Date Awarded


Document Type


Degree Name

Doctor of Philosophy (Ph.D.)


Virginia Institute of Marine Science


L. Donelson Wright


Mixing and dispersion in a partially mixed estuary are driven by, among other processes, the gravitational circulation, wind-events and tides. These processes are important in the vertical and horizontal distribution and exchange of salinity, oxygen, nutrients, and organisms. The objective of this research was to examine the temporal variability of the pycnocline in the mid-Chesapeake Bay in response to forcing at subtidal and higher frequencies as observed in the spring of 1993. The largest change in the pycnocline was associated with meteorological forcing. Up-Bay wind stress and subsequent abatement of the wind stress produced a 5 meter displacement of the mean pycnocline depth. The data suggest that the change in depth of the pycnocline was the result of the geostrophic response of the across-channel pycnocline tilt to wind-induced reversal of the along-channel residual circulation. The observed 2-4 meter semi-diurnal vertical oscillations of the pycnocline were attributed to a topographically induced internal tide. A simple tow-layer model was developed to determine the amplitude and phase of the pycnocline displacements associated with the internal tide. Model results and observed displacements of the pycnocline show good agreement. Other processes, such as an internal lateral seiche of semi-diurnal tidal frequency, advection of the along-channel density gradient and geostrophic adjustment were also investigated but were found to be inadequate in describing the observed oscillations. Finally, the smallest changes of the pycnocline were associated with high-frequency internal waves, fronts and homogeneous layers which were found to occur frequently throughout the field survey. A classification system based on the characteristics of the homogeneous layers observed was implemented to investigate source mechanisms and potential for transport and mixing. Indications are that the relatively short, thin layers were the result of local mixing, and that the relatively long, thick layers were inconsistent with local mixing.



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