Comparison of calculated energy flux of internal tides with
                        microstructure measurements

Falahat, Saeed; Nycander, Jonas; Roquet, Fabien; Thurnherr, Andreas M.; Hibiya, Taketoshi

doi:10.3402/tellusa.v66.23240

“…The remaining two-thirds of power input are ignored or surmised to participate in sustaining a constant background diffusivity of order 10 −5 m 2 s −1 (Simmons et al, 2004). This approach has two principal limitations: (i) the fraction of local dissipation is not uniform and actually depends on resolution and location (Falahat et al, 2014a;St Laurent & Nash, 2004;Vic et al, 2019) and (ii) constant background diffusivities disallow energy conservation and do not do justice to observed patterns of mixing rates (de Lavergne et al, 2019;Pollmann et al, 2017).…”

Section: 1029/2020ms002065mentioning

confidence: 99%

A parameterization of local and remote tidal mixing

Lavergne

¹

,

Vic

²

,

Madec

³

et al. 2020

Preprint

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Vertical mixing is often regarded as the Achilles' heel of ocean models. In particular, few models include a comprehensive and energy-constrained parameterization of mixing by internal ocean tides. Here, we present an energy-conserving mixing scheme which accounts for the local breaking of high-mode internal tides and the distant dissipation of low-mode internal tides. The scheme relies on four static two-dimensional maps of internal tide dissipation, constructed using mode-by-mode Lagrangian tracking of energy beams from sources to sinks. Each map is associated with a distinct dissipative process and a corresponding vertical structure. Applied to an observational climatology of stratification, the scheme produces a global three-dimensional map of dissipation which compares well with available microstructure observations and with upper-ocean finestructure mixing estimates. This relative agreement, both in magnitude and spatial structure across ocean basins, suggests that internal tides underpin most of observed dissipation in the ocean interior at the global scale. The proposed parameterization is therefore expected to improve understanding, mapping, and modeling of ocean mixing.Plain Language Summary When tidal ocean currents flow over bumpy seafloor, they generate internal tidal waves. Internal waves are the subsurface analog of surface waves that break on beaches. Like surface waves, internal tidal waves often become unstable and break into turbulence. This turbulence is a primary cause of mixing between stacked ocean layers-a key process regulating ocean currents and biology and a key ingredient of computer models of the global ocean. In this article, a three-dimensional global map of mixing induced by internal tidal waves is presented. This map incorporates a large variety of energy pathways from the generation of tidal waves to turbulence, accounting for the conservation of energy. The map is compared to available observations of turbulence across the globe and found to reproduce with good fidelity the main patterns identified in observations. This relatively good agreement suggests that internal tidal waves are the main source of turbulence in the subsurface ocean and implies that the map may serve a range of applications. In particular, the three-dimensional map provides an efficient and realistic means to represent mixing by internal tidal waves in global ocean models.

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“…The remaining two‐thirds of power input are ignored or surmised to participate in sustaining a constant background diffusivity of order 10 −5 m 2 s −1 (Simmons et al, ). This approach has two principal limitations: (i) the fraction of local dissipation is not uniform and actually depends on resolution and location (Falahat et al, ; St Laurent & Nash, ; Vic et al, ) and (ii) constant background diffusivities disallow energy conservation and do not do justice to observed patterns of mixing rates (de Lavergne et al, ; Eden et al, ; Pollmann et al, ).…”

Section: Introductionmentioning

confidence: 99%

A Parameterization of Local and Remote Tidal Mixing

Lavergne

¹

,

Vic

²

,

Madec

³

et al. 2020

J Adv Model Earth Syst

View full text Add to dashboard Cite

Vertical mixing is often regarded as the Achilles' heel of ocean models. In particular, few models include a comprehensive and energy-constrained parameterization of mixing by internal ocean tides. Here, we present an energy-conserving mixing scheme which accounts for the local breaking of high-mode internal tides and the distant dissipation of low-mode internal tides. The scheme relies on four static two-dimensional maps of internal tide dissipation, constructed using mode-by-mode Lagrangian tracking of energy beams from sources to sinks. Each map is associated with a distinct dissipative process and a corresponding vertical structure. Applied to an observational climatology of stratification, the scheme produces a global three-dimensional map of dissipation which compares well with available microstructure observations and with upper-ocean finestructure mixing estimates. This relative agreement, both in magnitude and spatial structure across ocean basins, suggests that internal tides underpin most of observed dissipation in the ocean interior at the global scale. The proposed parameterization is therefore expected to improve understanding, mapping, and modeling of ocean mixing.Plain Language Summary When tidal ocean currents flow over bumpy seafloor, they generate internal tidal waves. Internal waves are the subsurface analog of surface waves that break on beaches. Like surface waves, internal tidal waves often become unstable and break into turbulence. This turbulence is a primary cause of mixing between stacked ocean layers-a key process regulating ocean currents and biology and a key ingredient of computer models of the global ocean. In this article, a three-dimensional global map of mixing induced by internal tidal waves is presented. This map incorporates a large variety of energy pathways from the generation of tidal waves to turbulence, accounting for the conservation of energy. The map is compared to available observations of turbulence across the globe and found to reproduce with good fidelity the main patterns identified in observations. This relatively good agreement suggests that internal tidal waves are the main source of turbulence in the subsurface ocean and implies that the map may serve a range of applications. In particular, the three-dimensional map provides an efficient and realistic means to represent mixing by internal tidal waves in global ocean models.

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“…Very recently, Falahat et al (2014) compared the internal tide energy conversion rate based on an extension of the method of Nycander (2005) to observed dissipation data obtained over rough topographic features of the Mid-Atlantic Ridge (Polzin et al 1997). They found a reasonable correlation between the calculated conversion and observed dissipation, and the magnitude agrees well.…”

Section: Introductionmentioning

confidence: 93%

Global Calculation of Tidal Energy Conversion into Vertical Normal Modes

Falahat

¹

,

Nycander

²

,

Roquet

³

et al. 2014

Journal of Physical Oceanography

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A direct calculation of the tidal generation of internal waves over the global ocean is presented. The calculation is based on a semianalytical model, assuming that the internal tide characteristic slope exceeds the bathymetric slope (subcritical slope) and the bathymetric height is small relative to the vertical scale of the wave, as well as that the horizontal tidal excursion is smaller than the horizontal topographic scale. The calculation is performed for the M 2 tidal constituent. In contrast to previous similar computations, the internal tide is projected onto vertical eigenmodes, which gives two advantages. First, the vertical density profile and the finite ocean depth are taken into account in a fully consistent way, in contrast to earlier work based on the WKB approximation. Nevertheless, the WKB-based total global conversion follows closely that obtained using the eigenmode decomposition in each of the latitudinal and vertical distributions. Second, the information about the distribution of the conversion energy over different vertical modes is valuable, since the lowest modes can propagate over long distances, while high modes are more likely to dissipate locally, near the generation site. It is found that the difference between the vertical distributions of the tidal conversion into the vertical modes is smaller for the case of very deep ocean than the shallow-ocean depth. The results of the present work pave the way for future work on the vertical and horizontal distribution of the mixing caused by internal tides.

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Comparison of calculated energy flux of internal tides with microstructure measurements

Cited by 14 publications

References 51 publications

A parameterization of local and remote tidal mixing

A parameterization of local and remote tidal mixing

A Parameterization of Local and Remote Tidal Mixing

Global Calculation of Tidal Energy Conversion into Vertical Normal Modes

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