Lateral dispersion over the continental shelf: Analysis of dye release experiments

Lateral dispersion over the continental shelf was examined using dye studie s performed as a part of the Coastal Mixing and Optics experiment. Four exp eriments performed at intermediate depths, each lasting 2.5-5 days, were ex amined. In some cases the dye patches remained fairly homogeneous both vert ically and horizontally throughout an experiment. In other cases, significa nt patchiness was observed on scales ranging from 2 to 10 m vertically and a few hundred meters to a few kilometers horizontally. The observations sho wed that the dye distributions were significantly influenced by shearing an d straining on scales of 5-10 m in the vertical and 1-10 km in the horizont al. Superimposed on these larger-scale distortions were simultaneous increa ses in the horizontal second moments of the dye patches, with corresponding horizontal diffusivities based on a Fickian diffusion model of 0.3-4.9 m(2 ) s(-1). Analysis of the dye data in concert with shear estimates from ship board acoustic Doppler current profiler observations showed that the existi ng paradigms of shear dispersion and dispersion by interleaving water masse s cannot account for the observed diffusive spreading of the dye patches. T his result suggests that some other mechanisms provided an additional diffu sivity of order 0.2-4.6 m(2) s(-1) An alternative mechanism, dispersion by vortical motions caused by the relaxation of diapycnal mixing events, may e xplain the observed dispersion in some cases.