Transport properties in small-scale coastal flows: relative dispersion from VHF radar measurements in the Gulf of La Spezia

Lagrangian transport characteristics in the Gulf of La Spezia, a 5 x 10-km area along the western coast of Italy, are investigated using data collected from a very high frequency (VHF) radar system with 250mand 30-min resolution and two clusters of Coastal Dynamics Experiment surface drifters during 2 weeks in the summer of 2007. The surface drifters are found to follow the temporal and spatial evolution of the finite-scale Lyapunov exponents (FSLEs) computed by the VHF radar, indicating the precision of both the radar measurements and the diagnostic FSLE in mapping accurately the transport pathways. In light of this agreement, an analysis of the relative dispersion is conducted. It is found that the average FSLE value varies within a narrow range of 4 day(-1) <= lambda <= 7 day(-1) and displays an exponential regime over the entire extent of the measurements. The dynamical implication is that relative dispersion is controlled nonlocally, namely by slow, persistent, energetic mesoscale structures as opposed to the rapidly evolving high-gradient smallscale turbulent features. The value of the exponent is about an order of magnitude larger than those found in previous modeling studies and analysis of SCULP data in the Gulf of Mexico but somewhat smaller than that estimated from CLIMODE drifters in the Gulf Stream region. Scaling of the FSLE using a metric of resolved gradients of the Eulerian fields in the form of a positive Okubo-Weiss criterion is useful, but not as precise as in modeling studies. The horizontal flow convergence is found to have a small yet tangible effect on relative dispersion.