Coastal Circulation and Dispersion of Passive Tracers in the Red River Plume Region: Unveiling Seasonal- and Intra-seasonal Variability

Abstract. Surface current velocity measurements by high-frequency radars (HFR) were analyzed to characterize the surface circulation and to investigate the dispersion of passive tracers in the Red River (RR) plume region within the Gulf of Tonkin (GoT) from August to December 2024. The coastal circulation in the region, found to be strongly influenced by winds, tidal forcing, riverine input, and coastal bathymetry, exhibited a large spatio-temporal variability during the analysis period with the occurrence of small-scale structures, i.e., permanent submesoscale eddies. The dispersion under varying forcing conditions and an extreme event – the typhoon Yagi – was analyzed by particle tracking and Lagrangian diagnostics. The results revealed that the dispersion within the RR plume region predominantly followed a Richardson super-diffusive regime after 24 hours of tracking. Under the influence of typhoon Yagi, the dispersion quickly followed a ballistic regime after 12 hours of tracking, with the spreading rate ten times faster than that during normal conditions. In addition, the presence of Lagrangian Coherent Structures (LCSs), i.e., eddies next to the river outflow jets, coastal plume fronts, and zones of surface current convergence and divergence in the vicinity of river outlets, significantly influenced the dispersion behavior of tracers in the RR plume region.