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

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.