Role of Korea Strait Volume Transport in the East/Japan Sea Deep Water Circulation under the Mid-Holocene Climate Equilibrium

The East Sea, having its own meridional circulation system like that of the Atlantic Ocean but with a shorter periodicity, is known to be sensitive to climate change, with important implications for future changes in the ocean environment.

The meridional circulation system of the East Sea is known to be maintained, by 1) heat and salt supply from the Tsushima Warm currents, 2) ocean-atmosphere heat and freshwater exchange in the northern part of the East Sea, and 3) sea ice formation in the Tatar Strait, but it is not yet known which of these factors will most dominate the changes and variability of the East Sea circulation under different climate equilibria, and there has been no quantitative study of the path and strength of the deep circulation in the East Sea induced by each factor.

In this study, we explore the distinct characteristics of the East Sea’s deep circulation in the past compared to the present, to understand better the future climate change in the region. Through the simulations using the ocean regional model system (ROMS) with results from PMIP4/CMIP6 experiments as surface forcing, we investigate the factors influencing the response of deep circulation to the past by analyzing both the present and the past climate conditions. Specifically, we conducted quantitative analyses to investigate how the path and intensity of deep circulation in the East Sea vary due to the different conditions. This was achieved through model experiments (Surface bar, VT bar) in which we altered both the volume transport through the Korea Strait and the air-sea interaction through the surface.

The results reveal that the deep circulation of the East Sea was approximately 5% weaker 6000 years ago compared to the present. During this period, the seasonal temperature difference was larger than in the present climate, and the volume transport through the Korea Strait was higher. The weakening of the deep circulation is attributed to the higher temperatures in the southern part of the East Sea causing ocean stratification 6000 years ago. The heat transported by the Tsushima Warm currents as it entered the East Sea further intensified the stratification in the southern part, leading to a weakened deep circulation. In the northern part of the East Sea, increased density resulted from the increased salt delivered by the Eastern Korea Warm Current, strengthening the northern circulation along with the brine rejection. However, this effect is overwhelmed by surface warming through air-sea interaction in the East Sea, resulting in the net weakening of the meridional circulation. Also, the processes and characteristics of the linkages between sea ice formation, the variability of the volume transport and atmospheric conditions are analyzed. Finally, this study presents insights into the sensitivity of the East Sea meridional circulation system to future climate change with less uncertainty helped by better understanding paleoclimate in the region.