North Pacific Intermediate Water in the Kuroshio/Oyashio Mixed Water Region

The North Pacific Intermediate Water (NPIW) originates as a vertical salinity minimum in the mixed water region (MWR) between the Kuroshio and Oyashio, just east of Japan. Salinity minima in this region are examined and related to the water man structures, dynamical features, and winter mixed layer density of waters of Oyashio origin. Stations in the MWR are divided into five regimes, of which three represent source waters (from the Kuroshio, Oyashio, and Tsugaru Current) and two are mixed waters formed from these three inputs. Examination of NPIW at stations just east of the MWR indicates that the mixed waters in the MWR are the origin of the newest NPIW. Multiple salinity minima with much finestructure are seen throughout the MWR in spring 1989, with the most fragmented occurring around the large warm core ring centered at 37°N, 144°E, suggesting that this is a dominant site for salinity minimum formation. The density of the NPIW in the MWR is slightly higher than the apparent late winter surface density of the subpolar water. It is hypothesized that the vertical mixing that creates interfacial layers above the salinity minima also increases the density of the minima to the observed NPIW density. Transport of new intermediate water (26.65–27.4 σθ) eastward out of the MWR is about 6 Sv (Sv≡106 m3 s−1), of which roughly 45% is of Oyashio origin and the other 55% of Kuroshio origin. Therefore. the transport of subpolar water into the subtropical gyre in the western North Pacific is estimated to be about 3 Sv.