Persistent meanders and eddies lead to a quasi-steady Lagrangian transport pattern in a weak western boundary current

TL;DR

This study uses a 13-year simulation to reveal how persistent meanders and eddies in the Brazil Current create stable Lagrangian transport patterns, aiding in oil spill response and ecological connectivity assessments.

Contribution

It demonstrates that climatological Lagrangian Coherent Structures accurately depict transport pathways influenced by mesoscale features in a weak western boundary current.

Findings

cLCSs align with satellite and Eulerian data of the Brazil Current.

Persistent cyclonic and anticyclonic structures cause chevron-shaped deformations in transport patterns.

cLCSs effectively traced the 2011 Chevron oil spill and large-scale oil slicks reaching Brazilian beaches.

Abstract

The Brazil Current (BC) is a weak western boundary current described as a flow with intense mesoscale activity and relatively low volume transport. We use a 13-year simulation to show that the presence of persistent meanders and eddies leads to characteristic quasi-steady Lagrangian transport patterns, extracted through climatological Lagrangian Coherent Structures (cLCSs). The cLCSs position the surface expression of the BC core along the 2km isobath, in agreement with satellite sea-surface temperature and the model Eulerian mean velocity. The cLCSs deformation pattern responds to zonally persistent cross-shelf SSH transition from positive values near coastline to low values between 200m and 2km and back to positive offshore from the 2km isobath. Zonally-paired cyclonic and anticyclonic structures are embedded in this transition, causing the cLCSs to deform into chevrons. An transport barrier is identified close to the 200m isobath confirmed by limited inshore movement of drogued buoys and indicated by an along slope maxima of climatological strength of attraction (cp). We show that the persistent cyclonic and anticyclonic structures may induce localized cross-shelf transport. Regions of low cp coincide with large shelves and with stagnant synthetic trajectories. We show that cLCSs depict trajectories initiated at the location of Chevron spill (Nov. 2011) as compared to synthetic and satellite trajectories, and the outline of the oil from that accident. There is an agreement between the large-scale oil slicks reaching the Brazilian beaches (Aug. 2019 to Feb. 2020) and the cp at the coast. The identification and quantitative description of cLCSs improves the effectiveness of future emergency response to oil spills, contingency planning, rescue operations, larval and fish connectivity assessment, drifter launch strategies, waste pollutant dispersion and destination.