The penetration of the Northern Current over the Gulf of Lions (Mediterranean) as a downscaling problem

Coastal shelf models suffer from the difficulty of specifying both the initial field and the external circulation at the open boundary. Only basin scale models can have the right space-time variability to routinely provide such information. Do facto, such models have however a coarser resolution than the coastal model and the downscaling of the circulation to the high resolution grid is biased. Indeed the models are not necessarily based on the same physics and the interactions of the general circulation with the topography, in particular over the shelf break region, arc generally not correctly represented on the large-scale grid. Studying the response of the Northern Current to the atmospheric forcing in the region of Marseille (France) and its interactions with the continental shelf break, we evaluate in this paper the spurious consequences at short and medium range that can result from a crude interpolation of basin scale model outputs to initialize and force along its open boundaries a high resolution coastal model. We show how an analysis of these fields based on a 3D variational initialization technique can improve the results. (C) 2001 Ifremer/CNRS/IRD/Editions scientifiques et medicales Elsevier SAS.