Finite-Element Tridimensional Modeling of the Circulation in the Mururoa Atoll Lagoon

Blaise, Sébastien [UCL] Deleersnijder, Eric [UCL] Remacle, Jean-François [UCL] White, Laurent [UCL]

Finite-element tridimensional modeling of the circulation in the Mururoa atoll lagoon (French Polynesia). Over the last decade, increasing effort has been directed towards the development of marine models using unstructured meshes. Admittedly, unstructured meshes have much to offer for marine modelling. They allow for an accurate representation of the topography (e.g., islands, narrow straits) and bathymetry. The mesh can easily be refined in regions of interest or coarsened in those regions where the dynamics is less demanding. Finally, unstructured meshes set up in spherical geometry allow to avoid singularities at the poles, rendering those techniques potentially useful for ocean modelling. In this study, we present the application of an unstructured meshes finite-element model to the high-resolution simulation of the flow in the Mururoa atoll lagoon. This is a lagoon of approximately 20km long and 30m depth, separated from the ocean by a shallow connection (5-10m depth). Due to the predominance of wind forcing, an overturning circulation is induced in the lagoon: the surface water is driven by the wind, falls to the bottom when it arrives near the boundaries of the atoll, and comes back in the opposite direction of the wind near the sea bottom. As the circulation is highly variable on the vertical, its simulation requires a tridimensional model. For a better parameterisation of vertical mixing, a sophisticated turbulence closure (Mellor-Yamada level 2.5) is implemented in the model. The talk will first detail the general circulation in the Mururoa atoll lagoon. After that, the model details will be explained and finally results of simulations will be shown, using different diagnosis to represent the flow in the lagoon.