The primary contributions of this thesis include the first stages of development of a 2D, finitevolume, non-hydrostatic, sigma-coordinate code and beginning to apply the Dynamically Orthogonal field equations to study the sensitivity of internal tides to perturbations in the density field. First, we ensure that the 2D Finite Volume (2DFV) code that we use can accurately capture non-hydrostatic internal tides since these dynamics have not yet been carefully evaluated for accuracy in this framework. We find that, for low-aspect ratio topographies, the -coordinate mesh in the 2DFV code produces numerical artifacts near the bathymetry. To ameliorate these staircasing effects, and to develop the framework towards a moving mesh with free-surface dynamics, we have begun to implement a non-hydrostatic sigma-coordinate framework which significantly improves the representation of the internal tides for low-aspect ratio topographies. Finally we investigate the applicability of stochastic density perturbations in an internal tide field. We utilize the Dynamically Orthogonal field equations for this investigation because they achieve substantial model order reduction over ensemble Monte-Carlo methods.

A novel hydrostatic coupled-mode shallow water model (CSW) is developed and used to simulate tides in the greater Middle Atlantic Bight region. The model incorporates realistic stratification and topography, an internal tide generating function (ITGF) that provides internal tide forcing from existing surface tide parameters, and dynamical terms that describe linearized wave- mean-flow and mean-density interactions. Several idealized and realistic simulations are used to verify the model. These verification simulations include internal-tide interactions involving topographic coupling and mean-flow coupling, and comparisons with other simpler and more complex nonlinear primitive-equation models. Then, twenty-four simulations of internal tide generation and propagation in the greater Middle Atlantic Bight region are used to identify significant internal-tide interactions with the Gulf Stream. The simulations indicate that locally generated mode-1 internal tides can refract and/or reflect at the Gulf Stream. The redirected internal tides often re-appear at the shelfbreak, where they produce onshore energy fluxes that are intermittent (i.e., noncoherent) because meanders in the Gulf Stream alter their precise location, phase, and amplitude. These results provide an explanation for the anomalous onshore energy fluxes previously observed at the New Jersey Shelfbreak and linked with the generation of nonlinear internal waves.

Internal tides in the Middle Atlantic Bight region are noticeably influenced by the presence of the shelfbreak front and the Gulf Stream. To identify the dominant interactions of these waves with subtidal flows, vertical-mode momentum and energy partial differential equations are derived for small-amplitude waves in a horizontally and vertically sheared mean flow and in a horizontally and vertically variable density field. First, the energy balances are examined in idealized simulations with mode-1 internal tides propagating across and along the Gulf Stream. Next, the fully-nonlinear dynamics of regional tide-mean flow interactions are simulated with a primitive equation model, which incorporates realistic summer mesoscale features and atmospheric forcing. The summer shelfbreak front, which has horizontally variable stratification, decreases topographic internal-tide generation by about 10% and alters the wavelengths and arrival times of locally generated mode-1 internal tides on the shelf and in the abyss. The (sub)-mesoscale variability at the front and on the shelf, as well as the summer stratification itself, also alter the internal tide propagation. The Gulf Stream produces anomalous regions of O(20 mW m^-2) mode-1 internal-tide energy-flux divergence, which are explained by mean-flow terms in the mode-1 energy balance. Advection explains most tide-mean flow interaction, suggesting that geometric wave theory predicts mode-1 reflection and refraction at the Gulf Stream. Geometric theory predicts that offshore-propagating mode-1 internal tides that strike the Gulf Stream at oblique angles (more than thirty degrees from normal) are reflected back to the coastal ocean, preventing their radiation into the central North Atlantic.

Congratulations to Morgan Kane for successfully completing her RSI 2016 summer research with our MSEAS group at MIT. She completed a report and presentation on “Go with the flow: The Effect of Geophysical Flows on Transports”. She is from Mt. Hope High School, Bristol, RI.

Abstract

The purpose of this work is to develop a numerical model that calculates the efficient and safe routes for a vessel to take across the Atlantic Ocean. Existing software exists to perform this task, but it could be significantly improved by building on the experience gained with the open-source model VISIR-I (www.visir-model.net) and the oceanographic datasets of the Copernicus Marine Environment Monitoring Service (http://marine.copernicus.eu/). In particular, the VISIR model has been evolved into a new code in Python and the path optimization is now solved on a non-uniform unstructured grid. The new code will start employing the CMCC CGLORS reanalysis of ocean circulation at ¼ deg, while other relevant environmental fields will be added later on. The new code will be used for achieving the goals of H-2020 project AtlantOS Task 8.3, which includes the capacity to compute safe routes optimizing the economic cost of navigation through use of dynamic environmental information.

Biography

Gianandrea’s research activity aims to improve Maritime Transportation by means of Decision Support Systems. Together with colleagues of the TESSA and IONIO projects, he designed and implemented VISIR, a ship routing model for safer and more efficient navigation, and presently leads its scientific and operational development. As a model, VISIR’s source code is made publicly available following the guidelines of the Free and Open Source Software. As an operational system, VISIR already has an operational implementation in the Mediterranean Sea.