Dispersal and water age of the plumes from the four outlets of the Pearl River during the dry winter.

This study quantitatively investigates the impacts of hydrodynamic factors on the transport pathways and water age (WA) of river plumes from four outlets in the Pearl River Estuary during the dry winter season using the well-validated Coupled Ocean-Atmosphere-Wave-Sediment Transport (COAWST) model. With reduced discharge and strong downwelling-favorable winds, winter plumes are confined within the estuary with intense lateral overlap, favoring westward spreading and higher WA. River discharge dominates WA variability, with surface WA more sensitive than bottom WA. Strong northerly winds enhance seaward advection, while wave-induced Stokes drift reduces WA by ~0.5 days over the West Shoal but increases WA by >2 days in the southeastern estuary. Easterly winds exert weaker effects. Tides reduce bottom WA east of the West Shoal via enhanced vertical turbulent mixing and modify residual currents, which in turn increases surface WA in the Humen plume, especially during spring tides. Plume-plume interactions also regulate WA: the Humen plume reduces WA of lateral plumes by 1-7 days, and the Hengmen plume impedes mainstream plumes' westward intrusion and causes 1-2 days of local changes. As a fundamental proxy for the transport of all terrestrial inputs, the quantified WA distribution highlights the complex interactions of discharge, winds, waves, tides, and plume-plume interactions in modulating estuarine dynamics, with direct implications for pollutant transport, nutrient accumulation, and harmful algal bloom risk assessment in similar subtropical estuarine systems.