Multiscale Spatio-Temporal Variations of Suspended Sediment Fronts in the Semi-Enclosed Bohai Sea, China

Suspended sediment fronts (SSFs) significantly affect water quality, ecology, and geomorphological evolution of estuarine and coastal waters. Based on 10-year suspended sediment concentration data derived from satellite images, the multiscale spatio-temporal variability of SSFs in the Bohai Sea was studied. Frontal probability (FP) was used to quantify the intensity of frontal activity. The 10-year averaged FP was higher near the coast and decreased with increasing offshore distance. The arc-shaped distributions of FP near the Yellow River Estuary revealed two tidal shear frontal zones. In the eastern Bohai Sea, 13 finger-shaped high-FP areas reflected the spatial pattern and fine structures of tidal sand ridges. The complex topography and strong tidal dynamics provided the essential conditions for frontogenesis. The seasonality of frontal activity was dominantly regulated by the change in wind waves associated with monsoons and stratification. The direct or indirect effects of sediment discharges from the Yellow River and Liaohe River on seasonal and annual variations of frontal activity only existed within close proximity to the estuaries. Frontal activity generally showed an interannual weakening trend, especially in the Laizhou Bay (-0.27%/yr), while an opposite trend was identified in the Liaodong Bay (0.10%/yr). This phenomenon was related to the interannual variations of wind speeds. Meanwhile, wind speed anomalies caused by El Nino-Southern Oscillation events could explain the significant interannual fluctuations in frontal activity such that, frontal activity was significantly enhanced (suppressed) when El Nino (La Nina) events occurred, especially during 2015-2016 (2017-2018).