CTD-based assessment of salinity and stratification in a freshwater-fed microtidal bay using nested ROMS–SWAN simulations: Alfacs Bay (Ebro Delta, NW Mediterranean)

Shallow, restricted microtidal bays in deltaic environments pose a stringent test for coastal-scale modelling because hydrographic variability is controlled by freshwater discharges routed through channels, wind-driven exchange and sharp bathymetric gradients. The associated non-linear coupling between stratification, residual circulation and wave–current dynamics in shallow exchange corridors can limit predictability and lead to persistent, spatially structured errors in operational coastal simulations.

We assess hydrographic performance in Alfacs Bay (Ebro Delta, NW Mediterranean) using a high-resolution, nested COAWST configuration with two-way ROMS–SWAN coupling. Atmospheric forcing is provided by Spain’s National Meteorological Agency (AEMET). Model output is evaluated against a 2022 CTD dataset from seven fixed stations using rigorous space–time–depth collocation that preserves the vertical structure of temperature–salinity profiles. Performance is quantified using bias, RMSE and correlation, complemented by stratification diagnostics and regime-based analyses contrasting calm conditions with wind-driven events.

Across N = 2397 CTD–model collocations, temperature is reproduced with high fidelity (bias = +0.30 °C, RMSE = 1.32 °C, R = 0.99), indicating that seasonal-to-event-scale thermal variability is well captured. In contrast, salinity exhibits a systematic positive bias and low correlation (bias = +2.24 psu, RMSE = 2.69 psu, R = 0.30), consistent with an overly marine and weakly variable representation of inner-bay hydrography and degraded stratification dynamics. Guided by these error signatures, we conduct sensitivity experiments that vary freshwater discharge magnitude and its distribution across inflow pathways, and quantify the added impact of wave–current coupling on hydrography and exchange-relevant diagnostics during high-energy wind–wave events.

Overall, salinity/stratification emerges as the main skill-limiting component in this restricted shallow bay, motivating a process-oriented evaluation of freshwater routing, mixing and wave–current feedbacks to prioritise improvements in coupled coastal prediction.

Acknowledgements: This work has been funded by the contract 24263-COP-INNO USER 9000: COPERNICUS MARINE NATIONAL COLLABORATION PROGRAMME: EU COASTAL MONITORING DEMONSTRATORS. Lot no 1: FLORETHA: FLOoding and eRosion at the Ebro delta coasT and Harmful Algal bloom forecasting in its inner semi-enclosed bays.