Deep-ocean inertial subrange small bandwidth coherence and Ozmidov-frequency separation

TL;DR

This study investigates the inertial subrange in stratified ocean turbulence, revealing a separation near the Ozmidov frequency that distinguishes anisotropic from isotropic turbulence, based on high-resolution temperature sensor data.

Contribution

It provides new observational evidence of the inertial subrange structure and its separation near the Ozmidov frequency in stratified ocean turbulence environments.

Findings

Inertial subrange separates into two parts near the Ozmidov frequency.

Transition from anisotropic to isotropic turbulence is observed.

Differences in turbulence characteristics above seamounts and ridge-crest environments.

Abstract

The inertial subrange of turbulence in a density stratified environment is the transition from internal waves to isotropic turbulence, but it is unclear how to interpret its extension to anisotropic stratified turbulence. Knowledge about stratified turbulence is relevant for the dispersal of suspended matter in geophysical flows, such as in most of the ocean. For studying internal-wave-induced ocean-turbulence moored high-resolution temperature (T-)sensors are used. Spectra from observations on episodic quasi-convective internal wave breaking above a steep slope of large seamount Josephine in the Northeast-Atlantic demonstrate an inertial subrange that can be separated in two parts: A large-scale part with relatively coherent portions adjacent to less coherent portions, and a small-scale part that is smoothly continuous (to within standard error). The separation is close to the Ozmidov frequency, and coincides with the transition from anisotropic/quasi-deterministic stratified turbulence to isotropic/stochastic inertial convective motions as inferred from a comparison of vertical and horizontal co-spectra. These observations contrast with T-sensor observations of shear-dominated internal wave breaking in an equally turbulent environment above the slope of a small Mid-Atlantic ridge-crest, which demonstrate a stochastic inertial subrange throughout.