Southeast Florida Shelf circulation and volume exchange, observations of km-scale variability

Tides, wind and the Florida Current affect the dynamics of the South Florida continental shelf over a range of scales. Two field experiments in the winter (LARGO1) and summer (LARGO2) employed a high-frequency (HF) Ocean Surface Current Radar (OSCR), moored current meters and a boat-mounted Acoustic Doppler Current Profiler (ADCP) to study the circulation over the southeast Florida Shelf. The OSCR measured near-surface currents over an approximately 30 km by 30 km grid, with 1-km horizontal resolution, covering the shelf and extending offshore of the shelf break. By using observed 20-min surface vector field time series to backward track patches with defined volumes of shelf water, their recent origin was identified. The proportion of surface volume flux past various points on the shelf that originated offshore, along the shelf edge and inshore was then estimated. Onshore winds were linked to periods of transport of offshore water all the way to the inner shelf during both the summer and winter seasons. In LARGO2 the exchange of offshore water with water over the middle and inner shelf was closely linked to onshore and downcoast winds. Simulated particle trajectories revealed that Florida Current spin-off eddies caused off-shelf water to be advected onto the middle (LARGO1) or outer (LARGO2) shelf. However, because eddies had short residence times and small spatial scales their contribution to the overall exchange was much smaller than the wind.