| The coastal circulation on the shelf off southeast Florida is strongly related to the dynamics of the Florida Current. A strong highly baroclinic current is confined to flow over a rapidly changing 3-D topography. Superimposed upon the mean Florida Current are a great variety of motions spanning a large range of time and space scales. In addition to what has been known as spin off eddies, large-amplitude tidal velocity oscillations with amplitudes exceeding 0.5 ms-1 are observed. This phenomenon affects navigation and acoustics in the area and also induces cross-shelf exchange of the biological species and pollutants. Exploratory measurements conducted by NSU and USF during 1999-2002 as a part of the South Florida Ocean Measurement Center have shown that the large-amplitude velocity oscillations are of baroclinic nature and have period ~10 hrs. This time period does not coincide exactly with the semidiurnal M2 (12.42 hrs) or S2 (12 hrs) tidal constituents. In addition, these internal oscillations appear to be strongly modulated over time scales of several days, seasonally, and interannually. The data are consistent at least in general features with the Niiler’s model of a near-resonant cross-stream internal seiche. In his model, an abrupt change of the channel depth at the edge of the continental shelf is a critical factor in the exchange of energy between the surface and internal mode. Another critical factor is the density stratification, and the position of the stratified current with respect to the continental shelf break is of importance. The baroclinic oscillation becomes large if the period of surface tidal wave is close to the natural period of internal seiche in the channel (~10 hrs in this part of Florida Straits during summer months). The modulation of the 10 hr velocity oscillation on the seasonal and interannual time scale can be explained by the dependence of the channel resonant properties on the stratification and position of Gulf Stream. Stratification is also affected by the spin off eddies, which results in the modulation of the 10-hr velocity oscillation on shorter time scales (several days). Results of Mooers (1975) suggest that the effective local inertial period (27 hrs) may be substantially reduced due to significant vorticity in the western boundary of Gulf Stream, which provides an alternative explanation for the 10 hr peak. |
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