Subtropical mode water intrusion into the South China Sea through the Luzon strait revealed by radioactive dispersion from Fukushima power plant

The Luzon Strait is a critical passage connecting the North Pacific and the South China Sea (SCS). The intrusion of the subtropical mode water (STMW) from the North Pacific into the SCS via the Luzon Strait and its mechanisms has not yet been fully understood. Radiocesium ( ^137 Cs) released into the ocean following the Fukushima Daiichi Nuclear Power Plant accident in March 2011 may be a useful tracer of the long pathway of the STMW from the Kuroshio Extension to the SCS. This study employed a three-dimensional numerical ocean model, called Regional Ocean Modeling System, to analyze the transport of ^137 Cs with the STMW into the SCS via the Luzon Strait, specifically focusing on its path and travel time. The model results showed that ^137 Cs in the Luzon Strait increased via the STMW layer seven years after the accident. ^137 Cs transported through the STMW layer arrived earlier than surface circulation in the Luzon Strait. The net ^137 Cs flux into the SCS exhibits distinct temporal variability according to the Kuroshio intrusion pattern. A larger net intrusion of ^137 Cs was observed during the leaking pattern of the Kuroshio path than during the looping or leaping paths. Considering the role of the SCS as a gateway to downstream marginal seas such as the East China, Yellow, and Japan/East Seas, understanding these transport mechanisms is critical for evaluating the path and travel time of ^137 Cs and other tracers.