Thermocline circulation and ventilation of the Japan/East Sea, part II: A source water-mass mixing (SWAM) model

Abstract

The recently obtained high resolution conductivity-temperature-depth (CTD), Argo and bottle data combined with historical geochemical data are used in a source water-mass mixing (SWAM) model for the Japan/East Sea (JES) thermocline. The water-mass properties for resolving model source water types include two physical conservative tracers: potential temperature θ (°C) and salinity S (psu), a dynamical tracer: fN 2 (10 6 s −3) (where f is the Coriolis frequency and N 2 is the squared buoyancy frequency), dissolved oxygen, O 2 (μmol kg −1), and nutrient silicate, H 4 SiO 4 (μmol kg −1) and two conservative chemical tracers: initial phosphate, P O 4 0 (μmol kg −1) and NO (μmol kg −1). The mixing scheme comprises three source water masses: North Pacific Subtropical Water (NPSW) which is the only invasion water mass from the open North Pacific and two locally formed water masses, Tatar Strait Water (TSW) and Peter the Great Bay Water (PGBW). The SWAM model is performed on three neutral density surfaces σ N =25.8, 26.4 and 27.0 encompassing the thermocline from about 50 to 180 dbar. The model-derived mixing fraction provides a quantitative description of the source water masses. Results show that NPSW contributes to only about one third of the mixing ratio while PGBW mixing proportion is more than 50%. This implies that most of NPSW is actually transformed and renewed in the JES by winter convection and probable brine rejection when NPSW is considered as a sole input source and PGBW and TSW are the transformed end-members of NPSW. Also it means that what we see the JES recirculation is actually the dominant PGBW water recirculation rather than the NPSW. The high mixing fraction of PGBW explains why JES water content is highly ventilated and has a very high oxygen and renewal rate. It is found that the JES transport is contributed by NPSW for 0.72±0.13 and 1.11±0.16 Sv by PGBW and 0.27±0.05 Sv by TSW, respectively. A total annual mean transport with an error bar is thus 2.10±0.34 Sv, very close to the Tsushima Current transport at 2.3–2.4 Sv observed in the KTS with a possible error of ±0.5 Sv. Additional study was carried out on a year-to-year basis to examine the temporal variability of the water-mass property and mixing for the last three quarters of the last century from 1925 to 2000. It is found that NPSW has undergone no substantial variation in most of its properties except for silicate which decreases significantly and therefore retains its open ocean characteristics. PGBW has experienced strong increase in potential temperature, salinity and oxygen and decrease in silicate. The same situation occurred to TSW except for its less significant increase in oxygen. The increase in thermocline oxygen contrasts to the oxygen decrease in deep water found in previous studies and is most likely the consequence of shallow overturning circulation under a general warming trend since weakening of potential vorticity fN 2 is also detected. The temporal variability of the water-mass mixing fractions reveals a remarkable discovery—TSW was once the major contributor of the water-mass formation, mixing and transformation in the northern and western Japan Basin prior to the warming episode started in the 1950s with a high fraction of 70–80%. TSW contributed also significantly to the Ulleung and Yamato Basins at that time with a fraction of 30% and 40%, respectively, compared to 20% and 30% in 2000. It was only after the 1950 s that PGBW started to take over TSW becoming the major player in the JES thermocline circulation and ventilation till today consistent with the annual mean situation discussed above.