Seasonal Variability of Surface Currents in the Gulf of Tonkin Derived from HF Radar Observations

Three high-frequency radars (HFR), Long range Seasonde stations, are operating in the western Gulf of Tonkin (GoT) since 2012, providing hourly surface currents. In this study, we analyze the HFR data to study the seasonal variations of the surface current in the GoT. The comparison of different data processing and gap-filling schemes (OI, OMA and 2dVar) and in situ velocity measurements was performed. The variational interpolation (2dVar) of velocities revealed performance. The analysis of 2.5-year-long data (April 2014 - October 2016) shows strong seasonal variations of the surface circulation. EOF decomposition reveals a control of the surface circulation by the monsoon wind, which accounts for 62.5% of the total surface currents variability. In addition, near the shore, the geostrophic component was found to provide a larger contribution to the total current velocities (more than 500%). This indicates an important role of the density-driven currents in the dynamics of the GoT.