[1] On the basis of the Princeton Ocean Model, a robust diagnostic model is developed to calculate the residual currents in the Hiuchi-Nada, a semi-enclosed bay of the Seto Inland Sea. The input water temperature and salinity data are from six hydrographic surveys in 2002, and the model results in the summer show a clockwise eddy in the western part of the bay and an anticlockwise eddy in the eastern part. The magnitude of two eddies is $10 cm/s. The modeled flow pattern is consistent with early observations derived from moored arrays. The two eddies are persistent throughout the summer because winds are usually weak and cannot influence their existence. The tide-induced residual currents are appreciable only in the close vicinity of the Kurushima Strait and Bisan Strait, the two straits connecting the bay to the other bays, and have little affect on the two eddies that are inside the bay. Thus the two density-driven eddies in the Hiuchi-Nada are the basic pattern of circulation in the summer. Using the calculated residual currents, we examined the intrusion route of waters from the Kurushima Strait and Bisan Strait with passive tracer experiments. The results show that the water in the Kurushima Strait intrudes into the bay mainly through the middle and bottom layers while the water in the Bisan Strait intrudes mainly through the surface layer. Furthermore, the tracer cannot reach the center of the anticlockwise eddy in the eastern part of the bay. This indicates that the eddy prevents water exchange between its center and outer edges and provides favorable conditions for the formation of oxygen-deficient waters in the central part of the eddy.

The GNSS buoy system for early tsunami warnings has been under development for about 20 years. A small prototype buoy was first deployed in Sagami Bay, Japan, in 1997. Then, after a series of experiments aiming for operational use, the system was implemented as a part of national wave monitoring system NOWPHAS. The NOWPHAS system had set up more than 10 GNSS buoys around Japan by 2011, and it recorded the tsunami caused by the 11 March 2011 Tohoku-oki earthquake. The records were used to update the tsunami warning at the time of the 2011 Tohoku-oki earthquake. However, the buoys were placed less than 20 km from the coast, as the system used the baseline mode RTK-GPS algorithm, which was not far enough for effective evacuation of people. Thus, we began trying to improve the system by putting the buoys much farther from the coast. The new system employs a newly developed positioning algorithm, Precise Point Positioning with Ambiguity Resolution (PPP-AR), together with satellite data transmission. A series of experiments involving the new system successfully indicated changes in sea level with an accuracy of a few centimeters. Given the success of the experiments, we are trying to use the GNSS buoys not only to provide early tsunami warnings but also to monitor various other geohazards. For example, we are trying to use the GNSS-Acoustic system to continuously monitor crustal movements on the ocean floor, to monitor the ionosphere, and to monitor the atmosphere. Ancillary sensors on the buoys will be utilized for oceanographic monitoring as well.

[1] In this study gravitational circulation and its response to the variation in river discharge have been investigated in the Seto Inland Sea, Japan. Monthly hydrographic surveys have been conducted from 2002 to 2006 in Hiuchi-Nada, a basin located between two straits. Horizontal salinity gradients along the channel varied as the 9/10 power of the river discharge into one end of the basin. Significant volume fluxes which were proportional to the horizontal salinity gradients were found. The relationship between river discharge (R m 3 s À1 ) and the volume flux (Q m 3 s À1 ) was represented as Q = 13.9R 0.88 . When river discharge increased, vertical density gradients were intensified and then along-channel horizontal density gradients were gradually enhanced due to vertical mixing at the western strait. These gradients drove gravitational circulations along the channel. The gravitational circulations were not uniform across the channel and accompanied by horizontal circulations. In the northern part of the basin, low-salinity water at the eastern end of the basin was directed toward the west and high-salinity water was directed toward the east. The eastward flows arising from the intrusion of relatively heavy water at the western strait extended from the bottom to the surface. In the southern part, low-salinity water was directed toward the west. The westward flows arising from the intrusion of relatively light water at the eastern strait concentrated in the middle layer and formed a plume-like structure, accompanied by the eastward flow under the plume. These situations were responsible for the development of the bulk exchange flux.

The seawater exchange is closely related to the spreading of pollutant and the water quality in coastal seas. The Seto Inland Sea is a semi-enclosed coastal sea which has a series of broad basins and narrow straits, and the along-channel outflows from the strait located at the center of the channel toward east and west may play an important role in seawater exchange. In this study, flows along the boundary between Bisan Seto which is located at the center of the Seto Inland Sea and the adjacent basin called Harima Nada were measured using a ship-mounted Acoustic Doppler Current Profiler and residual flows were obtained by removing tidal currents. The survey results showed significant volume flux of the along-channel outflows from Bisan Seto toward Harima Nada. A numerical model based on Princeton Ocean Model was also used in order to analyze the driving forces of the along-channel outflow. The outflow was well reproduced using the model which was driven by tide and horizontal density gradient between the strait and basin, suggesting that these possibly induce the seawater exchange in the Seto Inland Sea.