Distribution and Modification of North Pacific Intermediate Water in the Kuroshio-Oyashio Interfrontal Zone

Abstract

A multiship CTD survey was conducted in the Kuroshio-Oyash interfrontal zone in the area of 30°?41°N, 140°E?180° from May to June 1992 to examine the distribution, modification process, and formation site of North Pacific Intermediate Water (NPIW). Low salinity Oyashio water and high salinity Kuroshio water merge along the Kuroshio Extension just off the east coast of Japan and together flow eastward. Eastward geostrophic transport and salinity across the Kuroshio strong current indicate that the mixture of the Kuroshio and Oyashia waters along the Kuroshio Extension can produce new NPIW. Transport estimate suggests that the new NPIW bifurcates into the interfrontal zone and into the recirculation of the subtropical gyre. A salinity minimum structure can he also created due to the isopycnal mixing along the Kuroshio Extension between the Oyashio and Kuroshio waters, which have different salinity and vertical velocity structures. The low-salinity Oyashio water seen along the Oyashio branches and the Kuroshio Extension has low potential vorticity (Q=??1fδ?/δz), suggesting that NPIW formation is primarily density driven due to sinking in the Okhotsk Sea.