Mixing and Formation of Layers by Internal Wave Forcing

Abstract

The energy pathways from propagating internal waves to the scales of irreversible mixing in the ocean are not fully described. In the ocean interior, the triadic resonant instability is an intrinsic destabilization process that may enhance the energy cascade away from topographies. The present study focuses on the integrated impact of mixing processes induced by a propagative normal mode‐1 over long‐term experiments in an idealized setup. The internal wave dynamics and the evolution of the density profile are followed using the light attenuation technique. Diagnostics of the turbulent diffusivity KT and background potential energy BPE are provided. Mixing effects result in a partially mixed layer colocated with the region of maximum shear induced by the forcing normal mode. The maximum measured turbulent diffusivity is 250 times larger than the molecular value, showing that diapycnal mixing is largely enhanced by small‐scale turbulent processes. Intermittency and reversible energy transfers are discussed to bridge the gap between the present diagnostic and the larger values measured in Dossmann et al. (). The mixing efficiency η is assessed by relating the BPE growth to the linearized KE input. One finds a value of Γ=12−19%, larger than the mixing efficiency in the case of breaking interfacial wave. After several hours of forcing, the development of staircases in the density profile is observed. This mechanism has been previously observed in experiments with weak homogeneous turbulence and explained by Phillips (1972) argument. The present experiments suggest that internal wave forcing could also induce the formation of density interfaces in the ocean.

Excitation and resonant enhancement of axisymmetric internal wave modes

“…The modal boundary condition is expressed by equation (17) as detailed before. For N < ω, the problem can be readily solved via a coordinate transformation: (r ′ = Γr, z ′ = z) so that equation (15) becomes:…”

Near-field internal wave beams in two dimensions

“…In one design, called GOAL (generator of oscillations as you like), the elements are in direct contact with the fluid (Dossmann etal. 2016, 2017; Brunet, Dauxois& Cortet 2019); in another, called ASWaM (arbitrary spectrum wave maker), they operate behind a neoprene sheet smoothing out the discretization (Dobra, Lawrie & Dalziel 2019).…”

“…Alternative designs have also been developed, in which each generating element, either plate, bar or rod, is controlled by an individual motor. In one design, called GOAL (generator of oscillations as you like), the elements are in direct contact with the fluid (Dossmann et al 2016(Dossmann et al , 2017Brunet, Dauxois & Cortet 2019); in another, called ASWaM (arbitrary spectrum wave maker), they operate behind a neoprene sheet smoothing out the discretization (Dobra, Lawrie & Dalziel 2019).…”

Experimental study on superharmonic wave generation by resonant interaction between internal wave modes

“…The internal wave generator comprised 50 plates of 6.5-mm thickness each, and they were independently driven by 50 different motors. More details of this wave generator, which was previously used in [39], can be found in [40]. Each plate executed constant amplitude oscillations along the horizontal such that the vertical profile of the horizontal displacement field at the wave generator location (x = 0) was given by (t denotes time)…”