Topical Area: Water Circulation and Currents (poster presentation)

W. Douglas Wilson, Elizabeth Johns, and Ryan H. Smith, NOAA/AOML/PhOD, Miami, FL; Thomas N. Lee, University of Miami/RSMAS, Miami, FL

As part of the South Florida Ecosystem Restoration Program, a physical oceanographic study of the circulation of Florida Bay and its connection with the surrounding waters of the Gulf of Mexico, the southwest Florida shelf, and the Atlantic Ocean is presently underway. The measurement program includes moored arrays equipped with current meters, bottom pressure sensors and conductivity/temperature sensors, satellite-tracked surface drifters, a shipboard Acoustic Doppler Current Profiler (to measure volume transport through the channels between the Florida Keys), and bimonthly interdisciplinary shipboard surveys with continuous underway thermosalinograph observations of surface salinity, temperature, and fluorescence. Results from the first two years of the study (1995-1997) have shown that there is a net southeastward flow of 1 to 4 cm/s which transports waters from the Gulf of Mexico and the Everglades through western Florida Bay and the channels of the Middle Florida Keys, on a time scale of 1 to 3 months depending on local wind forcing. This net flow, with a volume transport of 1000 to 2000 m³/s, has the potential to deliver harmful algal blooms and anthropogenic effects such as excess nutrient loading out to the environmentally sensitive coral reefs of the Florida Keys National Marine Sanctuary. The ongoing study now focuses on refining and quantifying the net flow from Florida Bay to the Atlantic and its response to seasonal and episodic meteorological forcing. In addition, new emphasis is placed on examining the fate of the freshwater discharges from the Everglades into the Gulf of Mexico via the numerous rivers of the southwest Florida coastline, and the relation of the river plume dispersion to regional wind and rainfall distributions.

The moored conductivity/temperature array (Figure 1) consists of 20 sensors positioned from the Florida Keys reef tract, through Florida Bay and around Cape Sable, extending northward off the mouths of the Shark, Broad, and Lostmans Rivers, to Indian Key just south of Marco Island, FL. The array is designed to resolve the three-dimensional structure of the river plumes, with emphasis on Shark River. In addition to the conductivity/temperature moorings, four moored upward-looking ADCP's are positioned west of Lostmans River and Cape Sable, with the offshore pair located 30 nm west of the Southwest Florida coast. The ADCP's provide a continuous measure of currents, and, paired with data from the other moored instrumentation, will allow a quantitative analysis of the freshwater discharge. Bottom pressure sensors are included on the moorings 30 nm offshore of Cape Sable, adjacent to Cape Sable, in western Florida Bay, and in the Atlantic, offshore of Long Key, FL. From these instruments a continuous measurement of sea level height and the slope of the sea surface can be obtained. The bulk of the array was deployed in September 1997, and recovered and redeployed in March/April 1998.

Results from the moored array will be compared with wind fields obtained from the array of CMAN weather stations and with regional rainfall indices obtained from the NEXRAD radar study (Marks and Willis), as well as individual rain gauge and river flow data, to determine the meteorological forcing mechanisms which drive the river plume dispersion. Preliminary results of such comparisons have indicated that the dispersion is highly variable, strongly correlated with both wind and rainfall patterns, with the rainfall determining the size of the freshwater plumes on a daily-to-weekly time scale, and the wind fields determining the direction of dispersion. Satellite-tracked surface drifters, which are deployed in the Shark River plume during the bimonthly shipboard surveys, have corroborated the meteorologically-driven forcing of the regional circulation.

In addition, the shipboard surveys yield measurements of temperature and conductivity continuously along the cruise track in the study area, and the mapped fields of surface salinity which can be obtained from these surveys complement the results of the moored array, which are continuous in time but not spatially continuous. To yield a three-dimensional view of the freshwater discharges, conductivity-temperature-depth (CTD) stations are taken along transects perpendicular to each river mouth, and show the extent to which the fresh water penetrates vertically into the water column. Early results from these observations have indicated that there is a high degree of variability in the vertical extent and mixing of the river plumes with the Gulf of Mexico coastal waters, probably driven by the ambient meteorological conditions and the synoptic background circulation as well as the strength of the river discharge itself.

It is expected that the moored array and the shipboard surveys will continue for at least two years, preferably longer, as there is considerable interannual variability in the larger-scale regional circulation driven by factors ranging from El NiZo cycles to interannual changes in the wind fields over the Gulf of Mexico and Atlantic Ocean, as well as large year-to-year variability in rainfall distributions over the Everglades. In order to quantify the regional circulation and river dispersion and their effects on the ecological health of Florida Bay and the Florida Keys National Marine Sanctuary, and to provide important boundary conditions and assessment of the numerical models which are presently being developed for the region, it will be necessary to make observations over the full range of interannual variability which occurs.