Observations indicate a seasonal variation of the Kuroshio subsurface water (KSSW) intrusion into the coastal region off the Zhejiang Province of China. The observed temperature and salinity distributions suggest a stronger intrusion in June and weaker intrusion in December. A hydrodynamic model well reproduces the seasonal variation of temperature and salinity characteristics and the current structure over the continental shelf. A passive tracer in the subsurface layer is used to investigate the movement of the KSSW. Tracer experiments confirm the seasonal variation of the KSSW intrusion, showing that the intrusion is stronger in summer and weaker in autumn and winter. Moreover, tracer concentrations suggest that in general it takes at least 1.5 months for the KSSW from east of Taiwan to reach the Zhejiang coastal area. Analyses of the momentum balances indicate that the KSSW's northward movement is regulated by the geostrophic current, while the shoreward intrusion results from the bottom Ekman effect. The seasonal changes in pressure gradients and vertical eddy viscosities contribute to the KSSW intrusion into the Zhejiang coastal area in June rather than in December. Sensitivity experiments show that the seasonal variation of the KSSW intrusion is mainly induced by the different wind stresses in summer and winter. The stronger southerly monsoon, the weaker and warmer Taiwan Warm Current, and the stronger and colder Kuroshio Current are favorable to the KSSW intrusion.

The Changjiang Estuary and adjacent sea area in the East China Sea suffer from frequent harmful algal blooms. However, the relative importance of riverine nutrient input from the Changjiang River and oceanic nutrient input from the Taiwan Warm Current and Kuroshio Current to the development and distribution of summer phytoplankton blooms in this area remains unclear. To address this problem, we deployed a coupled physical‐biological model. The coupled model successfully reproduces the main hydrographic and biogeochemical features in this domain. Both satellite observations and model results show two regions of elevated chlorophyll concentrated in this site. Simulated results show that harmful algal blooms in the region north of the Zhoushan Islands are mainly driven by riverine nutrients from the Changjiang River, while algal blooms in the region south of the Zhoushan Islands are mainly controlled by nutrients from the open ocean. Nutrient input, particularly phosphate, from the Kuroshio subsurface water contributes most to the accumulation of dinoflagellate biomass and chlorophyll in the southern region, while the Taiwan Warm Current has less influence. This has implications for nutrient control and land management practices: Although reducing riverine nutrient loads may significantly reduce phytoplankton growth north of the Zhoushan Islands, it will have little effect in the area to the south.

The Kuroshio Current, a vigorous western boundary current in the Pacific, is responsible for bringing high temperature, high salinity, and phosphate‐rich open ocean water into the East China Sea (ECS) when it flows past Taiwan and generates a branch current, known as the Kuroshio Branch Current (KBC), which intrudes onto the continental shelf of the ECS. Based on observations and numerical modeling, a 1.5‐layer reduced‐gravity model is evaluated and modified to simulate the flow pattern of the Kuroshio intrusion (KI) northeast of Taiwan. We revised the analytical solution derived by Hsueh (1992), https://doi.org/10.1029/92JC01401 to conserve the horizontal momentum balance in the along‐shelf and cross‐shelf directions. The new analytical solution not only explains the KI angle but also addresses the KBC recirculation mechanism. To sustain the steady‐state momentum balance in the area adjacent to the shelf break northeast of Taiwan, there must be a recirculation that plays the same role in vorticity dynamics as the combination of the joint effect of baroclinicity and relief, bottom friction, and nonlinear advection. Unlike previous studies, our results agree well with the observations and could account for seasonal variations in the KI direction. Moreover, the new model predicts a reasonable velocity maximum (0.42 m s⁻¹) over the shelf break. Satellite data and numerical results confirm that the new solution reflects a reasonable estimation for KI direction. This theory may potentially be applied to regions with trenches or valleys with completely different flow patterns by using similar analytical methods.

Dinoflagellates frequently cause harmful, large‐scale spring algal blooms in the Changjiang Estuary and adjacent seas of China. However, the population source of some dinoflagellates in this area remains unknown. We deployed an adjoint model and a coupled physical‐biological model to explore the possible nonlocal dinoflagellate population sources to the Changjiang Estuary and adjacent seas. Our simulation results revealed that the Taiwan Strait and the area east of Taiwan are two nonlocal source regions of May dinoflagellate blooms in the study area. Under different hydrodynamic and biochemical conditions, dinoflagellates from the Taiwan Strait directly triggered dinoflagellate blooms from south to north along the coast of China. Dinoflagellates from the area east of Taiwan first concentrated and bred in the offshore temperature frontal area and then spread southwestward, triggering dinoflagellate blooms along the Zhejiang coast. Due to the different influence mechanisms, it took longer for the initial dinoflagellate population in the Taiwan Strait to exert the same impact on dinoflagellate blooms in the study area as that from the area east of Taiwan. Moreover, the density and appearance time of the initial dinoflagellate population in the Taiwan Strait affected their biomass and bloom range in the study area. Even a small initial dinoflagellate population east of Taiwan could trigger a bloom in the study area. This study suggests the target domains for finding nonlocal dinoflagellate cysts and provides a strategy to predict dinoflagellate blooms in the Changjiang Estuary adjacent area by monitoring dinoflagellate biomass in the nonlocal source regions.