Hydrology and circulation in a coastal area off Marseille: Validation of a nested 3D model with observations

A high resolution 3D coastal ocean model has been applied to the forecast of the oceanic circulation off Marseille (from the Rhone River to the Cap Sicié). The realistic numerical simulations were forced by the Rhone River, meteorological fields and by the surrounding general circulation. A lot of observations providing the stratification and currents in the area were acquired at moorings and from vessel cruises during the last decade. In order to validate the nested forecasting system, quantitative comparisons between observations and model results were performed for the years 2007 and 2008. Statistics over long time series of temperature at moorings were computed to show the model skills in capturing the monthly to seasonal variability of the thermal structure. The comparison also demonstrated the model's ability to reproduce the observed features over the shelf, like the warming or cooling of the sea due to upwelling and downwelling events. The model also reproduced an intrusion of the Rhone River plume observed into the southern bay of Marseille during a flood event, inducing a salinity decrease at the Frioul station under westerlies and south-easterlies. Hence, although the wind constrains the coastal circulation, the general circulation is also of great influence, with intrusions of the Northern Current from the south of the domain observed and reproduced by the model in winter, due to the penetration of meanders over the narrow eastern part of the shelf. This process was better reproduced by the nested high resolution model than by the North Western Mediterranean Sea model. The nested model was also found to be more suitable to reproduce the plumes of diluted pollutants emanating from coastal discharges.

Research Highlights
► We reproduce circulation and hydrology of the bay of Marseille with a 3D ocean model.
► We use a nested model strategy to better reproduce the circulation over the shelf.
► We use statitistics and Taylor and target diagrams for the model validation.
► The model reproduces upwellings, intrusions of Rhone waters and Northern Current.