Subtropical Mode Water Identification and Vertical Mixing Quantification in the Western Pacific Boundary Current Area using Radiocesium

Abstract

Water originated from subtropical mode water (STMW*) promotes material/energy exchange and affects ecosystems in the western Pacific boundary current area (WPBCA), where it rises by the topography. However, water mixing blurs the ‘mode’ of STMW*, limiting its observations in the WPBCA. In this study, we provide the first direct evidences of STMW* in the WPBCA and quantify the related vertical mixing using 137Cs. The large retention rate of STMW* in the Tsushima Strait implies this part of STMW* experiences little vertical mixing. However, with strong vertical mixing in the Luzon Strait, less than half of STMW* was retained within a short time. Quantifying the contribution of nutrient-rich water in the subtropical gyre to shallow/euphotic layer in the WPBCA and marginal seas, based on observed diffusivity and chemical tracers, could improve ecosystem models, and be further extended to other global boundary current areas.