Éditeur(s) : Yale University
Résumé : Sea-surface temperature images of the coastal upwelling regions off Northwest Africa show that the core of upwelling is sometimes located far from the coast. This has been documented in three regions that share a common feature, namely a wide and shallow continental shelf. This upwelling feature plays a key role in the ecology of the Canary Current System. It creates an innerfront which provides retention for biological material, e.g. fish eggs and larvae, in the highly productive nearshore environment. An analytical model has been developed based on a two dimensional extension of Ekman's solution. The linear and steady response of a homogeneous ocean forced by an upwelling-favorable wind provides a mechanism for the upwelling separation from the coast. The merging of the surface and bottom Ekman layers induces a very weak cross-shore circulation and a "kinematic barrier" for the Ekman transport divergence. In the case of an alongshore wind, the barrier is located near the isobath h ≈ 0.4D, where D is the thickness of Ekman layers. This yields an upwelling cell which is essentially concentrated in the region 0.5D < h < 1.25D, with upwelling occurring preferentially near the isobath h ≈ 0.6D. It turns out that the cross-shore width of upwelling scales with D/S, the ratio of Ekman depth to bottom topographic slope. The application of this solution to real bathymetric profiles rationalizes, not only the offshore upwelling observations in Northwest Africa, but also the influence of topography on the cross-shelf structure of a wind-driven coastal upwelling. The model also quantifies the effect of the cross-shore wind component showing how it drives the nearshore pressure gradient adjustment and how it affects the upwelling. A linear numerical experiment reproduces the theoretical steady solution, thereby allowing investigation of the transient regime. Relaxation of the hypothesis in the numerical model validates the linear assumption of the theory and then allows investigation of the sensitivity to friction parameterizations and the influence of stratification. The latter leads to an "oscillation" of the upwelling cell with seaward migration driven by outcropping and homogeneization of the water column, and, coastal incursion driven by a "boundary layers splitting" process caused by shoreward advection of the isopycnal dome and stratification of the inner shelf.
Journal of Marine Research (Yale University), 2008-09 , Vol. 66 , N. 5 , P. 589-616

Droits : 2008 Yale University
http://archimer.ifremer.fr/doc/2008/publication-6334.pdf
DOI:10.1357/002224008787536790
http://archimer.ifremer.fr/doc/00000/6334/

Éditeur(s) : HAL CCSD
Résumé : International audience
This paper deals with the study of the kinematic dimension of scribbling activities executed by kindergarten children aged from 3 to 6 years old from three grades. For this purpose, three sigma-lognormal features, six classical ones and one hybrid feature related to visuo-motor skills are extracted from scribbles realized using five different Type Grib. The statistical analysis of these data illustrates that sigma-lognormal modeling can satisfactorily reconstruct kindergarten children’s scribbles. Moreover, this preliminary study confirms that there are significant differences between grades with respect to six of the features we studied, regardless of the nature of the scribbling movement made by children. These features are related to rapidity, fluidity and precision of linear and curvilinear movements used for scribbling tasks.
17th Biennial Conference of the International Graphonomics Society
Pointe-à-Pitre, Guadeloupe

hal-01165910
https://hal.univ-antilles.fr/hal-01165910
https://hal.univ-antilles.fr/hal-01165910/document
https://hal.univ-antilles.fr/hal-01165910/file/IGS_2015_submission_28.pdf

Éditeur(s) : HAL CCSD Wiley
Résumé : International audience
In this paper, the generalized Stokes problem αu−νΔu+∇p=f, ∇u=0, is considered, where ν denotes the kinematic viscosity, α is a positive parameter proportional to the inverse of the time-step, and the source term f gathers the external forcing, the nonlinear term, and the explicit part of the linear term. The system is derived from a time discretization for the Navier-Stokes equation governing the motion of a two-dimensional incompressible fluid. A domain decomposition numerical method is developed for solving the system. The original domain is divided into several nonoverlapping subdomains, and in each subdomain the concerned equations are discretized by certain finite element methods. A new feature of the method is that the pressure p is required to be piecewise linear rather than piecewise constant, so that the nonphysical oscillations in the solution using piecewise linear pressure are eliminated. However, a difficulty in using piecewise linear pressure is that the Stokes problem in each subdomain involves Dirichlet boundary conditions on pressure at the interface, which is not a real Stokes problem. To avoid the difficulty, a modification is proposed which removes the continuity conditions on the pressure across the interface. In this way, the Stokes problem in each subdomain has only Dirichlet boundary conditions on the velocity. Formulations and discussions for both the methods are presented, which shows that the modified method is simpler than the original one. Numerical experiments are displayed to show the efficiency of the methods, especially that the modified method has the same accuracy as the original one.
ISSN: 0749-159X

hal-00730200
https://hal.archives-ouvertes.fr/hal-00730200

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Recent global kinematic studies reveal that most of the trenches roll back but a significant number of them advance toward the upper plate. Those advancing trenches are mostly located in the Western Pacific and correspond to the subduction of very old, Mesozoic oceanic lithosphere. While retreating trenches are commonly explained by the slab pull action of the descending lithosphere, the origin of advancing trenches is still debated. Since this relationship is dependent upon the adopted reference frame, we select region where geological studies show the variability of trench migration style with time. The Izu-Bonin-Mariana (IBM) region represents a key example. The detailed reconstruction of the trench migration of the IBM subducting system reveals that after a long episode of asymmetric rollback, the IBM trench recently started advancing. We propose that this change from retreating to advancing trench mode results from the subduction of progressively older and stiffer lithospheric material. We test this hypothesis by means of two-dimensional (2-D) numerical models, reproducing the effects of the lithospheric aging during subduction. The result of our numerical tests shows that the entrance of old and stiff lithosphere forces the trench to advance because the increasing stiffness of the slab prevents the slab to unbend once it has subducted. We adapt this physical result to the IBM evolution, showing that the switch from trench retreat to trench advance can reconcile the shape of IBM slab as previously suggested. Finally we discuss the possibility that the subduction of the Cretaceous lithosphere might have triggered the change of the motion of the Pacific plate around 5 Ma.
ISSN: 0012-821X

hal-00455988
https://hal.archives-ouvertes.fr/hal-00455988
DOI : 10.1016/j.epsl.2009.09.042