Tidal Modulation of Two Interacting Mid-Field Bulges of the Pearl River Plume

River plumes are important pathways for transporting terrestrial materials to coastal oceans. Utilizing the Regional Ocean Modeling System, which has been well-validated against cruise observations and satellite data, we investigated the tidal modulation of the mid-field bulge in the Pearl River Estuary (PRE). Our findings reveal that the Pearl River plume exhibits two interacting bulges, forming a distinctive indented structure (i.e., isohalines curving toward shore) with periodic spring-neap tidal variation which is related to the tidal modulation of freshwater transport at the PRE mouth. Freshwater transport at the PRE mouth is primarily through two pathways: the West Shoal and the Lantau Channel. The Modaomen Estuary (adjacent to the PRE on its western side) and the West Shoal exhibit sustained downstream freshwater transport, forming a bulge in the west side of the PRE (the western bulge), while the freshwater within the Lantau Channel undergoes periodic anticyclonic motion modulated by tides, leading to the formation of a periodic indented structure. This process is governed by barotropic pressure gradient forces, manifesting as temporal variations in surface height gradients driven by freshwater accumulation. Furthermore, tidal forcing drives the fortnightly cyclic detachment of the bulge from the Lantau Channel (the eastern bulge) through its regulation on surface salinity distribution and vertical mixing intensity. Wind forcing suppresses the formation of this periodic indented structure in the plume by modulating freshwater flux. This study enhances our understanding of the periodic spring-neap tidal variation of the combined bulge and deepens our comprehension of the river plume's mid-field dynamics.