Seasonal variability of water residence time in the Subei Coastal Water, Yellow Sea: The joint role of tide and wind

Assessing the coastal water export rate is important for better solving the coastal environmental problems and developing environmental management strategies. To investigate the water export rate and its spatiotemporal variation in the Subei coastal water (SCW), a subregion of the Yellow Sea, this study applied water residence time (RT) as an indicator and studied the roles of tides, wind, and river discharge on the water export rate using numerical modeling and the RT adjoint method. RT had an annual and spatial mean of ∼80 days but presented highly spatial and seasonal heterogeneity. Spatially, high RT (∼100–300 days) occurred in the nearshore region, and its location varied in seasons. Temporally, RT was the shortest in the spring (∼55 days), longest in the summer (∼91 days), and median (∼81 days) in autumn and winter. Furthermore, the seasonal RT mismatched the strength of residual transport velocity (RTV) at the current season but instead relied on the combined effects of RTVs in the current and next seasons. Additional sensitivity experiments revealed that tides greatly increased the RT, the Changjiang River runoff slightly decreased the RT, while monsoon wind dominated the RT seasonal variation. As highlighted by an idealized analytical framework, increasing the magnitude ratio of the wind- and tidal-induced RTVs could amplify the RT seasonal variability. Ecologically, the shortest RT in spring might promote the rapid expansion of macroalgal blooms originating from the SCW into offshore waters, thereby contributing to green tide occurrence in the Yellow Sea. Therefore, the seasonality of RT should be considered in coastal management and scientific planning.