ocean_data_tools: A MATLAB toolbox for interacting with bulk freely-available oceanographic data

ocean_data_tools simplifies the process of extracting, formatting, and visualizing freely-available oceanographic data. A wealth of oceanographic data (from research cruises, autonomous floats, global ocean models, etc.) is accessible online. However, many oceanographers and environmental scientists (particularly those from subdisciplines not accustomed to working with large datasets) can be dissuaded from utilizing this data because of the overhead associated with determining how to batch download data and format it into easily-manipulable data structures. ocean_data_tools solves this problem by allowing the user to transform common oceanographic data sources into uniform structure arrays, call general functions on these structure arrays, perform custom calculations, and make graphics

Across-Scale Energy Transfer In The Southern Ocean

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

Contamination of Finescale Strain Estimates of Turbulent Kinetic Energy Dissipation by Frontal Physics

Finescale strain parameterization (FSP) of turbulent kinetic energy dissipation rate has become a widely used method for observing ocean mixing, solving a coverage problem where direct turbulence measurements are absent but CTD profiles are available. This method can offer significant value, but there are limitations in its broad application to the global ocean. 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 frequently occurs in the Southern Ocean. In this study we use direct turbulence measurements from Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES) and glider microstructure measurements from Autonomous Sampling of Southern Ocean Mixing (AUSSOM) to show that FSP can have large biases (compared to direct turbulence measurement) below the mixed layer when physics associated with T/S fronts are meaningfully present. We propose that the FSP methodology be modified to 1) include a density ratio (Rρ)-based data exclusion rule to avoid contamination by double diffusive instabilities in frontal zones such as the Antarctic Circumpolar Current, the Gulf Stream, and the Kuroshio, and 2) conduct (or leverage available) microstructure measurements of the depth-varying shear-to-strain ratio Rω(z) prior to performing FSP in each dynamically unique region of the global ocean

Shear turbulence in the high-wind Southern Ocean using direct measurements

The ocean surface boundary layer is a gateway of energy transfer into the ocean. Wind-driven shear and meteorologically forced convection inject turbulent kinetic energy into the surface boundary layer, mixing the upper ocean and transforming its density structure. In the absence of direct observations or the capability to resolve sub-grid scale 3D turbulence in operational ocean models, the oceanography community relies on surface boundary layer similarity scalings (BLS) of shear and convective turbulence to represent this mixing. Despite their importance, near-surface mixing processes (and ubiquitous BLS representations of these processes) have been under-sampled in high energy forcing regimes such as the Southern Ocean. With the maturing of autonomous sampling platforms, there is now an opportunity to collect high-resolution spatial and temporal measurements in the full range of forcing conditions. Here, we characterize near-surface turbulence under strong wind forcing using the first long-duration glider microstructure survey of the Southern Ocean. We leverage these data to show that the measured turbulence is significantly higher than standard shear-convective BLS in the shallower parts of the surface boundary layer and lower than standard shear-convective BLS in the deeper parts of the surface boundary layer; the latter of which is not easily explained by present wave-effect literature. Consistent with the CBLAST (Coupled Boundary Layers and Air Sea Transfer) low winds experiment, this bias has the largest magnitude and spread in shallowest 10% of the actively mixing layer under low-wind and breaking wave conditions, when relatively low levels of turbulent kinetic energy (TKE) in surface regim

Shear turbulence in the high-wind Southern Ocean using direct measurements

Author Posting. © American Meteorological Society, 2022. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 52(10), (2022): 2325–2341, https://doi.org/10.1175/jpo-d-21-0015.1.The ocean surface boundary layer is a gateway of energy transfer into the ocean. Wind-driven shear and meteorologically forced convection inject turbulent kinetic energy into the surface boundary layer, mixing the upper ocean and transforming its density structure. In the absence of direct observations or the capability to resolve subgrid-scale 3D turbulence in operational ocean models, the oceanography community relies on surface boundary layer similarity scalings (BLS) of shear and convective turbulence to represent this mixing. Despite their importance, near-surface mixing processes (and ubiquitous BLS representations of these processes) have been undersampled in high-energy forcing regimes such as the Southern Ocean. With the maturing of autonomous sampling platforms, there is now an opportunity to collect high-resolution spatial and temporal measurements in the full range of forcing conditions. Here, we characterize near-surface turbulence under strong wind forcing using the first long-duration glider microstructure survey of the Southern Ocean. We leverage these data to show that the measured turbulence is significantly higher than standard shear-convective BLS in the shallower parts of the surface boundary layer and lower than standard shear-convective BLS in the deeper parts of the surface boundary layer; the latter of which is not easily explained by present wave-effect literature. Consistent with the CBLAST (Coupled Boundary Layers and Air Sea Transfer) low winds experiment, this bias has the largest magnitude and spread in the shallowest 10% of the actively mixing layer under low-wind and breaking wave conditions, when relatively low levels of turbulent kinetic energy (TKE) in surface regime are easily biased by wave events.This paper is VIMS Contribution 4103. Computational resources were provided by the VIMS Ocean-Atmosphere and Climate Change Research Fund. AUSSOM was supported by the OCE Division of the National Science Foundation (1558639)