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Amemou, H., Kone, V., Aman, A., & Lett, C. (2020). Assessment of a Lagrangian model using trajectories of oceanographic drifters and fishing devices in the Tropical Atlantic Ocean. Prog. Oceanogr., 188, 102426.
Résumé: In the Tropical Atlantic Ocean, we assessed the accuracy of a Lagrangian model (Ichthyop) forced with velocity fields from a hydrodynamical model (CROCO) and two different remote sensing products (GlobCurrent and OSCAR) using trajectories of oceanographic drifters. Additionally, we evaluated the possibility to expand the drifters data using trajectories of GPS-buoy equipped drifting Fish Aggregating Devices (FADs). The observed and simulated trajectories were compared in terms of spatial distribution, velocity distribution and a nondimensional skill score. For the drifters and FADs, the GlobCurrent and OSCAR products lead to similar performances as the CROCO model-ouputs in the broad studied domain. In the Gulf of Guinea, however, the CROCO model performed significantly better than the other two because the parent solution of CROCO benefited from its communication with a child grid of finer resolution in this region. On average, the simulations lead to an underestimation of the drifter and FAD velocities, likely because the spatial resolutions of the forcing products were insufficient and the time frequency at which they were produced were too low to resolve the relevant oceanic processes properly. We found a low skill for all models to simulate FAD trajectories, possibly because of the devices vertical structure that prevent FADs from drifting like water parcels. Our results therefore suggest that in the Tropical Atlantic the FAD dataset may not be appropriate to use for corroborating Lagrangian simulations.
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Crochelet, E., Barrier, N., Andrello, M., Marsac, F., Spadone, A., & Lett, C. (2020). Connectivity between seamounts and coastal ecosystems in the Southwestern Indian Ocean. Deep-Sea Res. Part II-Top. Stud. Oceanogr., 176, 104774.
Résumé: Understanding larval connectivity patterns is critical for marine spatial planning, particularly for designing marine protected areas and managing fisheries. Patterns of larval dispersal and connectivity can be inferred from numerical transport models at large spatial and temporal scales. We assess model-based connectivity patterns between seamounts of the Southwestern Indian Ocean (SWIO) and the coastal ecosystems of Mauritius, La Reunion, Madagascar, Mozambique and South Africa, with emphasis on three shallow seamounts (La Pemuse [LP], MAD-Ridge [MR] and Walters Shoal [WS]). Using drifter trajectory and a Lagrangian model of ichthyo-plankton dispersal, we show that larvae can undertake very long dispersion, with larval distances increasing with pelagic larval duration (PLD). There are three groups of greater connectivity: the region between the eastern coast of Madagascar, Mauritius and La Reunion islands; the seamounts of the South West Indian Ridge; and the pair formed by WS and a nearby un-named seamount. Connectivity between these three groups is evident only for the longest PLD examined (360 d). Connectivity from seamounts to coastal ecosystems is weak, with a maximum of 2% of larvae originating from seamounts reaching coastal ecosystems. Local retention at the three focal seamounts (LP, MR and WS) peaks at about 11% for the shortest PLD considered (15 d) at the most retentive seamount (WS) and decreases sharply with increasing PLD. Information on PLD and age of larvae collected at MR and LP are used to assess their putative origin. These larvae are likely self-recruits but it is also plausible that they immigrate from nearby coastal sites, i.e. the southern coast of Madagascar for MR and the islands of La Reunion and Mauritius for LP.
Mots-Clés: behavior; Biophysical model; Connectivity; conservation; coral-reef fishes; Ichthyop; Lagrangian modelling; larvae dispersal; Larval drift; Larval duration; local retention; madagascar; marine populations; model; population connectivity; recruitment; Seamounts; Southwestern indian ocean; Surface drifters
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Hancke, L., Roberts, M. J., & Ternon, J. - F. (2014). Surface drifter trajectories highlight flow pathways in the Mozambique Channel. Deep-Sea Research Part II.Topical Studies in Oceanography, 100(No spécial), 27–37.
Résumé: The pattern of surface circulation in the Mozambique Channel was elucidated from the trajectories of 82 satellite-tracked drifters over the period 2000-2010 and complementary satellite-derived altimetry. Overall, the trajectories indicated that anticyclonic activity was mostly observed on the western side of the Channel, with cyclonic activity being more prevalent in the east. A lack of eddy activity was noted in the southeast corner of the Channel (i.e. SW of Madagascar). Drifter behaviour illustrated that surface water from the Comoros Basin, entrained into anticyclonic eddies during formation, can be retained and isolated for months whilst being transported southwards through the Channel. During a tropical cyclone weather event, a drifter was observed to switch between counter-rotating eddies indicating that horizontal mixing of the Ekman layer does occur. The drifters also illustrated and emphasised the flow field and transport between eddies (i.e. the interstitial flow) in the Mozambique Channel. Despite the dominance of southward propagating anticyclones, drifters were able to move north and south through the Channel in the frontal flow field between eddies within periods of 51-207 days. Cross-channel transport in both directions between the Madagascan and Mozambique shelf regions was similarly observed, with time spans of 19-30 days. Surprisingly, drifters from the southern limb of the East Madagascar Current were transported westward across the channel to the Mozambique shelf. This transport was similarly facilitated by the frontal flow field between eddies. It is hypothesised that the frontal zones between eddies and interstitial waters play an important role in distributing biota in the Mozambique Channel.
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