Florida Bay Circulation and Exchange Study

Topical Area: Water Circulation and Currents

Thomas Lee and Elizabeth Williams, University of Miami/RSMAS, Miami, FL; Elizabeth Johns and Doug Wilson, NOAA/AOML, Miami, FL

The goal of this project is to study of the interaction and exchange of Florida Bay with the connecting coastal waters of the Gulf of Mexico and the Atlantic in the Florida Keys. The research is designed to address several of the key scientific questions presented in the NOAA/COP Florida Bay Implementation Plan concerning circulation and water quality as critical to understanding the functioning of the ecosystem and future evolution from restoration actions. Observational methods consist of a combination of bi-monthly interdisciplinary surveys, in-situ moorings, shipboard Acoustic Doppler Current Profiler (ADCP) transport transects in the major Keys flow passages, and Lagrangian surface drifters to describe and quantify the circulation within the Bay as related to local forcing and coupling with the waters of the Atlantic and Gulf (Figure 1). These observations will also help to provide necessary boundary conditions for physical and biological models.

The moored array was deployed Sept. 1997 and will be maintained for two years. The array consists of 4 bottom mounted (ADCPís) equipped with nearsurface T/S sensors on the southwest Florida shelf offshore of Shark River (moorings A-D, Figure 1), a Shark River plume array of 9 T/C sensors to monitor changes in the Shark River discharge nearfield, a single current/T/C mooring near the western boundary of Florida Bay, and 3 current/T/C moorings positioned along the Florida Keys reef tract to measure interaction and exchange between the southwest shelf, Everglades discharge, Florida Bay, Keys coastal waters and the Florida Current. In addition there is a bottom pressure array to measure cross- and along-shelf pressure gradients on the southwest shelf, as well as the cross-Keys pressure gradient. Initial results from this array for the first 6 months will be presented.

Interdisciplinary surveys of T/S/Chl and nutrients are conducted with the RV Calanus every two months through the south Florida and Keys coastal waters from Miami to Dry Tortugas, southwest shelf to Naples, Everglades discharge and western Florida Bay. These data are used together with survey data through the shallow enclosed regions of Florida Bay made with a shallow catamaran to investigate the interaction and coupling between the different regions and the dispersion pathways for river and Florida Bay discharges. Initial results show the influence of the anomalous large winter rainfall associated with El Nino, causing decreased salinities in the southwest Florida nearshore waters during what is normally the dry season. The dispersion characteristics of these waters will be discussed.

Satellite tracked CODE surface drifters are deployed within the Shark River Discharge on the bi-monthly surveys. Synthesis of drifter trajectories indicate a net southerly flow from the Gulf of Mexico to the Florida reef tract through western Florida Bay that varies with season, stronger in the winter (3 to 4 cm/s) and weaker in summer (1 to 2 cm/s). Drifter trajectories are strongly influenced by local tide and wind forcing. A multiple linear regression model is used to explain approximately 70 to 80% of the subtidal variance of drifter currents due to local wind forcing. Drifter results to date indicate a preferred exchange route for Shark River discharge and western Florida Bay waters toward the reef tract through Long Key Channel. However, in the fall northeast winds can cause movement toward the southwest and the Dry Tortugas. The cause and variability of the residual background currents are being investigated.

Subtidal volume transport through Channels 5, 2 and Long Key Channel combined range from about 1000 m3/s toward the southeast to 200 m3/s toward Florida Bay. These subtidal flows are equivalent in magnitude to the mean river discharge onto the southeast U. S. shelf by all the rivers between Florida and Cape Hatteras. Approximately 500-700 m3/s flows through Long Key Channel alone when the flow is toward the southeast, which is about 100 times greater than the peak fresh water discharge out of Shark River. Also we found that most of the surface drifters deployed near Shark River and in the western part of Florida Bay were observed to exit Florida Bay toward the reef tract through Long Key Channel. Therefore it appears that waters from Shark River and the seagrass die-off region of western Florida Bay will be passively advected and dispersed toward the Florida Keys Marine Sanctuary primarily through Long Key Channel by this strong southeastward background flow. The advective/dispersal time-scale for materials in the Shark River Plume to reach the FKNMS is estimated from drifter trajectories at one to two months. However, at times when the local winds are intensified from the east or northeast a cross-key pressure gradient is set up that reverses the background flow toward Florida Bay.

Subtidal flows through the Keys passages are driven by cross-keys pressure gradients. The cross-keys pressure gradients measured at Tennessee Reef and in western Florida Bay were found to be dependent on local wind speed and direction do to the curving coastline of the Keys. Southeast winds typical of the Keys region cause a set-up of sea level in Florida Bay and set-down in the middle and northern Keys coastal waters and drive a southeast flow toward the reef tract through the Keys passages. East and northeast winds cause sea level to set-up in the Keys coastal waters and set-down in Florida Bay, driving subtidal flows toward Florida Bay through the passages.