EGU2020-8503
https://doi.org/10.5194/egusphere-egu2020-8503
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Internal tides / lee waves coupling : dynamics and impact on the ocean energy budget

Yvan Dossmann1 and Callum Shakespeare2
Yvan Dossmann and Callum Shakespeare
  • 1Université de Lorraine, LEMTA, Nancy, France (yvan.dossmann@univ-lorraine.fr)
  • 2Research School of Earth Sciences, The Australian National University, Canberra, Australia

Internal tides / lee waves coupling : dynamics and impact on the ocean energy budget

Yvan Dossmann,

LEMTA, Université de Lorraine, CNRS, Nancy, France.

Callum Shakespeare,

Climate and Fluid Physics, The Australian National University, Canberra, Australia

 

Usual parameterizations of mixing in global models quantify independently the contribution of internal tides -generated by barotropic flows- and lee waves -generated by quasi-steady flows- relying on a linear approach based on the theory of Bell [1]. However the combined effects of the tidal and quasi-steady flows causes a linear coupling between internal tides and lee waves that has been overlooked in internal wave mixing parameterizations over the last decades [2]. This coupling induces major changes in the internal wave dynamics that has dramatic global impacts on :

  • the energy fluxes to lee waves that is cancelled by 20 % on a global scale and up to 90 % in key areas of the Meridional Overturning Circulation as the Drake passage.

  • the generation of Doppler-shifted internal tides beyond the critical latitudes.

  • the existence of a net wave stress above abyssal hills comparable to the local wind stress.

An accurate description of the cascade from generation to mixing is a necessary step to define relevant parameterizations at the ocean scale and significantly reduce the large uncertainties due to partially represented processes.

The experimental campaign LATMIX led at ANU Canberra in 2019 has confirmed the dynamical effects of this linear coupling on internal wave propagation, energy fluxes and mixing based on high resolution density measurements with the light attenuation technique (LAT). Strong nonlinear processes such as the formation of horizontal vortices have been measured in the bottom boundary layer. The generation of these vortices is only observed when the steady and tidal forcings are combined, while different strong nonlinear structures are present in the case of a pure steady flow [3]. Mixing induced by nonlinear processes overcomes internal wave induced mixing in most relevant parameter regimes for the ocean. These results provide insights to better understand and represent (non-)linear internal wave processes and their impact on mixing at regional and global scales. I will present the main results of this experimental campaign and discuss their implications for the representation of internal wave induced mixing at regional and global scales.

References

[1] Bell, T. H. : Topographically generated internal waves in open ocean », Journal of Geophysical Research, vol. 80, p. 320–327, 1975.

[2] Shakespeare, C. : Interdependence of internal tide and lee wave generation at abyssal hills: global calculations », in Press, Journal of Physical Oceanography, 2020.

[3] Dossmann, Y.; G. Rosevear, M.; Griffiths, R. W.; McC. Hogg, A.; Hughes, G. O. and Copeland, M.: Experiments with mixing in stratified flow over a topographic ridge, Journal of Geophysical Research: Oceans 121 : 6961-6977, 2016.

 

How to cite: Dossmann, Y. and Shakespeare, C.: Internal tides / lee waves coupling : dynamics and impact on the ocean energy budget, EGU General Assembly 2020, Online, 4–8 May 2020, https://doi.org/10.5194/egusphere-egu2020-8503, 2020

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Presentation version 1 – uploaded on 01 May 2020
  • CC1: Comment on EGU2020-8503, Paul Pukite, 15 May 2020

    In reference to the Drake Passage, Woodworth and Hibbert (Ocean Science, vol. 14, no. 4, pp. 711–730, 2018) were able to extract the long-period tidal forcing from bottom-pressure readings. These long-period tides can then be used to calibrate the forcings used to drive ENSO and other climate dipole cycles. Crucially, impulses of an annual and semi-annual cycle are required to amplify the tidal cycles to generate the correct synchronizaton.  Interesting that the region near the southern Andes is an origin of gravity wave momentum transfer for the earth, and showing strong annual and semi-annual impulses (see the Rapp paper at this EGU https://meetingorganizer.copernicus.org/EGU2020/EGU2020-12918.htm)

    The annual impulses together with the tidal forcing create triad patterns with a characteristic mirror symmetry around the annual driving frequency

     

    For further references, see this :

    https://geoenergymath.com/2019/02/25/long-period-tides/