A tale of two eddies: Diagnosing coherent eddies through acoustic remote sensing

A 38 kHz vessel‐mounted acoustic Doppler current profiler is used to explore in detail the dynamics of an anticyclonic and a cyclonic eddy during two transits of the cruise vessel Explorer of the Seas from the Caribbean to New Jersey in July 2007. The radial scale of the two eddies is similar, but whereas the cyclone is strongly surface intensified, the anticyclone has its maximum expression with near–solid body rotation between 200 and 800 m depth. The anticyclone has a minimum in relative vorticity very close to −f at 800 m depth and the cyclone has a maximum of about +1.6 f close to the surface where f is the local Coriolis parameter. By integrating the momentum equation the geopotential anomaly field and hence the potential energy of the eddies can be determined quite accurately, which means that the kinetic and potential energy of the eddies can be determined purely through acoustic remote sensing. Given a density profile just outside the eddy one can integrate the gradient wind equation to obtain an estimate of the density and hence potential vorticity fields through the two eddies. The acoustic backscatter patterns in the eddies are quite distinct from the surrounding waters. The backscatter intensity of the main scattering layer at ∼600 m depth decreases by ∼10 dB in the core of both eddies. In the cyclonic eddy three identifiable scattering layers in the main thermocline show a strong tendency for the scattering layer to track the shoaling density structure toward the center of the eddy. Key Points Power of acoustic remote sensing of currents and biomass Quantitative assessment of eddy vorticity and energetics Highly structured biomass patterns in eddies