Sensitivity of phytoplankton bloom magnitude off the Yangtze River estuary to the Northwest Pacific subtropical high ridge position

In recent years, phytoplankton bloom magnitude (PBM) off the Yangtze River Estuary has gradually increased, disrupting the marine ecological balance. To address the operational need for seasonal forecasting of phytoplankton blooms, we combined reanalysis data and a high-resolution physical–biogeochemical model to investigate the relationship between the Northwestern Pacific Subtropical High (NWPSH) and PBM. Results indicate that when the June NWPSH ridge line is located at or north of 20°N, the June–August PBM is lower than when it is located south of 20°N; the mechanisms driving the PBM reduction differ among the three months. This reduction is mainly attributed to the decrease in photosynthetically active radiation (PAR) caused by persistent precipitation in June, and the declines in sea surface temperature (SST), buoyancy frequency (N 2 ), and nitrate (NO 3 ) concentration resulting from high-salinity, cold, and nutrient-poor seawater brought by anomalous westward currents. These factors together explain 92% of the variation in PBM during anomalous NWPSH years. In July, although reduced precipitation leads to anomalously high PAR, the anomalously strong southward currents carrying high-salinity, cold, and low-nutrient water further reduce SST, N 2 , and NO 3 concentration. These factors together explain 90% of the variation in PBM during anomalous NWPSH years. By August, despite the rebound in NO 3 due to persistent precipitation, PAR, SST, and N 2 remain anomalously low due to anomalous southward currents. Collectively, these factors explain 74% of the variation in PBM during anomalous NWPSH years. Through sensitivity experiments, when the June NWPSH ridge line is located south of 20°N, each 0.2° southward shift in latitude raises June–August PBM by approximately 4.5 mg/m 3 . Although this conclusion is not derived from an established observational relationship or operational forecasting rule, the results of idealized model experiments, based on the response characteristics of June–August PBM to the June NWPSH, can still provide a scientific support for the early warning of PBM.