The red Noctiluca scintillans (RNS) blooms break out near Pingtan Island in the northern Taiwan Strait from April to June regularly. It is essential to gain insight into their formation mechanism to accurately predict and provide early warnings for these blooms. Previous studies and observations showed that RNS blooms are the most likely to occur during shifting and weak winds. To explore this phenomenon further, we employed a high-resolution coastal model to investigate the hydrodynamics influencing RNS blooms around Pingtan Island from April to June 2022. The model results revealed that seawater exhibited weak circulation but strong stratification during RNS blooms. Residence time during both bloom and non-bloom periods were examined through numerical experiments by releasing passive neutrally-buoyant particles in three bays of Pingtan Island. The results showed a significantly longer fast-flushing phase during RNS blooms, indicating reduced flushing and self-purification within the bays. This hydrodynamic environment provided a favorable basis for RNS blooms breakout near Pingtan Island. The shifts and weakening of the northeast wind contributed substantially to weakening the flow field around Pingtan Island, and played a crucial role in creating the hydrodynamics conducive to RNS blooms. Our study offers fresh insights into the mechanisms underpinning RNS blooms formation near Pingtan Island, providing a vital framework for forecasting RNS blooms in this region.
 

Noctiluca scintillans, numerical modeling, hydrodynamic conditions, residence time, stratification