Date of Award

8-2012

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Oceanography

Advisor

Neal R. Pettigrew

Second Committee Member

Carol D. Janzen

Third Committee Member

Gregory P. Gerbi

Abstract

General circulation and transport exchange within Casco Bay, Maine occurs primarily through the three deepest channels separating the interior sections of the Bay from the outer Bay and adjacent Western Gulf of Maine shelf. Resource management in Casco Bay relies on an ability to predict and/or track oil spill trajectories, Red Tide events, nutrient plumes, pollution and other factors important to the estuarine environment. However, the exchange through these channels, the mean circulation and associated forcing mechanisms are not well understood. In this study, time series of current velocities, temperature, salinity and wind data collected during 2004 and 2005 are used to analyze the transport exchange patterns between interior and exterior sections of Casco Bay. Seasonal and episodic forcing of the observed transport is also examined. Subtidal volume transport in the southwestern section of interior Casco Bay appears to be largely balanced between Portland Channel and Hussey Sound. Inflow is generally restricted to depths below 12 m in Hussey Sound, while mean outflow typically occurs above 12m. Subtidal current in Portland Channel typically flows out of the southwestern section of the bay throughout the entire water column. Transport contributions from these two channels in and out of the southwestern lobe of interior Casco Bay sum to roughly 275 m3/s toward the inner bay – less than one standard deviation away from zero. Transport exchange with the northeastern interior section of Casco Bay is predominantly restricted to Broad Sound, although adjacent Luckse Sound may also contribute to the net transport. Transport into this section occurs predominately at depths below 24 m in Broad Sound. Surface current velocities in Broad Sound are highly variable. Time averaged, cross-sectionally integrated transport in Broad Sound was found to be 830 m3/s toward the northeastern section. Forcing conditions, such as wind and river discharge cause variation to the general transport in all channels, sometimes reversing the mean circulation. Typically, Portland Channel current behaves barotropically and is coherent with both surface density changes and along-channel wind stress. Hussey Sound has a characteristic estuarine two-layer current structure, with most of the current variance occurring in the stronger and deeper inflowing layer. This deeper mode is coherent with surface density changes, while the surface layer is coherent with wind stress. Broad Sound also has a two layered current structure which is coherent with along-shelf wind, surface density changes and wind oriented along the reach of Middle Bay. Broad Sound is uniquely sensitive to density changes, as the direction of the baroclinic flow was observed to reverse in the presence of the Kennebec River plume. Seasonal variability in current magnitude and structure was observed in all channels. This is especially true in Hussey Sound, where stratification during warmer months appears to enhance the depth and variability of the surface layer. Under mixed winter conditions, however, most of the current variance is contained in the bottom layer. Seasonal changes to wind and discharge forcing conditions make transport variability in all channels almost twice as large in spring and winter as in summer.

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