A numerical study of circulation and mixing in a macrotidal estuary: Cobscook Bay, Maine

Abstract

A numerical, three-dimensional coastal ocean model was ics. used to study the circulation and subsequent mixing of Cobscook Bay by the lunar semi-diurnal tide. The results showed strong ebbing and flooding currents along a main channel connected to the open boundary and a pair of counter rotating eddies that form as a result of the complex topography and coastal morphology. The eddy pair begins to form with the flooding tide and is persistent in the residual velocity Geld. Hourly simulations that tracked particle position, calculated by interpolating the velocities of the surrounding grid cells to the point where a particle was located, were used to estimate residence times in the bay. Residence times for individual particles varied with initial position and time of release. Few particles released from the arms of the bay escaped from the bay, regardless of release time, during the eight lunar day simulation. Regions close to the open boundary had a large fraction of particles escape and consequentially short residence times. It was not unusual for neighboring particles to follow divergent pathways that effectively become unpredictable. A tidal-residual residence time map was constructed as an average over 12 consecutive hourly maps. The tidal-averaged residence time for neutral surface particles A tidal-residual residence time map was constructed as an average over 12 consecutive hourly maps. The tidal-averaged residence time for neutral surface particles varied from less than one day in the main channel of the outer bay to more than eight days in the inner extremities of the arms of the bay. A conservative tracer was used that followed the some three-dimensional advection-diffusion equation as salinity. Continuous source points and sources that were active for only one lunar day were used to study the flushing of the bay. Mixing was strongest in regions with the highest residual velocities and was localized in regions with small residual velocities. Concentrations in the arms increased as each tidal cycle introduced more of the tracer into the arms of the bay without removing a significant portion of the existing water. The flooding tide, combined with the eddy pair, causes material to primarily accumulate in a southern arm of the bay.