Long-term Variation of Ventilation System in the East Sea (Japan Sea) Revealed by Heat Content Change and Water Mass Analysis

The semi-enclosed East Sea (ES) is called a miniature ocean with its own thermohaline circulation characterized by the formation of deep and intermediate water masses in the Japan Basin, southward discharge of those subsurface water masses towards the Ulleung and Yamato basins, and northward heat transport by the Tsushima Warm Current in the upper layer. Reports have been given of rapid changes of physical and biogeochemical properties associated with its ventilation system. We present results on upper ocean heat content variations and changes in water mass structure and properties from the analysis of historical and most recent hydrographic data. The analysis of non-seasonal heat content (HCA) variations in the upper 500 m from 1976 to 2007 highlights the 2-year lagged in-phase decadal-scale HCA variations in the eastern and western ES until 1995 followed by uncorrelated variations between two regions thereafter with pronounced interannual variations. Long-term trend of HCA in the entire ES shows an increasing trend, but with a large increase in the eastern part and relatively weaker but statistically significant decrease in the western part. The thickness variation of water warmer than 10°C mainly contributes to the HCA variation. Analyses of upper circulation in conjunction with climate indices suggest the importance of the wind-stress curl pattern represented by the Western Pacific index in the western ES and the influence of the Siberian High in the eastern ES. The thickness and temperature variation of 1-5°C representing the East Sea Intermediate Water (ESIW) is relatively minor contributor to the HCA variation in the upper 500 m. However, the thickness (temperature) of the ESIW has been increased (decreased) in the entire ES since 1992, which implies that the formation of the ESIW has been activated in recent decades. To investigate water mass changes in deeper than 500 m, we use full-depth CTD data obtained from CREAMS expeditions from 1993 to 2015. Temperature deeper than 1000 m has been increased about 0.03°C during 20 years and the depth of deep salinity minimum depth which is the lower (upper) limit of the East Sea Central Water (Deep Water) has been deepened. Other characteristics of water mass structure and property changes will be presented.