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Nonlinear internal waves are found in many parts of the world ocean. Their widespread distribution is a result of their origin in the barotropic tide and in the variety of ways they can be generated, including by lee waves, tidal beams, resonance, plumes, and the transformation of the internal tide. The differing generation mechanisms and diversity of generation locations and conditions all combine to produce waves that range in scale from a few tens of meters to kilometers, but with all properly described by solitary wave theory. The ability of oceanic nonlinear internal waves to persist for days after generation and the key role internal waves play in connecting large-scale tides to smaller-scale turbulence make them important for understanding the ocean environment.

Submarine melting has been implicated in the accelerated retreat of marine‐terminating glaciers globally. Energetic ocean flows, such as subglacial discharge plumes, are known to enhance submarine melting in their immediate vicinity. Using observations and a large eddy simulation, we demonstrate that discharge plumes emit high‐frequency internal gravity waves that propagate along glacier termini and transfer energy to distant regions of the terminus. Our analysis of wave characteristics and their correlation with subglacial discharge forcing suggest that they derive their energy from turbulent motions within the discharge plume and its surface outflow. Accounting for the near‐terminus velocities associated with these waves increases predicted melt rates by up to 70%. This may help to explain known discrepancies between observed melt rates and theoretical predictions. Because the dynamical ingredients—a buoyant plume rising through a stratified ocean—are common to many tidewater glacier systems, such internal waves are likely to be widespread. Plain Language Summary Recent acceleration in sea‐level rise has been attributed to the mass loss of glaciers that terminate in the ocean, such as those found in Greenland and Alaska. Warm ocean currents are thought to melt glacier ice, contributing to their loss of mass and retreat. We use moored instruments deployed with autonomous vehicles, as well as a computer simulation, to demonstrate how a previously unconsidered type of current, called an internal wave, is generated at marine‐terminating glaciers. We show that the strength of the waves is related to the amount of subglacial discharge that originates from surface melting occurring at higher elevations on the glacier. Internal waves may contribute to local ice melt, and ultimately glacier mass loss, by mixing warm water in a thin layer immediately adjacent to the glacier. Key Points First‐ever time series of water velocity in the calving zone of a glacier terminus, enabled by moorings deployed from a robotic vessel Energetic high‐frequency internal waves were emitted from the subglacial discharge plume and reproduced in a large eddy simulation Internal waves have the potential to significantly increase ambient melt rates by enhancing water velocity across the terminus

Vertical two‐dimensional numerical experiments incorporating Garrett‐Munk (GM) internal waves are conducted to investigate tide‐induced near‐field mixing over a finite‐amplitude sinusoidal seafloor, conventionally attributed to the breaking of high‐wavenumber internal tidal waves. Turbulent mixing is characterized by tidal excursion parameter (Te) and topographic steepness parameter (Sp) measuring tidal current strength and seafloor slope gradient, respectively. Under strong tidal currents (Te > 1), high‐wavenumber internal lee waves propagate upward from the seafloor. Even when Te and Sp are set to produce nearly the same upward energy flux, the vertical profile of mixing hotspots varies with Sp. For Sp ≳$\\mathit{\\gtrsim }$0.2, near‐inertial currents above the seafloor rapidly amplify by absorbing energy of internal lee waves from below, hindering their upward propagation and creating “short mixing hotspots.” For Sp < 0.2, these near‐inertial currents diminish, allowing internal lee waves to propagate upward and interact with the GM background internal waves, creating “tall mixing hotspots.” Plain Language Summary We perform numerical experiments to study how tidal currents cause mixing in the ocean near a rough seafloor. By considering the interaction between strong tidal currents and small‐scale rough seafloor features, we find that high‐frequency internal waves, different from those previously thought, propagate rapidly upward from the seafloor and interact with background waves, causing mixing above the seafloor. The pattern of this mixing changes with the steepness of the seafloor slope. If the seafloor slope is steep, the currents near the seafloor quickly absorb the energy of the upward‐propagating waves, creating a mixing region confined to the seafloor (short mixing hotspot). In contrast, if the seafloor slope is gentle, less energy is absorbed, allowing the upward‐propagating waves to travel higher and interact with background waves, creating a mixing region extending from the seafloor (tall mixing hotspot). These mixing processes have been overlooked due to limited knowledge of seafloor details. However, they are crucial because tall mixing hotspots can significantly affect global ocean circulation. Therefore, they deserve more attention in future studies. Key Points Strong tidal flows over rough seafloors induce internal lee waves that propagate upward while inducing mixing near the seafloor Over steep seafloors, inertial currents inhibit the upward propagation of internal lee waves, creating short mixing hotspots Over less steep seafloors, the lack of inertial currents allows internal lee waves to propagate upward, creating tall mixing hotspots

Recent microstructure observations in the Southern Ocean report enhanced internal gravity waves and turbulence in the frontal regions of the Antarctic Circumpolar Current extending a kilometer above rough bottom topography. Idealized numerical simulations and linear theory show that geostrophic flows impinging on rough small-scale topography are very effective generators of internal waves and estimate vigorous wave radiation, breaking, and turbulence within a kilometer above bottom. However, both idealized simulations and linear theory assume periodic and spatially uniform topography and tend to overestimate the observed levels of turbulent energy dissipation locally at the generation sites. In this study, we explore the downstream evolution and remote dissipation of internal waves generated by geostrophic flows using a series of numerical, realistic topography simulations and parameters typical of Drake Passage. The results show that significant levels of internal wave kinetic energy and energy dissipation are present downstream of the rough topography, internal wave generation site. About 30%–40% of the energy dissipation occurs locally over the rough topography region, where internal waves are generated. The rest of the energy dissipation takes place remotely and decays downstream of the generation site with an e -folding length scale of up to 20–30 km. The model we use is two-dimensional with enhanced viscosity coefficients, and hence it can result in the underestimation of the remote wave dissipation and its decay length scale. The implications of our results for turbulent energy dissipation observations and mixing parameterizations are discussed.

The sea surface temperature (SST) in the Southern Ocean (SO) exerts widespread effects on the climate. Yet there is a persistent warm SST bias in the SO across generations of coupled global climate models (CGCMs). Existing literature blames such bias primarily on the deficiencies of model‐simulated atmospheric processes or basin‐scale ocean circulations. In this study, we show that the warm SST bias in the SO can be mitigated by parameterizing the vertical mixing induced by wind‐driven near‐inertial internal waves (NIWs) in the thermocline. By representing NIW‐induced vertical mixing in the thermocline of an eddy‐resolving CGCM, the warm SST bias in the SO is significantly reduced due to the enhanced downward heat flux from the surface boundary layer to the ocean interior. Our findings provide a new pathway to alleviate the warm SST bias in the SO, helping in improving the fidelity of model projected future climate changes.

In this paper, we use asymptotic theory and numerical methods to study resonant triad interactions among discrete internal wave modes at a fixed frequency ($\\omega$) in a two-dimensional, uniformly stratified shear flow. Motivated by linear internal wave generation mechanisms in the ocean, we assume the primary wave field as a linear superposition of various horizontally propagating vertical modes at a fixed frequency $\\omega$. The weakly nonlinear solution associated with the primary wave field is shown to comprise superharmonic (frequency $2\\omega$) and zero frequency wave fields, with the focus of this study being on the former. When two interacting primary modes $m$ and $n$ are in triadic resonance with a superharmonic mode $q$, it results in the divergence of the corresponding superharmonic secondary wave amplitude. For a given modal interaction $(m, n)$, the superharmonic wave amplitude is plotted on the plane of primary wave frequency $\\omega$ and Richardson number $Ri$, and the locus of divergence locations shows how the resonance locations are influenced by background shear. In the limit of weak background shear ($Ri\\to \\infty$), using an asymptotic theory, we show that the horizontal wavenumber condition $k_m + k_n = k_q$ is sufficient for triadic resonance, in contrast to the requirement of an additional vertical mode number condition ($q = |m-n|$) in the case of no shear. As a result, the number of resonances for an arbitrarily weak shear is significantly larger than that for no shear. The new resonances that occur in the presence of shear include the possibilities of resonance due to self-interaction and resonances that occur at the seemingly trivial limit of $\\omega \\approx 0$, both of which are not possible in the no shear limit. Our weak shear limit conclusions are relevant for other inhomogeneities such as non-uniformity in stratification as well, thus providing an understanding of several recent studies that have highlighted superharmonic generation in non-uniform stratifications. Extending our study to finite shear (finite $Ri$) in an ocean-like exponential shear flow profile, we show that for cograde–cograde interactions, a significant number of divergence curves that start at $Ri\\to \\infty$ will not extend below a cutoff $Ri$ $\\sim O(1)$. In contrast, for retrograde–retrograde interactions, the divergence curves extend all the way from $Ri\\to \\infty$ to $Ri = 0.5$. For mixed interactions, new divergence curves appear at $\\omega = 0$ for $Ri\\sim O(10)$ and extend to other primary wave frequencies for smaller $Ri$. Consequently, the total ($\\text {cograde} + \\text {retrograde} + \\text {mixed}$) number of resonant triads is of the same order for small $Ri\\approx 0.5$ as in the limit of weak shear ($Ri\\to \\infty$), although it attains a maximum at $Ri\\sim O(10)$.

While nonlinear internal wave (NLIW) trains are known to influence near‐sea bed suspended sediment dynamics, the mechanisms remain a topic of debate. We present near‐sea bed observations of suspended sediment concentration C$C$and estimates of vertical sediment flux, at high vertical‐ and temporal‐resolution, during trains of NLIW of depression. We quantify the contributions of vertical advection and turbulent mixing to C$C$ . Vertical advection was important during the leading wave and the turbulent mixing flux was important over the entire wave train. Maximum C$C$was highly correlated with the maximum horizontal current speed squared and was only weakly correlated with the maximum vertical velocity. Boundary layer‐induced turbulence was thus inferred to be the key driver of net vertical sediment flux over wave trains of this type. Estimating the maximum total horizontal speed (i.e., wave‐induced plus background) is sufficient for modeling sediment vertical dynamics in shelf‐scale modeling studies.

The inversion of remote sensing signatures of internal solitary waves (ISWs) can retrieve dynamic characteristics in the ocean interior. However, the presence of ubiquitous large‐amplitude ISWs poses challenges to the commonly used weakly nonlinear methods for parameter retrieval. Through laboratory experiments, we establish a relationship between surface features and internal characteristics of ISWs by the remote sensing imaging mechanism. The results demonstrate that strong nonlinearity significantly influences the retrieval of ISWs, primarily manifested in the calculation of wave‐induced velocities and the applicability of ISW solutions. A fully nonlinear model Dubreil–Jacotin–Long equation is used in the retrieval and has been tested under different conditions. Mooring observations indicate that the determination of ISW parameters from satellite images is affected by the complexity of in situ stratification, but additional remote sensing information such as surface velocities enables us to perform retrievals even if the real‐time measurement of pycnocline depth is not available. Plain Language Summary Internal solitary waves (ISWs), as nonlinear internal waves, play an essential role in oceanic human activities and ocean mixing. The surface current induced by ISWs can create rough and smooth regions on the sea surface due to the modulated roughness, hence presenting alternating bright and dark stripes in satellite images. Satellites can observe ISWs over a wide range via surface manifestations, and the internal dynamics can be calculated from surface features using retrieval methods. However, the availability of retrieval methods still needs to be verified, facing the difficulty of matching mooring observations and satellite images of the same ISW in a short time interval. According to the proportional relation of remote sensing signatures and wave‐induced velocities, this study establishes the relationship between surface features and internal characteristics of ISWs in laboratory experiments. Strong nonlinearity significantly influences the retrieval of ISWs and a fully nonlinear model is well applied in retrieval. Then we test the retrieval in oceanic environments, mooring observations show the critical role of stratification in retrieval. This work provides a reliable dynamics model for the inversion of remote sensing signatures of ISWs into characteristics in the ocean interior. Key Points The relationship between surface features and internal parameters of internal solitary waves is established in laboratory experiments Strong nonlinearity significantly impacts the determination of wave parameters from the surface. A fully nonlinear model is well applied Accurate parameter determination is constrained by the complex oceanic stratification, but more remote sensing information can overcome it

The authors present inferences of diapycnal diffusivity from a compilation of over 5200 microstructure profiles. As microstructure observations are sparse, these are supplemented with indirect measurements of mixing obtained from (i) Thorpe-scale overturns from moored profilers, a finescale parameterization applied to (ii) shipboard observations of upper-ocean shear, (iii) strain as measured by profiling floats, and (iv) shear and strain from full-depth lowered acoustic Doppler current profilers (LADCP) and CTD profiles. Vertical profiles of the turbulent dissipation rate are bottom enhanced over rough topography and abrupt, isolated ridges. The geography of depth-integrated dissipation rate shows spatial variability related to internal wave generation, suggesting one direct energy pathway to turbulence. The global-averaged diapycnal diffusivity below 1000-m depth is O(10−4) m2 s−1 and above 1000-m depth is O(10−5) m2 s−1. The compiled microstructure observations sample a wide range of internal wave power inputs and topographic roughness, providing a dataset with which to estimate a representative global-averaged dissipation rate and diffusivity. However, there is strong regional variability in the ratio between local internal wave generation and local dissipation. In some regions, the depth-integrated dissipation rate is comparable to the estimated power input into the local internal wave field. In a few cases, more internal wave power is dissipated than locally generated, suggesting remote internal wave sources. However, at most locations the total power lost through turbulent dissipation is less than the input into the local internal wave field. This suggests dissipation elsewhere, such as continental margins.

Internal gravity waves can cause mixing in the radiative interiors of stars. We study this mixing by introducing tracer particles into 2D hydrodynamic simulations. Following the work of Rogers & McElwaine, we extend our study to different masses (3, 7, and 20 M ⊙) and ages (ZAMS, midMS, and TAMS). The diffusion profiles of these models are influenced by various parameters such as the Brunt–Väisälä frequency, density, thermal damping, the geometric effect, and the frequencies of waves contributing to these mixing profiles. We find that the mixing profile changes dramatically across age. In younger stars, we noted that the diffusion coefficient increases toward the surface, whereas in older stars the initial increase in the diffusion profile is followed by a decreasing trend. We also find that mixing is stronger in more massive stars. Hence, future stellar evolution models should include this variation. In order to aid the inclusion of this mixing in 1D stellar evolution models, we determine the dominant waves contributing to these mixing profiles and present a prescription that can be included in 1D models.