Observations and idealized models of dispersion on the southwestern Puerto Rican insular shelf

Drifter tracks indicate that the mean currents on the southwestern Puerto Rican insular shelf are driven primarily by an alongshore wind stress and an adverse pressure gradient. Observations during periods of low or variable wind indicated large particle dispersion associated with variable flow around island obstacles close to the release site. A simple analogous model is presented to show how periodically fluctuating flow causes dispersion at the junction of three channels. Away from the island obstacles, the particle dispersion is an order of magnitude less than that due to a vacillating flow around island s. A comparison is made between the observed dispersion away from islands and that obtained from an idealized dynamical model of wind driven flow. Using sinusoidal bathymetry with horizontal and vertical scales comparable to the larger scale features of insular shelf geometry results in modeled particle dispersion less than that observed in one day drifter experiments. Considering bathymetry with smaller horizontal scales resulted in particle dispersion rates comparable to observations. The model indicates that variable flow over bathymetric features is a process that contributes substantially to dispersion on the shallow insular shelf at time scales of the order of several days. Over such time scales, the model stretches and folds an initially uniform two-dimensional cloud of particles into a streaky distribution with fractal dimension 1.55 ± 0.05.