Thermocline circulation and ventilation of the East/Japan Sea, part I: Water-mass characteristics and transports

Abstract

Abstract The East/Japan Sea (EJS) has unique water-mass characteristics in the western Pacific marginal seas due to limited exchange with the open North Pacific. The major inflow of source water mass is North Pacific Subtropical Water (NPSW) carried by the Kuroshio branching and Tsushima Current. The locally formed cold/fresh waters from the Tatar Strait and Russia coast by winter convection mix with NPSW contributing to water-mass transformation, especially during winter when upper isopycnal surfaces outcrop and thermocline is ventilated. The geographic limit of the Korea/Tsushima Strait (KTS) with a sill depth of about 120-140 dbar confines the inflow of lower NPSW, and so the EJS thermocline layer is somewhat truncated with a rather thin layer for about 100 dbar. This study uses high resolution conductivity-temperature-depth (CTD) and Argo data with a third decimal or higher accuracy for temperature and salinity obtained mainly by Research Institute of Oceanography, Seoul National University through domestic and international collaboration with several Korean ocean research institutes and Russia and USA partners since early 1990s. The basin covered data were divided into summer and winter half-year representing seasonal difference since most CTD surveys were conducted purposely in summer and winter. Analysis is made for the upper layer, about 50-70 dbar, from the surface to the upper main thermocline and the thermocline layer down to about 150-180 dbar south of the subpolar front (SPF). The lower thermocline is defined slightly below the sill depth of the KTS, considering the deepening of NPSW after passing through the KTS. The thermocline layer is encompassed by three selected neutral density surfaces σN=25.8, 26.4 and 27.0 with a distance of about 40 dbar between two neighboring surfaces. The core of thermocline is followed by the σN=25.8 surface characterized by a salinity maximum and a minimum of potential vorticity. Winter convection is discussed and compared with three other major convection sites of the world's oceans, the Gulf of Lions, Labrador Sea and Greenland Sea, showing some common and distinctive features, especially the extremely low salinity of the EJS. Water-mass properties on neutral density surfaces are analyzed with the water-mass Turner angle (WTu) and circulation and transport are deducted from geostrophic calculations. From the 15-year mean hydrography, a basin-wide net annual mean transport of about 2.10±0.29 Sv (1 Sv=106 m3 s-1) is estimated with summer and winter transports of 2.56±0.36 and 1.63±0.23 Sv, respectively. This transport is slightly less than the annual mean transport of the Tsushima Current at the KTS, 2.4 Sv from cable and 2.3 Sv from other direct current meter and geostrophic methods but matches the ±14% error bar of ±0.29 Sv adjusted by ±150 dbar from the reference level of 800 dbar. This error bar is close to the error of ±0.34 Sv determined from water-mass conservation residual in a separated study. Three mechanisms are discovered to explain the seasonal difference in the Tsushima Current transports: the stronger winter Ekman pumping, outcropping and southward crossing flow. During winter, the Tsushima Current branches are imposed under strong wind stress curl in the Ulleung Basin and Yamato Basin, showing a doubling Ekman downwelling transport, partly weakening the Tsushima Current flow in the eastern boundary. Meanwhile the thermocline isopycnal surfaces outcrop in winter, reducing volume transport due to reduced space and thickness. The southward currents in the southern Ulleung Basin and Yamato Basin are perpendicular to the Tsushima Current branches west of Japan, which weakens the eastern boundary current in winter.