Northwest Pacific subtropical countercurrent on isopycnal surface in summer

Abstract

A three-dimensional absolute velocity field on the isopycnal surface in the northwest Pacific subtropical region in the summer is calculated from the Navy's Generalized Digital Environmental Model (GDEM) climatological temperature and salinity data on a 0.5° × 0.5° grid using the P-vector method. GDEM for the area was built from historical (1930-1997) temperature and salinity profiles. The basic current system, consisting of North Equatorial Current, Kuroshio and its recirculation, Subtropical Countercurrent (STCC), is identified. STCC occurs from the surface to the level of σθ = 25.8 between 18°N and 23.5°N, with a maximum velocity around 0.1 m/s. At σθ = 23.5, STCC originates east of Luzon Strait at around 122.5°E and meanders eastward. As σθ increases, its origin shifts toward northeast. Comparison between velocity and potential vorticity fields leads to the conclusion that the STCC core is above the southern boundary of the subtropical mode water.