Remote Radar Measurement of Shelf Currents off Key Largo, Florida, U.S.A.

Abstract

The coastal shelf of the Florida Keys is characterized by shallow and highly variable topography, where currents are influenced by tides, wind, and the very energetic offshore Florida Current system. The transient dynamics of variable scale are not easily captured with traditional fixed-mooring observation platforms. Consequently, an experiment combining moorings, Acoustic Doppler Current Profiler (ADCP) transects and a shore-based High-Frequency (HF) radar system (OSCR) was conducted to monitor the spatial and temporal variability of the flow field over the shelf and off-shore deeper waters. Both the wind- and tide-driven flows in the middle to inner shelf, called Hawk Channel, and two offshore eddy events of the Florida Current were sampled during the experiment.Qualitative agreement between OSCR, current meter moorings and ADCP transects was good in the shelf, however, at the inshore edge of the Florida Current there were significant differences. The ADCP captured the strong horizontal shear at the shelf edge, whereas OSCR smoothed the front over two kilometres due to its measurement cell size. These differences in the spatial sampling resolution in the high shear region prevented direct quantitative comparisons. Although there were significant local variations linked to topography, a major portion of the subtidal flow in the shelf was related to wind. Within the shelf both cross-shelf and along-shelf flows were highly spatially correlated, with r=0·82 andr =0·86, respectively.Two sub-mesoscale eddies that had characteristics consistent with the spin-off eddies described by Lee (1975), were observed to advect at velocities ranging from 0·53ms−1to 0·80ms−1along the inshore edge of the Florida Current. The two eddies had along-shelf length scales from 19km to 47km and cross-shelf scales of 15km and 25km. Each eddy consisted of a single cyclonic vortex rotating with tangential velocities of 0·30ms−1and 0·50ms−1respectively. The eddies had a strong signature at the shelf break, however they did not have any observable influence on flows over the inner-shelf.