Upscale Energy Transfer by the Vortical Mode and Internal Waves

Abstract

Diapycnal mixing in the ocean is sporadic yet ubiquitous, leading to patches of mixing on a variety of scales. The adjustment of such mixed patches can lead to the formation of vortices and other small-scale geostrophic motions, which are thought to enhance lateral diffusivity. If vortices are densely populated, they can interact and merge, and upscale energy transfer can occur. Vortex interaction can also be modified by internal waves, thus impacting upscale transfer. Numerical experiments were used to study the effect of a large-scale near-inertial internal wave on a field of submesoscale vortices. While one might expect a vertical shear to limit the vertical scale of merging vortices, it was found that internal wave shear did not disrupt upscale energy transfer. Rather, under certain conditions, it enhanced upscale transfer by enhancing vortex–vortex interaction. If vortices were so densely populated that they interacted even in the absence of a wave, adding a forced large-scale wave enhanced the existing upscale transfer. Results further suggest that continuous forcing by the main driving mechanism (either vortices or internal waves) is necessary to maintain such upscale transfer. These findings could help to improve understanding of the direction of energy transfer in submesoscale oceanic processes.

Resolving the Paradox of Oceanic Large-Scale Balance and Small-Scale Mixing

“…One can estimate that globally the small-scale dissipation is between 20% and 25% of the available energy, thus alleviating the long-standing issue in ocean and climate dynamics concerning the amount of energy dissipation. Performing modeling of such flows may mis-represent small-scale statistics, as shown for example in [32], but recent numerical experiments at moderate resolution using such a technique [33] do find an inverse cascade of energy for Boussinesq flows. The findings presented herein thus might help devise more realistic turbulence closures for the atmosphere and ocean.…”

The Impact of Submesoscale Physics on Primary Productivity of Plankton

“…Observationally, submesoscale processes are difficult to distinguish from internal waves (Bühler et al 2014) on account of their similar near-inertial timescales and the nonlinear interactions between the vorticity of the flow and the waves (Brunner-Suzuki et al 2014, Klein et al 2004. The strong vorticity of submesoscale dynamics is particularly effective in modifying near-inertial waves and their energy propagation (Kunze 1985), which has implications for the transfer of energy from one scale to another.…”

Cascades and transitions in turbulent flows

“…They have been observed in geophysical flows, e.g. where the atmosphere acts like a 2D flow at large scale and as a 3D flow at small scales [77][78][79][80][81][82][83][84][85] and in the ocean [86,87]. Similar behaviour has been attributed to astrophysical flows (like the atmosphere of Venus and Jupiter [88,89], and accretion discs [90]), in plasma flows [91] and in industrial applications (like in tokamak [92]) either due to the thinness of the layer, to fast rotation or to the presence of strong magnetic fields.…”