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The world's oceans are confronted with plastic pollution, 80% of which is of terrestrial origin, flowing mainly from the mismanaged waste of coastal populations and to a less extent, from river discharge. To study the fate of this pollution, we follow the trajectories of neutral plastic particles released continuously in numerical ocean simulations with two realistic source scenarios defined according to poorly managed waste from coastal populations and river discharge. The trajectories are three-dimensional and calculated for a period of 24 years by the Ariane Lagrangian tool from ocean currents simulated by a ¼° global ocean general circulation model (NEMO). The important particularity of the present model is that it is coupled with the WaveWatch III (WW3) wave model and consequently represents the Stokes drift in a consistent manner. The results are compared to trajectories calculated with an uncoupled NEMO simulation in which the Stokes drift is simply not considered. The results show that microplastics (as neutral particles) accumulate at the surface in the subtropical convergence zones of the Ekman transport before penetrating to depth and being strongly dispersed around 200 to 300 m depth over 40 degrees of latitude. At the end of the simulation, about 5.3% of the microplastics remains at the surface in these convergence zones and near the emission regions for the wave-coupled model, whereas only 2% remains for the uncoupled model. Our results indicate that waves may increase the retention of neutral plastic particles at the surface by a factor of two to three because of the upward vertical velocities induced by the divergence of Stokes transport in the surface layers. Plastic surface concentrations are maximal in the North Pacific and Indian Ocean basins. This result is due to the large discharge fluxes surrounding these basins of the northern hemisphere. The Mediterranean Sea exhibits also highly concentrations in microplastics due to high coastal population densities. This work shows the strong influence of waves (and Stokes drift) on the transport of plastic particles in the oceans, both on the retention of particles at the surface, the importance and location of convergence zones, and on the dispersion of neutral plastics at depth.

The California Current System is an eastern boundary upwelling system (EBUS) with high biological production along the coast. Oligotrophic offshore waters create cross-shore gradients of biological and physical properties, which are affected by intense mesoscale eddy activity. The influence of eddies on ecosystem dynamics in EBUS is still in debate. To elucidate the mechanisms that influence the dynamics of ecosystems trapped in eddies, and the relative contribution of horizontal and vertical advection in determining local production, we analyze a particular cyclonic eddy using Lagrangian particle-tracking analyses of numerical Eulerian. The eddy formed in a coastal upwelling system; coastal waters trapped in the eddy enabled it to leave the upwelling region with high concentrations of plankton and nutrients. The ecosystem was initially driven mainly by recycling of biological material. As the eddy moved offshore, production in its core was enhanced compared to eddy exterior waters through Ekman pumping of nitrate from below the euphotic zone; this Ekman pumping was particularly effective due to the shallow nitracline in the eddy compared to eddy exterior waters. Both eddy trapping and Ekman pumping helped to isolate and maintain the ecosystem productivity in the eddy core. This study shows the importance of cyclonic eddies for biological production in EBUS: they contribute both to the redistribution of the coastal upwelling ecosystem and are local regions of enhanced new production. Together, these processes impact cross-shore gradients of important biological properties.

International audience; We characterize eddy activity in the southern Benguela upwelling system with absolute sea level data deduced from remote-sensing measurements and numerical simulations run with a regional model, using a wavelet-based technique for identification and tracking of coherent structures. Statistics on eddy count, size, type and location are obtained along the continental slope of the upwelling system for a common period of interest shared by model and satellite data. We draw a comparison between mesoscale variability calculated by the regional model and ground truth accounted by satellite altimetry. We show that the presence of cyclonic eddies along the upwelling front over the South Benguela slope is recurrent throughout the year. Moreover, the agreement between model and observations is fair and validates the modeling approach. Tracking in the model of volume properties of continental slope, cyclonic eddies on their way toward the open ocean suggests that these eddies play a significant role in coastal–open ocean exchanges. With lifetimes of several months, these eddies can export significant volumes of nutrient-rich waters far from the coastal upwelling.

International audience; The migration duration of European eel (Anguilla anguilla) larvae (leptocephali) from the spawning areas in the Sargasso Sea to the European continental shelf remains highly controversial, with estimates varying from 6 months to more than 2 yr. We estimated the fastest migration period and the shortest distance travelled by eel larvae by simulating Lagrangian particles released in the Sargasso Sea and by simulating a range of larval behaviours (fixed-depth drift, vertical diurnal migration and active-depth selection to maximize current velocity). This enabled us to compute (i) a passive drift speed, and (ii) a hypothetic swimming speed needed for European eel larvae to cross the Atlantic in 6 months (i.e., the migration duration estimated from otolith daily growth increments). Our results show that the minimum travel time for an eel larva that is passively drifting was 10 months and 3 days. Active behaviours (vertical diurnal migration and rheotaxis) paradoxically increased the migration period. We found that for leptocephali to cross the Atlantic Ocean in 6 months, they would need to swim a minimum of 3.4 body lengths per second for 8200 km. No larvae have been observed with such swimming capabilities. These results provide evidence that leptocephali cannot cross the Atlantic in 6 months.

© 2015. American Geophysical Union. All Rights Reserved. The California Current System is an eastern boundary upwelling system (EBUS) with high biological production along the coast. Oligotrophic offshore waters create cross-shore gradients of biological and physical properties, which are affected by intense mesoscale eddy activity. The influence of eddies on ecosystem dynamics in EBUS is still in debate. To elucidate the mechanisms that influence the dynamics of ecosystems trapped in eddies, and the relative contribution of horizontal and vertical advection in determining local production, we analyze a particular cyclonic eddy using Lagrangian particle-tracking analyses of numerical Eulerian. The eddy formed in a coastal upwelling system; coastal waters trapped in the eddy enabled it to leave the upwelling region with high concentrations of plankton and nutrients. The ecosystem was initially driven mainly by recycling of biological material. As the eddy moved offshore, production in its core was enhanced compared to eddy exterior waters through Ekman pumping of nitrate from below the euphotic zone; this Ekman pumping was particularly effective due to the shallow nitracline in the eddy compared to eddy exterior waters. Both eddy trapping and Ekman pumping helped to isolate and maintain the ecosystem productivity in the eddy core. This study shows the importance of cyclonic eddies for biological production in EBUS: they contribute both to the redistribution of the coastal upwelling ecosystem and are local regions of enhanced new production. Together, these processes impact cross-shore gradients of important biological properties.

We investigate the coupling between the physics and new production variability during the period April 1992 to June 1995 in the equatorial Pacific via two cruises and simulations. The simulations are provided by a high-resolution Ocean General Circulation Model forced with satellite-derived weekly winds and coupled to a nitrate transport model in which biology acts as a nitrate sink. The cruises took place in September-October 1994 and sampled the western Pacific warm pool and the upwelling region further east. The coupled model reproduces these contrasted regimes. In the oligotrophic warm pool the upper layer is fresh, and nitrate-depleted, and the new production is low. In contrast, the upwelling waters are colder, and saltier with higher nitrate concentrations, and the new production is higher. Along the equator the Eastern edge of the warm pool marked by a sharp salinity front, also coincides with a "new production front". Consistent with the persistent eastward surface currents during the second half of 1994, these fronts undergo huge eastward displacement at the time of the cruises. The warm/fresh poll and oligotrophic region has an average new production of 0.9 mmol NO3/m2/d, which is almost balanced by horizontal advection from the central Pacific and by vertical advection of richer water from the nitrate reservoir below. In contrast, the upwelling mesotrophic region shows average new production of 2.1 mmol NO3/m2/d and the strong vertical nitrate input by the equatorial upwelling is balanced by the losses, through westward advection and meridional divergence of nitrate rich waters, and by the biological sink. (Résumé d'auteur)

[1] Results from a regional ocean model and numerical Lagrangian analyses are compared with in situ measurements to describe the properties and dynamics of Antarctic Intermediate Water (AAIW) in the region of the Cape Basin. The AAIW that originates in the South Atlantic (A-AAIW) at 8°W follows two branches. A southern branch, flowing mostly south of 40°S, is blocked by topography and is deflected westward without significant changes in its physical properties. A northern branch crosses the Cape Basin with strong modification of its physical properties. The AAIW that originates in the Indian Ocean (I-AAIW) flows into the Atlantic Ocean via the Agulhas Current and undergoes small physical changes in the Cape Basin. In the model, the salinity ranges of A-AAIW and I-AAIW cores that reach the southeast Atlantic are 34.2–34.5 and 34.5–34.6, respectively. The modeled AAIW distribution and behavior compare well with observations, despite a bias of +0.2 in salinity. To investigate the dynamical processes involved in the interocean exchanges of these AAIW varieties, we use diagnoses based on the Okubo-Weiss parameter and the directional variations of trajectories of particles transported by the model velocity field. Our results suggest that I-AAIW flows into the Cape Basin more within eddies, and particularly within cyclones, than A-AAIW. Once the mixing of both varieties operates, physical and behavioral differences fade and the resulting AAIW flows over the Walvis Ridge in a less turbulent way as part of the Benguela Current, with salinity between 34.55 and 34.6.