Contamination of Finescale Strain Estimates of Turbulent Kinetic Energy Dissipation by Frontal Physics

Abstract

Finescale strain parameterization (hereafter, FSP) of turbulent kinetic energy dissipation rate has become a widely used method for observing ocean mixing, solving a coverage problem where direct turbulence measurements are absent but CTD profiles are available. This method can offer significant value, but there are limitations in its broad application to the global ocean. FSP often fails to produce reliable results in frontal zones where temperature-salinity (T/S) intrusive features contaminate the CTD strain spectrum, as well as where the aspect ratio of the internal wave spectrum is known to vary greatly with depth, as frequently occurs in the Southern Ocean. In this study we use direct turbulence measurements from DIMES (Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean) and glider microstructure measurements from AUSSOM (Autonomous Sampling of Southern Ocean Mixing) to show that FSP can have large biases (compared to direct turbulence measurement) below the mixed layer when physics associated with T/S fronts are meaningfully present. We propose that the FSP methodology be modified to (1) include a density ratio (Rp)-based data exclusion rule to avoid contamination by double diffusive instabilities in frontal zones such as the Antarctic Circumpolar Current, the Gulf Stream, and the Kuroshio, and (2) conduct (or leverage available) microstructure measurements of the depth-varying shear-to-strain ratio Rw(Z) prior to performing FSP in each dynamically-unique region of the global ocean.

Parameterization of Shear‐To‐Strain Ratio Used in Finescale Parameterization

“…For instance, Liang et al (2021) reported an increase in R ω with depth based on observations conducted in the western Pacific, and established a relationship of R ω (z) = 10 z/b , where b = 2,500 m and z represents water depth. Ferris et al (2022) focused on the Drake Passage region and proposed a depth-dependent parameterization of R ω (z) = 6.5 + 3.5 tanh[ 2π(z 1500) 2000 ] , which smoothly transitions from the GM-assumed R ω = 3 above 1,000 m to the literature-supported R ω = 10 below 2,000 m. The GHP estimates based on this parameterization of R ω exhibit improved agreement with observations in the main thermocline and above, compared to those based on R ω = 10, although both approaches capture the bottomintensified feature of ε. These studies have greatly enriched our knowledge of R ω .…”

“…Ferris et al. (2022) focused on the Drake Passage region and proposed a depth‐dependent parameterization of $${R}_{\omega }(z)=6.5+3.5\hspace*{.5em}\mathrm{tanh}\left[\frac{2\pi (z-1500)}{2000}\right]$$, which smoothly transitions from the GM‐assumed R ω = 3 above 1,000 m to the literature‐supported R ω = 10 below 2,000 m. The GHP estimates based on this parameterization of R ω exhibit improved agreement with observations in the main thermocline and above, compared to those based on R ω = 10, although both approaches capture the bottom‐intensified feature of ε . These studies have greatly enriched our knowledge of R ω .…”