Influence of Kuroshio Path Variation South of Japan on Formation of Subtropical Mode Water

Abstract Distributions of subtropical mode water (STMW) in the northwestern part of the North Pacific Subtropical Gyre were investigated, using temperature–salinity profiles from 2005 to 2011, with particular reference to the Kuroshio meander and non-meander path states, south of Japan. In spring of meander years, warm STMW with a potential temperature of 19°–20°C (potential density anomaly of 24.6–24.9 kg m−3) was found in the Shikoku Basin, whereas cold STMW below 19°C was distributed throughout the southern region of Japan in non-meander years. The warm STMW was formed in a spatially isolated and warm winter mixed layer (ML) in the Shikoku Basin, where a local recirculation developed in association with the Kuroshio meander path; both the absence of horizontal mixing with a cold ML south of the Kuroshio Extension because of the spatially isolated ML and an increase in horizontal heat advection due to the westward flow associated with this local recirculation caused the ML warming in the Shikoku Basin. After the spring shoaling of the ML, the warm STMW was preserved under the seasonal pycnocline until midsummer at a depth of 100–250 dbar; its thickness was approximately half that of the cold STMW in the Shikoku Basin in non-meander years. The warm STMW was rapidly eroded between the late summer and the following winter.