FSLE analysis and validation of Lagrangian simulations based on satellite-derived GlobCurrent velocity data

We present an independent validation of the satellite surface velocity fields released by the ESA Data User Element GlobCurrent Project. The validation methodology is based on the analysis of Lagrangian numerical trajectories integrated from satellite-based sea surface currents. Two case studies were considered: Mediterranean Sea and North Atlantic Ocean. The Finite-Scale Lyapunov Exponents provide a rigorous, quantitative tool to evaluate Lagrangian simulations with respect to real drifter trajectories. An accuracy threshold scale can be identified as the scale above which the error size, propagating along a numerical trajectory, grows no faster than the relative separation between real drifters. Below this threshold, the error growth rate tends to diverge linearly as the error decreases. The mean error growth speed, at early stage, is found to be related to the kinetic energy of the missing scales of motion, not resolved by the GlobCurrent products. Established kinematic Lagrangian models are, also, exploited to compensate the energy gap between real and numerical trajectories in the unresolved scale range. Ultimately, GlobCurrent surface velocity fields are shown to have overall good 'Lagrangian skills' for large-scale transport and dispersion numerical simulations.