Topical Area: Water Circulation and Currents

Frank Aikman, III and William P. O’Connor, NOAA/National Ocean Service, Silver Spring, MD; George L. Mellor, Princeton University, Princeton, NJ

OBJECTIVE AND GOALS: The principal objective of this work is to develop, test, evaluate, and implement a Florida Shelf domain model that will provide accurate information on the boundary conditions to be used for forcing hydrodynamic models of Florida Bay. The Florida Shelf model will provide water level, current velocity, and (eventually) water density (temperature and salinity) information on time scales of hours.

Our long-range goal is to determine how open ocean forcing can be incorporated into a model of Florida Bay and to address the Bay's interaction and exchange with the adjacent shelf and Gulf of Mexico. This is a key component of the NOAA focus on the large-scale regional oceanographic, atmospheric, ecological, and fisheries context within which Florida Bay restoration will proceed. Secondarily, our goal is to initiate the extension of the NOAA Coastal Ocean Forecast System (COFS) development activity on the U.S. East Coast (Aikman et al., 1996) into the Gulf of Mexico. When this is accomplished we will have an operational nowcast and short-term forecast tool that should provide 3-dimensional and improved boundary condition information for Florida Bay.

RATIONALE: Any numerical model of the unique and complex Florida Bay hydrodynamics, such as the barotropic circulation model (RMA-10) being applied by the U.S. Army Corps of Engineers (COE), requires adequate open ocean boundary condition information. In fact, one of the most difficult aspects of limited-area bay modeling is the adequacy of the lateral boundary condition information available. For Florida Bay, the barotropic forcing from winds and tides is probably of primary importance, with the baroclinic forcing, for example, from meandering of the Gulf of Mexico Loop Current or the influence of Loop Current Eddies most likely being of secondary importance.

Regional observations within and around Florida Bay have not had sufficient spatial or temporal coverage to provide adequate boundary condition information. Very little is actually known about the relative importance of the boundary forcing from the east and south (Florida Keys) versus the west (West Florida Shelf) of Florida Bay. However, recently obtained and on-going measurements of currents, salinity, temperature, and bottom pressure (Tom Lee, University of Miami and Ned Smith, Harbor Branch Oceanographic Institute - both personal communications) will prove useful for both defining conditions at the boundary and for the evaluation of the boundary condition information derived from the National Ocean Service (NOS) model simulations.

METHODOLOGY: NOS is applying the Princeton Ocean Model (Blumberg and Mellor, 1987) for the Florida Shelf domain to simulate the barotropic boundary conditions for Florida Bay. The model grid resolution is 5' in latitude and longitude and the domain includes the entire Florida shelf (22.5 to 30^o N, 79 to 86^o W). Using tidal elevation boundary conditions modified from Schwiderski (1980) and wind forcing from the 29-km Eta atmospheric forecast model (Black, 1994), the Florida Shelf model has been calibrated for tides and is in the process of being evaluated for wind-plus-tide-driven simulations (O=Connor et al., 1998a). Output water level and barotropic current fields are provided to the COE to serve as boundary conditions for their barotropic model (RMA-10) of Florida Bay. Analyzed (Eta 48-km) wind fields will be tested and evaluated using the Florida Shelf model to see if this offers an improvement over the Eta forecast winds, and the assimilation of water level gauge data into the barotropic model will be tested for the Florida Shelf domain.

STATUS and ACCOMPLISHMENTS: A 14-month (1 September 1995 to 31 October 1996) barotropic Florida Shelf model simulation of wind-plus-tide-forced water level and currents has been completed and we are in the process of evaluating the results using NOS water level gauge data, observations of bottom pressure and both moored and drifting buoy current meter data from Tom Lee (University of Miami) and Ned Smith (Harbor Branch Oceanographic Institute). This includes estimates of the cross-Florida Bay sea level slope. The results indicate that the barotropic model water levels are in close agreement with the observations around Florida Bay (rms difference 0.08m; correlation coefficient 0.9), that the model cross-Florida Bay sea level slope is in qualitatively good agreement with the observations, and that the observed cross-Florida Bay sea level slope is primarily a barotropic feature.

The hourly water level and barotropic current fields of the 14-month simulation have been interpolated to the Florida Bay model (RMA-10) grid and delivered to the COE in July 1997 for testing as boundary conditions for their barotropic model simulations (Keu Kim, U.S. Army Corps of Engineers, Waterways Experiment Station - personal communication).

Evaluation results and plans for further 2-dimensional simulations and 3-dimensional model development and simulations were presented at the Florida Bay Physical Science Research Team meeting on September 4-5, 1997, in Miami, FL; at the 5th Estuarine and Coastal Modeling Conference on October 22-24, 1997 (O’Connor et al., 1998a); and at the 2nd AMS Conference on Coastal Atmospheric and Oceanographic Prediction and Processes on January 11-16, 1998, (O’Connor et al., 1998b).

FUTURE PLANS: Analyzed Eta 48-km winds will next be evaluated using the Florida Shelf model to see how much of an improvement this is compared to using strictly forecast winds. In addition, analyzed wind fields will soon be available from the Rapid Update Cycle (RUC-2) model which will have the advantage of an hourly data assimilation cycle (versus 3 hours for the Eta model) and will be of 40 km resolution. This model is to become available in the late spring of 1998 and we plan to test these wind fields with the Florida Shelf model. NOS is also examining the regional availability of real-time meteorological observations and will consider developing our own analyzed wind fields for the region.

We will develop and test the assimilation of water level gauge data into the barotropic Florida Shelf model using a number of different nudging techniques. Depending on the results, such techniques could be used for operational nowcasting using the real-time NOS water level gauge data available in the region.

A fully 3-dimensional version of the Princeton Ocean Model is being tested at Princeton University which will encompass the extension of the COFS into the Gulf of Mexico. This model will be used to determine the importance of baroclinicity in the coastal circulation on the West Florida Shelf and to evaluate the baroclinic effects on Florida Bay currents and water levels. If required, 3-dimensional boundary condition information (including estimates of the 3-dimensional salinity, temperature and density) could be provided to the COE for Florida Bay, as well as for other regional studies associated with Florida Bay, the West Florida Shelf, and the Florida Keys National Marine Sanctuary.

REFERENCES:

Aikman, F. III, G. L. Mellor, T. Ezer, D. Sheinin, P. Chen, L. Breaker, K. Bosley, and D. B. Rao. 1996. Towards an operational nowcast/forecast system for the U.S. East Coast. In: Modern Approaches to Data Assimilation in Ocean Modeling. P. Malanotte-Rizzoli (Editor). Elsevier Oceanography Series, 61, 347-376.

Black, T. L. 1994. The new NMC mesoscale Eta model: Description and Forecast Examples. Weather and Forecasting, 9, 265-278.

Blumberg, A.F., and G.L. Mellor. 1987. A description of a three-dimensional coastal ocean circulation model. Three-Dimensional Coastal Ocean Models, 4, edited by N. Heaps. American Geophysical Union, 208p.

O’Connor, W.P., F. Aikman III, E.J. Wei, and P.H. Richardson. 1998a. Comparison of observed and forecasted sea levels along the West Florida coast. Estuarine and Coastal Modeling, Proceedings, 5th International Conference, October 22-24, 1997, Alexandria, VA (in press).

O’Connor, W.P., F. Aikman III, E.J. Wei, and P.H. Richardson. 1998b. Comparison of observed and forecasted sea levels for the Texas coast near Galveston Bay. Preprints of the 2nd Conference on Coastal Atmospheric and Oceanographic Prediction and Processes, American Meteorological Society, January 11-16, 1998, Phoenix, AZ, 23-29.

Schwiderski, E. W. 1980. On Charting Global Ocean Tides. Reviews of Geophysics and Space Physics, 18(1), 243-268.