Effects and mechanisms of tidal forcing on the frontal divergence/convergence of the Changjiang River plume

Ocean frontal regions are pivotal for physical and biogeochemical processes, particularly in estuarine and coastal regions, where their dynamic processes are often associated with high-frequency phytoplankton blooms. In strongly tidal estuarine regions, the horizontal divergence in frontal regions is significantly regulated by tidal forcing. In this study, a high-resolution hydrodynamic model based on the Regional Ocean Modeling System was utilized to investigate the seasonal, spring-neap tides, and high-low water slack characteristics of horizontal divergence in the Changjiang River plume water frontal region. A divergence tendency equation was employed to diagnose the dynamic mechanisms and driving factors.Results indicated that the horizontal divergence in the frontal regions exhibited a periodic characteristic, with positive divergence at high water slack and negative divergence at low water slack. This indicated that horizontal divergence consistently occurred during high water slack, while horizontal convergence was prevalent during low water slack. This characteristic was robust, persisting regardless of seasonal changes or spring-neap tidal cycles. The divergence tendency term (DVT) also followed a periodic pattern: positive during flood tides and negative during ebb tides, indicating increased horizontal divergence during flood tides and reduced divergence during ebb tides. The horizontal deformation term (HDF) and horizontal viscosity term (HVISC) were negligible, while the pressure gradient term (PRG) was consistently important. During spring tides, the driving factors for horizontal divergence showed no seasonal differences. In the northern frontal region, the PRG and vertical viscosity term (VVISC) dominated the DVT, while in the southern region, the divergence change term (DVC), VDF (vertical deformation), PRG, and VVISC jointly dominated the DVT. In contrast, during neap tides, the driving factors were seasonally regulated by changes in frontal position and shape, primarily influencing the VVISC. In the northern region, PRG remained consistently significant. If the VVISC was also significant, other terms became negligible; if the VVISC was insignificant, the VDF, DVC, and occasionally the Coriolis term (COR) collectively played a regulatory role. In the southern region, the VDF, DVC, and PRG were consistently dominant, while the importance of VVISC exhibited seasonal variations. The driving factors differed significantly between spring and neap tides in the northern region but were similar in the southern region, with the only distinction being the seasonal variation in the VVISC.