From basin to sub-basin scale assessment and intercomparison of numerical simulations in the Western Mediterranean Sea

• Quantitative evaluation and intercomparison of simulations in western Mediterranean
• Sub-basin scale assessment approach highlighting local and regional biases
• Realistic model representation of general observed ocean circulation and variability
• Specific simulation errors in the circulation of Balearic and Alboran Seas
• Regional and seasonal simulation errors in water masses properties

This study describes a quantitative evaluation of simulations in the western Mediterranean Sea at basin and sub-basin scales. The Mediterranean Forecasting System and the Mercator-Océan simulations provide operational ocean forecasts and hindcasts in the Mediterranean, and are also used as initial and boundary conditions for regional models. In this context, hindcast simulations from 2009 to 2012 are compared with available multi-platform observations at various spatial and temporal scales to evaluate their performance. Both simulations reproduce well the observed mean conditions and variability over the last years. The sub-basin scale analyses of the three-dimensional ocean structures and water mass properties reveal seasonal and regional temperature and salinity errors at the surface in both simulations, as well as significant biases at intermediate and deep layers in the Mediterranean Forecasting System. The simulated surface geostrophic velocities are weaker than those derived from altimetry, and circulation biases persist in the Balearic Sea. Additionally, the seasonal existence of the Alboran gyres is not well reproduced in either simulation. The identification of regional simulation biases is essential to advance from global to regional and local scale forecasting, in particular, improving the representation of the local physical processes and their interactions with the sub-basin dynamics and the general circulation.