Anisotropy in Coastal Ocean Relative Dispersion Observations

J. C. Ohlmann, L. Romero, E. Pallàs-Sanz and P. Perez-Brunius

DOI: https://doi.org/10.1029/2018GL081186

Abstract
Horizontal relative dispersion is investigated with Lagrangian drifter and dye tracer observations on relatively small scales (~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.

Keywords: dispersion, submesoscale, coastal

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