Estimating surface water mixing ratios using salinity and potential alkalinity in the Kuroshio‐Oyashio mixed water regions

Abstract

AbstractWe used salinity and potential alkalinity data from hydrographic observations to investigate surface mixing ratios in the Kuroshio‐Oyashio mixed water region in the western North Pacific. In addition to mixing between the Kuroshio Extension (KE) and Oyashio, we assessed freshwater input/removal. A mixing scenario with three end‐members was assumed in the surface layer ≤100 m. The results indicate that water masses near the sea surface in the Kuroshio‐Oyashio mixed water region were mainly the result of mixing between the KE and Oyashio. The freshwater contribution was approximately 2.2% at depth 10 m. The volume of freshwater estimated from this percentage was consistent with surface water budgets estimated from reanalysis precipitation and evaporation data. The estimated mixing ratio of the KE (rk) along the quasi‐stationary jet in the western North Pacific, which splits from the KE and flows northeastward toward the subarctic region, decreased downstream from 95% to 27% in only 42 days, suggesting that water properties were changed rapidly by mixing. Correlation between rk around the quasi‐stationary jet and nutrients concentration was significantly negative in the layer where photosynthesis was negligible, indicating that the mixing between the KE and Oyashio is an important determinant of the horizontal distribution of nutrients in this area.