Measuring the Dissipation Rate of Turbulent Kinetic Energy in Strongly Stratified, Low‐Energy Environments: A Case Study From the Arctic Ocean

Abstract

We compare estimates of the turbulent dissipation rate, ε, obtained independently from coincident measurements of shear and temperature microstructure in the southeastern Beaufort Sea, a strongly stratified, low‐energy environment. The measurements were collected over 10 days in 2015 by an ocean glider equipped with microstructure instrumentation; they yield 28,575 shear‐derived and 21,577 temperature‐derived ε estimates. We find agreement within a factor of 2 from the two types of estimates when ε exceeds 3 × 10−11 W/kg, a threshold we identify as the noise floor of the shear‐derived estimates. However, the temperature‐derived estimates suggest that the dissipation rate is lower than this threshold in 58% of our observations. Further, the noise floor of the shear measurements artificially skews the statistical distribution of ε below 10−10 W/kg, that is, in 70% of our observations. The shear measurements overestimate portions of the geometric mean vertical profile of ε by more than an order of magnitude and underestimate the overall variability of ε by at least 2 orders of magnitude. We further discuss uncertainties that arise in both temperature‐ and shear‐derived ε estimates in strongly stratified, weakly turbulent conditions, and we demonstrate how turbulence spectra are systematically modified by stratification under these conditions. Using evidence from the temperature‐gradient spectral shapes and from the observed ε distributions, we suggest that the temperature‐derived dissipation rates are reliable to values as small as 2 × 10−12 W/kg, making them preferable for characterizing the turbulent dissipation rates in the weakly turbulent environment of this study.

Laboratory study of the properties of frazil ice particles and flocs in water of different salinities

“…In a similar series of experiments in the same tank, McFarlane et al (2015) found that the tank-averaged turbulent kinetic energy dissipation rate was ∼ 3.4 × 10 −2 m 2 s −3 at a propeller speed of 325 rpm, and this fell within the range of dissipation rates estimated for rivers in Alberta. Dissipation rates in the ocean range from ∼ 10 −2 to 10 −9 m 2 s −3 (Banner and Morrison, 2018;Wang and Liao, 2016), with a reported lower range in the polar regions ranging from ∼ 10 −3 to 10 −10 m 2 s −3 (Chanona et al, 2018;Scheifele et al, 2018). Therefore, the dissipation rate generated in these experiments falls within the upper limits of values observed in the ocean.…”

“…2. This method yields values of the freezing point that are within 0.01 • C or better (Mair et al, 1941;She et al, 2016). The freezing point measurements made during each repeated experiment at a given salinity were averaged and the mean freezing points (± standard deviation) for salinities of 15 ‰, 25 ‰ and 35 ‰ C. C. Schneck et al: Properties of frazil and flocs in different salinities were −0.89 ± 0.02, −1.48 ± 0.02 and −2.09 ± 0.02 • C, respectively.…”

“…Typical supercooling curves showing the difference between the freezing point temperature T f in freshwater and saline water in a confined vessel. Adapted fromShe et al (2016).…”

Pan-Arctic Ocean Primary Production Constrained by Turbulent Nitrate Fluxes

“…Parameterizations of this kind, relying on models of internal wave breaking, are most useful away from boundaries, in scenarios of perennial stratification where year-round background fluxes dominate (Randelhoff and Guthrie, 2016), and less so to characterize near-surface mixing. Other promising avenues are approaches based on turbulence structure functions (Wiles et al, 2006), high-frequency Acoustic Doppler Current Profiler measurements, or microstructure sensors deployed on moorings and gliders (Scheifele et al, 2018).…”

A methodology for Thorpe scaling 512 Hz fast thermistor data from buoyancy-driven gliders to estimate turbulent kinetic energy dissipation rate in the ocean

“…8 ai. A study in 2018 [9] suggest that a lower limit for reliable values derived from temperature is no smaller than 2 × 10 −12 W kg −1 (in weakly turbulent envirnoments), nearly three orders greater than the second peak seen in Fig. 8 aii.…”