Abstract:The Southern Ocean plays a critical role in global climate, yet the mixing properties of the circulation in this part of the ocean remain poorly understood. Here dispersion in the vicinity of the Southern Antarctic Circumpolar Current Front, one of the branches of the Antarctic Circumpolar Current, is studied using 10 pairs of surface drifters deployed systematically across the frontal jet and its flanks. Drifter pairs were deployed with an initial separation of 13 m and report their position every hour. The s… Show more
“…The relative dispersion ( D 2 ( t )), which is a benchmark of the mean square separation of particle pairs, is defined by where i and j refer to each drifter of a pair in the cluster of N drifter pairs and x i , j ( t ), y i , j ( t ) correspond to the coordinates in the Universal Transverse Mercator (UTM) projection at the time t . Typically, pairs or clusters of drifters that are initially separated by a small distance δ and were deployed at a certain time t 0 are used for the analysis. Various studies used “chance pairs,” which are deployed randomly but are simultaneously present in a given study area, to increase the number of drifter pairs for Lagrangian statistics (Dräger‐Dietel et al, 2018; LaCasce, 2010; Lumpkin & Elipot, 2010; Ollitrault et al, 2005; van Sebille et al, 2015). We have avoided the use of chance pairs in this study to overcome the issue of biased dispersion rates that can be produced for submesoscale dispersion (LaCasce & Ohlmann, 2003; Lumpkin & Elipot, 2010).…”
We examine the relative dispersion and the contribution of tides on the relative diffusivities of surface drifters in the North Sea. The drifters are released in two clusters, yielding 43 pairs, in the vicinity of a tidal mixing front in the German Bight, which is located in the southeastern area of the North Sea. Both clusters indicate decreasing dispersion when crossing the tidal mixing front, followed by exponentially increasing dispersion with e‐folding times of 0.5 days for Cluster 1 and 0.3 days for Cluster 2. A transition of the dispersion regimes is observed at scales of the order of the Rossby radius of deformation (10 km). After that, the relative dispersion grows with a power‐law dependency with a short period of ballistic dispersion (quadratic growth), followed by a Richardson regime (cubic growth) in the final phase. Scale‐dependent metrics such as the relative diffusivities are consistent with these findings, while the analysis of the finite‐scale Lyapunov exponents (FSLEs) shows contradictory results for the submesoscales. In summary, the analysis of various statistical Lagrangian metrics suggests that tracer stirring at the submesoscales is nonlocal and becomes local at separation scales larger than 10 km. The analysis of meridional and zonal dispersion components indicates anisotropic dispersion at the submesoscales, which changes into isotropic dispersion on the mesoscales. Spectral analysis of the relative diffusivity gives evidence that semidiurnal and shallow‐water tides influence relative diffusivity at the mesoscales, especially for drifter separations above 50 km.
“…The relative dispersion ( D 2 ( t )), which is a benchmark of the mean square separation of particle pairs, is defined by where i and j refer to each drifter of a pair in the cluster of N drifter pairs and x i , j ( t ), y i , j ( t ) correspond to the coordinates in the Universal Transverse Mercator (UTM) projection at the time t . Typically, pairs or clusters of drifters that are initially separated by a small distance δ and were deployed at a certain time t 0 are used for the analysis. Various studies used “chance pairs,” which are deployed randomly but are simultaneously present in a given study area, to increase the number of drifter pairs for Lagrangian statistics (Dräger‐Dietel et al, 2018; LaCasce, 2010; Lumpkin & Elipot, 2010; Ollitrault et al, 2005; van Sebille et al, 2015). We have avoided the use of chance pairs in this study to overcome the issue of biased dispersion rates that can be produced for submesoscale dispersion (LaCasce & Ohlmann, 2003; Lumpkin & Elipot, 2010).…”
We examine the relative dispersion and the contribution of tides on the relative diffusivities of surface drifters in the North Sea. The drifters are released in two clusters, yielding 43 pairs, in the vicinity of a tidal mixing front in the German Bight, which is located in the southeastern area of the North Sea. Both clusters indicate decreasing dispersion when crossing the tidal mixing front, followed by exponentially increasing dispersion with e‐folding times of 0.5 days for Cluster 1 and 0.3 days for Cluster 2. A transition of the dispersion regimes is observed at scales of the order of the Rossby radius of deformation (10 km). After that, the relative dispersion grows with a power‐law dependency with a short period of ballistic dispersion (quadratic growth), followed by a Richardson regime (cubic growth) in the final phase. Scale‐dependent metrics such as the relative diffusivities are consistent with these findings, while the analysis of the finite‐scale Lyapunov exponents (FSLEs) shows contradictory results for the submesoscales. In summary, the analysis of various statistical Lagrangian metrics suggests that tracer stirring at the submesoscales is nonlocal and becomes local at separation scales larger than 10 km. The analysis of meridional and zonal dispersion components indicates anisotropic dispersion at the submesoscales, which changes into isotropic dispersion on the mesoscales. Spectral analysis of the relative diffusivity gives evidence that semidiurnal and shallow‐water tides influence relative diffusivity at the mesoscales, especially for drifter separations above 50 km.
“…Drijfhout et al 1996, Blanke and Raynaud 1997, Döös et al 2008 and ocean dynamics (e.g. van Sebille et al 2012b, Ypma et al 2015. It is also the most commonly used framework to compute the pathways and distributions of plastic particles in the ocean (e.g.…”
Marine plastic debris floating on the ocean surface is a major environmental problem. However, its distribution in the ocean is poorly mapped, and most of the plastic waste estimated to have entered the ocean from land is unaccounted for. Better understanding of how plastic debris is transported from coastal and marine sources is crucial to quantify and close the global inventory of marine plastics, which in turn represents critical information for mitigation or policy strategies. At the same time, plastic is a unique tracer that provides an opportunity to learn more about the physics and dynamics of our ocean across multiple scales, from the Ekman convergence in basin-scale gyres to individual waves in the surfzone. In this review, we comprehensively discuss what is known about the different processes that govern the transport of floating marine plastic debris in both the open ocean and the coastal zones, based on the published literature and referring to insights from neighbouring fields such as oil spill dispersion, marine safety recovery, plankton connectivity, and others. We discuss how measurements of marine plastics (both in situ and in the laboratory), remote sensing, and numerical simulations can elucidate these processes and their interactions across spatio-temporal scales. Environ. Res. Lett. 15 (2020) 023003 E van Sebille et al Environ. Res. Lett. 15 (2020) 023003 E van Sebille et al References Acha E M, Mianzan H W, Iribarne O, Gagliardini D A, Lasta C and Daleo P 2003 The role of the Rı́o de la Plata bottom salinity front in accumulating debris Mar. Pollut. Bull. 46 197-202 Acha E M, Piola A, Iribarne O and Mianzan H 2015 Ecological Processes at Marine Fronts: Oases in the Ocean (Berlin: Springer) Aliani S and Molcard A 2003 Hitch-hiking on floating marine debris: macrobenthic species in the Western Mediterranean Sea Hydrobiologia 503 59-67 Allen J 1985 Principles of Physical Sedimentology (Berlin: Springer) Alpers W 1985 Theory of radar imaging of internal waves Nature 314 245-7 Alsina J M and Cáceres I 2011 Sediment suspension events in the inner surf and swash zone. Measurements in large-scale and high-energy wave conditions Coast. Eng. 58
“…The availability of trajectories of over 22,000 dFADs has revealed a considerable number of probable beaching events, and in many cases their deployment and drift pattern prior to beaching 16 . However, given the high level of fine-scale variability in ocean circulation, small variations in the initial deployment location may lead to varied trajectories and final positions over time 17 . Therefore, quantifying the connectivity between areas of high beaching and the surrounding ocean in a statistically robust way would require observations from many more dFADs.Figure 1Number of beaching events (1,320 in total) per 1° grid cell across 2016 and 2017.…”
In an effort to increase purse seine fishing efficiency for tropical tunas, over 30,000 drifting Fish Aggregating Devices (dFADs) are deployed every year by fishers in the Western and Central Pacific Ocean (WCPO). The use of dFADs also impacts ecosystems, in particular through marine pollution and dFAD beaching. This paper presents the first estimate of dFAD beaching events in the WCPO (>1300 in 2016–2017) and their distribution. Lagrangian simulations of virtual dFADs, released subject to contrasting deployment distributions, help us determine the relative importance of operational versus environmental drivers of dFADs drifting to beaching areas. The highest levels of beaching, occurring on Papua New Guinea and Solomon Islands, are likely a result of the prevailing westward oceanic circulation and subsequent local processes driving dFADs towards land. Similarly, high beaching rates in Tuvalu appear to be due to the general circulation of the WCPO. In contrast, beaching in Kiribati Gilbert Islands appear to be more strongly related to dFAD deployment strategy. These findings indicate that reducing beaching events via changes in deployment locations may be difficult. As such, management approaches combining dFAD deployment limits, the use of biodegradable dFADs, recoveries at-sea close to sensitive areas and/or beached dFAD removal should be considered.
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