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

Abstract

The Brazil Current (BC) is a weak western boundary current flowing along the Southwestern Atlantic Ocean. It is frequently described as a flow with intense mesoscale activity and relatively low volume transport between 5.0 and 10.0 Sv. We use a 13-year eddy-resolving primitive-equation simulation to show that the presence of persistent meanders and eddies leads to characteristic quasi-steady Lagrangian transport patterns, aptly extracted through climatological Lagrangian Coherent Structures (cLCS). The cLCS position the surface expression of the BC core along the 2000-m isobath, in excellent visual agreement with high resolution satellite sea-surface temperature and the model Eulerian mean velocity. The cLCS deformation pattern also responds to zonally persistent cross-shelf SSH transition from positive (high) values near coastline to low (negative) values between 200- and 2000-m and back to positive (high) offshore from the 2000-m isobath. Zonally-paired cyclonic and anticyclonic structures are embedded in this transition, also causing the cLCS to deform into chevrons. An efficient transport barrier is identified close to the 200-m isobath confirmed by limited inshore movement of drogued drifters and accurately indicated by an along slope maxima of climatological strength of attraction. We also show that the persistent cyclonic and anticyclonic structures may induce localized cross-shelf transport. Regions of low climatological strength of attraction coincide with large shelves and with stagnant synthetic trajectories. We also show that cLCS accurately depict trajectories initiated at the location of Chevron’s spill (November 2011) as compared to synthetic and satellite-tracked trajectories, and the outline of the oil from that accident. There is also an agreement between the large-scale oil slicks reaching the Brazilian beaches (from August 2019 to February 2020) and the strength of climatological attraction at the coast. Our work also clarifies the influence of persistent mesoscale structures on the regional circulation. The identification and quantitative description of climatological Lagrangian coherent structures is expected to improve the effectiveness of future emergency response to oil spills, contingency planning, rescue operations, larval and fish connectivity assessment, drifter launch strategies, waste pollutant and marine debris dispersion and destination.

On the Advection of Upwelled Water on the Western Yucatan Shelf

“…Trajectories from the east move across weak cLCS that are deformed as chevrons within the Yucatan Shelf, while trajectories from the northwest move along strong cLCS until they meet within the initial position polygon (Figure 12). The advection of cool SST coincides with most trajectories (Figure 10C), in crossing weak cLCS that deformed as chevrons and indicate the direction of weak transport, like in Gouveia et al (2021), where along-slope SST advection happens through weak cLCS deformed as chevrons. cLCSs with relatively strong climatological attraction indicate transport barriers along which transport is often directed (see also Climatological Lagrangian Coherent Structures section for more details on interpreting cLCS).…”

“…However, the averaging of the Cauchy-Green tensor formally destroys the transport-barrier property of LCS Haller (2015)). Thus, the interpretation of cLCS requires careful comparison with additional Eulerian and Lagrangian information, for example, described in Gouveia et al (2021) where it is shown that cLCS may deform as chevrons. When cLCS deform as chevrons, they do not represent transport barriers, but instead, they identify a jet-like structure, as is the case in this study where cLCS identifies a coastal jet-like current.…”

“…The different interpretations for cLCS follow the fact that cLCS can be thought of as a superposition of 7-day LCS from the climatological velocity, with the integration initiating at different times (see, e.g., Appendix C of Duran et al, 2018). In the Supporting Information (Appendix A) of Duran et al (2018), it is shown that the deformation due to along-path horizontal divergence is negligible, and therefore, the patterns identified by cLCS are due to large-scale two-dimensional flow (see also Supporting Information in Gouveia et al, 2021). This does not exclude the possibility of a vertical circulation due to superimposed upwelling, as an example.…”

Western boundary currents drive sun-coral (Tubastraea spp.) coastal invasion from oil platforms

“…2 ). In fact, the Lagrangian transport patterns in this region described by Gouveia et al 53 produce a cross-shelf transport barrier to floating particles. However, there is one place where particles can reach the coast south of 24° S. Our simulations also show an important zonal transport of sun-coral larvae towards the coast that coincides with this latitude in Fig.…”

“…1 ). The region is characterized by the presence of quasi-steady Lagrangian transport structures driven by semi-permanent eddies and meanders 53 . These mesoscale features originate from the combination of the abrupt change in the coastline orientation, the bathymetric gradient, and also by the baroclinically unstable nature of the BC 54 , 55 .…”