Tidal-Driven Water Residence Time in the Bohai and Yellow Seas: The Roles of Different Tidal Constituents

Water residence time (WRT) is a crucial parameter for evaluating the rate of water exchange and it serves as a timescale for elucidating hydrodynamic processes, pollutant dispersion, and biogeochemical cycling in coastal waters. This study investigates the tidal-driven WRT patterns in the Bohai and Yellow Seas (collectively known as BYS) by employing a tidal model in conjunction with an adjoint WRT diagnostic model and explores the influence of tidal constituents on WRT. The findings indicate that the tidal-driven WRT in the BYS is approximately 2.11 years, exhibiting a significant spatially heterogeneous distribution. The WRT pattern shows a strong correlation with the pattern of tidal-driven Lagrangian residual currents (LRCs). Semidiurnal tides have a more pronounced effect on WRT than diurnal tides. Semidiurnal tides significantly reduce WRT across the entire BYS, while diurnal tides predominantly influence WRT in the Bohai Sea (BS). The M2 tidal constituent is the most influential in decreasing WRT and enhancing water exchange, owing to its dominant energy contribution within the tidal system. In contrast, the S2 tidal constituent has a minimal effect; however, its interaction with the M2 tidal constituent plays a significant role in reducing the WRT. The K1 and O1 constituents exert more localized effects on WRT, particularly in the central BS, where their energy ratios relative to M2 are relatively high. Although the amplitude of the S2 constituent exceeds that of K1 and O1, its contribution to LRC—and consequently to WRT—is limited due to the overlapping tidal wave with M2. This research contributes to a deeper understanding of the influence of tidal dynamics on long-term water transport and associated timescales, which are vital for enhancing predictions of material transport and ecosystem dynamics in tidal-dominated environments.