The role of anisotropy in oceanic lithosphere from 'cradle to grave'

<p>The oceanic crust and lithosphere are commonly treated as geologically simple, their fundamental properties encapsulated by the 1D model of layered oceanic crust and the plate-cooling model of lithospheric thickness we learnt as undergraduates. The question of directionality or anisotropy in the behaviour and deeper structure of oceanic plates is relatively rarely considered, despite formation processes, such as rifting and seafloor spreading, and surface topography, such as abyssal hills, that are clearly highly anisotropic. In this presentation, we bring together evidence from a variety of sources from regional studies of rifting and volcanism to numerical modelling and global analyses of bathymetry and gravity data. We show how anisotropy is imprinted into the oceanic lithosphere at formation, both in the early rifting phases and at mature spreading centres, and how that anisotropic signature persists for many millions of years, potentially strengthened by preferential alignment of mineral phases as the moving plates cool and thicken. We then consider how this directionality impacts later deformation, volcanism, and eventually subduction.</p>

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

<p><em>Internal tides / lee waves coupling : dynamics and impact on the ocean energy budget</em></p><p>Yvan Dossmann,</p><p>LEMTA, Université de Lorraine, CNRS, Nancy, France.</p><p>Callum Shakespeare,</p><p>Climate and Fluid Physics, The Australian National University, Canberra, Australia</p><p> </p><p>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 :</p><ul><li> <p>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.</p> </li> <li> <p>the generation of Doppler-shifted internal tides beyond the critical latitudes.</p> </li> <li> <p>the existence of a net wave stress above abyssal hills comparable to the local wind stress.</p> </li> </ul><p>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.</p><p>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.</p><p>References</p><p>[1] Bell, T. H. : Topographically generated internal waves in open ocean », <em>Journal of Geophysical Research</em>, vol. 80, p. 320–327, 1975.</p><p>[2] Shakespeare, C. : Interdependence of internal tide and lee wave generation at abyssal hills: global calculations », <em>in Press</em>, <em>Journal of Physical Oceanography</em>, 2020.</p><p>[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, <em>Journal of Geophysical Research: Oceans </em>121 : 6961-6977, 2016.</p><p> </p>

Interdependence of Internal Tide and Lee Wave Generation at Abyssal Hills: Global Calculations

The generation of internal waves at abyssal hills has been proposed as an important source of bottom-intensified mixing and a sink of geostrophic momentum. Using the theory of Bell, previous authors have calculated either the generation of lee waves by geostrophic flow or the generation of the internal tide by the barotropic tide, but never both together. However, the Bell theory shows that the two are interdependent: that is, the presence of a barotropic tide modifies the generation of lee waves, and the presence of a geostrophic (time mean) flow modifies the generation of the internal tide. Here we extend the theory of Bell to incorporate multiple tidal constituents. Using this extended theory, we recalculate global wave fluxes of energy and momentum using the abyssal-hill spectra, model-derived abyssal ocean stratification and geostrophic flow estimates, and the TPX08 tidal velocities for the eight major constituents. The energy flux into lee waves is suppressed by 13%–19% as a result of the inclusion of tides. The generated wave flux is dominated by the principal lunar semidiurnal tide (M2), and its harmonics and combinations, with the strongest fluxes occurring along midocean ridges. The internal tide generation is strongly asymmetric because of Doppler shifting by the geostrophic abyssal flow, with 55%–63% of the wave energy flux (and stress) directed upstream, against the geostrophic flow. As a consequence, there is a net wave stress associated with generation of the internal tide that reaches magnitudes of 0.01–0.1 N m−2 in the vicinity of midocean ridges.

Nonpropagating Form Drag and Turbulence due to Stratified Flow over Large-Scale Abyssal Hill Topography

Drag and turbulence in steady stratified flows over “abyssal hills” have been parameterized using linear theory and rates of energy cascade due to wave–wave interactions. Linear theory has no drag or energy loss due to large-scale bathymetry because waves with intrinsic frequency less than the Coriolis frequency are evanescent. Numerical work has tested the theory by high passing the topography and estimating the radiation and turbulence. Adding larger-scale bathymetry that would generate evanescent internal waves generates nonlinear and turbulent flow, driving a dissipation approximately twice that of the radiating waves for the topographic spectrum chosen. This drag is linear in the forcing velocity, in contrast to atmospheric parameterizations that have quadratic drag. Simulations containing both small- and large-scale bathymetry have more dissipation than just adding the large- and small-scale dissipations together, so the scales couple. The large-scale turbulence is localized, generally in the lee of large obstacles. Medium-scale regional models partially resolve the “nonpropagating” wavenumbers, leading to the question of whether they need the large-scale energy loss to be parameterized. Varying the resolution of the simulations indicates that if the ratio of gridcell height to width is less than the root-mean-square topographic slope, then the dissipation is overestimated in coarse models (by up to 25%); conversely, it can be underestimated by up to a factor of 2 if the ratio is greater. Most regional simulations are likely in the second regime and should have extra drag added to represent the large-scale bathymetry, and the deficit is at least as large as that parameterized for abyssal hills.