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Observations of turbulence and mixing in the southeastern Beaufort Sea Scheifele, Benjamin

Abstract

In this thesis, I use a novel set of hydrography and turbulence measurements from the southeastern Beaufort Sea to i. compare estimates of the turbulent kinetic energy dissipation rate, ε, obtained independently from shear and temperature microstructure measurements; ii. characterize turbulence and mixing in the Amundsen Gulf region of the southeastern Beaufort Sea; and iii. describe the characteristics of tracer diffusion in an oceanic flow as it transitions between fully turbulent and nearly-laminar. I collected the measurements over 10 days in 2015 using an ocean glider measuring temperature, conductivity, and pressure on O(10)-cm scales and shear and temperature on O(1)-mm turbulent scales. The two independent ε estimates agree within a factor of 2 when ε exceeds 3 × 10⁻¹¹ W kg⁻¹, but diverge by up to two orders of magnitude at smaller values. I identify the noise floor of the shear measurements as the primary reason for this divergence and, therefore, suggest that microstructure temperature measurements are preferable for estimating ε in low energy environments like the Beaufort Sea. I find that turbulence is typically weak in Amundsen Gulf: ε has a geometric mean value of 2.8 × 10⁻¹¹ W kg⁻¹ and is less than 1 × 10⁻¹⁰ W kg⁻¹ in 68% of observations. Turbulent dissipation varies over five orders of magnitude, is bottom enhanced, and is primarily modulated by the M2 tide. Stratification is strong and frequently damps turbulence, inhibiting diapycnal mixing in up to 93% of observations. However, a small number of strongly turbulent mixing events disproportionately drive net buoyancy fluxes. Heat fluxes are modest and nearly always below 1 W m⁻². Finally, I use the turbulence measurements to demonstrate how tracer diffusion in the ocean transitions continuously between turbulent diffusion and near-molecular diffusion as turbulence weakens and stratification strengthens. I use the buoyancy Reynolds number, ReB, to quantify the relative energetic contributions of potential and kinetic energy to the flow dynamics and find that present models for tracer diffusion are accurate to within a factor of 3 when ReB > 10. However, contrary to expectations, I find that significant enhanced tracer diffusivity at turbulent scales remains present when ReB is below unity.

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Attribution-NonCommercial-NoDerivatives 4.0 International

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