A tidally dependent plume bulge at the Pearl River Estuary mouth

Both observations and numerical simulations have proven the existence of a plume bulge at the trumpet-like Pearl River Estuary (PRE). The formation dynamic of the PRE plume bulge during the southeasterly wind period is investigated with numerical model simulations based on the Regional Ocean Modelling System (ROMS). The results indicate that the formation and evolution of the PRE plume bulge is strongly modulated by the tidal cycle. The bulge is sustained by the strong jet flow along the longitudinal Lantau Channel, which is induced by the ebb current, and transports freshwater offshore. Its size is significantly modulated by spring-neap cycle, being large during spring tide and very small during neap tide. Besides, the existence of abundant freshwater in the estuary is a precondition for the formation of bulge. The ebb jet flow could be strengthened by lateral buoyancy forcing, which is induced by the accumulated freshwater in the estuary, thus guarantees the offshore extension of bulge. The input momentum flux by the river runoff, however, makes no significant contribution. The work indicates a new mechanism of plume bulge formation at a wide estuary and evaluates the impact of different forcings. •A strong ebb jet along the Lantau Channel can maintain the bulge of Pearl River Estuary.•The ebb jet is discontinuous and weak in neap, making the bulge being modulated by tide.•The directly river discharge motivated flow has little contribution to the bulge formation.•The runoff maintains the freshwater accumulation and strengthens the ebb jet.