The Kuroshio intrusion from the Luzon Strait significantly affects the South China Sea (SCS) ecosystems, especially during the Northeast Monsoon, a time when field observations are notably scarce and where vertical mixing induced by strong winds can obscure the effects of the Kuroshio intrusion. In this study, we address these gaps by reanalyzing data from 20 cruises (5338 samples) in the SCS between 2004 and 2015. Additionally, we carried out two dedicated field cruises during the Northeast and the Southwest Monsoon in 2018, respectively. Our field observations from both cruises revealed a consistent unimodal relationship between total chlorophyll a (Chla) concentrations in the upper 50 m of the water column and the Kuroshio intrusion index. Specifically, we observed that a stronger Kuroshio intrusion during the Northeast Monsoon significantly enhanced Chla concentrations. This enhanced Chla concentration during the Northeast Monsoon was primarily driven by increases in Synechococcus and nanophytoplankton, contrasting with the dominance of Prochlorococcus during Southeast monsoon. Furthermore, our analysis of long-term remote sensing data corroborated these findings, demonstrating a consistent pattern wherein Kuroshio intrusion leads to elevated Chla concentrations, particularly during the Northeast Monsoon. Importantly, we identified a significant positive correlation between the intensity of Kuroshio intrusion and the magnitude of Chla increase. However, our findings also suggest a concerning trend: weakening Kuroshio intrusion intensity over time, potentially diminishing future biogeochemical effects on SCS ecosystems. Continued monitoring and research are crucial to understand and mitigate these impacts.
 

Kuroshio intrusion,South China Sea,Chlorophyll a