Downscaling from Oceanic Global Circulation Model towards Regional and Coastal Model using spectral nudging techniques: application to the Mediterranean Sea and IBI area models

In order to favour the downstream from the European MyOcean service of large scale ocean forecasts to the national service of coastal ocean forecasts, studies of new methods improving the downscaling between global or regional ocean models and coastal ones are in progress. In the framework of PREVIMER and MyOcean projects respectively, IFREMER and Mercator Ocean studied in parallel the impact of the spectral nudging method. In the present work, the performance of spectral nudging was assessed using two regional hydrodynamic models: one of Mediterranean Sea and one of the Iberia-Biscay-Ireland (IBI) area. They are both forced by a coarser global circulation model (GCM) (PSY2V4/Mercator Ocean). This technique prevents large and unrealistic departures between the global circulation model (GCM) driving fields and the regional model fields at the GCM spatial scales. Regarding the Med Sea application, the model’s temperature T and salinity S are spectrally nudged towards PSY2V4 solution using nudging terms in the model tracer equations: a semi prognostic approach has been chosen. The simulated fields are then compared with those estimated by no-nudged model and confronted to observations. Results show that the spectral nudging is able to constrain error growth in large-scale circulation without significant damping of the meso-scales eddy fields. Regarding the IBI application, T, S as well as current speed U, V have been nudged towards PSY2V4 solution: the increments are directly added as source and sink terms in the model prognostic equations. PSY2V4, nudged IBI and no-nudged IBI models as well as the present operational IBI model weekly restarted by PSY2V4 are compared and confronted to observations. The large scale and meso-scale structures are better represented off the shelf area where IBI is nudged and the bias significantly decreases on the shelf where the high frequency and high resolution physics of free IBI is better resolved than the assimilated PSY2V4 model one.