Numerical study on tidal duration asymmetry and shallow-water tides within multiple islands: An example of the Zhoushan Archipelago

Tidal duration asymmetry (TDA) in Zhoushan Archipelago (ZA), East China, was investigated with a three-dimensional hydrodynamic model based on the finite volume community ocean model (FVCOM). The model was evaluated against observed sea surface elevations, currents, and harmonic constants, and the evaluation indicated that the model agreed well with the observations. As TDA is caused by high-frequency harmonics of the main astronomical tides, the generation mechanisms for the main shallow-water tides (M₄, M₆, MS₄, and MS₆) generated by nonlinearity were explored first. Shallow-water tides are primarily generated by the nonlinear advection and interactions in multi-island areas and shallow-waters with the M₂ and S₂ tides at the open boundary, and the S₂ tide suppresses (or promotes) the generation of M₄/M₆ (or MS₄/MS₆) tides. The model results indicated that tides were flood-dominant in ZA, and the strength of asymmetry increased from the deep sea to the shallow-water area and the multi-island area. To explore the mechanisms leading to the observed TDA, seven sensitivity experiments were conducted by modifying the bottom friction term, removing the islands, removing the advection term in the model equations, and changing the water depth in the Luotou Channel. The sensitivity analysis results indicated that TDA was strongly related to the above four factors. Specifically, in the Luotou Channel area and Hangzhou Bay, decreasing the bottom friction term decreased TDA, and removing the islands enhanced TDA. In addition, the advection term also played an important role in determining TDA, especially at the entrance of the Luotou Channel. Furthermore, TDA was strengthened when the water depth decreased.