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Ocean variability in the form of coherent mesoscale eddies is still poorly understood despite more than a century of persistent interest and undeniable geophysical significance. These eddies propagate for thousands of miles and distribute vast amounts of nutrients and energy throughout the world’s oceans. Most intriguing is how analytical theories and physical arguments suggest that larger vortices, those with radii greater than 75 km, should become unstable and break down in the span of months, yet they are observed to last for years. This research explores two phenomena that contribute to the longevity of these features. First is their ability to adjust to ambient large-scale flows—a feature that determines the eddy’s endurance. The second aspect is the role small-scale, irregular topography plays in vortex dynamics. We demonstrate that rough seafloor stabilizes surface-intensified vortices by restricting the motion in the deep layer, thereby allowing the upper ring to perpetuate unhindered. Using an analytical parameterization, we show how this "sandpaper effect" can be accounted for in quasi-geostrophic and full Navier-Stokes models. This research will ultimately allow the U.S. Navy to improve global ocean forecasts without dramatically increasing the spatial resolution., Civilian, Department of the Navy, Approved for public release. Distribution is unlimited.

Hundreds of sites in littoral and inland waters across the United States have been reported as possibly containing underwater munitions, a safety hazard for the general public. Therefore, it is critical to determine and predict munition location and depth to implement remediation strategies. The mobility of munitions may be influenced by currents, waves, and seafloor slope. Burial depth may be affected by scour, sediment accretion, wave-induced liquefaction, and bedform migration. Here, environmental conditions and morphological evolution are investigated by examining observational data and modeling analyses to better understand the physical processes influencing the burial and mobility of munitions. The environmental models (Delft3D) are validated using observations from field experiments. A coupled Delft3D-object model capable of predicting the mobility and burial of objects on a sandy seafloor is presented. Although the object model limitation considers only cylindrical objects on flat seafloor, ignoring pitch and yaw movements, the coupled Delft3D-object model predictions agree well with observations. In addition, processes such as sediment transport, wave-induced liquefaction, and sand wave migration are examined. The findings show that environmental conditions from Delft3D can be used as a forcing term by other models (e.g., object and wave-induced liquefaction models), which is a valuable tool for predicting the fate of munitions., SERDP, Capitao-de-Corveta, Brazilian Navy, Approved for public release. Distribution is unlimited.

Intrusions are found throughout the ocean and are suspected to significantly transport salt, heat, nutrients, and pollutants laterally in ways that can have large impacts on large-scale ocean circulation. We perform the first fully resolved simulations that allow for the natural development of intrusions at an arbitrary inclination. This allows us to characterize their small-scale mixing and their lateral transport. We use these results to develop a less computationally expensive model that parameterizes the small-scale mixing of microscale double-diffusive processes in a multi-dimensional simulation. Such studies have been limited to one-dimensional simulations in the past. We test this parameterized model by comparing the results to a comparable fully resolved simulation. This allows us to perform simulations on more physically relevant scales for intrusions, which will allow us to explore the dependencies of heat and salt transport on our governing parameters of density ratio, aspect ratio, and isothermal slope. We find that intrusion-driven transport decreases with increasing density ratio and that the results depend strongly on the aspect ratio of the domain. Both fully resolved simulations and some parameterized simulations are dominated by collective instabilities. However, other parameterized simulations, particularly those at low density ratios, show the development of direct mode intrusions, and they evolve into thermohaline staircases.

Icebergs present an inherent risk to ship navigation, as collision with an iceberg can have catastrophic consequences. Icebergs are also a crucial source of freshwater flux to the polar oceans, where they can influence ambient water circulation and deep water formation. Prediction of iceberg distribution is thus important for both safe operations in bergy waters and in global climate circulation models. To model the drift of an iceberg, its deterioration—that is, the loss of mass due to thermal (melting) and dynamic (calving) processes—must also be known, since drift rate is dependent upon both the size and shape of the iceberg. In this thesis, we use the state-of-the-art North American Ice Service (NAIS) Iceberg Drift and Deterioration Model v2.0 to identify key deterioration mechanisms and determine major sources of uncertainty. We find the wave erosion parameters have the greatest impact on iceberg prediction, specifically to the form of the iceberg perimeter eroded by wave action. In contrast, the model is insensitive to parameterizations of surface melting. We therefore suggest that future work focus on better resolution and validation of modelled iceberg deterioration.

Advances in engineering and technology have made acoustic detection of submarines increasingly difficult. Using hydrodynamic signatures created by propagating submarines is an alternative method for submarine detection. Detection based on hydrodynamic signatures may also offer unique tactical advantages, given the tendency of wakes to persist for long timescales. Artificial neural networks trained on velocity field data show promise to automate detection. We used numerical simulations to generate velocity data on wake, jet, and convective turbulence. All these forms of turbulence have similar characteristics in the velocity field, yet their spectra reveal subtle differences that could be exploited for wake identification purposes. We then trained a convolutional neural network with the simulation results and demonstrated that neural networks can classify turbulent flows based on small-scale features with high accuracy. In particular, we find that the developed algorithms can successfully identify wakes in 92% of cases, which implies that the AI-based technology is viable and ready for the transition to the analysis of the experimental and field data., Office of Naval Research, 875 North Randolph Street, Arlington, VA 22203, Lieutenant Commander, United States Navy, Approved for public release. Distribution is unlimited.

The recent development of various masking and noise-reducing technologies for submarines, aimed at the reduction of their acoustic signatures, makes it critical to advance and implement alternative detection methods. To meet this objective, this study explores the prospects of identification of turbulent wakes that are necessarily generated by propagating objects in the ocean. This project is based on a series of high-resolution numerical simulations that were performed using the CFD (computational fluid dynamics) software OpenFOAM. In particular, the analysis is focused on the sea-surface signatures of submarine wakes and the role played by the sub-surface mixed layer in the vertical transmission of hydrodynamic wake signals. In the course of this investigation, we systematically explore the sensitivity of wake dynamics and its detection potential to the mixed layer depth, the location of the submersible, and its speed. Our findings are expected to ultimately offer valuable operational guidance for Undersea Warfare.

In this study, the numeral simulations were conducted using OpenFOAM to investigate how changes in mixed layer depth (MLD), speed and depth of a submerged body (SB) affect observable signatures of the SB moving beneath a mixed layer. We studied the effect of these factors on both surface and interior temperature perturbations. This study has shown that the wake generated by an SB in the presence of a mixed layer has a greater surface temperature signature than that without it. Furthermore, the deeper the MLD, the greater the thermal signal. This is because when the mixed layer is present, the region is more weakly stratified than it would be without a mixed layer, and the reduced buoyancy force permits fluid entering the mixed layer to penetrate farther. Through variation in speed and depth of SB, we found that faster SB results in stronger turbulence, greater temperature change, and larger areas of surface wake penetration. Also, deeper SB motion levels result in greater surface temperature signals by dredging up colder water from the SB surroundings to the surface. This study confirmed the possibility of detection through surface temperature changes formed by wakes inevitably generated by submerged objects., Office of Naval Research, Washington, DC 20375, Dae-wi, Republic of Korea Navy, Approved for public release. Distribution is unlimited.

Hundreds of sites in littoral and inland waters across the United States have been reported as possibly containing underwater munitions, a safety hazard for the general public. Therefore, it is critical to determine and predict munition location and depth to implement remediation strategies. The mobility of munitions may be influenced by currents, waves, and seafloor slope. Burial depth may be affected by scour, sediment accretion, wave-induced liquefaction, and bedform migration. Here, environmental conditions and morphological evolution are investigated by examining observational data and modeling analyses to better understand the physical processes influencing the burial and mobility of munitions. The environmental models (Delft3D) are validated using observations from field experiments. A coupled Delft3D-object model capable of predicting the mobility and burial of objects on a sandy seafloor is presented. Although the object model limitation considers only cylindrical objects on flat seafloor, ignoring pitch and yaw movements, the coupled Delft3D-object model predictions agree well with observations. In addition, processes such as sediment transport, wave-induced liquefaction, and sand wave migration are examined. The findings show that environmental conditions from Delft3D can be used as a forcing term by other models (e.g., object and wave-induced liquefaction models), which is a valuable tool for predicting the fate of munitions.

Advances in engineering and technology have made acoustic detection of submarines increasingly difficult. Using hydrodynamic signatures created by propagating submarines is an alternative method for submarine detection. Detection based on hydrodynamic signatures may also offer unique tactical advantages, given the tendency of wakes to persist for long timescales. Artificial neural networks trained on velocity field data show promise to automate detection. We used numerical simulations to generate velocity data on wake, jet, and convective turbulence. All these forms of turbulence have similar characteristics in the velocity field, yet their spectra reveal subtle differences that could be exploited for wake identification purposes. We then trained a convolutional neural network with the simulation results and demonstrated that neural networks can classify turbulent flows based on small-scale features with high accuracy. In particular, we find that the developed algorithms can successfully identify wakes in 92% of cases, which implies that the AI-based technology is viable and ready for the transition to the analysis of the experimental and field data.

Though thermohaline staircases exist in a large region of the Arctic, the interactions of such staircases with shear and turbulence are still largely unexplored. We perform a series of two- and three-dimensional simulations with and without shear and demonstrate the capacity of shear to both form and disrupt Arctic staircases. Both isotropic shear and unidirectional oscillating shear are considered. Shear is shown to disrupt staircases when the Richardson number falls below 1/4. For isotropic shear, this process works by breaking down layers individually, which leads to the gradual merging of layers, whereas the unidirectional shear tends to break down interfaces more evenly. For weaker shear (Richardson numbers greater than 2), the spontaneous development of layers is observed.

This study explores the long-term impact of a weak initial departure from circular symmetry in coherent equivalent-barotropic vortices on their dynamics and evolution. An algorithm is developed which makes it possible to construct models of vortices that initially propagate with prescribed velocity. These solutions are used as the initial conditions for a series of numerical simulations. Simulations indicate that seemingly minor perturbations of dipolar form can control the propagation of vortices for extended periods, during which they translate over distances greatly exceeding their size. The numerical results are contrasted with the linear model, which assumes that the non-axisymmetric component of circulation in the vortex interior is relatively weak. The linear solutions reflect the self-propagation tendencies of coherent vortices to a much lesser degree, which underscores the role of fundamentally nonlinear mechanisms at play. The remarkable ability of quasi-monopolar vortices to retain the memory of weak initial perturbations helps to rationalize the wide range of the observed propagation velocities of coherent long-lived vortices in the ocean.

This study explores the dynamics of diffusive convection, which is realized in regions where cool and fresh water-masses rest on top of those that are warm and salty. This type of convection is often observed in the Arctic Ocean and is characterized by the development of fine-scale steps in vertical salinity and temperature profiles known as thermohaline staircases. The Arctic staircases control the rate of upward heat transfer from waters of Atlantic origin, thereby influencing the melting of sea-ice and the polar climate in general. This thesis aims to utilize numerical modeling to define conditions that are favorable or unfavorable for creating thermohaline staircases and provide an explanation as to why they are not constantly prevalent throughout the Arctic. In particular, the presented high-resolution simulations explore the role of vertical shear associated with internal waves that are ubiquitous in the World Ocean. While previous investigations suggested that that the vertical shear could adversely affect staircases, this effect has not been quantified and physical mechanisms at play are still poorly understood. The present study addresses these unresolved problems in the theory of double-diffusive convection, concurrently shedding light on the mechanics of heat transfer in high-latitude oceans., Lieutenant, United States Navy, Approved for public release. distribution is unlimited

With advancing technology, submarines are getting quieter, and detection methods may need to include non-acoustical means. Wake detection is one such method that can be used for submarine detection. An internally heated submerged body such as a submarine travelling through stratified ocean columns generates a warm wake. Using the Massachusetts Institute of Technology General Circulation Model (MITgcm), we conducted a series of large eddy simulations to model the quantitative and qualitative effects of the warm wake produced by a towed spherical submerged body with a heating source. In this study, we studied the effects of varying the heating rate and noted that there are observable differences in the thermal anomaly between these cases during the early wake. At later times, heating of the wake does not seem to have a substantial effect on the thermal and turbulent characteristics of the wake. Distinctive loop shapes form in slices of the turbulent dissipation rate in all cases that may be used to easily identify submarine wakes. We also observed that the thermal anomaly is correlated to the submerged body's velocity and to higher background salinity gradients for comparable density stratification.

This study explores the control of mesoscale variability by topographic features with lateral scales that are less than the scale of the eddies generated by baroclinic instability. These dynamics are described using a combination of numerical simulations and an asymptotic multiscale model. The multiscale method makes it possible to express the system dynamics by a closed set of equations written entirely in terms of mesoscale variables, thereby providing a physical basis for the development of submesoscale parameterization schemes. The submesoscale topography is shown to influence such fundamental properties of mesoscale variability as the meridional eddy-induced transport and eddy kinetic energy. It is argued that the adverse influence of submesoscale topography on baroclinic instability is ultimately caused by the homogenization tendency of potential vorticity in the bottom density layer. The multiscale model formally assumes a substantial separation between the scales of interacting flow components. However, the comparison of asymptotic solutions with their submesoscale-resolving numerical counterparts indicates that the multiscale method is remarkably accurate even when scale separation is virtually non-existent.

This study characterizes the detectability of a stratified wake in the presence of background internal waves using numerical simulations. To achieve this, we conducted numerical simulations on Department of Defense high-performance computing systems using the Massachusetts Institute of Technology general circulation model, which we adapted for wake simulations. The internal waves were generated by forcing fluid motion over the seafloor with the frequency of semi-diurnal tides, while the topography is simulated based on a data-based stochastic model. By varying the magnitude of the forcing, we were able to perform a series of simulations with distinct initial intensities of the internal wave field in the range that reflects typical conditions in the ocean. The submerged body is characterized as an ellipsoid, whose depth and velocity were varied in order to analyze the strength of the signatures in relation to the background noise. Our analysis reveals limited influence of the wave field on the wake’s persistence. We also estimate the noise for the turbulent and thermal dissipation rates by taking a time average over a tidal cycle prior to the introduction of the submerged body. Using this noise, we show that, for our parameters, the wake from a submerged body would be undetectable after 30–60 minutes of passage. Finally, we observed that an interaction exists between the wake and the boundary layer that results in turbulence entrainment between both regions of the domain., Lieutenant Commander, United States Navy, Approved for public release. distribution is unlimited