[A-OS16] Frontiers of Ocean Mixing Research

[AOS16-P02] Contamination of finescale strain estimates of turbulent kinetic energy dissipation by frontal physics

*Laur Ferris¹, Donglai Gong², Sophia Merrifield³, Louis St. Laurent⁴ (1.University of Alaska Fairbanks, 2.Virginia Institute of Marine Science, 3.Scripps Institution of Oceanography, 4.Applied Physics Laboratory - University of Washington)

Keywords:turbulence, parameterizations, finescale, double diffusive instability

Finescale strain parameterization (FSP) of the 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 offers value, but there are limitations in its broad application to the global ocean; particularly at intense frontal regions such where adjacent warm/salty and cold/fresh waters create double diffusive instability. 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), a 2017-2018 glider program, to show that FSP can have biases of up to 8 orders of magnitude below the mixed layer when physics associated with T/S fronts are meaningfully present. 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). We propose that the FSP methodology be modified to (1) include a density ratio (R_ρ)-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 R_ω(z) prior to performing FSP in each dynamically-unique region of the global ocean.