing than Rayleigh-wave inversion because it possesses the following three advantages. (1) Numerical modeling results suggest the independence of P-wave velocity makes Love-wave dispersion curves simpler than Rayleigh waves. A complication of “Mode kissing” is an undesired and frequently occurring phenomenon in Rayleigh-wave analysis that causes mode misidentification. This phenomenon is less common in dispersion images of Love-wave energy. (2) Real-world examples demonstrated that dispersion images of Love-wave energy have a higher signal-to-noise ratio and more focus than those generated from Rayleigh waves. This advantage is related to the long geophone spreads commonly used for SH-wave refraction surveys, images of Love-wave energy from longer offsets are much cleaner and sharper than for closer offsets, which makes picking phase velocities of Love waves easier and more accurate. (3) Real-world examples demonstrated that inversion of Love-wave dispersion curves is less dependent on initial models and more stable than Rayleigh waves. This is due to Love-wave’s independence of P-wave velocity, which results in fewer unknowns in the MALW method compared to inversion methods of Rayleigh waves. This characteristic not only makes Love-wave dispersion curves simpler but also reduces the degree of nonuniqueness leading to more stable inversion of Love-wave dispersion curves.

Surface Alfven waves in solar flux tubes

Abstract: Magnetohydrodynamic (MHD) waves are ubiquitous in the solar atmosphere. Alfven waves and magneto-sonic waves are particular classes of MHD waves. These wave modes are clearly different and have pure properties in uniform plasmas of infinite extent only. Due to plasma non-uniformity MHD waves have mixed properties and cannot be classified as pure Alfven or magneto-sonic waves. However, vorticity is a quantity unequivocally related to Alfven waves as compression is for magneto-sonic waves. Here, we investigate MHD waves superimposed on a one-dimensional non-uniform straight cylinder with constant magnetic field. For a piecewise constant density profile we find that the fundamental radial modes of the non-axisymmetric waves have the same properties as surface Alfven waves at a true discontinuity in density. Contrary to the classic Alfveen waves in a uniform plasma of infinite extent, vorticity is zero everywhere except at the cylinder boundary. If the discontinuity in density is replaced with a continuous variation of density, vorticity is spread out over the whole interval with non-uniform density. The fundamental radial modes of the non-axisymmetric waves do not need compression to exist unlike the radial overtones. In thin magnetic cylinders the fundamental radial modes of the non-axisymmetric waves with phase velocities between the internal and the external Alfven velocities can be considered as surface Alfven waves. On the contrary, the radial overtones can be related to fast-like magneto-sonic modes.

Double-Ridge Internal Tide Interference and Its Effect on Dissipation in Luzon Strait

Abstract: Luzon Strait between Taiwan and the Philippines features two parallel north–south-oriented ridges. The barotropic tides that propagate over these ridges cause strong internal waves and dissipation. The energy dissipation mechanisms and the role of the baroclinic wave fields in this dissipation are investigated using numerical simulations with the Massachusetts Institute of Technology general circulation model (MITgcm). The model is integrated over two-dimensional configurations along a zonal transect at 20.6°N for a maximum duration of a spring–neap cycle. Nearly all dissipation occurs at the steep ridge crests due to high-mode turbulent lee waves with horizontal scales of several kilometers and vertical scales of hundreds of meters. The spatial structure and timing of the predicted velocities and dissipation agree with observations and confirm the existence of these lee waves. The lee wave strength is greatly affected by the internal waves generated at the other ridge. When semidiurnal barotropic...

The Unpredictable Nature of Internal Tides on Continental Shelves

Abstract: Packets of nonlinear internal waves (NLIWs) in a small area of the Mid-Atlantic Bight were 10 times more energetic during a local neap tide than during the preceding spring tide. This counterintuitive result cannot be explained if the waves are generated near the shelf break by the local barotropic tide since changes in shelfbreak stratification explain only a small fraction of the variability in barotropic to baroclinic conversion. Instead, this study suggests that the occurrence of strong NLIWs was caused by the shoaling of distantly generated internal tides with amplitudes that are uncorrelated with the local spring-neap cycle. An extensive set of moored observations show that NLIWs are correlated with the internal tide but uncorrelated with barotropic tide. Using harmonic analysis of a 40-day record, this study associates steady-phase motions at the shelf break with waves generated by the local barotropic tide and variable-phase motions with the shoaling of distantly generated internal tides. ...

Global Modeling of Internal Tides Within an Eddying Ocean General Circulation Model

Abstract: : Ocean tides, and the atmospherically forced oceanic general circulation and its associated mesoscale eddy field, have long been run separately in high-resolution global models. They are now being simulated concurrently in a high-resolution version of the HYbrid Coordinate Ocean Model (HYCOM). The incorporation of horizontally varying stratification with the addition of atmospheric forcing yields internal tides (internal waves of tidal frequency) in high-latitude, low-stratification regions that are qualitatively different from those in earlier global internal tide models, in which atmospheric forcing and horizontally variable stratification were absent. The internal tides in the new concurrent HYCOM simulations compare well with those measured in along-track satellite altimeter data. The new concurrent simulations demonstrate that the wavenumber spectrum of sea surface height --a measure of the energy contained in different length scales is dominated in some locations by internal tides and in others by mesoscale eddies. Tidal kinetic energies in the new concurrent simulations compare well with those in current-meter observations, as long as sufficient spatial averaging is performed. The new concurrent simulations are being used in the planning of future-generation satellite altimeters, in the provision of boundary conditions for coastal ocean models, and in studies of ocean mixing.

Global M 2 internal tide and its seasonal variability from high resolution ocean circulation and tide modeling

Abstract: [1] The present study describes a model simulation, where ocean tide dynamics are simulated simultaneously with the ocean circulation. The model is forced by a lunisolar tidal forcing described by ephemerides and by daily climatological wind stress, heat, and fresh water fluxes. The horizontal resolution is about 0.1 and thus, the model implicitly resolves meso-scale eddies and internal waves. In this model simulation the global M2 barotropic to baroclinic tidal energy conversion amounts to 1.2 TW. We show global maps of the surface signature of the M2 baroclinic tide and compare it with an estimate obtained from 19 years of satellite altimeter data. Further, the simulated seasonality in the low mode internal tide field is presented and, as an example, the physical mechanisms causing the non-stationarity of the internal tide generated in Luzon Strait are discussed. In general, this study reveals the impact of inter-annual changes of the solar radiative forcing and wind forced ocean circulation on the generation and propagation of the low mode internal tides. The model is able to simulate non-stationary signals in the internal tide field on global scales which have important implications for future satellite altimeter missions. Citation: Muller, M., J. Y. Cherniawsky, M. G. G. Foreman, and J.-S. von Storch (2012), Global M2 internal tide and its seasonal variability from highresolution ocean circulation and tide modeling, Geophys. Res. Lett., 39, L19607, doi:10.1029/2012GL053320.

Simulating the Long-Range Swell of Internal Waves Generated by Ocean Storms

Abstract: USAGE Permission is granted to copy this article for use in teaching and research. Republication, systematic reproduction, or collective redistribution of any portion of this article by photocopy machine, reposting, or other means is permitted only with the approval of The Oceanography Society. Send all correspondence to: info@tos.org or The Oceanography Society, PO Box 1931, Rockville, MD 20849-1931, USA. Oceanography THE OffICIAL MAGAzINE Of THE OCEANOGRAPHY SOCIETY

Inferring dynamics from the wavenumber spectra of an eddying global ocean model with embedded tides

Abstract: [1] The slopes of the wavenumber spectra of sea surface height (SSH) and kinetic energy (KE) have been used to infer “interior” or surface quasi-geostrophic (QG or SQG) dynamics of the ocean. However, inspection of spectral slopes for altimeter SSH in the mesoscale band of 70 to 250 km shows much flatter slopes than the QG or SQG predictions over most of the ocean. Comparison of altimeter wavenumber spectra with spectra from an eddy resolving global ocean circulation model (the HYbrid Coordinate Ocean Model, HYCOM, at 1/12.5° equatorial resolution), which has embedded tides, suggests that the flatter slopes of the altimeter SSH may arise from three possible sources: (1) presence of strong internal tides, (2) shift of the inertial sub-range to smaller scales and (3) altimeter noise. Artificially adding noise to the model tends to flatten the spectra for low KE regions. Near internal tide generating regions, spectral slopes in the presence of internal waves are much flatter than QG or SQG predictions. Separating the variability into high and low frequency (around periods of 2 days), then a different pattern emerges with a flat high-frequency wavenumber spectrum and a steeper low-frequency wavenumber spectrum. For low mesoscale KE, the inertial sub-range, defined by the nearly flat enstrophy band, moves to smaller scales and the mesoscale band of 70 to 250 km no longer represents the inertial sub-range. The model wavenumber spectra are consistent with QG and SQG theory when internal waves and inertial sub-range shifts are taken into consideration.

An evaluation of the barotropic and internal tides in a high‐resolution global ocean circulation model

Abstract: [1] Global comparisons of barotropic and internal tides generated in an eddy-resolving ocean circulation model are made with tidal estimates obtained from altimetric sea surface heights and an altimetry-constrained tide model. As far as we know, our Hybrid Coordinate Ocean Model (HYCOM) simulations shown here and in an earlier paper are the only published high-resolution global simulations to contain barotropic tides, internal tides, the general circulation, and mesoscale eddies concurrently. Comparing the model barotropic tide with a global data-assimilative shallow water tide model shows that the global tidal elevation differences are approximately evenly split between discrepancies in tidal amplitude and phase. Both the model and observations show strong generation of internal tides at a limited number of “hot spot” regions with propagation of beams of energy for thousands of kilometers away from the sources. The model internal tidal amplitudes compare well with observations near these energetic tidal regions. Averaged over these regions, the model and observation internal tide amplitude estimates agree to approximately 15% for the four largest semidiurnal constituents and 23% for the four largest diurnal constituents. Away from the hot spots, the comparison between the model and altimetric amplitude is not as good due, in part, to two problems, errors in the model barotropic tides and overestimation of the altimetric tides in regions of strong mesoscale eddy activity. Examining the general energy distribution of the simulated internal tide is an important first step in the evaluation of internal tides in HYCOM.

Trapped Core Formation within a Shoaling Nonlinear Internal Wave

Abstract: Large-amplitude (100–200 m) nonlinear internal waves (NLIWs) were observed on the continental slope in the northern South China Sea nearly diurnally during the spring tide. The evolution of one NLIW as it propagated up the continental slope is described. The NLIW arrived at the slope as a nearly steady-state solitary depression wave. As it propagated up the slope, the wave propagation speed C decreased dramatically from 2 to 1.3 m s−1, while the maximum along-wave current speed Umax remained constant at 2 m s−1. As Umax exceeded C, the NLIW reached its breaking limit and formed a subsurface trapped core with closed streamlines in the coordinate frame of the propagating wave. The trapped core consisted of two counter-rotating vortices feeding a jet within the core. It was highly turbulent with 10–50-m density overturnings caused by the vortices acting on the background stratification, with an estimated turbulent kinetic energy dissipation rate of O(10−4) W kg−1 and an eddy diffusivity of O(10−1) m2...

Poincaré wave–induced mixing in a large lake

Abstract: A 10,000-km2 hypoxic ‘dead zone’ forms, during most years, in the central basin of Lake Erie. To investigate the processes driving the hypoxia, we conducted a 2-yr field campaign where the mixing in the lake interior during the stratification period was examined using current meters and temperature-loggers data, as well as > 600 temperature microstructure profiles, from which turbulent mixing was computed. Near-inertial Poincare waves drive shear instability, generating ∼ 1-m amplitude and 10-m wavelength high-frequency internal waves with ∼ 1-mdensity overturns that lead to an increase in turbulent dissipation by one order of magnitude. The instabilities are associated with enhanced vertical shear at the crests and troughs of the Poincare waves and may be correlated with the local gradient Richardson number. Poincare wave–induced mixing should be an important factor when the Burger number < 0.25. The strong diapycnal mixing induced by the Poincare wave activity will also significantly modify the energy-flux paths. For example, we estimate that, in Lake Erie, 0.85% of the wind energy is transferred to the lake interior (below the surface layer); of this, 40% is dissipated in the interior metalimnion and 60% is dissipated at the bottom boundary. In smaller lakes, 0.42% of wind energy is transferred to the deeper water, with 90% dissipated in the boundary and 10% in the interior metalimnion. © 2012, by the Association for the Sciences of Limnology and Oceanography, Inc.

On the steady-state fully resonant progressive waves in water of finite depth

Abstract: The steady-state fully resonant wave system, consisting of two progressive primary waves in finite water depth and all components due to nonlinear interaction, is investigated in detail by means of analytically solving the fully nonlinear wave equations as a nonlinear boundary-value problem. It is found that multiple steady-state fully resonant waves exist in some cases which have no exchange of wave energy at all, so that the energy spectrum is time-independent. Further, the steady-state resonant wave component may contain only a small proportion of the wave energy. However, even in these cases, there usually exist time-dependent periodic exchanges of wave energy around the time-independent energy spectrum corresponding to such a steady-state fully resonant wave, since it is hard to be exactly in such a balanced state in practice. This view serves to deepen and enrich our understanding of the resonance of gravity waves.

Direct observations of microscale turbulence and thermohaline structure in the Kuroshio Front

Abstract: [1] Direct observations of microstructure near the Kuroshio Front were conducted in August 2008 and October 2009. These show negative potential vorticity (PV) in the mixed layer south of the front, where directly measured turbulent kinetic energy dissipation rates are an order magnitude larger than predicted by wind-scaling. These elevated dissipation rates scale better with an empirical scaling, which considers local wind and Ekman buoyancy flux driven by downfront wind. Near-zero PV in the thermocline under the Kuroshio mainstream is observed at 200–300 m depth, with dissipation exceeding open ocean thermocline values by factors of 10–100. Overall, the large turbulent dissipation rates measured in the Kuroshio can be categorized into two groups, one characterized by low Richardson number along the Kuroshio Front thermocline, and the other characterized by high stratification away from the Kuroshio mainstream. The former is attributed to mixing by unbalanced frontal ageostrophic flows, and the latter is attributed to internal wave breaking. On average, both groups appear in regions of large horizontal density gradients. Observed thermohaline structure shows low salinity tongues from the surface to over 300 m depth and deep cold tongues, extending upward from 500 to 100 m depth in a narrow (20 km) zone, suggesting down and upwelling driven by geostrophic straining, which is confirmed by Quasigeostrophic-Omega equation solutions. This implies that adiabatic along isopycnal subduction and diabatic diapycnal turbulent mixing acting in tandem at the Kuroshio Front likely contribute to NPIW formation.

Enhancement of biological productivity by internal waves: observations in the summertime in the northern South China Sea

Abstract: Streaks of elevated concentrations of surface chlorophyll a (Chl_a) of various spacing were found to be associated with internal waves in their transmission zone and dissipation zone in the summertime in the deep open northern South China Sea. At an anchored station in the dissipation zone north of the Dongsha Atoll with a water depth of ca. 600 m, undulations of the mixed layer depth with an amplitude of ca. 30 m and a periodicity of ca. 12 h were observed, and they were accompanied by similar undulation in the isotherm and isopleth of the nutrients. These observations are consistent with the enhancement of vertical mixing by internal waves and the resulting transfer of cold, nutrient-rich subsurface water to the surface mixed layer to fuel biological productivity. In the transmission zone and dissipation zone, respectively, the summertime (May–October) average sea surface temperature was 0.5 and 0.8 °C lower and Chl_a was 19 and 43 % higher than those in a nearby subregion that was minimally affected by internal waves. The mean net primary productivity was elevated by 15 and 37 %. These results indicate that the enhancement of biological activity by internal waves is not confined to the shallow waters on the shelf. The effect can be detected in all phases of the internal waves although it may be especially prominent in the dissipation zone where mixing between subsurface and surface waters is more effective.

Boundary-layer-separation-driven vortex shedding beneath internal solitary waves of depression

Abstract: We investigate global instability and vortex shedding in the separated laminar boundary layer beneath internal solitary waves (ISWs) of depression in a two-layer stratified fluid by performing high-resolution two-dimensional direct numerical simulations. The simulations were conducted with waves propagating over a flat bottom and shoaling over relatively mild and steep slopes. Over a flat bottom, the potential for vortex shedding is shown to be directly dependent on wave amplitude, for a particular stratification, owing to increase of the adverse pressure gradient ( for leftward propagating waves) beneath the trailing edge of larger amplitude waves. The generated eddies can ascend from the bottom boundary to as high as 33 % of the total depth in two-dimensional simulations. Over sloping boundaries, global instability occurs beneath all waves as they steepen. For the slopes considered, vortex shedding begins before wave breaking and the vortices, shed from the bottom boundary, can reach the pycnocline, modifying the wave breaking mechanism. Combining the results over flat and sloping boundaries, a unified criterion for vortex shedding in arbitrary two-layer continuous stratifications is proposed, which depends on the momentum-thickness Reynolds number and the non-dimensionalized ISW-induced pressure gradient at the point of separation. The criterion is generalized to a form that may be readily computed from field data and compared to published laboratory experiments and field observations. During vortex shedding events, the bed shear stress, vertical velocity and near-bed Reynolds stress were elevated, in agreement with laboratory observations during re-suspension events, indicating that boundary layer instability is an important mechanism leading to sediment re-suspension.

A stochastic parameterization of non‐orographic gravity waves: Formalism and impact on the equatorial stratosphere

Abstract: [1] A formalism is proposed to represent a broadband spectrum of Gravity Waves (GWs) via the superposition of a large ensemble of statistically independent monochromatic ones. To produce this large ensemble at a reasonable numerical cost, we use the fact that the life cycles of the waves needed to be parameterized in General Circulation Models (GCMs) have time scales that largely exceed the time step of the model. We can therefore launch few waves with characteristics chosen randomly at each time step, and make them having an effect on a longer time scale by applying an AR1 relation between the gravity waves drag at a given time and that at the next time step. The stochastic GW parameterization is applied to a GCM in the tropics, and its additional drag causes a realistic Quasi-Biennial Oscillation (QBO). The more realistic wind structure also results in a better representation of the large scale equatorial waves, like the Rossby Gravity Waves (RGWs) with periods around 4–5 day.

Detailed internal wave mixing above a deep-ocean slope

Abstract: 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.

Modeling Semidiurnal Internal Tide Variability in the Southern California Bight

Abstract: The Regional Oceanic Modeling System (ROMS) is applied in a nested configuration with realistic forcing to the Southern California Bight (SCB) to analyze the variability in semidiurnal internal wave generation and propagation. The SCB has a complex topography with supercritical slopes that generate linear internal waves at the forcing frequency. The model predicts the observed barotropic and baroclinic tides reasonably well, although the observed baroclinic tides feature slightly larger amplitudes. The strongest semidiurnal barotropic to baroclinic energy conversion occurs on a steep sill slope of the 1900-m-deep Santa Cruz Basin. This causes a forced, near-resonant, semidiurnal Poincare wave that rotates clockwise in the basin and is of the first mode along the radial, azimuthal, and vertical directions. The associated tidal-mean, depth-integrated energy fluxes and isotherm oscillation amplitudes in the basin reach maximum values of about 5 kW m−1 and 100 m and are strongly modulated by the sprin...

Squall Lines and Convectively Coupled Gravity Waves in the Tropics: Why Do Most Cloud Systems Propagate Westward?

Abstract: The coupling between tropical convection and zonally propagating gravity waves is assessed through Fourier analysis of high-resolution (3-hourly, 0.5°) satellite rainfall data. Results show the familiar enhancement in power along the dispersion curves of equatorially trapped inertia–gravity waves with implied equivalent depths in the range 15–40 m (i.e., pure gravity wave speeds in the range 12–20 m s−1). Here, such wave signals are seen to extend all the way down to zonal wavelengths of around 500 km and periods of around 8 h, suggesting that convection–wave coupling may be important even in the context of mesoscale squall lines. This idea is supported by an objective wave-tracking algorithm, which shows that many previously studied squall lines, in addition to “2-day waves,” can be classified as convectively coupled inertia–gravity waves with the dispersion properties of shallow-water gravity waves. Most of these disturbances propagate westward at speeds faster than the background flow. To under...

Spontaneous inertia-gravity-wave generation by surface-intensified turbulence

Abstract: The spontaneous generation of inertia-gravity waves (IGWs) by surface-intensified, nearly balanced motion is examined using a high-resolution simulation of the primitive equations in an idealized oceanic configuration. At large scale and mesoscale, the dynamics, which is driven by baroclinic instability near the surface, is balanced and qualitatively well described by the surface quasi-geostrophic model. This however predicts an increase of the Rossby number with decreasing spatial scales and, hence, a breakdown of balance at small scale; the generation of IGWs is a consequence of this breakdown. The wave field is analysed away from the surface, at depths where the associated vertical velocities are of the same order as those associated with the balanced motion. Quasi-geostrophic relations, the omega equation in particular, prove sufficient to separate the IGWs from the balanced contribution to the motion. A spectral analysis indicates that the wave energy is localized around dispersion relation for free IGWs, and decays only slowly as the frequency and horizontal wavenumber increase. The IGW generation is highly intermittent in time and space: localized wavepackets are emitted when thin filaments in the surface density are formed by straining, leading to large vertical vorticity and correspondingly large Rossby numbers. At depth, the IGW field is the result of a number of generation events; away from the generation sites it takes the form of a relatively homogeneous, apparently random wave field. The energy of the IGW field generated spontaneously is estimated and found to be several orders of magnitude smaller than the typical IGW energy in the ocean.

The Cascade of Tidal Energy from Low to High Modes on a Continental Slope

Abstract: The linear transfer of tidal energy from large to small scales is quantified for small tidal excursion over a near-critical continental slope. A theoretical framework for low-wavenumber energy transfer is derived from “flat bottom” vertical modes and evaluated with observations from the Oregon continental slope. To better understand the observations, local tidal dynamics are modeled with a superposition of two idealized numerical simulations, one forced by local surface-tide velocities and the other by an obliquely incident internal tide generated at the Mendocino Escarpment 315 km southwest of the study site. The simulations reproduce many aspects of the observed internal tide and verify the modal-energy balances. Observed transfer of tidal energy into high-mode internal tides is quantitatively consistent with observed turbulent kinetic energy (TKE) dissipation. Locally generated and incident simulated internal tides are superposed with varying phase shifts to mimic the effects of the temporally ...

Kinetic energy dynamics in forced, homogeneous, and axisymmetric stably stratified turbulence

Abstract: Direct numerical simulations with up to 4096×4096×2048 grid points are used to study the dynamics of kinetic energy in forced, homogeneous, and axisymmetric stably stratified flow with unit Prandtl number. No mean shear or internal waves are introduced into the flows, forcing represents persistent large horizontal motions with a small amount of noise, and turbulence develops in response. Three different stratification levels result in buoyancy Reynolds numbers ranging from 11 to 230, and multiple cross-checks for consistency in various dimensionless ratios show that the flows are consistent with theory for strong stratification. The balance equations for the horizontal and vertical contributions to kinetic energy are examined in terms of two-dimensional spectra. Downscale cascades of horizontal energy are evident at both the horizontal and the vertical, as is upscale transfer of energy at large horizontal scales but small vertical scales. Vertical energy also cascades down scale in both directions, so the...

Eddy-Modulated Internal Waves and Mixing on a Midocean Ridge

Abstract: 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 . In contr...

Shoaling of large-amplitude nonlinear internal waves at Dongsha Atoll in the northern South China Sea

Abstract: Shoaling of large-amplitude (∼100 m) nonlinear internal waves over a steep slope (∼3°) in water depths between 100 m and 285 m near Dongsha Atoll in the northern South China Sea is examined with an intensive array of thermistor moorings and a bottom mounted Acoustic Doppler Current Profiler. During the 44 h study period in May 5–7, 2008, there were four groups of large internal waves with semidiurnal modulation. In each wave group a rapid transition occurred during the shoaling, such that the front face of the leading depression wave elongated and plunged to the bottom and the rear face steepened and transformed into a bottom-trapped elevation wave. The transitions occur in water depths of 200 m and deeper, and represent the largest documented internal wave shoaling events. The observations repeatedly capture the detailed temperature and velocity structures of the incident plunging waves. Strong horizontal convergence and intense upward motion are found at the leading edge of transformed elevation waves, suggesting flow separation near the bottom. The observations are compared with the previous observations and model studies. The implication of the shoaling internal waves on coral reef ecology also is discussed.

On the recovery of solitary wave profiles from pressure measurements

Abstract: We derive an explicit formula that permits the recovery of the profile of an irrotational solitary water wave from pressure data measured at the flat bed of the fluid domain. The formula is valid for the governing equations and applies to waves of small and large amplitude.

On the effects of frontogenetic strain on symmetric instability and inertia–gravity waves

Abstract: The dynamics of symmetric instability and two-dimensional inertia–gravity waves in a baroclinic geostrophic flow undergoing frontogenesis is analysed. A frontogenetic strain associated with a balanced deformation field drives an ageostrophic circulation and temporal variations in the basic state that significantly affect the properties of perturbations to the background flow. For stable stratification, perturbations to the basic state result in symmetric instability or inertia–gravity waves, depending on the sign of the Ertel potential vorticity and the magnitude of the Richardson number of the geostrophic flow. The kinetic energy (KE) of both types of motion is suppressed by frontogenetic strain due to the vertical shear in the ageostrophic circulation. This is because the perturbation streamlines tilt with the ageostrophic shear causing the disturbances to lose KE via shear production. The effect can completely dampen symmetric instability for sufficiently strong strain even though the source of KE for the instability (the vertical shear in the geostrophic flow) increases with time. Inertia–gravity waves in a baroclinic flow undergoing frontogenesis simultaneously lose KE and extract KE from the deformation field as they decay. This is because the horizontal velocity of the waves becomes rectilinear, resulting in a Reynolds stress that draws energy from the balanced flow. The process is most effective for waves of low frequency and for a geostrophic flow with low Richardson number. However, even in a background flow that is initially strongly stratified, frontogenesis leads to an exponentially fast reduction in the Richardson number, facilitating a rapid energy extraction by the waves. The KE transferred from the deformation field is ultimately lost to the unbalanced ageostrophic circulation through shear production, hence the inertia–gravity waves play a catalytic role in loss of balance. Given the large amount of KE in low-frequency inertia–gravity waves and the ubiquitous combination of strain and baroclinic geostrophic currents in the ocean, it is estimated that this mechanism could play a significant role in the removal of KE from both the internal wave and mesoscale eddy fields.