Coastal circulation and dispersion of passive tracers in the Red River plume region: unveiling seasonal- and intra-seasonal variability

Abstract. The Red River (RR) plume region of the Gulf of Tonkin (GoT) plays an important role in driving coastal dynamic variability and regulating sediment and nutrient transport and budgets, and is therefore vital for coastal ecosystems and maritime activities. High-frequency radar (HFR) surface current measurements were used to characterize surface circulation and assess passive-tracer dispersion from August to December 2024, improving understanding of particle transport, dispersal, and fate in this region. 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., 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 approached a Richardson super-diffusive regime after 24 h of tracking. Under the influence of typhoon Yagi, the dispersion was approximated by a ballistic regime after 12 h of tracking, with the spreading rate 10 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. Overall, this study provides new insights into how coastal circulation and material dispersal in the RR plume respond across a wide range of weather conditions, including extreme events.