Relative Dispersion on the Inner Shelf: Evidence of a Batchelor Regime

Abstract

Oceanographic relative dispersion (based on drifter separations r ) has been extensively studied, mostly finding either Richardson–Obukhov ( ) or enstrophy cascade [ ] scaling. Relative perturbation dispersion (based on perturbation separation r − r 0 , where r 0 is the initial separation) has a Batchelor scaling ( ) for times less than the r 0 -dependent Batchelor time. Batchelor scaling has received little oceanographic attention. GPS-equipped surface drifters were repeatedly deployed on the Inner Shelf off of Pt. Sal, California, in water depths ≤ 40 m. From 12 releases of ≈18 drifters per release, perturbation and regular relative dispersion over ≈4 h are calculated for 250 ≤ r 0 ≤ 1500 m for each release and the entire experiment. The perturbation dispersion is consistent with Batchelor scaling for the first 1000–3000 s with larger r 0 yielding stronger dispersion and larger Batchelor times. At longer times, and scale-dependent diffusivities begin to suggest Richardson–Obukhov scaling. This applies to both experiment averaged and individual releases. For individual releases, nonlinear internal waves can modulate dispersion. Batchelor scaling is not evident in as the correlations between initial and later separations are significant at short time scaling as ~ t . Thus, previous studies investigating are potentially aliased by initial separation effects not present in the perturbation dispersion . As the underlying turbulent velocity wavenumber spectra is inferred from the dispersion power law time dependence, analysis of both and is critical.