Persistence and Robustness of Lagrangian Coherent Structures

Abstract. Lagrangian coherent structures (LCS) are transient features in ocean circulation that describe particle transport, revealing information about transport barriers and accumulation or dispersion regions. Various methods exist to infer LCS from surface current fields provided by ocean circulation models. Generally, Lagrangian trajectories as well as LCS analysis inherit the uncertainty from the underlying ocean model, bearing substantial uncertainties as a result of chaotic and turbulent flow fields. In addition, velocity fields and resulting LCS evolve rapidly. In this study, finite time Lyapunov exponents (FTLE) are used to detect LCSs in surface current predictions from a regional ocean forecast system. We investigate the uncertainty of LCS at any given time using an ensemble prediction system (EPS) to propagate velocity field uncertainty into the LCS analysis. We evaluate variability of FTLE fields in time and across the ensemble at fixed times. Averaging over ensemble members can reveal robust FTLE ridges, i.e. FTLE ridges that exist across ensemble realisations. Time averages reveal persistent FTLE ridges, i.e. FTLE ridges that occur over extended periods of time. We find that LCS are generally more robust than persistent. Large scale FTLE ridges are more robust and persistent than small scale FTLE ridges. Averaging of FTLE field is effective at removing chaotic, short-lived and unpredictable structures and may provide the means to employ LCS analysis in forecasting applications that require to separate uncertain from certain flow features.