| 05 Nov 2025
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.
This study analyzes surface current velocity data obtained by high-frequency (HFR) ocean radar in the Red River (RR) plume region within the Gulf of Tonkin, characterizing surface circulation and investigating the dispersion of passive tracers.
It describes the dispersion processes in this sea area in detail for several cases, which is an interesting study.
The HFR data and drifter data generally agree. However, there is some question as to whether the accuracy of the HFR current velocity data remains the same even during periods of large river discharge.
Although the paper makes no mention of it, large river discharge leads to lower salinity and reduced electrical conductivity of the seawater.
Since ocean radar waves propagate along the sea surface, low seawater electrical conductivity causes greater radiowave attenuation, reducing the signal-to-noise ratio (SNR) of the Doppler spectrum data.
I think this results in reduced current velocity accuracy.
The comparison with drifter data was conducted during November and December, when river outflow is small.
Therefore, the accuracy of the HFR velocity data is likely better than that during periods of large river discharge.
This study performs 2D interpolation of current velocity vectors. When the number of measurement data used for the interpolation differs, the accuracy of the interpolated data also varies.
Particularly when river discharge is high, there may be many missing HFR radial velocity measurements. Consequently, the resulting current field is primarily based on 2D interpolation results and is represented as a uniform flow, which may affect the analysis results.
It is necessary to discuss how the number of Doppler spectra sufficient to determine the radial velocity differs between periods of low river discharge and periods of high discharge, and how this difference affects the accuracy of the 2D interpolated current velocity vectors.
Other comments:
(1) Around Line 165:
The explanation was unclear.
Is this simply calculating the distance between the actual buoy and the virtual buoy every 30 minutes for up to 48 hours after buoy deployment?
The explanation here seems different.
t_i is time and N is the number of buoys, but are they adding these in Equation 1?
(2) Figure 6: Where is the location? What about the confidence interval? Wouldn't the rotary spectrum be better?