Date of Award

2015

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Oceanography

First Advisor

Lewis M. Rothstein

Abstract

The overall objective of this dissertation is to establish a framework for the dynamical understanding of a number of circulation features found in Rhode Island coastal waters. Observations reveal a cyclonic, near-surface circulation around the periphery of Rhode Island Sound (RIS) that occurs in summer stratified conditions and disappears in winter when solar insolation and wind stirring result in strong vertical mixing. The dynamics of this circulation is investigated through a series of numerical simulations. We attribute the summer intensification of this `periphery current' to a continuous topographically rectified tidal residual current, and to a circulation produced by seasonal deep thermohaline gradients often observed from May to September.

The depth-averaged vorticity is found to be most useful for diagnosing the sources of the topographically-rectified tidal residual current, and that diagnostic is also applied toward our understanding of another feature of Rhode Island coastal circulation; a pair of opposite sign headland eddies that are found around Montauk Point in the Block Island Sound (BIS). The RIS cyclonic residual current is attributed to the nonlinear transfer of vorticity generated by the Coriolis torque when tidal currents oscillate, which is opposed by bottom frictional torque associated with topographic gradients and velocity shear. In addition to bottom frictional torque, we show that the nonlinear advection of vorticity is important in generating the eddy pair in BIS. The effect of bottom thermal fronts on the circulation is illustrated through process-oriented, reduced-physics experiments by varying the surface solar radiation.

Other important features of the Rhode Island coastal circulation are found in the region between BIS and Long Island Sound (LIS), including a buoyancy-driven coastal current along the southern shore of LIS and periodically detached freshwater patches to the south of Montauk Point and in central BIS. Buoyant discharge coming out of LIS, mainly from the Connecticut River, forms a seasonal plume front outside BIS between the coast of Long Island and the denser shelf waters. The front's seasonal variability and its response to tides, winds and surface heating are investigated through a series of process-oriented numerical experiments. In winter and spring, the plume front is intermediate with a large surface offshore extension detached from the bottom, while in summer, the front is bottom-advected with most of the width of the front in contact with the bottom, featured by steep isopycnals. Strong summer insolation together with weak buoyant discharge and weak winds generates the narrowest and steepest summer front. In addition, small changes in tidal currents over the spring-neap cycle cause significant, monthly fluctuations in turbulent mixing and vertical stratification in central BIS, modulating the freshwater incursions that generate episodic freshwater patches moving downstream along the southern shore of Long Island and towards RIS in the gap between Block Island and Point Judith. Observational evidence of these detached patches of fresh water is also discussed in this study.

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