Drifting Speed of Lagrangian Fronts and Oil Spill Dispersal at the Ocean Surface

Abstract

Due to its dire impacts on marine life, public health, and socio-economic services, oil spills require an immediate response. Effective action starts with good knowledge of the ocean dynamics and circulation, from which Lagrangian methods derive key information on the dispersal pathways present in the contaminated region. However, precise assessments of the capacity of Lagrangian methods in real contamination cases remain rare and limited to large slicks spanning several hundreds of km. Here we address this knowledge gap and consider two medium-scale (tens of km wide) events of oil in contrasting conditions: an offshore case (East China Sea, 2018) and a recent near-coastal one (East Mediterranean, 2021). Our comparison between oil slicks and Lagrangian diagnostics derived from near-real-time velocity fields shows that the calculation of Lagrangian fronts is, in general, more robust to errors in the velocity fields and more informative on the dispersion pathways than the direct advection of a numerical tracer. The inclusion of the effect of wind is also found to be essential, being capable of suddenly breaking Lagrangian transport barriers. Finally, we show that a usually neglected Lagrangian quantity, the Lyapunov vector, can be exploited to predict the front drifting speed, and in turn, its future location over a few days, on the basis of near-real-time information alone. These results may be of special relevance in the context of next-generation altimetry missions that are expected to provide highly resolved and precise near-real-time velocity fields for both open ocean and coastal regions.

Sub-frontal niches of plankton communities driven by transport and trophic interactions at ocean fronts

“…The HI was computed daily at 4 km resolution, based on the ESA CCI/C3S product, but is limited by cloud cover. The second is based on the detection of stirring structures and regions of convergence, quantified by backward in time finite-size Lyapunov exponents (FSLEs; D 'Ovidio et al, 2004;Fifani et al, 2021). The FSLEs were computed from horizontal velocities combining geostrophic currents derived from altimetry and Ekman currents derived from wind-stress data (Chabert et al, 2021), but its accuracy with fine-scale structures is limited by the low resolution of altimetry data.…”

Transport Barriers in Geophysical Flows: A Review

“…Moreover, the oil undergoes evaporation, emulsification, biodegradation, and other processes acting simultaneously, changing its physicochemical properties. The FTLE/FSLE diagnostics have been used to detect locations of HTBs and analyze their role in the dispersion of oil spills after catastrophic accidents in the Gulf of Mexico in April 2010 [89,90], in the East China Sea in 2018, and in the Mediterranean Sea in 2021 [91].…”

“…The method for predicting the speed of drifting Lagrangian fronts, situated along HTBs, has been proposed in Ref. [91]. This method uses the theoretical property of zero flux across TBs.…”

An ocean front dataset for the Mediterranean sea and southwest Indian ocean

“…Beside the study of front-marine life interactions 10 , investigating trophic relationships 70 and the accumulation of fish larvae 71 , NOMAD 51 could also be employed to monitor marine pollutants such as microplastics 72 , 73 and oil spills 74 along fronts. One could also examine relationships between surface thermal fronts and low-level winds at submesoscales from Radar Scatterometer data; as it has only been done at mesoscales so far 75 – 77 .…”