Distribution and seasonal change of the Tsugaru warm current water off Rokkasho

The first commercial spent fuel reprocessing plant in Japan is being installed in Rokkasho-mura, Aomori Prefecture. Decontaminated liquid effluents in its operation will be released into a sea. In accessing the environmental impact of radionuclides discharged into a sea, it is important that the patterns of water movements around the discharge outlet are clarified. This area off Rokkasho is an open coast, where the Tsugaru Warm Current Water (TWC), the cold Oyashio and the warm Kuroshio Extension meet. Therefore, it is considered that complicated water circulations will be formed around the region of the wastewater outlet. Current structures of the coastal water near the ocean outlet were investigated by use of mooring current meters/ADCPs, a towing-ADCP, and some CTD observations. In addition, extensive observations with CTD and a shipboard ADCP were made in detail around the off Rokkasho (Shimokita Peninsula) to evaluate the distribution and the seasonal change of the TWC. These observations were carried out five times in September 1997 to August 1999. Gyre mode and coastal mode of the TWC experimentally pointed out by Conlon are found by those investigations. In the gyre mode, the large eddy more than 100 km in diameter is found in the east part of the Tsugaru Strait, which has the vertical structure of 1,000 m in depth. From the current measurements by shipboard ADCP, the velocity of the TWC was more than three knots and the width of its fastest region about 30km at that mode. On the other hand, in the coastal mode, the TWC flows along the continental slope off Rokkasho (ca five miles off the coast) and is about 400m thick in depth. The TWC affects the layers below the sill depth of the Tsugaru Strait. In the gyre mode the TWC flows northward along the slope off Rokkasho, however, around the coastal zone standing near to the outlet, southward flow was observed predominantly. At the coastal mode, the northward flow was mostly observed around the coastal area, and the TWC flows southward or southeastward along the slope off Rokkasho. Since tidal currents are not major components in the coastal region according to the mooring current measurements, flow in the coastal zone near the facilities may be strongly affected by the flow patterns of the TWC. (author)