The influence of mesoscale eddies on shallow water acoustic propagation

Abstract

Acoustic propagation measurements in 150 m depth on the Florida escarpment observe the effects of the passage of a cyclonic eddy. As the stream core of the Florida Current meanders, the eddy is formed and propagates along the shelf edge. The sequence over a roughly a fortnight is as follows: ahead of the eddy, warm surface water and cold bottom water are swept onto the terrace forming a steep thermocline and corresponding strong downward refracting C(z). The gradient produce intense, focused RBR arrivals and the thermocline becomes a duct for internal waves to propagate shoreward. At first, the internal wave energy is minimal and propagation is stable and coherent. As the internal tides attempt to propagate on shelf, the sound speed field and the acoustic signals become increasingly variable. The variability reaches a crescendo as the 200 m long internal tide is blocked from propagating on to the narrower shelf and begins to break and overturn producing small-scale variability. As the eddy passes, nearly iso-thermal conditions are restored along with quiescent internal wave fields and reduced signal variability. Here, the effects are quantized with data from fixed-system acoustic and oceanographic measurements demonstrating that the mesoscale determines acoustic propagation conditions days in advance.