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Mardi 7 Octobre - On the retention, dispersion and transport of tracers by ocean currents: a Lagrangian perspective

by SEMSOU last modified Sep 30, 2014 12:46 PM
When Oct 07, 2014
from 11:00 AM to 12:00 PM
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Vincent Rossi, Postdoc at CSIC, Universitat de les Illes Baleares


 On the retention, dispersion and transport of tracers by ocean currents: a Lagrangian perspective

Although the Eulerian and Lagrangian descriptions of fluid motion are theoretically equivalent, the
latter is well-suited to investigate the retention, dispersion and transport of tracers by the ocean
circulation. First a Lagrangian methodology is developed to optimally identify the 3D shape of an
Agulhas Ring from a simulated velocity field of the South Atlantic ocean. It allows us to track the
spatio-temporal evolution of an eddy and to measure the proportion of water remaining in its core,
suggesting it is more retentive than its Eulerian equivalent. The second example focuses on larval
dispersal and oceanic connectivity, two crucial factors for structuring marine populations and designing
Marine Protected Areas (MPAs). Using an approach coupling Lagrangian trajectories and new tools
from Network Theory, we study dispersal by ocean currents in the Mediterranean basin. Passive
larvae of different pelagic durations and seasons are advected in a simulated surface flow from which a
network of connected areas is constructed. Hydrodynamical provinces extracted from this network are
delimited by frontiers which match multiscale oceanographic features. By examining the repeated
occurrence of such boundaries, we identify the spatial scales and geographic structures that control
larval dispersal across the entire seascape. We also analyze novel connectivity metrics for the existing
marine reserves and we discussed our results in the context of ocean biogeography and MPAs design.
In the last example, Lagrangian trajectories are employed to estimate the future long-term pathways of
the radioactive water accidentally released in the North Pacific ocean following the 2011 Fukushima
disaster. With a half-life of 30.1 years, Cesium-137 has the potential to travel large distances within the
ocean. Based on an ensemble of eddy-resolving simulations, our results suggest that the contaminated
plume would have been rapidly diluted by June 2011 due to the energetic Kuroshio Current/Extension.
The differential intrusion of the plume over the northwestern American shelves in 2014-2017 can be
explained by both local and basin-scale processes. Despite some uncertainties in the source function,
we found that Fukushima-derived Cs-137 will penetrate the interior ocean and spread to other oceanic
basins over the next three decades.

Collaborators: E. Ser-Giacomi, C. Lopez, E. Hernandez-Garcia, G. Froyland, C. Horenkamp, A. Sen Gupta, E. Van Sebille, N. Santitissadeekorn, M. England, V. Garcon.

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