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.

In simulations of the two-dimensional Ising model, we examine heterogeneous nucleation induced by a small impurity consisting of a line of l fixed spins. As l increases, we identify a limit of stability beyond which the metastable phase is not defined. We evaluate the free energy barrier for nucleation of the stable phase and show that, contrary to expectation, the barrier does not vanish on approach to the limit of stability. We also demonstrate that our values for the height of the barrier yield predictions for the nucleation time (from transition state theory) and the size of the critical cluster (from the nucleation theorem) that are in excellent agreement with direct measurements, even near the limit of stability.

Powell Lake contains a deep layer of relic seawater separated from the ocean since the last ice age. Permanently stratified and geothermally heated from below, this deep layer is an isolated geophysical domain suitable for studying double-diffusive convection. High-resolution CTD and microstructure measurements show several double-diffusive staircases (Rρ = 1.6 to 6) in the deep water, separated vertically by smooth high-gradient regions with much larger density ratios. The lowest staircase contains steps that are laterally coherent on the basin scale and have a well-defined vertical structure. On average, temperature steps in this staircase are 4 mK, salinity steps are 2 mg kg−1, and mixed layer heights are 70 cm. The CTD is capable of measuring bulk characteristics of the staircase in both temperature and salinity. Microstructure measurements are limited to temperature alone, but resolve the maximum temperature gradients in the center of selected laminar interfaces. Two different algorithms for characterizing the staircase are compared. Consistent estimates of the steady-state heat flux (27 mW m−2) are obtained from measurements above and below the staircase, as well as from microstructure measurements in the center of smooth interfaces. Estimates obtained from bulk interface gradients underestimate the steady-state flux by nearly a factor of 2. The mean flux calculated using a standard 4/3 flux law parameterization agrees well with the independent estimates, but inconsistencies between the parameterization and the observations remain. These inconsistencies are examined by comparing the underlying scaling relationship to the measurements.

This study uses CTD and microstructure measurements of shear and temperature from 348 glider profiles to characterize turbulence and turbulent mixing in the southeastern Beaufort Sea, where turbulence observations are presently scarce. We find that turbulence is typically weak: the turbulent kinetic energy dissipation rate, ε, has a median value (with 95% confidence intervals) of 2.3 (2.2, 2.4) × 10−11 W kg−1 and is less than 1.0×10−10 W kg−1 in 68% of observations. Variability in ε spans five orders of magnitude, with indications that turbulence is bottom enhanced and modulated in time by the semidiurnal tide. Stratification is strong and frequently damps turbulence, inhibiting diapycnal mixing. Buoyancy Reynolds number estimates suggest that turbulent diapycnal mixing is unlikely in 93% of observations; however, a small number of strongly turbulent mixing events are disproportionately important in determining net buoyancy fluxes. The arithmetic mean diapycnal diffusivity of density is 4.5 (2.3, 14) ×10−6 m2 s−1, three orders of magnitude larger than that expected from molecular diffusion. Vertical heat fluxes are modest at O(0.1) W m−2, of the same order of magnitude as those in the Canada Basin double-diffusive staircase, however, staircases are generally not observed. Despite significant heat present in the Pacific Water layer in the form of a warm-core mesoscale eddy and smaller, O(1) km, temperature anomalies, turbulent mixing was found to be too low to release this heat to shallower depths.

The strongly-stratified halocline of the Arctic Ocean is known to be remarkably resistant to turbulent mixing (Fer, 2009;Rainville & Winsor, 2008;Shaw & Stanton, 2014). This insulates the Arctic sea ice from significant vertical turbulent fluxes of heat sourced in deeper waters, such as the Atlantic Water core. However, it has recently been recognised that a significant warming of the Arctic halocline has been taking place in the Beaufort Sea, through the subduction of warm Pacific-origin waters in the Chukchi Sea. Timmermans et al. (2018) describe how this heat is "archived" in the halocline as it spreads through the Beaufort Sea, due to the highly stable and low-turbulence conditions there. Transport processes acting within the halocline influence the fate of this stored heat and whether it contributes to a warming of the sea ice. Even weak changes in mixing in the halocline are expected to have an impact on the general circulation of the Arctic Ocean (Spall, 2020) and in projected changes in Arctic Ocean primary productivity (Randelhof et al., 2020). Therefore, mixing rates and processes in the halocline must be better quantified and better understood to determine the immediacy and scale of the ensuing Arctic Ocean changes.In this study, we report on results based on observations made by an underwater glider equipped with specialized instrumentation to measure turbulent microstructure in the Amundsen Gulf region of the Beaufort Sea. Using this platform, we are able to quantify the unique thermal structure of the halocline in high vertical resolution and gain insight into its associated mixing mechanisms. We find that the dissipation rate of small-scale temperature variance is significantly enhanced relative to expectations given the extremely low levels of turbulence. We propose that this is the signature of a mixing mechanism first described by Stomel and Fedorov (1967) and Stern (1967) and quantified dynamically by Joyce (1977) and Davis (1994) in which the production of intrusive structure by lateral advection is balanced by its vertical diffusion by molecular and/or weakly turbulent mixing rates. The