Global Abyssal Mixing Inferred from Lowered ADCP Shear and CTD Strain Profiles

2006 ◽  
Vol 36 (8) ◽  
pp. 1553-1576 ◽  
Author(s):  
Eric Kunze ◽  
Eric Firing ◽  
Julia M. Hummon ◽  
Teresa K. Chereskin ◽  
Andreas M. Thurnherr
Keyword(s):  
Internal Wave ◽  
Wave Spectrum ◽  
Wave Interaction ◽  
Deep Ocean ◽  
Eddy Diffusivity ◽  
Small Scale ◽  
Tidal Dissipation ◽  
Turbulent Dissipation ◽  
Active Turbulence ◽  
Parametric Subharmonic Instability

Abstract Internal wave–wave interaction theories and observations support a parameterization for the turbulent dissipation rate ɛ and eddy diffusivity K that depends on internal wave shear 〈Vz2〉 and strain 〈ξz2〉 variances. Its latest incarnation is applied to about 3500 lowered ADCP/CTD profiles from the Indian, Pacific, North Atlantic, and Southern Oceans. Inferred diffusivities K are functions of latitude and depth, ranging from 0.03 × 10−4 m2 s−1 within 2° of the equator to (0.4–0.5) × 10−4 m2 s−1 at 50°–70°. Diffusivities K also increase with depth in tropical and subtropical waters. Diffusivities below 4500-m depth exhibit a peak of 0.7 × 10−4 m2 s−1 between 20° and 30°, latitudes where semidiurnal parametric subharmonic instability is expected to be active. Turbulence is highly heterogeneous. Though the bulk of the vertically integrated dissipation ∫ɛ is contributed from the main pycnocline, hotspots in ∫ɛ show some correlation with small-scale bottom roughness and near-bottom flow at sites where strong surface tidal dissipation resulting from tide–topography interactions has been implicated. Average vertically integrated dissipation rates are 1.0 mW m−2, lying closer to the 0.8 mW m−2 expected for a canonical (Garrett and Munk) internal wave spectrum than the global-averaged deep-ocean surface tide loss of 3.3 mW m−2.

Parametric instability of internal gravity waves

1975 ◽  
Vol 67 (4) ◽  
pp. 667-687 ◽  
Author(s):  
A. D. McEwan ◽  
R. M. Robinson
Keyword(s):  
Internal Wave ◽  
Large Scale ◽  
Wave Spectrum ◽  
Parametric Instability ◽  
Mathieu Equation ◽  
Internal Gravity Waves ◽  
Small Scale ◽  
Preliminary Calculation ◽  
Theoretical Predictions ◽  
Oceanic Internal Wave

A continuously stratified fluid, when subjected to a weak periodic horizontal acceleration, is shown to be susceptible to a form of parametric instability whose time dependence is described, in its simplest form, by the Mathieu equation. Such an acceleration could be imposed by a large-scale internal wave field. The growth rates of small-scale unstable modes may readily be determined as functions of the forcing-acceleration amplitude and frequency. If any such mode has a natural frequency near to half the forcing frequency, the forcing amplitude required for instability may be limited in smallness only by internal viscous dissipation. Greater amplitudes are required when boundaries constrain the form of the modes, but for a given bounding geometry the most unstable mode and its critical forcing amplitude can be defined.An experiment designed to isolate the instability precisely confirms theoretical predictions, and evidence is given from previous experiments which suggest that its appearance can be the penultimate stage before the traumatic distortion of continuous stratifications under internal wave action.A preliminary calculation, using the Garrett & Munk (197%) oceanic internal wave spectrum, indicates that parametric instability could occur in the ocean at scales down to that of the finest observed microstructure, and may therefore have a significant role to play in its formation.

scholarly journals Global Characterization of the Ocean’s Internal Wave Spectrum

2020 ◽  
Vol 50 (7) ◽  
pp. 1871-1891 ◽  
Author(s):  
Friederike Pollmann
Keyword(s):  
Internal Wave ◽  
Gravity Wave ◽  
Spectral Characteristics ◽  
Wave Spectrum ◽  
Internal Gravity Wave ◽  
Ocean Dynamics ◽  
Small Scale ◽  
Northwest Pacific ◽  
Spatial And Temporal Variability ◽  
Spectral Parameters

AbstractA key ingredient of energetically consistent ocean models is the parameterized link between small-scale turbulent mixing, an important energy source of large-scale ocean dynamics, and internal gravity wave energetics. Theory suggests that this link depends on the wave field’s spectral characteristics, but because of the paucity of suitable observations, its parameterization typically relies on a model spectrum [Garrett–Munk (GM)] with constant parameters. Building on the so-called “finestructure method,” internal gravity wave spectra are derived from vertical strain profiles obtained from Argo floats to provide a global estimate of the spatial and temporal variability of the GM model’s spectral parameters. For spectral slopes and wavenumber scales, the highest variability and the strongest deviation from the model’s canonical parameters are observed in the North Atlantic, the northwest Pacific, and the Southern Ocean. Internal wave energy levels in the upper ocean are well represented by the GM model value equatorward of approximately 50°, while they are up to two orders of magnitude lower poleward of this latitude. The use of variable spectral parameters in the energy level calculation hides the seasonal cycle in the northwest Pacific that was previously observed for constant parameters. The global estimates of how the GM model’s spectral parameters vary in space and time are hence expected to add relevant detail to various studies on oceanic internal gravity waves, deepening the understanding of their energetics and improving parameterizations of the mixing they induce.

scholarly journals Inferences and Observations of Turbulent Dissipation and Mixing in the Upper Ocean at the Hawaiian Ridge

2007 ◽  
Vol 37 (3) ◽  
pp. 476-494 ◽  
Author(s):  
Joseph P. Martin ◽  
Daniel L. Rudnick
Keyword(s):  
Energy Density ◽  
Internal Wave ◽  
Dissipation Rate ◽  
Internal Tide ◽  
Eddy Diffusivity ◽  
Internal Tides ◽  
Upper Ocean ◽  
Turbulent Dissipation ◽  
Hawaiian Ridge ◽  
Wave Induced

Abstract The Hawaiian Ridge is one of the most energetic generators of internal tides in the pelagic ocean. The density and current structure of the upper ocean at the Hawaiian Ridge were observed using SeaSoar and Doppler sonar during a survey extending from Oahu to Brooks Banks and up to 200 km from the ridge peak. Survey observations are used to quantify spatial changes in internal-wave-induced turbulent dissipation and mixing. The turbulent dissipation rate of kinetic energy ɛ and diapycnal eddy diffusivity Kρ are inferred from an established parameterization using internal wave shear as input. At the Kauai Channel (KC) and French Frigate Shoals/Brooks Banks sites, ɛ and Kρ decay away from the ridge with maxima exceeding minima by 5 times. At both sites, average Kρ is everywhere greater than the canonical open-ocean value of 10−5 m2 s−1. Along the ridge, ɛ and Kρ vary by up to 100 times and are largest at sites of largest numerical model internal tide energy density. In the eastern KC, Kρ > 10−3 m2 s−1 is typical in a patch more than 200 m thick located above the path of an M2 internal tide ray. An upper limit on the dissipation rate from M2 internal tides to turbulence within 50 km of the Hawaiian Ridge is roughly estimated to be in the range of 4–9 GW. At KC, the depth-integrated internal wave energy density and dissipation rate are positively correlated. Potential density inversions occur near the main ridge axis at significant topographic features. Average Kρ is larger inside inversions.

Nonlinear energy transfer and the energy balance of the internal wave field in the deep ocean

1976 ◽  
Vol 74 (2) ◽  
pp. 375-399 ◽  
Author(s):  
Dirk J. Olbers
Keyword(s):  
Energy Transfer ◽  
Energy Balance ◽  
Internal Wave ◽  
Wave Field ◽  
Wave Spectrum ◽  
Fluid Motion ◽  
Deep Ocean ◽  
Nonlinear Interactions ◽  
Nonlinear Energy Transfer ◽  
Spectral Models

The source function describing the energy transfer between the components of the internal wave spectrum due to nonlinear interactions is derived from the Lagrangian of the fluid motion and evaluated numerically for the spectral models of Garrett & Munk (1972a, 1975). The characteristic time scales of the transfer are found to be typically of the order of some days, so that nonlinear interactions will play an important role in the energy balance of the wave field. Thus implications of the nonlinear transfer within the spectrum for generation and dissipation processes are considered.

Influence of the Distortion of Vertical Wavenumber Spectra on Estimates of Turbulent Dissipation using the Finescale Parameterization: Eikonal Calculations

2021 ◽  
Author(s):  
ANNE TAKAHASHI ◽  
TOSHIYUKI HIBIYA ◽  
ALBERTO C. NAVEIRA GARABATO
Keyword(s):  
Internal Wave ◽  
Wave Energy ◽  
Mesoscale Eddy ◽  
Wave Interaction ◽  
Interaction Theory ◽  
Turbulent Dissipation ◽  
Energy Cascades ◽  
Circumpolar Current ◽  
Noise Contamination ◽  
The Antarctic

AbstractThe finescale parameterization, formulated on the basis of a weak nonlinear wave–wave interaction theory, is widely used to estimate the turbulent dissipation rate, ε. However, this parameterization has previously been found to overestimate ε in the Antarctic Circumpolar Current (ACC) region. One possible reason for this overestimation is that vertical wavenumber spectra of internal wave energy are distorted from the canonical Garrett-Munk spectrum and have a spectral “hump” at low vertical wavenumbers. Such distorted vertical wavenumber spectra were also observed in other mesoscale eddy-rich regions. In this study, using eikonal simulations, in which internal wave energy cascades are evaluated in the frequency-wavenumber space, we examine how the distortion of vertical wavenumber spectra impacts on the accuracy of the finescale parameterization. It is shown that the finescale parameterization overestimates ε for distorted spectra with a low-vertical-wavenumber hump because it incorrectly takes into account the breaking of these low-vertical-wavenumber internal waves. This issue is exacerbated by estimating internal wave energy spectral levels from the low-wavenumber band rather than from the high-wavenumber band, which is often contaminated by noise in observations. Thus, in order to accurately estimate the distribution of ε in eddy-rich regions like the ACC, high-vertical-wavenumber spectral information free from noise contamination is indispensable.

Internal Wave Generation by a Combination of Tidal and Steady Currents

2021 ◽  
Author(s):  
Xiaolin Bai ◽  
Kevin Lamb ◽  
José da Silva
Keyword(s):  
Internal Wave ◽  
Wave Generation ◽  
Satellite Observations ◽  
Internal Tides ◽  
Small Scale ◽  
Western Boundary ◽  
Boundary Current ◽  
Reanalysis Dataset ◽  
Amazon Shelf ◽  
Internal Wave Generation

<p>In the presence of topography, two main contributors for internal wave energy are tide-topography interaction transferring energy from the barotropic tide to internal tides, and lee wave generation when geostrophic currents or eddying abyssal flows interact with topography. In the past few decades, many studies considered the respective contribution of the oscillating flows or steady background flows, but few investigations have considered both.  </p><p>In this talk, we consider the joint effects of tidal and steady currents to investigate internal wave generation and propagation on the Amazon shelf, a hotspot for internal solitary wave (ISW) generation. The Amazon Shelf is off the mouth of the Amazon River in the southwest tropical Atlantic Ocean, affected by strong tidal constituents over complex bottom bathymetry and a strong western boundary current, the North Brazilian Current (NBC). Both satellite observations and numerical modelling are used in this study. Satellite observations provide a clear visualization of the wave characteristics, such as temporal and spatial distributions, propagating direction and its relation to background currents. Based on parameters from satellite observations and reanalysis dataset, we set up a model to numerically investigate the dynamics of the ISW generation. We demonstrate that the small-scale topography contributes to a rich generation of along-shelf propagating ISW, which significantly contribute to the ocean mixing and potentially cause sediment resuspension. Moreover, the ISW-induced currents also contribute to the sea surface wave breaking as observed by satellite measurements. In addition, statistics based on a decade of satellite images and numerical investigations on seasonal variations of the ISWs and the NBC improve our understanding of the generation and evolution of these nonlinear internal waves in the presence of background currents.</p>

scholarly journals Spring Mixing: Turbulence and Internal Waves during Restratification on the New England Shelf

2005 ◽  
Vol 35 (12) ◽  
pp. 2425-2443 ◽  
Author(s):  
J. A. MacKinnon ◽  
M. C. Gregg
Keyword(s):  
New England ◽  
Internal Waves ◽  
Mixed Layer ◽  
Wave Interaction ◽  
Heat Fluxes ◽  
Surface Mixed Layer ◽  
Turbulent Dissipation ◽  
Surface Heat Fluxes ◽  
Bottom Boundary Layers ◽  
Diapycnal Diffusivity

Abstract Integrated observations are presented of water property evolution and turbulent microstructure during the spring restratification period of April and May 1997 on the New England continental shelf. Turbulence is shown to be related to surface mixed layer entrainment and shear from low-mode near-inertial internal waves. The largest turbulent diapycnal diffusivity and associated buoyancy fluxes were found at the bottom of an actively entraining and highly variable wind-driven surface mixed layer. Away from surface and bottom boundary layers, turbulence was systematically correlated with internal wave shear, though the nature of that relationship underwent a regime shift as the stratification strengthened. During the first week, while stratification was weak, the largest turbulent dissipation away from boundaries was coincident with shear from mode-1 near-inertial waves generated by passing storms. Wave-induced Richardson numbers well below 0.25 and density overturning scales of several meters were observed. Turbulent dissipation rates in the region of peak shear were consistent in magnitude with several dimensional scalings. The associated average diapycnal diffusivity exceeded 10−3 m2 s−1. As stratification tripled, Richardson numbers from low-mode internal waves were no longer critical, though turbulence was still consistently elevated in patches of wave shear. Kinematically, dissipation during this period was consistent with the turbulence parameterization proposed by MacKinnon and Gregg, based on a reinterpretation of wave–wave interaction theory. The observed growth of temperature gradients was, in turn, consistent with a simple one-dimensional model that vertically distributed surface heat fluxes commensurate with calculated turbulent diffusivities.

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