Mixing and formation of layers by internal wave forcing

TL;DR

This study investigates how internal wave forcing induces mixing and layer formation in the ocean, revealing significant turbulent diffusivity enhancements and the potential for density staircase development over long-term idealized experiments.

Contribution

It provides new insights into the long-term effects of internal wave-induced mixing and the formation of density layers, using detailed diagnostics and idealized experimental setups.

Findings

Turbulent diffusivity up to 250 times molecular value.

Development of density staircases after several hours of forcing.

Mixing efficiency $ta$ is 12-19% higher than in wave breaking cases.

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 idealised setup. The internal wave dynamics and the evolution of the density profile are followed using the light attenuation technique. Diagnostics of the turbulent diffusivity $K_{T}$ 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, Experiments in Fluids, 57(8), 132 (2016). The mixing efficiency $\eta$ is assessed by relating the $BPE$ growth to the linearized $KE$ input. One finds a value of $\Gamma=12-19\%$ 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 argument. The present experiments suggest that internal wave forcing could also induce the formation of density interfaces in the ocean.