Temporal variation of deep water transport in the southwestern East/Japan Sea

Southward discharge of deep water masses that are formed in the Japan Basin in the northern East Sea occurs towards the Ulleung and Yamato basins through deep channels. The Ulleung Interplain Gap (UIG) is a unique conduit for deep water exchange between the Japan and Ulleung basins. The UIG is about 140 km long and 75 km wide, and deeper than 2000 m shoaling and slightly broadening towards the southwest. Deployment of a mooring, EC1, has been in operation since 1996 with a deployment of an array of five moorings including EC1 across the UIG for 16.5 months from 2002 to 2004. The mooring array reveals the two-way deep circulation in the UIG, broad and weak inflow towards the Ulleung Basin in the western UIG and narrow and strong compensating outflow in the eastern UIG. The inflow transport shows the maximum at EC1. The array also allows us to make an index of inflow transport below 1800 m based on measured abyssal current at EC1. Long-term mean inflow transport below 1800 m is about 0.16 Sv. The intraseasonal variability of the transport on timescales of 15~50 days mainly due to topographic Rossby waves accounts for more than 50% of total transport variance and can lead to about 1 Sv of transport flutuations. Seasonal and interannual variability lead to about 0.1 Sv transport variation. The transport tends to increase in summer seasonally with a secondary maximum in March. The transport varies interannually on timescales of about 3~4 years. The annual mean inflow transport shows record-length minimum in 2013 followed by an almost continuous increase of the transport until mid-2015. There is no long-term trend of the inflow transport from this 20-year long data in the UIG.