Intermittent intrusions of Changjiang Diluted water in Hangzhou Bay modulated by intraseasonal winds and tropical cyclones

It is generally recognised that the north-easterly monsoon leads to greater intrusion of Changjiang Diluted Water (CDW) into Hangzhou Bay in winter than in summer, which strongly influences the hydrography of Hangzhou Bay. However, anomalously lower salinity (<10) and higher dissolved inorganic nitrogen (DIN) concentrations (1.9 mg L⁻¹) were observed in central Hangzhou Bay in summer (August 2019) than the salinity (>15) and DIN concentrations (1.5 mg L⁻¹) in winter (March 2022). A high-resolution, well-validated model has further revealed intermittent intrusions of CDW, characterised as intraseasonal (30–120 d) and episodic variations, resulting in a unique low-salinity and high-nutrient water mass in the north-central bay during summer. The intermittent intrusions are driven by the combined effects of the Changjiang discharge, intraseasonal winds, tropical cyclones, and tidal residual currents. The Changjiang discharge amplifies the effect of CDW intrusion on a seasonal scale, which is responsible for the large (>10) decline in summer salinity in the initial intrusion area of the northern bay mouth. The intraseasonal variation in the south-westerly monsoon in summer causes intermittent intrusion of the CDW, accompanied by variations in the Taiwan Warm Current. Intrusion of CDW is greatly enhanced during episodic events of strong northerly winds caused by tropical cyclones. This episodic intrusion can cause a decline in salinity in the initial intrusion area of up to 6 in approximately one week. After the CDW intrudes into the north central bay, the westward tidal residual current along the northern coast continuously transports the CDW toward the head of Hangzhou Bay.