Master Internship in Geophysical Fluid Dynamics
Unité de Mécanique de Lille
Ocean Sciences (OS)
LAGRANGIAN DISPERSION IN STRATIFIED UPPER-OCEAN TURBULENCE
Oceanic motions at scales between 10 and 500 km are quasi-horizontal due to the pronounced stratification of seawater and to the Coriolis force and are characterized by quasi-2D turbulence. At scales around 300 km, coherent structures (almost circular vortices) with typical depth of 1000 m contain most of the kinetic energy in the ocean. At scales around 10 km, lie filamentary structures associated with strong gradients of physical properties (such as temperature) which play an important role in both physical and biogeochemical budgets. They are found mainly in the mixed layer (the first 100 m of depth), a weakly stratified layer on top of a more stratified one (the “thermocline”). Two mechanisms leading to the generation of these fine scales have been proposed: (i) the formation of fronts and filaments through the stirring by larger-scale 2D vortices, (ii) more 3D and smaller scale mixed-layer instabilities. A simple quasi-geostrophic two-layer model allows to account for both mechanisms, either separately or simultaneously. Previous results based on this model indicate that mixed-layer instabilities deeply affect the turbulence field, in terms of energy pathways across scales and flow structure in physical space.
The consequence on the spreading process of passive particles advected by the turbulent flow, however, is not known. The issue is relevant for both fundamental studies of turbulent transport and for applications. In particular, the study of tracer particle dispersion in such flows can give hints about the capabilities of Lagrangian reconstruction methods, used to improve the spatial resolution of satellite observations, in different (geographical and/or seasonal) situations.
To investigate this problem we will perform direct numerical simulations with and without mixed layer; the resulting turbulent flows will be used to advect synthetic tracer particles. The focus will be on Lagrangian dispersion, which will be analysed using different statistics at the surface and at depth to explore the impact of mixed-layer instabilities. This study is a contribution to the SWOT satellite mission that will measure ocean surface flows at high resolution, and is funded by CNES.
4 to 6 months internship, preferably starting in February/March 2019
Candidate having good knowledge of fluid mechanics or dynamical systems and an interest for numerical methods; education: Master in Fluid Mechanics, Physics, Applied Mathematics. Good knowledge of oral and written English is required. Knowing Fortran, C or Python would be a plus.
To apply candidates should send their CV and a letter of motivation to firstname.lastname@example.org