The Role of Subtropical Mode Water in the Subsurface Transport of Fukushima-derived 137Cs into the South China Sea

The Luzon Strait serves as a critical conduit between the Western North Pacific and the South China Sea (SCS), through which water-mass exchange plays a key role in regulating regional heat budgets and primary productivity. While surface exchange processes have been known well, subsurface intrusion dynamics—particularly those associated with Subtropical Mode Water (STMW)—remain poorly understood. In this study, we investigate the pathways and transport timescales of STMW intrusion through the Luzon Strait by employing radiocesium ¹³⁷Cs released during the 2011 Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident as a transient tracer. A three-dimensional Regional Ocean Modeling System (ROMS) was used to simulate the long-term dispersion of ¹³⁷Cs from the North Pacific into the SCS. The results show that the ¹³⁷Cs within the STMW layer reached the Luzon Strait approximately seven years after the accident, notably earlier than surface circulation. The net flux of ¹³⁷Cs into the SCS exhibits seasonal variability, with enhanced inflow during winter, primarily driven by horizontal advection and variations in Kuroshio intrusion behavior. A comparison of different intrusion modes indicates that the leaking path yields a substantially larger net inflow of radiocesium into the SCS than either the looping or leaping paths. Given that the SCS serves as a gateway to downstream marginal seas—including the East China Sea, Yellow Sea, and Japan/East Sea—these findings provide important insights into basin-scale transport processes of Pacific-derived tracers and their potential ecological implications.