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2002 LAPCOD Meeting
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Drifter trajectories connecting the coastal seas around South Florida
Elizabeth Williams, Thomas Lee, Villy Kourafalou
Rosenstiell School of Marine and Atmospheric Science, University of Miami
ewilliams@rsmas.miami.edu,villy@rsmas.miami.edu(Abstract received 10/30/2002 for session A)
ABSTRACT
During the last seven years years, over forty nearsurface CODE-type drifters have been released in bimonthly intervals in the Shark River Plume off the southwestern tip of the Florida mainland (25.35 N, 81.23 W). The drifter trajectories show that there is a strong link between the South Florida coastal waters (southwest Florida shelf, Florida Bay, the Florida Keys coastal zone and the Dry Tortugas), as the preferred pathways generally follow a southeastward route through western Florida Bay and the passages between the Keys, then westward along the reef tract to the Tortugas. The route through western Florida Bay is driven primarily by local wind forcing and by a mean sea level slope between the Gulf of Mexico and the Atlantic. The westward route along the reef tract is induced by the prevailing westward component in the local wind and by recirculating gyres and eddies north of the Florida Current. Seasonal variability is also detected, mainly associated with a shift in wind direction from northeasterly in fall and winter to southeasterly in summer. The wind magnitude is also seasonally modified, with strongest winds in the fall and winter seasons, associated with cold front passages. Enhanced stratification during summer and early fall allows a faster response of the drifters to the prevailing offshore component of the wind stress. Consequently, the most direct pathways to the Tortugas occur during the strong northeasterlies in fall, while the longest pathways to the Tortugas occur in summer, due to the southeasterlies that, although weak, effectively cause a northward drift reaching up to 27 N. However, multiple pathways can be seen in any season, reflecting high frequency changes in wind forcing.
Wind and current measurements are employed to compliment the Lagrangian data. It is found that seasonal changes in the regional wind forcing produce seasonal differences in the strength and variability of the currents on the west Florida shelf, as measured by moored Acoustic Doppler Current Profilers (ADCPs). Current amplitudes are greater in winter than in summer, following the enhanced wind stress. There is also a directional seasonal pattern in the measured surface currents which are more southward in the fall, winter, and spring seasons, changing to northward in the summer, due to the shift of summer winds to southeasterly.
A simple multiple regression model was employed to analyze the relationship of the seasonal wind components to velocity components that were computed by drifter trajectories. The analysis took place in the coastal areas which are not directly influenced by strong large-scale currents like those in the Loop Current and in the Straits of Florida. It was found that approximately 70 to 80% of the subtidal variance of drifter derived currents on the southwest Florida shelf and western Florida Bay is due to local wind forcing.
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2002 LAPCOD Meeting, Key Largo, Florida, December 12-16, 2000