Showing papers on "Internal wave published in 2013"

Simulation of the seeding of equatorial spread F by circular gravity waves

Abstract: [1] The Naval Research Laboratory three-dimensional simulation code SAMI3/ESF is used to study the response of the postsunset ionosphere to circular gravity waves. We model the coupling of both circular (local) and plane wave (nonlocal) gravity waves to the bottomside F layer as a mechanism for triggering equatorial plasma bubbles. Results support the hypothesis that nonplane gravity waves can more strongly couple to the F layer than plane gravity waves. Results also show that the coupling of the seed wave to the F layer depends on the (nonlocal) growth rate and the local electron density at the position of the seed wave.

Interpreting Energy and Tracer Spectra of Upper-Ocean Turbulence in the Submesoscale Range (1–200 km)

Abstract: Submesoscale (1–200km) wavenumber spectra of kinetic and potential energy and tracer variance are obtained from in situ observations in the Gulf Stream region and in the eastern subtropical North Pacific. In the Gulf Stream region, steep kinetic energy spectra at scales between 200 and 20km are consistent with predictions of interior quasigeostrophic–turbulence theory, both in the mixed layer and in the thermocline. At scales below 20km, the spectra flatten out, consistent with a growing contribution of internal-wave energy at small scales. In the subtropical North Pacific,theenergyspectraareflatterand inconsistentwith predictionsofinteriorquasigeostrophic–turbulence theory. The observed spectra and their dependence on depth are also inconsistent with predictions of surface quasigeostrophic–turbulencetheoryfortheobservedoceanstratification.Itappearsthatunbalancedmotions,most likely internal tides at large scales and the internal-wave continuum at small scales, dominate the energy spectrum throughout the submesoscale range. Spectra of temperature variance along density surfaces, which are not affected by internal tides, are also inconsistent with predictions of geostrophic-turbulence theories. Reasons for this inconsistency could be the injection of energy in the submesoscale range by small-scale baroclinic instabilities or modifications of the spectra by coupling between surface and interior dynamics or by ageostrophic frontal effects.

Internal waves and turbulence in the Antarctic Circumpolar Current

Abstract: This study reports on observations of turbulent dissipation and internal wave-scale flow properties in a standing meander of the Antarctic Circumpolar Current (ACC) north of the Kerguelen Plateau. The authors characterize the intensity and spatial distribution of the observed turbulent dissipation and the derived turbulent mixing, and consider underpinning mechanisms in the context of the internal wave field and the processes governing the waves’ generation and evolution.The turbulent dissipation rate and the derived diapycnal diffusivity are highly variable with systematic depth dependence. The dissipation rate is generally enhanced in the upper 1000–1500 m of the water column, and both the dissipation rate and diapycnal diffusivity are enhanced in some places near the seafloor, commonly in regions of rough topography and in the vicinity of strong bottom flows associated with the ACC jets. Turbulent dissipation is high in regions where internal wave energy is high, consistent with the idea that i...

Overturning circulation driven by breaking internal waves in the deep ocean

Abstract: A global estimate of the water-mass transformation by internal wave-driven mixing in the deep ocean is presented. The estimate is based on the energy conversion from tidal and geostrophic motions into internal waves combined with a turbulent mixing parameterization. We show that internal wave-driven mixing in the deep ocean can sustain 20–30 Sv of water-mass transformation. One third or more of this transformation is attributed to lee waves generated by geostrophic motions flowing over rough topography, primarily in the Southern Ocean. While these results are uncertain due to many assumptions, poorly constrained parameters and data noise that enter in the calculation, the result that lee wave-driven mixing plays an important role in the abyssal ocean circulation is likely robust. The implication is that lee wave-driven mixing should be represented in ocean and climate models, but currently it is not. Citation: Nikurashin, M., and R. Ferrari (2013), Overturning circulation driven by breaking internal waves in the deep ocean, Geophys. Res. Lett., 40, doi:10.1002/grl.50542.

Parametric Subharmonic Instability of the Internal Tide at 29°N

Abstract: Observational evidence is presented for transfer of energy from the internal tide to near-inertial motions near 298N in the Pacific Ocean. The transfer is accomplished via parametric subharmonic instability (PSI), which involvesinteractionbetweena primarywave (the internaltide in this case) andtwo smaller-scale waves of nearly half the frequency. The internal tide at this location is a complex superposition of a low-mode waves propagating north from Hawaii and higher-mode waves generated at local seamounts, making application of PSI theory challenging. Nevertheless, a statistically significant phase locking is documented between the internal tide and upward- and downward-propagating near-inertial waves. The phase between those three waves is consistent with that expected from PSI theory. Calculated energy transfer rates from the tide to nearinertial motions are modest, consistent with local dissipation rate estimates. The conclusion is that while PSI does befall the tide near a critical latitude of 298N, it does not do so catastrophically.

Near-Inertial Waves in Strongly Baroclinic Currents

Abstract: An analysis and physical interpretation of near-inertial waves (NIWs) propagating perpendicular to a steady, two-dimensional, strongly baroclinic, geostrophic current are presented. The analysis is appropriate for geostrophic currents with order-one Richardson numbers such as those associated with fronts experiencing strong, wintertime atmospheric forcing. This work highlights the underlying physics behind the properties of the NIWs using parcel arguments and the principles of conservation of density and absolute momentum. Baroclinicity introduces lateral gradients in density and vertical gradients in absolute momentum that significantly modify the dispersion and polarization relations and propagation of NIWs relative to classical internal wave theory. In particular, oscillations at the minimum frequency are not horizontal but, instead, are slanted along isopycnals. Furthermore, the polarization of the horizontal velocity is not necessarily circular at the minimum frequency and the spiraling of th...

Modern internal waves and internal tides along oceanic pycnoclines: Challenges and implications for ancient deep-marine baroclinic sands

Abstract: Thus far, the subject of deep-marine sands emplaced by baroclinic currents associated with internal waves and internal tides as potential reservoirs has remained an alien topic in petroleum exploration. Internal waves are gravity waves that oscillate along oceanic pycnoclines. Internal tides are internal waves with a tidal frequency. Internal solitary waves (i.e., solitons), the most common type, are commonly generated near the shelf edge (100–200 m [328–656 ft] in bathymetry) and in the deep ocean over areas of sea-floor irregularities, such as mid-ocean ridges, seamounts, and guyots. Empirical data from 51 locations in the Atlantic, Pacific, Indian, Arctic, and Antarctic oceans reveal that internal solitary waves travel in packets. Internal waves commonly exhibit (1) higher wave amplitudes (5–50 m [16–164 ft]) than surface waves (2 m [6.56 ft]), (2) longer wavelengths (0.5–15 km [0.31–9 mi]) than surface waves (100 m [328 ft]), (3) longer wave periods (5–50 min) than surface waves (9–10 s), and (4) higher wave speeds (0.5–2 m s–1 [1.64–6.56 ft s–1]) than surface waves (25 cm s–1 [10 in. s–1]). Maximum speeds of 48 cm s–1 (19 in. s–1) for baroclinic currents were measured on guyots. However, core-based sedimentologic studies of modern sediments emplaced by baroclinic currents on continental slopes, in submarine canyons, and on submarine guyots are lacking. No cogent sedimentologic or seismic criteria exist for distinguishing ancient counterparts. Outcrop-based facies models of these deposits are untenable. Therefore, potential exists for misinterpreting deep-marine baroclinic sands as turbidites, contourites, basin-floor fans, and others. Economic risks associated with such misinterpretations could be real.

Revisiting internal waves and mixing in the Arctic Ocean

Abstract: [1] To determine whether deep background mixing has increased with the diminishment of the Arctic sea ice, we compare recent internal wave energy and mixing observations with historical measurements. Since 2007, the North Pole Environmental Observatory has launched expendable current probes (XCPs) as a part of annual airborne hydrographic surveys in the central Arctic Ocean. Mixing in the upper 500 m is estimated from XCP shear variance and Conductivity-Temperature-Depth (CTD) derived Brunt-Vaisala frequency. Internal wave energy levels vary by an order of magnitude between surveys, although all surveys are less energetic and show more vertical modes than typical midlatitude Garrett-Munk (GM) model spectra. Survey-averaged mixing estimates also vary by an order of magnitude among recent surveys. Comparisons between modern and historical data, reanalyzed in identical fashion, reveal no trend evident over the 30 year period in spite of drastic diminution of the sea ice. Turbulent heat fluxes are consistent with recent double-diffusive estimates. Both mixing and internal wave energy in the Beaufort Sea are lower when compared to both the central and eastern Arctic Ocean, and expanding the analysis to mooring data from the Beaufort Sea reveals little change in that area compared to historical results from Arctic Internal Wave Experiment. We hypothesize that internal wave energy remains lowest in the Beaufort Sea in spite of dramatic declines in sea ice there, because increased stratification amplifies the negative effect of boundary layer dissipation on internal wave energy.

Experimental study of parametric subharmonic instability for internal plane waves

Abstract: Internal waves are believed to be of primary importance as they affect ocean mixing and energy transport. Several processes can lead to the breaking of internal waves and they usually involve nonlinear interactions between waves. In this work, we study experimentally the parametric subharmonic instability (PSI), which provides an efficient mechanism to transfer energy from large to smaller scales. It corresponds to the destabilization of a primary plane wave and the spontaneous emission of two secondary waves, of lower frequencies and different wave vectors. Using a time-frequency analysis, we observe the time evolution of the secondary waves, thus measuring the growth rate of the instability. In addition, a Hilbert transform method allows the measurement of the different wave vectors. We compare these measurements with theoretical predictions, and study the dependence of the instability with primary wave frequency and amplitude, revealing a possible effect of the confinement due to the finite size of the beam, on the selection of the unstable mode.

A stochastic parameterization of the gravity waves due to convection and its impact on the equatorial stratosphere

Abstract: [1] A formalism is proposed to parameterize the gravity waves due to convection in general circulation models with a stratosphere It is based on a stochastic approach, where a large ensemble of monochromatic gravity waves is built up by launching a few waves at each time step, and by adding the effect of these waves, to that of the waves launched before, during the same day The frequency and horizontal wave numbers of each wave are chosen randomly with fixed probability distribution, but the wave amplitude is directly related to precipitation, which is converted into heating rate Linear theory is then used to predict the gravity wave generated by the heating rate Off‒line tests are carried out using reanalysis and global precipitation data These tests demonstrate that the scheme launches gravity wave momentum fluxes that are much more erratic in amplitude than when uniform sources are considered Consequently, the scheme tends to produce momentum flux deposition at lower levels than for the case when uniform sources are considered We verify that the parameterization, when included in a general circulation model with vertical resolution in the stratosphere δz≈ 500m, is able to produce a quasi‒biennial oscillation, without being detrimental to other aspects of the model climatology, like the semiannual oscillation and the behavior of the extratropics

A Global Model for the Diapycnal Diffusivity Induced by Internal Gravity Waves

Abstract: An energetically consistent model for the diapycnal diffusivity induced by breaking of internal gravity waves is proposed and tested in local and global settings. The model [Internal Wave Dissipation, Energy and Mixing (IDEMIX)] is based on the spectral radiation balance of the wave field, reduced by integration over the wavenumber space, which yields a set of balances for energy density variables in physical space. A further simplification results in a single partial differential equation for the total energy density of the wave field. The flux of energy to high vertical wavenumbers is parameterized by a functional derived from the wave–wave scattering integral of resonant wave triad interactions, which also forms the basis for estimates of dissipation rates and related diffusivities of ADCP and hydrography fine-structure data. In the current version of IDEMIX, the wave energy is forced by wind-driven near-inertial motions and baroclinic tides, radiating waves from the respective boundary layers ...

A Stability Criterion for Two-Fluid Interfaces and Applications

Abstract: We derive a new stability criterion for two-fluid interfaces that ensures the existence of “stable” local solutions that do not break down too fast due to Kelvin–Helmholtz instabilities. It can be seen both as a two-fluid generalization of the Rayleigh–Taylor criterion and as a nonlinear version of the Kelvin stability condition. We show that gravity can control the inertial effects of the shear up to frequencies that are high enough for the surface tension to play a relevant role. This explains why surface tension is a necessary condition for well-posedness while the (low frequency) main dynamics of interfacial waves are unaffected by it. In order to derive a practical version of this criterion, we work with a nondimensionalized version of the equations and allow for the possibility of various asymptotic regimes, such as the shallow water limit. This limit being singular, we have to derive a new symbolic analysis of the Dirichlet–Neumann operator that includes an infinitely smoothing “tail” accounting for the contribution of the bottom. We then validate our criterion by comparison with experimental data in two important settings: air–water interfaces and internal waves. The good agreement we observe allows us to discuss the scenario of wave breaking and the behavior of water-brine interfaces, and to propose a formula for the maximal amplitude of interfacial waves. We also show how to rigorously justify two-fluid asymptotic models used for applications and how to relate some of their properties to Kelvin–Helmholtz instabilities.

The geography of semidiurnal mode-1 internal-tide energy loss

Abstract: [1] The semidiurnal mode-1 internal tide receives 0.1–0.3 TW from the surface tide and is capable of propagating across ocean basins. The ultimate fate of mode-1 energy after long-distance propagation is poorly constrained by existing observations and numerical simulations. Here, global results from a two-dimensional semi-analytical model indicate that topographic scattering is inefficient at most locations deeper than 2500 m. Next, results from a one-dimensional linear model with realistic topography and stratification create a map of mode-1 scattering coefficients along the continental margins. On average, mode-1 internal tides lose about 60% of their energy upon impacting the continental margins: 20% transmits onto the continental shelf, 40% scatters to higher modes, and 40% reflects back to the ocean interior. These analyses indicate that the majority of mode-1 energy is likely lost at large topographic features (e.g., continental slopes, seamounts, and mid-ocean ridges), where it may drive elevated turbulent mixing.

Tropical storm‐induced near‐inertial internal waves during the Cirene experiment: Energy fluxes and impact on vertical mixing

Abstract: [1] Near-inertial internal waves (NIW) excited by storms and cyclones play an essential role in driving turbulent mixing in the thermocline and interior ocean. Storm-induced mixing may be climatically relevant in regions like the thermocline ridge in the southwestern Indian Ocean, where a shallow thermocline and strong high frequency wind activity enhance the impact of internal gravity wave-induced mixing on sea surface temperature. The Cirene research cruise in early 2007 collected ship-borne and mooring vertical profiles in this region under the effect of a developing tropical cyclone. In this paper, we characterize the NIW field and the impact of these waves on turbulent mixing in the upper ocean. NIW packets were identified down to 1000 m, the maximum depth of the measurements. We estimated an NIW vertical energy flux of up to 2.5 mW m−2 within the pycnocline, which represents about 10% of the maximum local wind power input. A non-negligible fraction of the wind power input is hence potentially available for subsurface mixing. The impact of mixing by internal waves on the upper ocean heat budget was estimated from a fine-scale mixing parameterization. During the first leg of the cruise (characterized by little NIW activity), the average heating rate due to mixing was ~0.06 °C month−1 in the thermocline (23–24 kg m−3 isopycnals). During the second leg, characterized by strong NIW energy in the thermocline and below, this heating rate increased to 0.42 °C month−1, indicative of increased shear instability along near inertial wave energy pathways.

Eddy-Induced Modulation of Turbulent Dissipation over Rough Topography in the Southern Ocean

Abstract: Mesoscale eddies are universal features of the ocean circulation, yet the processes by which their energy is dissipated remain poorly understood. One hypothesis argues that the interaction of strong geostrophic flows with rough bottom topography effects an energy transfer between eddies and internal waves, with the breaking of these waves causing locally elevated dissipation focused near the sea floor. This study uses hydrographic and velocity data from a 1-yr mooring cluster deployment in the Southern Ocean to test this hypothesis. The moorings were located over a small (~10 km) topographic obstacle to the east of Drake Passage in a region of high eddy kinetic energy, and one was equipped with an ADCP at 2800-m depth from which internal wave shear variance and dissipation rates were calculated. Examination of the ADCP time series revealed a predominance of upward-propagating internal wave energy and a significant correlation (r = 0.45) between shear variance levels and subinertial near-bottom cur...

Effects of Kuroshio intrusions on nonlinear internal waves in the South China Sea during winter

Abstract: [1] During winter the Kuroshio tends to cross Luzon Strait, penetrating the northeastern South China Sea where it forms energetic mesoscale structures. Luzon Strait is also a site where westward-propagating large-amplitude internal waves are generated. We describe observations of these waves acquired in the deep basin of the South China Sea during the winter of 2010−2011, with the goal of assessing the influence of mesoscale variability on their properties. Combining tidal current simulations with an internal wave generation and evolution model, we obtain time series of deviations between our observations and the model simulations. These deviations are analyzed in terms of mesoscale variability based on the data-assimilated HYbrid Coordinate Ocean Model (HYCOM) simulations. We find that simplified models of nonlinear internal wave response to changes in horizontal stratification gradients and vertical shear provide at best weak simulations of amplitude modulation. In contrast to these results, deviations of internal wave arrival time, occurring up to 2 h early during Kuroshio inflows, are quite well simulated when derived from integration of the first internal mode phase speed along two-dimensional ray-traced paths using HYCOM simulations of velocity and density fields together with bathymetry. Refraction of the internal waves by Kuroshio intrusions can lead to substantial distortion of the paths, sufficient to provide a potential explanation for the apparent suppression of waves during previous winter measurements. Our results suggest that the internal wave field can be a sensitive indicator of mesoscale variability in Luzon Strait and the northeastern South China Sea.

Observations of Near-Inertial Internal Gravity Waves Radiating from a Frontal Jet

Abstract: Shipboard ADCP and towed CTD measurements are presented of a near-inertial internal gravity wave radiating away from a zonal jet associated with the Subtropical Front in the North Pacific. Three-dimensional spatial surveys indicate persistent alternating shear layers sloping downward and equatorward from the front. As a result, depth-integrated ageostrophic shear increases sharply equatorward of the front. The layers have a vertical wavelength of about 250 m and a slope consistent with a wave of frequency 1.01f. They extend at least 100 km south of the front. Time series confirm that the shear is associated with a downward-propagating near-inertial wave with frequency within 20% of f. A slab mixed layer model forced with shipboard and NCEP reanalysis winds suggests that wind forcing was too weak to generate the wave. Likewise, trapping of the near-inertial motions at the low-vorticity edge of the front can be ruled out because of the extension of the features well south of it. Instead, the authors...

Measurements of shoaling internal waves and turbulence in an estuary

Abstract: [1] The shoaling of horizontally propagating internal waves may represent an important source of mixing and transport in estuaries and coastal seas Including such effects in numerical models demands improvements in the understanding of several aspects of the energetics, especially those relating to turbulence generation, and observations are needed to build this understanding To address some of these issues in the estuarine context, we undertook an intensive field program for 10 days in the summer of 2008 in the St Lawrence Estuary The sampling involved shore-based photogrammetry, ship-based surveys, and an array of moorings in the shoaling region that held both conventional and turbulence-resolving sensors The measurements shed light on many aspects of the wave shoaling process Wave arrivals were generally phase-locked with the M2 tide, providing hints about far-field forcing In the deeper part of the study domain, the waves propagated according to the predictions of linear theory In intermediate-depth waters, the waves traversed the field site perpendicularly to isobaths, a pattern that continued as the waves transformed nonlinearly Acoustic Doppler velocimeters permitted inference of the turbulent energetics, and two main features were studied First, during a period of shoaling internal waves, turbulence dissipation rates exceeded values associated with tidal shear by an order of magnitude Second, the evolving spectral signatures associated with a particular wave-shoaling event suggest that the turbulence is at least partly locally generated Overall, the results of this study suggest that parameterizations of wave-induced mixing could employ relatively simple dynamics in deep water, but may have to handle a wide suite of turbulence generation and transport mechanisms in inshore regions

Exact geophysical waves in stratified fluids

Abstract: When studying water waves travelling over an inviscid fluid at the Earth's surface there are additional Coriolis and centrifugal forces which influence the motion of the fluid particles. In particular, for waves propagating near the Equator the geophysical wave problem can be modelled by the so-called f-plane approximation. In this paper, we provide an explicit exact solution to the edge wave problem for stratified geophysical flows in the equatorial f-plane approximation.

Electromagnetic Ion Cyclotron Waves in the Magnetosphere

Abstract: Electromagnetic ion cyclotron (EMIC) waves are generated in the equatorial region of the plasmasphere-magnetosphere by internal wave-particle interaction with ring current ions. In ground observations they are observed as Pc 1-2 (0.1-5 Hz) waves, and one group of waves exhibits a spectral fine structure that has been classically explained by bouncing packet field-aligned propagation. An unstructured class of Pc 1-2 waves, including intervals of pulsations with diminishing period, lacks a fine structure pattern and is the dominant emission observed in the middle and outer magnetosphere by satellites. Poynting flux observations show that wave energy propagates unidirectionally away from the equatorial region, which does not support the bouncing wave packet paradigm. The cold/cool magnetospheric plasma has a profound effect on the generation and spectral properties of EMIC waves. The waves are often observed at geostationary orbit within the outer magnetosphere extension of plasmaspheric plasma plumes seen by the IMAGE spacecraft in the extreme ultraviolet (EUV) imager instrument and associated with subauroral proton arcs seen by the far ultraviolet (FUV) imager. This provides evidence in support of a ring current loss mechanism induced by pitch angle scattering of protons by EMIC waves. Characteristic frequencies introduced into the cold/cool plasma by heavy O + and He + ions determine the EMIC wave spectra and these may be used in plasma diagnostic studies. Outstanding issues considered include the possible role of the ionospheric Alfven resonator in EMIC wave generation, and the modulation of EMIC waves by Pc 3-5 long-period ULF waves.

The Latitudinal Dependence of Shear and Mixing in the Pacific Transiting the Critical Latitude for PSI

Abstract: Turbulent mixing rates are inferred from measurements spanning 25°–37°N in the Pacific Ocean. The observations were made as part of the Internal Waves Across the Pacific experiment, designed to investigate the long-range fate of the low-mode internal tide propagating north from Hawaii. Previous and companion results argue that, near a critical latitude of 29°N, the internal tide loses energy to high-mode near-inertial motions through parametric subharmonic instability. Here, the authors estimate mixing from several variations of the finescale shear–strain parameterization, as well as Thorpe-scale analysis of overturns. Though all estimated diffusivities are modest in magnitude, average diffusivity in the top kilometer shows a factor of 2–4 elevation near and equatorward of 29°N. However, given intrinsic uncertainty and the strong temporal variability of diffusivity observed in long mooring records, the meridional mixing pattern is found to be near the edge of statistical significance.

Study of the propagation direction of the internal waves in the South China Sea using satellite images

Abstract: Internal wave propagation carries considerable vertical shear which can lead to turbulence and mixing. Based on the analysis of more than 2 500 synthetic aperture radar (SAR) and optical satellite images, the internal wave propagation in the whole South China Sea was investigated systematically. The results show that (1) in the northeastern South China Sea, most internal waves propagate westward from the Luzon Strait and are diffracted by coral reefs near the Dongsha Islands. Some impinge onto the shelf and a few are reflected; (2) in the northwestern South China Sea, most internal waves are generated at the shelf and propagate northwestward or westward to the coast; (3) in the western South China Sea, most internal waves propagate westward to the Vietnamese coast, except a few propagate southward to the deep sea; and (4) in the southern South China Sea, most internal waves propagate southwestward to the coast. Some propagate southeastward to the coast of Kalimantan Island, and a few propagate southeastward because of the influence of the Mekong River.

High‐frequency internal waves near the Luzon Strait observed by underwater gliders

Abstract: [1] Tidal flow through the Luzon Strait produces large internal waves that propagate westward into the South China Sea and eastward into the Pacific. Underwater gliders gathered sustained observations of internal waves during seven overlapping missions from April 2007 through July 2008. A particular focus is the high-frequency internal waves, where the operational definition of high involves periods shorter than a glider profile taking 3–6 h. Internal wave vertical velocity is estimated from measurements of pressure and glider orientation through two methods: (1) use of a model of glider flight balancing buoyancy and drag along the glider path and (2) high-pass filtering of the observed glider vertical velocity. By combining high-frequency vertical velocities from glider flight with low-frequency estimates from isopycnal depth variations between dives, a spectrum covering five decades of frequency is constructed. A map of the standard deviation of vertical velocity over the survey area shows a decay from the Luzon Strait into the Pacific. The growth of high-frequency vertical velocity with propagation into the South China Sea is observed through two 2-week time series stations. The largest observed vertical velocities are greater than 0.2 m s−1 and are associated with displacements approaching 200 m. The high-frequency waves are observed at regular intervals of 1 day as they ride on diurnal tidal internal waves generated in the Strait.