The Xiamen Symposium on Marine Environmental Sciences (XMAS 2027)

The Zhe-Min coastal region of China, a principal fisheries area, frequently experiences harmful algal blooms (HABs) driven by the nutrient-rich southward current of the Zhe-Min Coastal Current (ZMCC). In spring 2025, however, routine monitoring (since 2001) recorded an unprecedented absence of large-scale HABs. This study investigates the dynamical mechanisms underpinning this suppression. Our analysis indicates that a persistent, record-breaking anomalous northeastward sea surface current inhibited southward nutrient transport to the Fuzhou–Wenzhou coast. Dynamical decomposition shows that this anomalous current resulted from distinct regional processes: off Fuzhou it manifested as a barotropic response to an anomalous cross-shelf sea-level gradient associated with an extremely weak northeasterly monsoon, while off Wenzhou it was predominantly baroclinic, driven by cross-shelf temperature gradients linked to frequent marine heatwaves. The observed thermal extremes are largely attributable to the record-breaking northward shift of the subtropical high-pressure system during the spring of 2025. Moreover, the interaction between a coastal marine cold spell and offshore heatwaves in March promoted cross-shelf transport that exported coastal nutrients offshore. These findings indicate that climate–related shifts—notably weakened monsoonal forcing and increased frequency of extreme thermal events—can act synergistically to form a compound hazard chain that substantially alters coastal physical–biological coupling, however, in this instance, suppressed large-scale HABs outbreak. Future research should pay more attention to the impacts generated by the coupling processes among such extreme events.