Water masses and properties at 165°E in the western Pacific

Abstract

Full‐depth conductivity‐temperature‐depth‐oxygen profiler (CTDO2) data at 165°E in 5°S–40°N in summer 1991 (World Ocean Circulation Experiment (WOCE) Hydrographic Program P13C) and 27°–48°N in spring 1993 (P13J) are analyzed. Distributions of salinity and temperature are characterized by the fronts associated with ocean currents, such as the Kuroshio Extension front at 34°–35°N, the Kuroshio Bifurcation front at 38°–40°N, the subarctic front at 40°–42°N, other salinity fronts in the tropical waters, and some oxygen fronts. Dissolved oxygen increases upward sharply, called oxycline, just above the intermediate oxygen minimum layer. The processes of water‐mass extension are clarified mainly from oxygen and potential vorticity (PV). The North Pacific Tropical Water (NPTW) forms at the sea surface in 18°–31°N from winter to summer, and the winter‐formed NPTW of 23.0–25.0σθ (where σθ is potential density) extends to 12°N by the subtropical ocean gyre, so that it occupies a layer of 23.0–25.0σθ in 12°–31°N. The Subtropical Mode Water (STMW) forms at the surface in 29°–34°N in winter and extends to 25°N by the ocean circulation; it occupies a layer around 25.4σθ between the main and seasonal pycnoclines in 25°–34°N. The North Pacific Intermediate Water (NPIW) extends to 34°–38°N on 26.83σθ by the Kuroshio Extension and its northern branch and extends to 27°–33°N on 26.77σθ and to 15°–22°N on 26.61σθ by westward currents; it occupies 15°–38°N in the intermediate subtropical gyre. The NPIW is located in the oxycline with similar PV to the STMW. The boundary of the subtropical and tropical gyres is concluded to be around 12°N (14°–16°N) in a surface (intermediate) layer <25.0σθ (>26.0σθ). Bottom waters are also examined.