Showing papers on "Internal wave published in 2022"

Experimental observations of internal wave turbulence transition in a stratified fluid

Abstract: Recent developments of the weak turbulence theory applied to internal waves exhibit a power-law solution of the kinetic energy equation close to the oceanic Garrett & Munk spectrum, confirming weakly nonlinear wave interactions as a likely explanation of the observed oceanic spectra. However, finite-size effects can hinder wave interactions in bounded domains, and observations often differ from theoretical predictions. This article studies the dynamical regimes experimentally developing in a stratified fluid forced by internal gravity waves in a pentagonal domain. We find that by changing the shape and increasing the dimensions of the domain, finite-size effects diminish and wave turbulence is observed. In this regime, the temporal spectra decay with a slope compatible with the Garrett-Munk spectra. Different regimes appear by changing the forcing conditions, namely discrete wave turbulence, weak wave turbulence, and strongly stratified turbulence. The buoyancy Reynolds number Re b marks well the transitions between the regimes, with weak wave turbulence occurring for 1 (cid:46) Re b (cid:46) 3 . 5 and strongly non-linear stratified turbulence for higher Re b .

Identification of Inertial Modes in the Solar Convection Zone

Abstract: The observation of global acoustic waves (p modes) in the Sun has been key to unveiling its internal structure and dynamics. A different kind of wave, known as sectoral Rossby modes, has been observed and identified, which potentially opens the door to probing internal processes that are inaccessible through p-mode helioseismology. Yet another set of waves, appearing as retrograde-propagating, equatorially antisymmetric vorticity waves, has also been observed but their identification remained elusive. Here, through a numerical model implemented as an eigenvalue problem, we provide evidence supporting the identification of those waves as a class of inertial eigenmodes, distinct from the Rossby-mode class, with radial velocities comparable to the horizontal ones deep in the convective zone but still small compared to the horizontal velocities toward the surface. We also suggest that the signature of tesseral-like Rossby modes might be present in recent observational data.

Decadal observations of internal wave energy, shear, and mixing in the western Arctic Ocean

Abstract: As Arctic sea ice declines, wind energy has increasing access to the upper ocean, with potential consequences for ocean mixing, stratification, and turbulent heat fluxes. Here, we investigate the relationships between internal wave energy, turbulent dissipation, and ice concentration and draft using mooring data collected in the Beaufort Sea during 2003–2018. We focus on the 50–300 m depth range, using velocity and CTD records to estimate near-inertial shear and energy, a finescale parameterization to infer turbulent dissipation rates, and ice draft observations to characterize the ice cover. All quantities varied widely on monthly and interannual timescales. Seasonally, near-inertial energy increased when ice concentration and ice draft were low, but shear and dissipation did not. We show that this apparent contradiction occurred due to the vertical scales of internal wave energy, with open water associated with larger vertical scales. These larger vertical scale motions are associated with less shear, and tend to result in less dissipation. This relationship led to a seasonality in the correlation between shear and energy. This correlation was largest in the spring beneath full ice cover and smallest in the summer and fall when the ice had deteriorated. When considering interannually averaged properties, the year-to-year variability and the short ice-free season currently obscure any potential trend. Implications for the future seasonal and interannual evolution of the Arctic Ocean and sea ice cover are discussed.

Mean and Turbulent Characteristics of a Bottom Mixing‐Layer Forced by a Strong Surface Tide and Large Amplitude Internal Waves

Abstract: We present 15 days of both mean and turbulent field observations bottom mixing-layer at a gently sloping 250 m deep continental shelf site, energized by tides and nonlinear internal waves (NLIWs). The tidal frequency forcing was due to the combined effects of the barotropic tide and a mode-1 internal tide (IT), while the NLIWs were predominantly mode-1 waves of depression. The bottom mixing-layer thickness varied at both semidiurnal and sub-tidal ∼O(10)d frequencies, with an average thickness of around 10 m. Compression and expansion of the mixing-layer by both the IT and NLIWs affected the mean velocity profiles in the mixing-layer, while the phasing between the barotropic and baroclinic flows led to an asymmetry in mean velocity profiles between periods of rising and falling isotherms. With the exception of periods of flow reversal, the turbulent kinetic energy balance and turbulent stress observations were consistent with the existence of an inertial-sublayer with thickness of approximately 10%–15% of the mixing-layer thickness ( ∼1 m), even beneath NLIWs. In the outer portion of the mixing-layer—that is, above the inertial-sublayer—NLIWs modulated the local turbulence spectra. We discuss the observations in the context of a predictive model for mixing-layer thickness. The analysis suggests that the high-frequency variability in mixing-layer thickness was dominated by internal wave pumping, though strength of the ambient stratification and the frequency of the forcing were important controls on the time-averaged (sub-tidal) variation.

Interannual variability of internal tides in the Andaman Sea: an effect of Indian Ocean Dipole

Abstract: A marginal sea in the north eastern Indian Ocean, the Andaman Sea, has been known for the presence of high-amplitude internal waves since the nineteenth century. In this study, we explored the interannual variations of the internal wave activity in this complex region. We found that the Dipole Mode Index, which represents the Indian Ocean Dipole (IOD), influences the circulation in the Andaman Sea, which in turn impacts its density stratification on interannual scales. Ocean Reanalysis System 5 data (1993-2018) is used to see an increasing trend in the sub-surface stratification, whereas it showed a decreasing trend in the near-surface waters. Numerical model simulations carried out from 2009 to 2018 have shown that the interannual variability in the generation of semidiurnal internal tides is governed by distinct parameters (tidal forcing and stratification) at different sites in different months. Enhanced upwelling (downwelling) is observed during positive (negative) IOD events. Sensitivity experiments conducted between extreme IOD events (2006 and 2016) revealed an increase in internal tide generation from positive IOD to negative IOD. Furthermore, a sharp decrease in local baroclinic dissipation is seen during negative IOD, increasing baroclinic flux into the Andaman Sea. An increase in the strength of positive IOD could lead to enhanced diapycnal mixing due to strong local dissipation, whereas an increase in the intensity of negative IOD could result in amplified propagation of internal waves.

Development of the yearly mode-1 M2 internal tide model in 2019

Abstract: The yearly mode-1 M2 internal tide model in 2019 is constructed using sea-surface height measurements made by six concurrent satellite altimetry missions: Jason-3, Sentinel-3A, Sentinel-3B, CryoSat-2, Haiyang-2A and SARAL/AltiKa. The model is developed following a three-step procedure consisting of two rounds of plane wave analysis with a spatial bandpass filter in between. Prior mesoscale correction is made on the altimeter data using AVISO gridded mesoscale fields. The model is labeled Y2019, because it represents the one-year-coherent internal tide field in 2019. In contrast, the model developed using altimeter data from 1992–2017 is labeled MY25, because it represents the multi-year-coherent internal tide field in 25 years. Thanks to the new mapping technique, model errors in Y2019 are as low as those in MY25. Evaluation using independent altimeter data confirms that Y2019 reduces slightly less variance (∼6%) than MY25. Further analysis reveals that the altimeter data from five missions (without Jason-3) can yield an internal tide model of almost same quality. Comparing Y2019 and MY25 shows that mode-1 M2 internal tides are subject to significant interannual variability in both amplitude and phase, and their interannual variations are a function of location. Along southward internal tides from Amukta Pass, the energy flux in Y2019 is two times large and the phase speed is about 1.1% faster. This mapping technique has been applied successfully to 2017 and 2018. This work demonstrates that yearly internal tides can be observed by concurrent altimetry missions and their interannual variations can be determined.

Hydrodynamic behavior of a circular floating solar pond with an entrapped two-layer fluid

Abstract: The resonant behavior in a moonpool of a floating circular solar pond, with an entrapped two-layer fluid, is studied. The problem is solved by applying a domain-decomposition method using eigenfunction matching. The surface- and internal-wave elevations and the hydrodynamic coefficients of a typical floating solar pond under forced heave or surge motion are computed. The effects of density stratification on surface-wave elevation, added mass, and damping coefficients are analyzed. A collection of resonance frequencies of surface and internal waves is examined, together with the corresponding variations of modal shapes. For heave resonance, the surface and internal waves are characterized by axisymmetric sloshing modes, as opposed to antisymmetric sloshing modes under surge resonances. A frozen-mode approximation method that treats the moonpool fluid as a density-stratified solid is developed to estimate piston-mode frequencies. Non-dimensional resonance frequencies corresponding to antisymmetric and axisymmetric sloshing modes are estimated based on the standing-wave approximation and reciprocity relations between surface and internal wavenumbers. Satisfactory agreement between the estimated resonance frequencies and those computed by eigenfunction matching method is achieved. It is found that the first resonance of the internal wave, rather than higher-order resonances, is more likely to affect the surface-wave behavior, whereas resonances of the surface-wave modes have significant effects on the internal waves. Parametric analyses are performed to study the effects of geometry configurations of the pond. It is found that the resonance frequencies of internal waves under forced heave or surge motion decrease with an increasing density ratio.

Separating Energetic Internal Gravity Waves and Small‐Scale Frontal Dynamics

Abstract: Oceanic fronts with lateral scales less than 20 km are now known to be one of the major contributors to vertical heat fluxes in the global ocean, which highlights their potential impact on Earth's climate. However, frontal dynamics with time scales less than 1 day, whose contribution to vertical heat fluxes is thought to be significant, are obscured by energetic internal gravity waves (IGWs). In this study, we address this critical issue by separating IGWs and frontal dynamics using an approach based on their respective vertical scales of variability. Results using a numerical model with a horizontal grid spacing of 500 m confirm that it is possible to recover frontal dynamics at short time scales as well as associated intense vertical velocities and vertical heat fluxes. This opens up new possibilities for a more accurate estimation of the vertical exchanges of any tracers between the surface and the ocean interior.

Observations of coherent transverse wakes in shoaling nonlinear internal waves

Abstract: Space- and time-continuous seafloor temperature observations captured the three-dimensional structure of shoaling nonlinear internal waves (NLIWs) off of La Jolla, California. NLIWs were tracked for hundreds of meters in the cross- and along-shelf directions using a fiber optic Distributed Temperature Sensing (DTS) seafloor array, complemented by an ocean-wave-powered vertical profiling mooring. Trains of propagating cold-water pulses were observed on the DTS array inshore of the location of polarity transition predicted by weakly nonlinear internal wave theory. The subsequent evolution of the temperature signatures during shoaling was consistent with that of strongly nonlinear internal waves with a large Froude number, highlighting their potential to impact property exchange. Unexpectedly, individual NLIWs were trailed by a coherent, small-scale pattern of seabed temperature variability as they moved across the mid- and inner shelf. A kinematic model was used to demonstrate that the observed patterns were consistent with a transverse instability with an along-crest wavelength of ∼10 m – a distance comparable to the cross-crest width of the wave-core – and with an inferred amplitude of several meters. The signature of this instability is consistent with the span-wise vortical circulations generated in three-dimensional direct numerical simulations of shoaling and breaking nonlinear internal waves. The coupling between the small-scale transverse wave-wake and turbulent wave-core may have an important impact on mass, momentum, and tracer redistribution in the coastal ocean.

Investigation of Internal Tides Variability in the Andaman Sea: Observations and Simulations

Abstract: The Andaman Sea in the Indian Ocean is known for the presence of large-amplitude Internal waves (IW). Internal tides (IT) are IW of tidal frequency whose temporal variability is unknown in this region. Therefore, we used in-situ observations collected at (10.5°N, 94°E) from March 2017 to February 2018. The analysis shows that the kinetic energy of semidiurnal IT is dominant to that of diurnal IT by a factor of 4–5. The ellipticity of both semidiurnal and diurnal motions is dominated by rectilinear zonal flow, indicating generation at the slopes of the Andaman and Nicobar islands. Maximum isopycnal displacement reached 46 m for semidiurnal IT. The semidiurnal IT displayed significant seasonal variability—Stronger in summer and autumn but weaker in spring and winter, whereas the diurnal IT are relatively stronger in summer and winter. Furthermore, salinity plays a dominant role in controlling the near-surface stratification, whereas the temperature variations control the subsurface stratification. This led to the formation of a strong double pycnocline during autumn and winter. Baroclinic coherent semidiurnal (diurnal) variance accounts for 49% (27%) of the semidiurnal (diurnal) motions. Model simulations carried out for March, June, September, and December of 2017 using the Massachusetts Institute of Technology General Circulation Model showed significant seasonal variability in the generation and dissipation of IT. The isopycnal displacement near the generation sites is about 88 m. The experiments suggest that the presence of the Andaman Nicobar Ridge contributes nearly 89% to the total IT generation.

Instabilities in internal gravity waves

Abstract: Internal gravity waves are propagating disturbances in stably stratified fluids, and can transport momentum and energy over large spatial extents. From a fundamental viewpoint, internal waves are interesting due to the nature of their dispersion relation, and their linear dynamics are reasonably well-understood. From an oceanographic viewpoint, a qualitative and quantitative understanding of significant internal wave generation in the ocean is emerging, while their dissipation mechanisms are being debated. This paper reviews the current knowledge on instabilities in internal gravity waves, primarily focusing on the growth of small-amplitude disturbances. Historically, wave-wave interactions based on weakly nonlinear expansions have driven progress in this field, to investigate spontaneous energy transfer to various temporal and spatial scales. Recent advances in numerical/experimental modeling and field observations have further revealed noticeable differences between various internal wave spatial forms in terms of their instability characteristics; this in turn has motivated theoretical calculations on appropriately chosen internal wave fields in various settings. After a brief introduction, we present a pedagogical discussion on linear internal waves and their different two-dimensional spatial forms. The general ideas concerning triadic resonance in internal waves are then introduced, before proceeding towards instability characteristics of plane waves, wave beams and modes. Results from various theoretical, experimental and numerical studies are summarized to provide an overall picture of the gaps in our understanding. An ocean perspective is then given, both in terms of the relevant outstanding questions and the various additional factors at play. While the applications in this review are focused on the ocean, several ideas are relevant to atmospheric and astrophysical systems too.

Distribution and Source Sites of Nonlinear Internal Waves Northeast of Hainan Island

Abstract: The distribution and source sites of nonlinear internal waves (NLIWs) northeast of Hainan Island were investigated using satellite observations and a wavefront propagation model. Satellite observations show two types of NLIWs (here referred to as type-S and type-D waves). The type-S waves are spaced at a semidiurnal tidal period and the type-D waves are spaced at a diurnal tidal period. The spatial distribution of the two types of NLIWs displays a sandwich structure in which the middle region is influenced by both types of NLIWs, and the northern and southern regions are governed by the type-S and type-D waves, respectively. Solving the wavefront model yields good agreement between simulated and observed wavefronts from the Luzon Strait to Hainan Island. We conclude that the NLIWs originate from the Luzon Strait.

Mixing in the Southern Ocean

Abstract: Southern Ocean mixing helps to establish properties of the global ocean both by blending waters from the northern basins and through local water-mass formation. Air-sea fluxes of heat, momentum, freshwater, and gas are responsible for mixing and transformation of water properties at the ocean surface. Transient storms and submesoscale motions influence the timing and magnitude of upper-ocean mixing and exchange with the interior. In the Southern Ocean, along-isopycnal stirring by eddies connects the ocean surface with the mid-depth to deep interior. Interior mixing is largely adiabatic, but spatially heterogeneous in both vertical and horizontal directions, with eddy diffusivities estimated to vary by a factor of four or more, from roughly 700 to 2800 m2 s−1. Regions of strong stirring and strain are concentrated above and downstream of major bathymetric features. Small-scale turbulent mixing in the ocean interior is driven by internal waves, generated through flow–topography interactions, and turbulent mixing is enhanced in the bottom boundary layer. The marginal ice zones around Antarctica and processes on the Antarctic shelf are mixing regimes unique to the Southern Ocean that remain frontiers of current research.

Flow dynamics for coupled surface and internal deep-water waves

Abstract: Abstract This article determines the fluid motion underlying coupled linear internal and surface waves in a deep-water two-fluid-layer model (with the lower layer being of infinite depth). A detailed Eulerian description of the wave-field kinematics for coupled linear travelling waves is achieved using phase-plane analysis. The qualitative motion of individual fluid particles is elucidated through analysis of the relevant nonlinear dynamical systems from the Lagrangian viewpoint.

Internal Gravity Waves in the Earth’s Ionosphere

Abstract: The theory of low-frequency internal gravity waves (IGWs) is readdressed in the stable stratified weakly ionized Earth’s ionosphere. The formation of dipolar vortex structures and their dynamical evolution as well as the emergence of chaos in the wave–wave interactions are studied both in the presence and absence of the Pedersen conductivity. The latter is shown to inhibit the formation of solitary vortices and the onset of chaos.

The lifecycle of topographically-generated internal waves

Abstract: The dissipation and mixing associated with topographically generated internal waves form a critical component of ocean physics. In this chapter, we present a review of the basic components of their lifecycle, describing their generation, propagation, and interactions with mean flows, topography, and other internal gravity waves. Because of their established importance for ocean mixing, we focus on the distinct energy pathways towards dissipation of internal tides and lee waves. We close the chapter with a discussion of outstanding questions in this area of research.

Interpreting consequences of inadequate sampling of oceanic motions

Abstract: Inadequate sampling of oceanic motions, which commonly occurs for both oceanic measurements and simulations, can cause peculiar spectral features and potentially leads to misinterpretations. Here, we combine an extremely high‐frequency moored velocity record and a high‐resolution numerical simulation with the basic signal‐processing theory to quantitatively explore how varying sampling rates affect the ability to represent oceanic motions, especially internal gravity waves (IGWs). The moored measurements and simulations demonstrate that hourly sampling is sufficient to capture horizontal internal tidal velocities, but inadequate to faithfully characterize the vertical velocity for nearly all frequencies. The daily‐sampled model simulation shows a complicated frequency‐wavenumber spectral pattern of IGWs. Due to contrasting periodicities in time and space, temporal subsampling tends to retain the total variance of original IGWs, while spatial subsampling directly induces spectral energy loss. This study sheds light on data applications of the upcoming satellite altimetry missions.

The lifecycle of surface-generated near-inertial waves

Abstract: Our current knowledge of the lifecycle of the most energetic surface-generated internal waves in the ocean, wind-driven near-inertial waves (NIWs), is reviewed. The review covers the three stages in a NIW's lifecycle: formation as near-inertial motions in the mixed layer by the winds, propagation into the ocean interior as a NIW, and demise through turbulent dissipation and mixing, wave-wave interactions, and/or absorption into mean flows. A main goal of the review is to provide an update of our understanding of the processes that influence the wind-work on near-inertial motions, and those that govern the interactions of NIWs with mesoscale eddies and fronts. The impacts of NIWs on ocean mixing, both in the vertical and horizontal, are discussed.

On the Origins of the Oceanic Ultraviolet Catastrophe

Abstract: We provide a first-principles analysis of the energy fluxes in the oceanic internal wavefield. The resulting formula is remarkably similar to the renowned phenomenological formula for the turbulent dissipation rate in the ocean which is known as the Finescale Parameterization. The prediction is based on the wave turbulence theory of internal gravity waves and on a new methodology devised for the computation of the associated energy fluxes. In the standard spectral representation of the wave energy density, in the two-dimensional vertical wavenumber - frequency domain, the energy fluxes associated with the steady state are found to be directed downscale in both coordinates, closely matching the Finescale-Parameterization formula in functional form and in magnitude. These energy transfers are composed of a `local' and a `scale-separated' contributions; while the former is quantified numerically, the latter is dominated by the Induced Diffusion process and is amenable to analytical treatment. Contrary to previous results indicating an inverse energy cascade from high frequency to low, at odds with observations, our analysis of all non-zero coefficients of the diffusion tensor predicts a direct energy cascade. Moreover, by the same analysis fundamental spectra that had been deemed `no-flux' solutions are reinstated to the status of `constant-downscale-flux' solutions. This is consequential for an understanding of energy fluxes, sources and sinks that fits in the observational paradigm of the Finescale Parameterization, solving at once two long-standing paradoxes that had earned the name of `Oceanic Ultraviolet Catastrophe'.

Mode coupling and intensity fluctuation of sound propagation over continental slope in presence of internal waves

Abstract: The topographic variation underwater of the continental slope is one of the main causes that trigger the formation of internal waves, and the continental slope internal waves are ubiquitous in the ocean. The horizontal variation of waveguide environment caused by the internal wave and the continental slope can both lead to acoustic normal mode coupling, and then generates sound field fluctuation. Most of the existing research work is to study the single perturbation factor of either the internal waves or the continental slope on acoustic mode coupling and intensity fluctuation, while it is hard to find some research work that takes into account both the internal waves and the topographic variations as influencing factors. In this paper, numerical simulations are performed to allow the sound waves propagate through the internal waves in the downhill direction, using the acoustic coupled normal-mode model and four waveguide environments:thermocline, internal wave, continental slope and continental slope internal wave; And the mode coupling and intensity fluctuation characteristics and their physical mechanisms are studied, by comparing and analyzing the simulation results of the four different waveguide environment constructed. Some conclusion are obtained as follows:The intra-mode conduction coefficients are symmetric with the center of the internal wave, while the inter-mode coupling coefficients are antisymmetric with it; As the sound waves propagate toward or away from the center of the internal wave, the acoustic mode coupling becomes enhanced or weakened, and the coupling coefficients curves for large mode oscillate; The impacts of internal wave perturbation make the energy transferred from the smaller modes to the larger modes, which increases the sound field intensity attenuation; The number of the waveguide modes increases and the mode intensity attenuation is reduced, when the sound waves propagate downhill; The whole intensities of all modes for the continental slope internal wave environment is greater than the internal wave environment and less than the continental environment, and the energy transfer between mode groups is stronger than for individual effect of internal wave or continental slope, which lead to more energy transfer from the smaller to larger mode groups and an energy increase of the sound field above the thermocline.