Date of Award

2017

Document Type

Open Access Dissertation

Department

Marine Science

First Advisor

George Voulgaris

Abstract

The dynamic region extending from the inner- to outer-shelf is the nearshore region where an accurate understanding of the development of cross-shore flows is important as they control the exchange of constituents (e.g., pollutants, larvae, heat and biota) between the coastline and the open ocean. These flows are driven by a variety of forces, including wind and buoyancy. The wind forcing relevant to shelf circulation broadly falls within the synoptic scale of storm events, consisting of frontal systems (cold and warm fronts) and tropical storms. The buoyancy on the shelf is characterized by a vertical stratification or a pronounced cross-shore density gradient. In chapter 2, an event based, phase averaged climatological analysis is applied to a location along the South Atlantic Bight, off South Carolina, an area prone to cyclogenesis occurrence and passages of atmospheric fronts. The average state of the storm events and their variability are represented by the temporal evolution of atmospheric pressure, air temperature, wind velocity and wave directional spectral energy. The effectiveness of this analysis method is further verified by numerically simulating the wave conditions driven by the characteristic wind forcing and comparing the results with the wave climatology that corresponds to each storm type. High level of consistency found in the comparison indicates that, this analysis method can be used for accurate creation of oceanic atmospheric forcing that preserves the event time history. An application example of this analysis method is provided in Chapter 3 to study the long-term storm-induced sediment flux. Based on the storm climatology, a series of storm events are reconstructed with variable wind forcing and duration. The sediment fluxes induced by these storm events are quantified by using numerical modeling. The correlation between the storm intensity and sediment flux is then applied to estimate the accumulated sediment flux induced by all realistic storm events. In Chapter 4, the shelf response to these three climatologically defined storms is investigated numerically using Long Bay, a typical Carolina embayment with curved coastline as a study case. The analysis focuses on examining how the regionally defined cross-shore wind component, isobath divergence / convergence and coastline orientation affect the cross-shore circulation under stratified shelf conditions. The simulation results show that, the regionally defined offshore directed wind component promotes upwelling during the developing stage of cold front and enhances mixing during the decaying stage. During warm front and tropical storm, the cross-shore wind component becomes insignificant. At the southern side of the embayment, isobath divergence enhances upwelling, as it increases bottom Ekman transport and induces an onshore directed geostrophic transport. By examining the sea level along the curved coastline, the crossshore length scales on which the locally defined along-/cross-shore wind component acts are about 20 km seaward from the coastline. In contrast to the vertical stratification, a cross-shore density gradient provides the available potential energy (APE) to fuel the baroclinic instability. Chapter 5 studies the coupled wind forcing and baroclinic instability on shelf circulation. Multiple groups of runs are carried out and each condition is run under purely wind-driven and coupled wind-instability-driven shelf conditions. Under downwelling conditions, when no instability is present, the inner/mid shelf boundary location/depth is parameterized using a multi-parametric quantity where f if Coriolis parameter, is shelf slope, B is buoyancy flux and is wind forcing. The relationship is found to hold for when instabilities are present under downwelling conditions. In this case, increase of wind stress, and/or decrease of the magnitude of heat loss rate tend to inhibit the instability and shift the cross-shore exchange into patterns when no instability is present. Under upwelling conditions, the intensity of instability decreases as the wind stress increases, and/or the heat loss rate decreases. This is primarily caused by the enhancement of the turbulent frictional dissipation, while the extra supply of APE by upwelling seemingly becomes unimportant for the instability.

Rights

© 2017, Xiaodong Wu

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