Surface circulation characterization along the middle-south coastal region of Vietnam from high-frequency radar and numerical modelling

Abstract. Coastal water dynamics along the Vietnam Middle-South Coastal region (VMSC), part of the South China Sea, is highly complex with large spatio-temporal variability whose drivers are not yet well understood. For the first time, high-resolution surface current data from high-frequency radar (HFR) measurements were obtained in this region during the early (transition) phase of the Asian summer monsoon. The data were used to compare with simulation results from a circulation model, SYMPHONIE, and ultimately to optimise the wind forcing in the model. Both modelling and HFR were able to show the spatial and temporal evolution of the surface circulation, but some discrepancies were found between model and HFR data on some days, coinciding with the evolution of the wind. Two methods were used to optimise the wind forcing, namely the Ensemble Perturbation Smoother (EnPS) and the wind correction method using wind-driven surface currents (EkW). Both methods achieved a significant reduction (~36–40 %) in the error of the surface current velocity fields compared to the measured data. Optimised winds obtained from the two methods were compared with satellite wind data for validation. The results show that both optimisation methods performed better in the far field, where topography no longer affects the coastal surface circulation. The optimisation results revealed that the surface circulation is not only driven by winds but also by other factors such as intrinsic ocean variability which is not entirely controlled by boundary conditions. This indicates the potential usefulness of large velocity datasets and other data fusion methods to effectively improve modelling results.