Variability of current speed and dispersion in the Western Mediterranean Sea

Current speed and dispersion in the Alboran and Balearic Seas of the Western Mediterranean Sea are analyzed from extensive drifter datasets from three field campaigns. For each experiment, drifters were released at multiple depths. This extensive dataset yields new insights into the roles of depth, sampling season, and flow feature on the variability of mean linear speed and full-flow and turbulent dispersion. Mean speed was found to vary primarily with flow feature, with the six jets being fastest, the three submesoscale eddies slowest, and the three mesoscale vortices in between on average, albeit with significant overlap in the distributions. Moreover, data from three different sampling seasons suggests that similar flow features could be slower earlier in the year. Surprisingly, depth dependence from 0.4 m to 100 m was found to be minimal. Full-flow and turbulent dispersion also showed some depth-dependence, but with minimal consistent pattern. Flow type and seasonality, on the other hand, had similar impacts on dispersion as on mean speed, with greater dispersion for the later deployments and for jets over vortical flows. Drifters in all flows and at all depths exhibited a ballistic dispersion regime (proportional to t 2 ) in early times, whereby the period lasted longest in jets (especially coastal jets) and shortest in the submesoscale eddies. After this initial period, mesoscale vortices and submesoscale eddies limit further dispersion, resulting in a leveling off and finally oscillations around a shallow (full-flow) or no (turbulent) trend. The results highlight the importance of flow feature variability in ocean transport dynamics.