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

• Published in: Journal of geophysical research. Oceans 2018-08, Vol.123 (8), p.5459-5480
• Main Authors: Scheifele, Benjamin, Waterman, Stephanie, Merckelbach, Lucas, Carpenter, Jeffrey R.
• Format: Article
• Language: English
• Subjects: Arctic Ocean; Beaufort Sea; Case studies; Density stratification; Estimates; Geophysics; Gliders; Ice environments; Instrumentation; Kinetic energy; Microstructure; ocean microstructure glider; ocean turbulence; Oceans; Shear; shear microstructure; Stratification; Temperature; Temperature effects; temperature microstructure; Turbulence; Turbulence spectra; Turbulent kinetic energy; Underwater gliders; Vertical profiles
• ISSN: 2169-9275; 2169-9291
• DOI: 10.1029/2017JC013731

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. Key Points Dissipation rate estimates in low‐energy environments are subject to increased uncertainty when stratification is strong Shear microstructure measurements may overestimate dissipation rates in low‐energy environments Resulting biases in geometrically averaged dissipation rates may exceed an order of magnitude