# Ship Route Oil Spill Modeling: A Case Study of the Northeast Brazil Event, 2019
## 4 Discussion  
Numerical experiments designed to help to understand oil dispersion at sea have confirmed that a leak in a mobile source is the most plausible explanation for the environmental disaster on the northeastern and part of southeastern coasts of Brazil in the second half of 2019, particularly in light of the total volume reaching the shore and the latitudinal distribution. A tanker ship crossing a zonal system of surface currents transversely would result in a greater spreading of suspended particles compared to a fixed spill source within the same time frame. This hypothesis is also supported by [13], which explored non-floating oil (subsurface dispersion) originating from a moving vessel. Another study [34] also demonstrated that the oil, which was not identified in satellite imagery, likely dispersed to the subsurface after a period of approximately 7 days. The current study was able to emphasize the importance of conducting oil dispersion experiments on mobile spill sources rather than the more common type of studies that focuses on fixed sources (e.g., natural seeps and oil exploration platforms). The environmental and socioeconomic risks of spills originating from mobile sources are much greater. The probability of the broad spreading and advection of oil from a mobile source via distinct surface current systems is higher, allowing the oil to reach a greater diversity of coastal environments and consequently exacerbating the associated impacts. The methodology and results developed herein to better understand this specific event are applicable to other events that may occur in the future, such as possible cases of ships deliberately discharging oil into international waters for various reasons, tank cleaning, and navigation safety, among others. Many of these offshore activities, which are carried out by ships along their navigation routes, result in orphan oil spills with no identified origin. The oil dispersion modeling of mobile sources, combined with the reverse modeling of surface particles, satellite images (when available), high-resolution spatiotemporal meteoceanographic forcings, and the use of unmanned autonomous vehicles, will enable an increased surveillance of Brazilian jurisdictional waters and more precise identification of polluters. Although the analysis of surface circulation off the Brazilian coast was not the focus of this study, the distribution of oil arrival along the coast observed in experiment 22 is consistent with the expected pattern of surface circulation for the region. The annual mean bifurcation of the southern branch of the South Equatorial Current (0-100 m) near the Brazilian continental margin occurs between 10\({}^{\circ}\) S and 14\({}^{\circ}\) S, but seasonal variability  
Figure 6: Probabilistic analysis of the occurrence of surface oil considering the scenario simulated in experiment 22 with a volume of 3 internal tanks (42,753 m\({}^{3}\)). shows that, in the winter months (July to August), the bifurcation occurs between 17\({}^{\circ}\) S and 16\({}^{\circ}\) S [47], reaching the southernmost latitude in July and the northernmost latitude in November. Thus, oil dispersion on the sea surface from a transversely mobile source to the bifurcation system would result in greater north-south spreading. Additionally, mesoscale activities already identified in the region [48; 49] might be capable of trapping oil patches and advecting them to more distant regions. As also reported by [34], the physicochemical characteristics of the oil, primarily based on its density and viscosity, resulted in subsurface sinking approximately one week after the spill, as demonstrated by the inversion in the percentage of oil (%) present on the surface and in the subsurface (Figure 5). Indeed, such behavior is expected not only due to the oil's characteristics but also because of weathering processes, especially emulsification, which can triple the volume of spilled oil and increase its viscosity by over a thousand times [50].