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Horizontal relative dispersion is investigated with Lagrangian drifter and dye tracer observations on relatively small scales (similar to 100 to 850 m) in coastal waters. Anisotropy is quantified with an aspect ratio of the spreading in two orthogonal directions. Individual observations generally appear highly anisotropic. However, the ensemble mean computed in a coordinate system aligned with bathymetry indicates only weak anisotropy due to averaging highly anisotropic observations over a wide range of principal-axis directions. The strong anisotropy is preserved when the mean is computed in a principle-axis coordinate system. In fact, the ensemble mean in principle-axis coordinates gives antidispersion or convergence in the minor-axis direction. This result suggests gradients in buoyant materials such as spilled oil and other contaminants are not necessarily smoothed as the standard eddy-diffusivity parameter suggests. Flow kinematics computed with clusters of four drifters indicate that approximately 72% of energy in the observed dispersing flows can be attributed to organized submesoscale structures.

Plain Language Summary Recent theoretical and numerical ocean circulation studies suggest that relatively small-scale motions can spread passive tracers, such as spilled oil and other contaminants, in preferential directions. Further, through converging horizontal currents, these small-scale flows can serve to increase tracer concentrations. However, such preferential spreading is not well observed in nature. This study uses measurements of coastal ocean circulation made with water-following drifters and dye tracer on relatively small scales (similar to 100 to 850 m) to examine relative dispersion anisotropy. The measurements clearly show that spreading occurs in preferential directions that are time varying. Measurements also show a strong propensity for buoyant tracers to accumulate, thereby increasing concentrations. The results are important for improving the verity of numerical models that predict the movement and dilution of freely drifting contaminants in the ocean.