Doctor of Philosophy (Ph.D.)
Virginia Institute of Marine Science
Marjorie A Friedrichs
Courtney K Harris
Yinglong J Zhang
Submarine canyons can impact regional oceanography and provide a conduit for shelf-slope exchanges via topographically induced processes such as canyon upwelling and downwelling. Between Virginia and New York in the Mid-Atlantic Bight (MAB), there are five major shelf-incising canyons (Norfolk, Washington, Baltimore, Wilmington, and Hudson canyons) and four sizable slope-confined canyons (Accomac, Spencer, Lindenkhol, and Toms canyons). Canyon upwelling and downwelling at these canyons have not been well studied. Consequently, the associated hydrographic variability and dynamics inside the canyons and on the adjacent shelf are largely unknown, and the integrated impact of submarine canyons on MAB shelf hydrography and shelf-slope exchanges is poorly understood. This dissertation was inspired by a glider-based survey at Wilmington Canyon, and mainly used numerical modeling simulations for the MAB with an unstructured-grid three-dimensional ocean model to answer these questions. Using glider observations and realistic numerical modeling for February 25–March 7, 2016, Chapter 2 investigated the hydrographic variability associated with the Wilmington Canyon and adjacent outer shelf and shelf-break amid two consecutive events of upwelling and downwelling. Then focusing on the same Wilmington Canyon upwelling and downwelling events and using a realistically forced numerical modeling simulation as well as one without tidal forcing, Chapter 3 tested the dynamical drivers of winds and tides in canyon upwelling and downwelling events, also compared the differences in dynamics between the canyon and adjacent shelf-break. Chapter 4 presented the occurrences and integrated impact of 12 cycles of spatially coherent upwelling and downwelling through nine canyons between Virginia and New York, based on a realistic numerical modeling experiment and a No-Canyon experiment for January 1–April 14, 2016. Chapter 4 also analyzed the frequency of canyon upwelling and downwelling based on a 22-year record (1994–2015) of sea surface elevation. This dissertation found that the submarine canyons in the MAB induced significant spatial and temporal hydrographic variabilities on the shelf and enhanced shelf-slope water exchange. Due to favorable winds, sea surface tilts, and shelf-scale background ocean currents, as well as canyon topography, cycles of spatially coherent multi-canyon upwelling and downwelling frequently occurred in the MAB. Plumes of upwelled slope water from the canyons intruded the outer and mid shelf during the upwelling episodes, and dense water from the shelf retreated into the canyons during the downwelling episodes. For each cycle of upwelling and downwelling, upwelled water from the canyons caused shelf-scale anomalies of low temperature, high density, and northeastward velocity on the MAB outer shelf that were significant when averaged over the course of the season. This thesis suggests that submarine canyons are an integral part of MAB oceanography and deserve further investigations.
© The Author
Wang, Haixing, "Impact Of Canyon Upwelling And Downwelling In The Mid-Atlantic Bight" (2021). Dissertations, Theses, and Masters Projects. William & Mary. Paper 1627407406.