Stephanie Waterman

Assistant Professor

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Graduate Student Supervision

Doctoral Student Supervision (Jan 2008 - Nov 2019)
Observations of turbulence and mixing in the southeastern Beaufort Sea (2020)

In this thesis, I use a novel set of hydrography and turbulence measurements from the southeastern Beaufort Sea toi. 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; andiii. 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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Spatial and temporal variability of internal wave-driven mixing in the Arctic Ocean (2020)

The Arctic Ocean is a unique oceanographic environment that sits at the frontier of the impacts of climate change. In light of the ongoing dramatic changes observed in the Arctic Ocean, there has been a growing interest in improving our understanding of turbulent ocean mixing rates, which play an integral role in setting numerous oceanographic properties. However, scarcity of direct turbulence measurements in the Arctic Ocean inhibits our ability to robustly quantify the space-time variability of mixing in this region and understand the mechanisms that underpin it. This thesis addresses this issue by employing a finescale parameterization of turbulent dissipation to estimate turbulent mixing metrics from three unique Arctic Ocean datasets that span a wide range of distinct space and time scales. Key results include the following. First, estimated internal wave-driven dissipation rates span multiple orders of magnitude, both across large geographic domains and temporally on local scales. Despite this wide variability, dissipation rates display distinct regional differences, with estimated turbulent metrics that are consistently higher on the Canadian Arctic shelf than in the central basins. Dissipation rate time series also vary systematically at key tidal frequencies and on seasonal time scales, but exhibit no interannual trends on periods of up to 16 years. Additionally, a characterization of mixing regimes reveals large-scale spatial structure in the distribution of turbulent, non-turbulent, and marginal mixing regimes. Non-turbulent conditions are most prevalent, but wide variability implies that turbulent mixing occurs in all regions at least some of the time. Finally, dissipation rate estimates from each dataset provide consistent, statistically-significant evidence that tidal forcing and stratification strength modulate turbulence more strongly than wind speed, topographic roughness, or sea ice cover; however, the correlations between each of these metrics and turbulence are generally weak. Overall, the primary contribution of this thesis is the provision of an improved statistical characterization of turbulent metrics in the Arctic Ocean on unprecedented spatial and temporal scales. This characterization puts more limited mixing measurements into a broader context and further provides a valuable observational baseline that can be used to inform Arctic Ocean modelling studies.

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