Transient bidirectional flow in Yangtze River Estuary: Driving mechanisms and spatiotemporal variations

Study region The Yangtze River Estuary, China, located at the confluence of the Yangtze River and the East China Sea. Study focus This study combined observations and numerical modeling to investigate the occurrence, driving mechanisms, impacts, and spatiotemporal characteristics of transient bidirectional flow (TBF) events in the Yangtze River Estuary. New hydrological insights for the region In the Yangtze River Estuary, we identified TBF events characterized by instantaneous surface-seaward and bottom-landward currents that persisted over multiple tidal cycles. This differs from classical estuarine exchange flow, whose instantaneous flow field is usually vertically unidirectional under tidal dominance. Although similar instantaneous bidirectional flow structures have been observed previously in estuaries, their impacts, governing mechanisms, and spatiotemporal characteristics remain insufficiently understood. Our results show that TBF significantly influenced water column stratification and enhanced the net transport of water and salt. Momentum analysis and sensitivity experiments revealed its driving mechanisms: during ebb tides, strong northerly winds enhanced saltwater intrusion, generating a persistent landward baroclinic pressure gradient that reversed the bottom ebb current landward; during flood tides, the combined effect of river discharge and the salt wedge raised the upstream water level, producing a strong seaward barotropic pressure gradient that reversed the surface flood current seaward.