Lagrangian dispersion in the Red River plume region, Northeast Vietnam, from drifter observations and modeling

The transport and spatial distribution of materials within river plumes and neighboring waters are strongly influenced by the plume morphology and fronts bounding the plume area. Thus, identifying locations of plume fronts and assessing frontal mixing activity is particularly important for understanding the spatial distribution of biogeochemical tracers and the connectivity between source (river mouth) and offshore regions. Statistical properties of horizontal dispersion in the Red River (RR) plume region were obtained from drifter experiments, conducted in the Gulf of Tonkin during summer 2022, and also from realistic numerical modeling. Adopting a Lagrangian perspective, a method is proposed to characterize the RR plume morphology based on the extraction of Lagrangian Coherent Structures (LCS) from the surface flow field using Finite-Size Lyapunov Exponents (FSLE). Maxima of the attracting FSLE field (FSLE ridge lines) enabled us to identify zones of current convergence and shear in the flow, which are essential for characterizing the dynamics associated with plumes and mapping the fronts delimiting them. It became clear that the body of the RR plume is formed by the individual river plumes of the RR deltaic system. FSLE ridge lines allowed identification of the offshore expansion of individual plumes and revealed a variety of forms, semi-circular or hook-like filaments, characterizing the plume shape. The ballistic regime of dispersion was found inside the plume region, with the dominant direction perpendicular to the coastal flow direction revealing anisotropic shear dispersion. It was observed at scales of 0.3 to 3 km (submesoscale range), with a maximum value of 10–13 day⁻¹, as derived from the analysis of the scale dependent relative dispersion of real drifters. In model simulations, the relative dispersion appeared scale independent in the range below 2 km, indicating a significant effect of grid resolution on the model's representation of lateral mixing.