Horizontal dispersion within an anticyclonic mesoscale eddy

Ship-mounted ADCP and buoy data are used to fit an azimuthal velocity profi le to a N.E. Atlantic mesoscale eddy chosen as a site for a Lagrangian biog eochemical 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 held allows observation s of a sulphur hexafluoride (SF,) tracer release within the eddy to be put into a non-translating, non-rotating frame. Analysis of the transformed dat a suggests that the patch of tracer, though spreading, remained coherent th roughout the nine-day survey, A linear regression on the square of the radi al patch width versus time gives an estimate for the effective horizontal d iffusion coefficient of 22 +/- 10 m(2) s(-1). This is consistent with previ ous estimates of diffusion rates at the O(10 km) lengthscale of the patch. Theory predicts a corresponding along-streamline spreading time of similar to 14 days. This implies that the tracer patch mixed little with the surrou nding waters during the first five days of the survey, suggesting that biog eochemical processes were little affected by lateral mixing during this per iod. 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 SF, concentrations. Using a model of a;circular eddy with the calculated ve locity profile, the dispersion of the initial,patch of tracer is simulated for different diffusivities, the results confirming the estimate of along-s treamline mixing time and demonstrating the enhancement of azimuthal diffus ion by the radial shear of the flow. (C) 2001 Elsevier Science Ltd. All rig hts reserved.