The East Sea (Japan Sea), a semi-enclosed deep marginal sea, having a much faster turnover timescale (10–100 years) compared to the thousands of years required for the global ocean, is an important natural laboratory for investigating deep-ocean ventilation and long-term hydrographic variability in a present and future warming climate. This study examines long-term changes in deep circulation and hydrographic properties in the Ulleung Interplain Gap (UIG), a key deep-water passage connecting the Japan Basin and the Ulleung Basin in the East Sea. Based on moored current and hydrographic observations at the center of UIG for 26 years from 1996 to 2023, we identify a significant intensification of deep-water transports through the UIG since the late 1990s. The volume transport below 1500 m across the western UIG (WT₁₅) was estimated from the along-channel current measurements at the UIG center, using a linear relationship between the equatorward currents at the UIG center and equatorward volume transport calculated from current measurements at five moorings (U1–U5) across the UIG from November 2002 to April 2004. The WT₁₅ exhibits a significant linear trend of equatorward intensification with a rate of 0.0254 ± 0.0031 Sv per decade for the 26 years, suggesting a strengthening of abyssal overturning circulation. During the same period, potential temperature in the depth of 1500–2500 m steadily increased at a rate of approximately 0.017 °C per decade, indicating warming of deep waters entering the Ulleung Basin from the Japan Basin. Hydrographic analyses further reveal gradual changes in water-mass structure: the upper boundary of Central Water (CW) and the interface between CW and Deep Water (DW) deepened whereas the boundary between Deep Water (DW) and Bottom Water (BW) remained relatively stable. These results suggest a long-term intensification of deep circulation together with deep-water warming and structural changes of water-masses in the East Sea, highlighting the sensitivity of marginal-sea overturning circulation to ongoing climate change.

East Sea (Japan/East Sea),Meridional overturning circulation,abyssal circulation,deep transport