Horizontal dispersion within an anticyclonic mesoscale eddy

Ship-mounted ADCP and buoy data are used to fit an azimuthal velocity profile to a N.E. Atlantic mesoscale eddy chosen as a site for a Lagrangian biogeochemical survey, the cruise forming part of the UK Plankton Reactivity in the Marine Environment (PRIME) initiative. Together with the buoy-derived locus of the eddy centre, the ADCP-derived velocity field allows observations of a sulphur hexafluoride (SF6) tracer release within the eddy to be put into a non-translating, non-rotating frame. Analysis of the transformed data suggests that the patch of tracer, though spreading, remained coherent throughout the nine-day survey. A linear regression on the square of the radial patch width versus time gives an estimate for the effective horizontal diffusion coefficient of 22±10 m2 s−1. This is consistent with previous estimates of diffusion rates at the O(10 km) lengthscale of the patch. Theory predicts a corresponding along-streamline spreading time of ∼14 days. This implies that the tracer patch mixed little with the surrounding waters during the first five days of the survey, suggesting that biogeochemical processes were little affected by lateral mixing during this period. The theory is inapplicable at later times because the area was struck by a storm at the start of the sixth day, which resulted in the halving of SF6 concentrations. Using a model of a circular eddy with the calculated velocity profile, the dispersion of the initial patch of tracer is simulated for different diffusivities, the results confirming the estimate of along-streamline mixing time and demonstrating the enhancement of azimuthal diffusion by the radial shear of the flow.