Assessment of a NEMO-based downscaling experiment for the North-Western Mediterranean region: Impacts on the Northern Current and comparison with ADCP data and altimetry products

The aim of this paper is to assess the impact of numerical resolution on the simulation of the circulation in the North-Western Mediterranean Sea by comparing two realistic model configurations of 1/64° and 1/16°, the low resolution configuration (LRC) being also used at the high resolution configuration (HRC) open boundaries. The study mainly focuses on the Northern Current (NC), the major feature of the regional circulation in the area, influencing important processes such as meanders giving birth to eddies and NC intrusions on the shelf, strongly conditioning the interactions between coastal and off-shore waters. Both configurations use the NEMO code with the LRC covering the entire Mediterranean basin and the HRC being restricted to the North-Western Mediterranean Sea. The two numerical configurations are assessed by using hull-mounted ADCP data and geostrophic velocities derived from AVISO altimetry at key locations along the NC path. The two configurations develop a different solution regarding the current dynamics but both indicate an important variability of the NC position along its general path. The LRC simulations systematically exhibit a NC shifted too far off the coast and also spuriously displaced seaward along the Gulf of Lions shelf edge while high resolution simulations better fit the remote and in situ observations. In addition, the HRC is able to simulate more realistic coastal features such as a confined coastal jet in good agreement with the ADCP measurements. As satisfactory agreement is reached with the various observations used at different space and time scales, the development of this NEMO high resolution configuration stands as a promising experiment for various process studies or operational oriented applications.