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

Abstract

Lagrangian transport characteristics in the Gulf of La Spezia, a 5 × 10-km area along the western coast of Italy, are investigated using data collected from a very high frequency (VHF) radar system with 250 m and 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 ≤ λ ≤ 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.

Transit and residence times in the Adriatic Sea surface as derived from drifter data and Lagrangian numerical simulations

“…Observations of ocean currents and dispersion with surface drifters have been used effectively to describe and quantify the transport characteristics from the global scale (Niiler et al, 2003) to marginal seas (Falco et al, 2000;Lacorata et al, 2001) and selected coastal areas (Haza et al, 2010;Veneziani et al, 2007, Choukroun et al, 2010. In particular, the Adriatic Sea, a semi-enclosed basin of the Mediterranean, connected to the Ionian Sea through the Otranto Channel (Fig.…”

Lagrangian simulation and tracking of the mesoscale eddies contaminated by Fukushima-derived radionuclides

“…In spite of nonstationarity of the velocity field, some of them may exist for weeks and much more. The hyperbolic points and their attracting and repelling manifolds were recently identified with the help of drifter's tracks in the Gulf of La Spezia in the northwestern Mediterranean Sea (Haza et al, 2010), in the Gulf of Lion (Nencioli et al, 2011), in the Gulf of Mexico (Olascoaga et al, 2013) and in the northwestern Pacific (Prants et al, 2016).…”

Lagrangian descriptors and the assessment of the predictive capacity of oceanic data sets

“…Consistently Lagrangian descriptors have been shown to produce similar outputs for slightly modified velocity fields Mancho, 2010, 2012). A second grid size effect discussed by Hernández-Carrasco et al (2011) -the F-grid -is related to the adjustment of a parameter, δ, that measures the initial separation of trajectories on which FSLEs depend. However, this does not influence our results as LDs do not depend on such a parameter.…”

“…For instance, the correlation between Lagrangian structures obtained from radar data in different ocean areas is discussed by Shadden et al (2009) and Haza et al (2010). Kuznetsov et al (2002) have examined the role played by Lagrangian structures in the Gulf of Mexico.…”