Abnormally Increased Summer Bottom SSC: A Case Study in the Northern East China Sea

Abstract

An anomalous bottom high suspended sediment concentration (SSC) in summer, which even higher than bottom values in winter, has been widely observed on several stratified continental shelves. However, the in-depth mechanism remains insufficiently constrained. Combining winter and summer field observations in the northern East China Sea, offshore wave records, and regional and idealized Finite-Volume Community Ocean Model (FVCOM) experiments, this study elucidates how seasonal stratification regulates bottom SSC. The synchrony between bottom SSC and tidal currents, together with the similarity between the summer particle size distribution and nearby muddy seabed sediment, indicates that current induced resuspension is a major source of the elevated summer bottom SSC. Field observations further reveal the enhanced baroclinic tidal currents in summer. Typhoon waves were unlikely to dominate resuspension during the observation period. Regional FVCOM experiments show little seasonal difference under barotropic simulations, whereas the baroclinic simulation produces stronger summer currents with a clearer vertical structure. Idealized experiments indicate that the presence of a thermocline is the main condition for current enhancement and that thermocline depth exerts a stronger control on the velocity profile than its intensity. These results suggest that summer stratification can increase bottom SSC by suppressing turbulent mixing, reducing tidal kinetic energy dissipation, strengthening baroclinic current responses, and enhancing near bottom bed shear stress. The findings reveal that stratification fundamentally affects current dynamics and bottom SSC seasonality, providing a new potential mechanism of material distribution for stratified and muddy continental shelf seas.