River plumes are important pathways for transporting terrestrial materials to coastal oceans. Traditional plume theory suggests that a bulge may form in the mid-field under certain conditions. Utilizing the Regional Ocean Modeling System, which has been well-validated against cruise observations and satellite data, we investigated the tidal modulation of the bulge in the Pearl River Estuary (PRE). Our results reveal that, under weak wind forcing, tidal forcing modulates the bulge into an indented structure with periodic neap‐spring variation. The bulge in the PRE undergoes a periodic process of growth, detachment, downstream movement, and dissipation, leading to the formation of an indented structure between the newly formed and the older bulge. We further investigated the development and detachment mechanisms of the bulge under weak wind conditions, finding that tidal asymmetry caused by the obstruction and diversion of the ebb flow by the islands outside the estuary mouth enhances the islands' ability to trap freshwater. This tidal modulation of the islands' trapping capability is responsible for the growth and downstream movement of the newly formed bulge in the PRE. Tidal forcing causes periodic spatial heterogeneity in offshore residual currents and periodic changes in turbulent mixing intensity, causing the detachment of the bulge at the estuary mouth. This study enhances our understanding of the periodic neap-spring tidal variation of the bulge and tidal modulation of the mid-field plume in the PRE.
 

tidal modulation, bulge, Pearl River plume