Transport of North Pacific Intermediate Water across Japanese WOCE sections

Abstract

Under the auspices of the World Ocean Circulation Experiment (WOCE) Hydrographic Program, Japan carried out a total of four sections in the North Pacific, P2 at 30°N, P13 at 165°E, P9 at 137°E, and P8 at 130°E. These Japanese and several other WOCE sections are used to estimate the transport of North Pacific Intermediate Water (NPIW) in the subtropical gyre. Previous studies speculate about 3 Sv (1 Sv = 106 m3 s−1) of NPIW transport, originating mainly in Okhotsk Sea, entering the subtropical gyre southward via the mixed water region in the east of Japan. The total subpolar input is 3.2 Sv when another source of NPIW in the Gulf of Alaska, estimated at 0.2 Sv in this study, is added. This transport from the Japanese WOCE sections (a mean of 2.8 ± 0.2 Sv) compares well with several other sections (a mean of 2.5 ± 0.2 Sv). From a total of five meridional and two zonal sections, a mean transport of 2.7 ± 0.2 Sv is found in the defining NPIW layer between σN = 26.5 and 27.4 neutral density surfaces. The transport shows a steady southward and westward increase. About 3 Sv transport is found mainly in the southern (south of 25°N) and western (west of the dateline) sections. Closer to the subarctic‐tropical frontal zone and in the eastern subtropical gyre, NPIW transport is less than 2 Sv. This implies that a lower subpolar input of about 2 Sv is sufficient for subtropical ventilation. Recirculation, entrainment with the upper and lower waters, and epineutral and dianeutral mixing likely add about 1 Sv to the NPIW transport. To interpret the transport distribution, a simple water mass mixing scheme is applied, which includes two formation sources, Okhotsk Intermediate Water (OIW) and Gulf of Alaska Intermediate Water (GAIW) in the subpolar regions, and a transformed aged NPIW (aNPIW) in the northwestern subtropical gyre southeast of Japan. The aNPIW mixing fraction increases southward and westward, while OIW and GAIW fractions decrease. This suggests that after NPIW source water parcels enter the subtropical gyre, they largely recirculate within the wind‐driven gyre and thus become aged. Therefore one usually sees very low oxygen and CFC values in the NPIW layer in the subtropical gyre.