Seasonal Sea‐Level Variability Regulates Salt‐Marsh Edge Retreat

Abstract Seasonal variations in salt marsh edge retreat are often attributed to fluctuations in nearshore wave forcing. This study, conducted along the central coast of Jiangsu, China, demonstrates that monsoon‐driven seasonal sea‐level variability, rather than offshore wave conditions, exerts the primary control on retreat rates. UAV and GNSS‐RTK surveys from 2020 to 2022 reveal accelerated marsh edge retreat during summer–autumn, coinciding with elevated sea levels. Idealized wave modeling shows that higher sea levels reduce energy dissipation across nearby tidal flats, facilitating greater wave energy transmission to the marsh edge. Field observations further show seasonal changes in tidal flat elevation, with erosion in summer–autumn and deposition in winter–spring. These morphological changes appear to result from sea‐level‐driven variations in nearshore wave forcing, where enhanced summer wave action erodes the tidal flat, increasing water depth and further reducing dissipation, thereby reinforcing wave energy transmission to the marsh edge. These findings highlight the dominant role of seasonal sea‐level variability in driving lateral marsh retreat, while suggesting that tidal flat morphological adjustments may amplify the seasonal erosion response by reinforcing nearshore hydrodynamic contrasts.