Observations of the Near-Field Yellow River Plume, Multiple Fronts and Their Biogeochemical Effects

Abstract

River plumes transport fresh water into the ocean, and exhibit intense convergent and turbulent processes in the frontal regions, ultimately influencing coastal ecosystems. Satellite images show multiple fronts can be generated in river plumes. Although some efforts have been done to study the generation of multiple fronts inside river plumes by theoretical, numerical modeling and laboratory methods, their generation mechanisms and effects on biogeochemical processes remain to be clarified through in situ observational evidence. In the present study, the near-field Yellow River plume, multiple fronts and their biogeochemical effects are investigated through moored, shipboard, and remote sensing observations. Numerical model simulations are conducted to help study the tidal movement of the Yellow River plume. The results show that the near-field Yellow River plume is generally advected back and forth in the east‒west direction under the effects of tidal currents. Following the plume leading front, a turbulent bore is observed to be generated and mix near-surface water with low turbidity and high concentration of dissolved oxygen into the water column. Behind the turbulent bore, a series of subfronts with distances of O (∼100 m) are formed. The generation mechanisms of these subfronts are found to be associated with internal shear instabilities. Under the effects of cross-front shear currents and vertical mixing near the subfronts, a three-layer structure of turbidity and surface low-turbidity stripes form inside the plume. Multiple fronts also have significant effects on the accumulation of surface materials.