Measurements of drifter cluster dispersion

Abstract

The motion of clusters of drifters in stratified coastal waters was studied. Wave‐like motion dominated the single‐particle statistics and had length scales larger than the cluster dimensions; consequently it was largely filtered out of motion relative to the cluster centroid. But the stochastic motion causing eddy diffusion seemed to be equally present in both single‐particle motion and motion relative to the cluster centroid. The single‐particle kinetic energy was 10 and 2 times the kinetic energy of motion relative to the centroid in summer and winter, respectively. The relative motion had longer Lagrangian integral time‐scales and smaller Eulerian spatial correlation scales than the single‐particle motion. Integral length scales of relative motion were of 0.1 and 0.2 times the standard deviation of drifter positions about the centroid for summer and winter ensembles, respectively. The y component of ensemble‐averaged relative eddy diffusivity in summer (0.21 m² s^?1) was much larger than that in winter (0.036 m² s^?1) whereas the summer x component (0.066 m² s^?1) was similar to that in winter (0.087 m² s^?1). The dispersion of individual clusters can vary considerably from that expected from the ensemble‐averaged eddy diffusivity. The cluster dispersion was intermittent, with long quiescent periods of gradual cluster deformation and short events causing rapid cluster deformation. In quiescent periods the centroid motion and velocity gradients were consistent with the kinematics of internal waves.