Saltwater intrusion in the Yangtze Estuary poses an increasing threat to freshwater security under climate change and basin-scale reservoir regulation. Although cold surges are well-known major drivers of saltwater intrusion, typhoons with more complex wind directions can also exert substantial impacts, necessitating a systematic investigation of wind-direction effects. In this study, a validated three-dimensional estuarine and coastal ocean model (UFDECOM-i) was applied to examine hydrodynamics and saltwater intrusion under steady winds from eight directions at 5, 10, 15 m/s, imposed for 5 days under neap-tide and dry-season runoff (10,000 m³/s) conditions. Results identify four distinct response patterns: (1) Northerly winds induce a horizontal circulation characterized by inflow through the North Channel and outflow through the South Channel, intensifying both frontal intrusion in the North Channel and saltwater spillover from the North Branch into the South Branch (SSO). (2) Southerly winds trigger the opposite circulation, enhancing frontal intrusion in the South Channel while weakening SSO. (3) Westerly winds exert a weaker effect on the horizontal circulation but generate nonlinear responses to frontal intrusion in the North Channel, which is intensified at moderate wind speeds and suppressed under stronger winds. (4) Easterly winds suppress frontal intrusion while significantly exacerbating SSO. Dynamically, these distinct responses stem from the complex interactions and differential contributions of remote wind-driven large-scale shelf processes and local wind-driven estuarine dynamics. Northerly winds dominate the saltwater intrusion via large-scale shelf processes, including landward Ekman transport and coastal Kelvin waves. The saltwater intrusion under southerly winds relies on both large-scale oceanic forcing and local dynamic adjustment. In contrast, the responses to zonal winds are governed primarily by local dynamic processes: the saltwater intrusion under westerly winds depends on the competition between wind-induced strain and mixing, whereas easterly winds weaken exchange flow through intense vertical mixing. A key finding is that meridional winds exert a greater impact on saltwater intrusion than zonal winds due to their tighter linkage with large-scale shelf processes, advancing the frontal intrusion of the 0.45 psu surface isohaline landward by up to ~50 km, compared with a maximum increase of ~20 km under westerly winds. This study systematically clarifies wind-direction effects on saltwater intrusion in the Yangtze Estuary, highlighting the critical role of wind direction in estuarine-shelf coupling for large estuaries in the Pacific-Asian marginal seas. These finddings providing scientific support for freshwater resources management and coastal disaster prevention in Pacific-Asian marginal sea regions.

Yangtze Estuary,Saltwater intrusion,Wind direction,Estuarine-shelf coupling,Water and salt transport