Doctor of Philosophy (Ph.D.)
Virginia Institute of Marine Science
Louis St. Laurent
John M Klinck
Alison M Macdonald
Marjorie AM Friedrichs
Numerous physics are responsible for forward energy cascade at oceanic fronts but their roles are not fully clear. This dissertation investigates wind-sheared turbulence in the ocean surface boundary layer (OSBL), internal wave interactions in the ocean interior, and instability-driven turbulence in energetic jets; with attention paid to the parameterizations used to quantify them. At the OSBL, meteorological forcing injects turbulent kinetic energy (TKE), mixing the upper ocean and rapidly transforming its density structure. In the absence of direct observations or capability to resolve sub-grid scale turbulence in ocean models, the community relies on boundary layer scalings (BLS) of shear and convective turbulence to represent this mixing. Despite the importance of near-surface mixing, ubiquitous BLS representations of these processes have been under-assessed in high energy forcing regimes such as the Southern Ocean. Glider microstructure from AUSSOM (Autonomous Sampling of Southern Ocean Mixing), a long-duration glider mission, is leveraged to show BLS of shear turbulence exhibits a consistent bias in estimating TKE dissipation rates in the OSBL. In the interior, finescale strain parameterization (FSP) of the TKE dissipation rate has become a widely used method for observing mixing, solving a coverage problem where only CTD profiles are available. However there are limitations in its application to intense frontal regions where adjacent warm/salty and cold/fresh waters create double diffusive instability. Direct turbulence measurements from DIMES (Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean) and AUSSOM are used to show FSP can have biases of up to 8 orders of magnitude below the mixed layer when physics associated with T/S fronts are present. FSP often fails to produce reliable results in frontal zones where temperature-salinity (T/S) intrusive features contaminate the CTD strain spectrum, as well as where the aspect ratio of the internal wave spectrum is known to vary greatly with depth (as in the Southern Ocean). We propose that the FSP methodology be modified to include a density ratio-based data exclusion rule to avoid contamination by double diffusive instabilities in frontal zones. At energetic frontal jets, symmetric instability (SI) has gained momentum for explaining enhanced turbulence. Submesoscale frontal instabilities are well-established by idealized analytical and numerical studies to be a significant source of TKE in the global ocean. However, observations of TKE dissipation enhanced by SI are few, and it is unknown to what order in the real ocean this process is active. AUSSOM measured elevated TKE dissipation rates throughout the core of the Polar Front (PF). Motivated by this finding, we use a 1-km Regional Ocean Modeling System hindcast to investigate the role of SI in energy cascade and Southern Ocean mixing. We extend popular overturning instability criteria for application to ageostrophic flows. SI of the centrifugal/inertial variety is widespread along the northern continental margins of the Antarctic Circumpolar Current due to topographic shearing of the anticyclonic side of PF-associated jets but is notably absent (above 1-km scale) from the open-ocean mixed layer. Contrarily, modeled velocity fields are strongly indicative of critical layers and other internal wave interactions dominating the open-ocean elevated TKE budget even at energetic fronts.
© The Author
Ferris, Laur, "Across-Scale Energy Transfer In The Southern Ocean" (2022). Dissertations, Theses, and Masters Projects. William & Mary. Paper 1673281632.