Topical Area: Water Circulation and Currents

John D. Wang and Brian K. Haus, Applied Marine Physics and Center for Marine and Environmental Analysis, Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida

As part of a NOAA Coastal Ocean Program study of the effects of natural and anthropogenic stressors on the South Florida Ecosystem, a numerical hydrodynamic model has been developed for the offshore waters including Hawk Channel and the coral reef tract. The model is used to provide a quantitative characterization of the physical parameters influencing or controlling the principal ecosystem components in the seascape mosaic of the study area, which consists of a more limited domain from Fowey Rocks to Long Key. The model is particular useful for the purpose of assessing the spatial variability within the study area. Both short term forcings from episodic events and the longer term seasonal and interannual variations are important to the stress regimes.

Field data including moored current meters deployed by the Harbor Branch Oceanographic Institute, shipboard profiling of currents, salinity, and temperature, and Ocean Surface Current Radar (OSCR) remote sensing provide model calibration fields. A deployment of OSCR to the offshore waters between Pacific and Carrysfort Reefs during the first two weeks of March produced detailed surface current fields at 700 points within a 30 km x 30 km area near north Key Largo. The moored current meter recordings at Pacific Reef and two midshelf locations in Hawk Channel, contemporaneous Acoustic Doppler Current Profiler velocity transects, and hydrographic temperature/salinity sections supply information on vertical structure and together with the OSCR fields give the most detailed description yet of the flow and density field over a significant portion of the shelf. Analysis of these data and numerical modeling in the coming year should result in a better understanding of the processes influencing transport pathways and rates from the shoreline to the offshore shelf break.

A vertically integrated 2-D finite element hydrodynamic model has been adapted to the study area. The model attains a horizontal resolution of approximately 1500 m nearshore and 3000 m offshore. The model bottom topography is specified from a digital data base of depth soundings. The tidal elevation is prescribed along the offshore open boundary located in depths of approximately 700 m. Radiation conditions are imposed at the two cross-shore open boundaries to allow outward passage of waves from the domain interior. To further reduce spurious wave reflection, a sponge layer with artificially enhanced eddy viscosity is implemented along all open boundaries. The model which includes all of south Biscayne Bay is forced with observed winds from the CMAN stations at Fowey Rocks, Molasses Reef, and Sombrero Key. Freshwater inflows from canals in Biscayne Bay are also imposed on the model.

The hydrodynamic response in the middle and inner shelf is dominated by tides and wind. The outer edge of the shelf is strongly influenced by the Florida Current and flows to the north most of the time. The reef tract and Hawk Channel are therefore located in strong shear zones with ample opportunities for mixing and exchange. The implications of these conditions for the coral reef ecosystems and fish resources will receive additional attention in the coming year. We also plan to use a similar methodology to study in more detail the area immediately to the southwest including the Triangles, Conch and Molasses Reefs.