Intrusion Mechanism And Ecological Effect Of The Kuroshio Subsurface Water To The Changjiang Estuary And Its Adjacent Area

The Kuroshio subsurface water intrusion is an important pathway for the water and material exchanges between the Kuroshio Current and the East China Sea continental shelf.However,the key dynamical mechanism and influencing factors driving the Kuroshio subsurface water to reach as far as the Zhejiang coastal area remain unclear.Moreover,there is no consensus on the relative contributions of the oceanic nutrients and riverine nutrients input from the Changjiang River.A high-resolution three-dimensional hydrodynamic model is established based on ROMS?Regional Ocean Model System?model in the East China Sea.The model results,passive tracer,and observations are used to study the spatial and tempal variations,dynamical mechanisms,and influencing factors of the Kuroshio subsurface water intrusion to the Zhejiang coastal area.The model well reproduces the seasonal variations of temperature and salinity characteristics and the current structure over the East China Sea continental shelf.The observed and simulated temperature and salinity distributions suggest a seasonal variation of the Kuroshio subsurface water intrusion into the coastal region off the Zhejiang Province of China,which is stronger in June and weaker in December.A passive tracer in the subsurface layer is used to investigate the movement of the Kuroshio subsurface water.Tracer experiments confirm the seasonal variation of the Kuroshio subsurface water 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 Kuroshio subsurface water from east of Taiwan to reach the Zhejiang coastal area.Analyses of the momentum balances indicate that the Kuroshio subsurface water'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 bottom frictions contribute to the Kuroshio subsurface water intrusion into the Zhejiang coastal area in June rather than in December.Sensitivity experiments show that the seasonal variation of the Kuroshio subsurface water intrusion is mainly induced by the different wind stresses in summer and winter.It is also indicated that the Kuroshio subsurface water intrusion would be intensified when there is the stronger southerly monsoon,the weaker and warmer Taiwan Warm Current,or the stronger and colder Kuroshio Current;otherwise the intrusion would be weakened.A coupled physical-biogeochemical model ROMS-CoSiNE?Carbon,Silicate and Nitrogen Ecosystem?is established to assess the relative importance of riverine nutrients input from the Changjiang River and the oceanic nutrients to the development and distribution of summer phytoplankton blooms in the Changjiang Estuary and the adjacent sea area.The coupled model successfully reproduces the main hydrographic and biogeochemical features of the region.Both satellite observations and model results show two high-chlorophyll regions in the study domain,located around the Changjiang Estuary north of the Zhoushan Islands and in the Zhejiang coastal area south of the Zhoushan Islands,respectively.Numerical experiments 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 oceanic nutrients.The oceanic nutrients input into the ECS via two pathways separated by the Taiwan Island,the Taiwan Warm Current and the Kuroshio Current.Nutrients inputs,particularly phosphate,from the Kuroshio Current play the major role in the high chlorophyll concentration in the southern region,while the nutrients from Taiwan Warm Current have little influence on the chlorophyll concentrations in the study region.The distributions of the hydrological condition and nutrients in the Kuroshio Current also indicate that the PO₄ from the Kuroshio subsurface water is the most important nutrient to the high chlorophyll in the southern region of the Changjiang estuary and its adjacent area.