Radiation and Dissipation of Internal Waves Generated by Geostrophic Motions Impinging on Small-Scale Topography: Application to the Southern Ocean

Abstract Recent estimates from observations and inverse models indicate that turbulent mixing associated with internal wave breaking is enhanced above rough topography in the Southern Ocean. In most regions of the ocean, abyssal mixing has been primarily associated with radiation and breaking of internal tides. In this study, it is shown that abyssal mixing in the Southern Ocean can be sustained by internal waves generated by geostrophic motions that dominate abyssal flows in this region. Theory and fully nonlinear numerical simulations are used to estimate the internal wave radiation and dissipation from lowered acoustic Doppler current profiler (LADCP), CTD, and topography data from two regions in the Southern Ocean: Drake Passage and the southeast Pacific. The results show that radiation and dissipation of internal waves generated by geostrophic motions reproduce the magnitude and distribution of dissipation previously inferred from finescale measurements in the region, suggesting that it is one of the primary drivers of abyssal mixing in the Southern Ocean.

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Downstream Propagation and Remote Dissipation of Internal Waves in the Southern Ocean

Journal of Physical Oceanography ◽

10.1175/jpo-d-18-0134.1 ◽

2019 ◽

Vol 49 (7) ◽

pp. 1873-1887 ◽

Cited By ~ 6

Author(s):

Kaiwen Zheng ◽

Maxim Nikurashin

Keyword(s):

Energy Dissipation ◽

Southern Ocean ◽

Internal Wave ◽

Linear Theory ◽

Internal Waves ◽

Turbulent Energy ◽

Wave Radiation ◽

Small Scale ◽

Length Scale ◽

Geostrophic Flows

AbstractRecent 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.

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Modeling and Analysis of Internal-Tide Generation and Beamlike Onshore Propagation in the Vicinity of Shelfbreak Canyons

Journal of Physical Oceanography ◽

10.1175/jpo-d-13-0179.1 ◽

2014 ◽

Vol 44 (3) ◽

pp. 834-849 ◽

Cited By ~ 11

Author(s):

Weifeng G. Zhang ◽

Timothy F. Duda ◽

Ilya A. Udovydchenkov

Keyword(s):

Internal Wave ◽

Internal Waves ◽

Internal Tide ◽

Propagation Model ◽

Internal Tides ◽

Wave Radiation ◽

Beam Radiation ◽

Control Simulation ◽

Conversion Rates ◽

The Cross

Abstract A hydrostatic numerical model with alongshore-uniform barotropic M2 tidal boundary forcing and idealized shelfbreak canyon bathymetries is used to study internal-tide generation and onshore propagation. A control simulation with Mid-Atlantic Bight representative bathymetry is supported by other simulations that serve to identify specific processes. The canyons and adjacent slopes are transcritical in steepness with respect to M2 internal wave characteristics. Although the various canyons are symmetrical in structure, barotropic-to-baroclinic energy conversion rates Cυ are typically asymmetrical within them. The resulting onshore-propagating internal waves are the strongest along beams in the horizontal plane, with the stronger beam in the control simulation lying on the side with higher Cυ. Analysis of the simulation results suggests that the cross-canyon asymmetrical Cυ distributions are caused by multiple-scattering effects on one canyon side slope, because the phase variation in the spatially distributed internal-tide sources, governed by variations in the orientation of the bathymetry gradient vector, allows resonant internal-tide generation. A less complex, semianalytical, modal internal wave propagation model with sources placed along the critical-slope locus (where the M2 internal wave characteristic is tangent to the seabed) and variable source phasing is used to diagnose the physics of the horizontal beams of onshore internal wave radiation. Model analysis explains how the cross-canyon phase and amplitude variations in the locally generated internal tides affect parameters of the internal-tide beams. Under the assumption that strong internal tides on continental shelves evolve to include nonlinear wave trains, the asymmetrical internal-tide generation and beam radiation effects may lead to nonlinear internal waves and enhanced mixing occurring preferentially on one side of shelfbreak canyons, in the absence of other influencing factors.

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Radiation and Dissipation of Internal Waves Generated by Geostrophic Motions Impinging on Small-Scale Topography: Theory

Journal of Physical Oceanography ◽

10.1175/2009jpo4199.1 ◽

2010 ◽

Vol 40 (5) ◽

pp. 1055-1074 ◽

Cited By ~ 103

Author(s):

Maxim Nikurashin ◽

Raffaele Ferrari

Keyword(s):

Internal Waves ◽

Bottom Topography ◽

Drake Passage ◽

Wave Radiation ◽

Small Scale ◽

Ocean Bottom ◽

Inertial Oscillations ◽

Weakly Nonlinear ◽

Weakly Nonlinear Theory ◽

Internal Wave Generation

Abstract Observations and inverse models suggest that small-scale turbulent mixing is enhanced in the Southern Ocean in regions above rough topography. The enhancement extends O(1) km above the topography, suggesting that mixing is supported by the breaking of gravity waves radiated from the ocean bottom. In this study, it is shown that the observed mixing rates can be sustained by internal waves generated by geostrophic motions flowing over bottom topography. Weakly nonlinear theory is used to describe the internal wave generation and the feedback of the waves on the zonally averaged flow. Vigorous inertial oscillations are driven at the ocean bottom by waves generated at steep topography. The wave radiation and dissipation at equilibrium is therefore the result of both geostrophic flow and inertial oscillations differing substantially from the classical lee-wave problem. The theoretical predictions are tested versus two-dimensional high-resolution numerical simulations with parameters representative of Drake Passage. This work suggests that mixing in Drake Passage can be supported by geostrophic motions impinging on rough topography rather than by barotropic tidal motions, as is commonly assumed.

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

Journal of Physical Oceanography ◽

10.1175/jpo-d-12-0222.1 ◽

2013 ◽

Vol 43 (11) ◽

pp. 2288-2308 ◽

Cited By ~ 28

Author(s):

J. Alexander Brearley ◽

Katy L. Sheen ◽

Alberto C. Naveira Garabato ◽

David A. Smeed ◽

Stephanie Waterman

Keyword(s):

Southern Ocean ◽

Internal Wave ◽

Ocean Circulation ◽

Wave Energy ◽

Bottom Topography ◽

Temporal Correlation ◽

Drake Passage ◽

Wave Radiation ◽

Bottom Current ◽

Turbulent Dissipation

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 current speeds. Periods of strong near-bottom flow coincided with increased convergence of eddy-induced interfacial form stress in the bottom 1500 m. Predictions of internal wave energy radiation were made from theory using measured near-bottom current speeds, and the mean value of wave radiation (5.3 mW m−2) was sufficient to support the dissipated power calculated from the ADCP. A significant temporal correlation was also observed between radiated and dissipated power. Given the ubiquity of strong eddy flows and rough topography in the Southern Ocean, the transfer from eddy to internal wave energy is likely to be an important term in closing the ocean energy budget.

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Internal Wave Generation by a Combination of Tidal and Steady Currents

10.5194/egusphere-egu21-8477 ◽

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

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. &#160;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.

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Improving Estimates of the Antarctic Circumpolar Current Streamlines in Drake Passage

Journal of Physical Oceanography ◽

10.1175/2007jpo3834.1 ◽

2008 ◽

Vol 38 (5) ◽

pp. 1000-1010 ◽

Cited By ~ 15

Author(s):

Yueng-Djern Lenn ◽

Teresa K. Chereskin ◽

Janet Sprintall

Keyword(s):

Southern Ocean ◽

Antarctic Circumpolar Current ◽

Acoustic Doppler Current Profiler ◽

Drake Passage ◽

Meridional Overturning Circulation ◽

Dynamic Height ◽

Current Profiler ◽

Meridional Overturning ◽

The Mean ◽

Circumpolar Current

Abstract Accurately resolving the mean Antarctic Circumpolar Current (ACC) is essential for determining Southern Ocean eddy fluxes that are important to the global meridional overturning circulation. Previous estimates of the mean ACC have been limited by the paucity of Southern Ocean observations. A new estimate of the mean surface ACC in Drake Passage is presented that combines sea surface height anomalies measured by satellite altimetry with a recent dataset of repeat high-resolution acoustic Doppler current profiler observations. A mean streamfunction (surface height field), objectively mapped from the mean currents, is used to validate two recent dynamic height climatologies. The new streamfunction has narrower and stronger ACC fronts separated by quiescent zones of much weaker flow, thereby improving on the resolution of ACC fronts observed in the other climatologies. Distinct streamlines can be associated with particular ACC fronts and tracked in time-dependent maps of dynamic height. This analysis shows that varying degrees of topographic control are evident in the preferred paths of the ACC fronts through Drake Passage.

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The Impact of a Variable Mixing Efficiency on the Abyssal Overturning

Journal of Physical Oceanography ◽

10.1175/jpo-d-14-0259.1 ◽

2016 ◽

Vol 46 (2) ◽

pp. 663-681 ◽

Cited By ~ 43

Author(s):

Casimir de Lavergne ◽

Gurvan Madec ◽

Julien Le Sommer ◽

A. J. George Nurser ◽

Alberto C. Naveira Garabato

Keyword(s):

Potential Energy ◽

Internal Waves ◽

Bottom Water ◽

Internal Tides ◽

Mixing Efficiency ◽

Small Scale ◽

Antarctic Bottom Water ◽

Energy Fraction ◽

Geothermal Heating ◽

The Impact

AbstractIn studies of ocean mixing, it is generally assumed that small-scale turbulent overturns lose 15%–20% of their energy in eroding the background stratification. Accumulating evidence that this energy fraction, or mixing efficiency Rf, significantly varies depending on flow properties challenges this assumption, however. Here, the authors examine the implications of a varying mixing efficiency for ocean energetics and deep-water mass transformation. Combining current parameterizations of internal wave-driven mixing with a recent model expressing Rf as a function of a turbulence intensity parameter Reb = εν/νN2, the ratio of dissipation εν to stratification N2 and molecular viscosity ν, it is shown that accounting for reduced mixing efficiencies in regions of weak stratification or energetic turbulence (high Reb) strongly limits the ability of breaking internal waves to supply oceanic potential energy and drive abyssal upwelling. Moving from a fixed Rf = 1/6 to a variable efficiency Rf(Reb) causes Antarctic Bottom Water upwelling induced by locally dissipating internal tides and lee waves to fall from 9 to 4 Sverdrups (Sv; 1 Sv ≡ 106 m3 s−1) and the corresponding potential energy source to plunge from 97 to 44 GW. When adding the contribution of remotely dissipating internal tides under idealized distributions of energy dissipation, the total rate of Antarctic Bottom Water upwelling is reduced by about a factor of 2, reaching 5–15 Sv, compared to 10–33 Sv for a fixed efficiency. The results suggest that distributed mixing, overflow-related boundary processes, and geothermal heating are more effective in consuming abyssal waters than topographically enhanced mixing by breaking internal waves. These calculations also point to the importance of accurately constraining Rf(Reb) and including the effect in ocean models.

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Internal Waves and Mixing near the Kerguelen Plateau

Journal of Physical Oceanography ◽

10.1175/jpo-d-15-0055.1 ◽

2015 ◽

Vol 46 (2) ◽

pp. 417-437 ◽

Cited By ~ 9

Author(s):

Amelie Meyer ◽

Kurt L. Polzin ◽

Bernadette M. Sloyan ◽

Helen E. Phillips

Keyword(s):

Internal Wave ◽

Wave Field ◽

Internal Waves ◽

Large Scale ◽

Polar Front ◽

Frontal Region ◽

Small Scale ◽

Kerguelen Plateau ◽

Flow Strength ◽

Internal Wave Field

AbstractIn the stratified ocean, turbulent mixing is primarily attributed to the breaking of internal waves. As such, internal waves provide a link between large-scale forcing and small-scale mixing. The internal wave field north of the Kerguelen Plateau is characterized using 914 high-resolution hydrographic profiles from novel Electromagnetic Autonomous Profiling Explorer (EM-APEX) floats. Altogether, 46 coherent features are identified in the EM-APEX velocity profiles and interpreted in terms of internal wave kinematics. The large number of internal waves analyzed provides a quantitative framework for characterizing spatial variations in the internal wave field and for resolving generation versus propagation dynamics. Internal waves observed near the Kerguelen Plateau have a mean vertical wavelength of 200 m, a mean horizontal wavelength of 15 km, a mean period of 16 h, and a mean horizontal group velocity of 3 cm s−1. The internal wave characteristics are dependent on regional dynamics, suggesting that different generation mechanisms of internal waves dominate in different dynamical zones. The wave fields in the Subantarctic/Subtropical Front and the Polar Front Zone are influenced by the local small-scale topography and flow strength. The eddy-wave field is influenced by the large-scale flow structure, while the internal wave field in the Subantarctic Zone is controlled by atmospheric forcing. More importantly, the local generation of internal waves not only drives large-scale dissipation in the frontal region but also downstream from the plateau. Some internal waves in the frontal region are advected away from the plateau, contributing to mixing and stratification budgets elsewhere.

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Strong Internal Waves Generated by the Interaction of the Kuroshio and Tides over a Shallow Ridge

Journal of Physical Oceanography ◽

10.1175/jpo-d-18-0238.1 ◽

2019 ◽

Vol 49 (11) ◽

pp. 2917-2934 ◽

Cited By ~ 2

Author(s):

Eiji Masunaga ◽

Yusuke Uchiyama ◽

Hidekatsu Yamazaki

Keyword(s):

Internal Wave ◽

Internal Waves ◽

Energy Flux ◽

Wave Energy ◽

High Frequency ◽

Deep Ocean ◽

Internal Tides ◽

Navier Stokes ◽

Wave Energy Flux ◽

The Kuroshio

AbstractThe Kuroshio and tides significantly influence the oceanic environment off the Japanese mainland and promote mass/heat transport. However, the interaction between the Kuroshio and tides/internal waves has not been examined in previous works. To investigate this phenomenon, the two-dimensional high-resolution nonhydrostatic oceanic Stanford Unstructured Nonhydrostatic Terrain-Following Adaptive Navier–Stokes Simulator (SUNTANS) model was employed. The results show that strong internal tides propagating upstream in the Kuroshio are generated at a near-critical internal Froude number (Fri = 0.91). The upstream internal wave energy flux reaches a magnitude of 12 kW m−1, which is approximately 3 times higher than that of internal waves without the Kuroshio. On the other hand, under supercritical conditions, the Kuroshio suppresses the internal wave energy flux. The interaction of internal tides and the Kuroshio also generates upstream propagating high-frequency internal waves and solitary wave packets. The high-frequency internal waves contribute to the increase in the total internal wave energy flux up to 40% at the near-critical Fri value. The results of this study suggest that the interaction of internal tides and the Kuroshio enhances the upstream propagating internal tides under the specified conditions (Fri ~ 1), which may lead to deep ocean mixing and transport at significant distances from the internal wave generation sites.

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The temporal variability of near-inertial internal waves in an internal tide beam

10.5194/egusphere-egu21-792 ◽

2021 ◽

Author(s):

Jonas Löb ◽

Monika Rhein

Keyword(s):

Internal Wave ◽

Internal Waves ◽

Temporal Variability ◽

Internal Tide ◽

Internal Tides ◽

Inertial Waves ◽

Energy Fluxes ◽

Wind Event ◽

Mode 2 ◽

Inertial Energy

Low mode internal waves in the stratified ocean are generated by the interaction between barotropic tides and seafloor topography and by the wind field in the near-inertial range. They are crucial for interior mixing and for the oceanic energy pathways, since they carry a large portion of the energy of the entire internal wave field. Long-term observations of energy fluxes of internal waves are sparse. The aim of this work is to study the temporal variability of wind generated low mode near-inertial internal waves inside an internal tide beam emanating from seamounts south of the Azores. For this, 20 months of consecutive mooring observations are used to calculate the mode 1 and mode 2 near-inertial energy fluxes as well as kinetic and potential energies. The gathered time series of near-inertial internal wave energy flux is not steady due to its intermittent forcing and is neither dominated by either mode 1 or mode 2. It shows a peak induced by a distinct strong wind event which is directly linked to wind-power input into the mixed layer north-east of the mooring location, and allows a comparison between the wind event and a background state. Furthermore, indications of non-linear interactions of the near-inertial waves with the internal tides in the form of resonant triad interaction and non-linear self-interaction have been found. This study provides new insights on the relative importance of single wind events and reinforces the assumption of a global non-uniform distribution of near-inertial energy with emphasis in regions where these events occur often and regularly. It furthermore displays its importance to be adequately incorporated into ocean general circulation models and in generating ocean mixing estimates by near-inertial waves as a similarly important component next to the internal tides.

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