Quantifying the Water Contribution of Subtropical Mode Water and Related Isopycnal/Diapycnal Water Mixing in the Western Pacific Boundary Current Area Using Radiocesium: A Significant Nutrient Contribution From Subtropical Pacific Gyre to the Marginal Region

Abstract Subtropical mode water (STMW) plays a pivotal role in material exchange and influences marine ecosystems in the shallow layers of the western Pacific boundary current area (WPBCA, including the Kuroshio and its adjacent marginal seas), one of the typical western boundary areas in the global ocean. However, since the Kuroshio with high velocity prevents direct intrusion of STMW into the WPBCA, the STMW could only be transported into the WPBCA from the upstream and/or source Kuroshio region. In the region, water mixing blurs the “mode” of STMW and limits its observation. In this study, we measured 137Cs in the WPBCA, and identified the distribution of STMW‐derived water (STMW*) in a certain layer (σθ ∼ 25.2–25.8 kg m−3). In this layer, two different mixing strengths were quantified during the advection of STMW* from the North Pacific to the WPBCA. Quantification based on 137Cs, rare earth elements, temperature, and salinity indicated a significant decrease in the STMW* signature (<50%) in the layer of Luzon Strait, confirming strong vertical mixing due to topographic constraint. In contrast, in the same layer of Tsushima Strait, where nutrients accounted for 15.3% of the total nutrient flux to the Sea of Japan, 57% ± 20% of the water comes from the STMW* of subtropical gyre, indicating weak mixing during the long‐path transport. Quantifying the contribution of subsurface nutrient‐rich water in the subtropical gyre to shallower layers of WPBCA, using direct measured chemical tracers, could be extended to other areas and further to couple with ecosystem models.