Results from the continuous measurement with the oceanographic observation tower of the Saga University show the detailed seasonal variation in structure of the lower Chikugo river estuary. Although the Chikugo river estuary was considered to be a well-mixed type estuary, our results showed that it sometimes appears to be moderately-mixed type or strong-stratified type estuary. It was quantitatively evaluated that salinity had larger effects on the seasonal variation in density stratification than temperature. After the flood in the rainy season, low saline water discharge from the river to sea surface produced a strong-stratified type. Moreover, fluctuations of stratification with spring-neap tidal cycle, mixed (or stratification was weaken) in spring and stratified (or stratification was strengthen) in neap, were revealed. For January to June and October to December, the spring-neap tidal cycle in stratification appeared only in the former period despite the non-significant differences in river discharge, tidal oscillation, wind speed, water temperature, and salinity between both periods. During the latter period, the water column was almost usually well mixed. It would be caused by the seasonal change in surface heat flux, i.e., in the latter period the vertical convection by surface cooling as well as tidal mixing play roles in mixing force, while in the former period surface heating supply buoyancy to the water column in the estuary.

Structure of the sea and sound propagation in the sea near the Sanriku coast are analyzed on 18, 19 in July, 2007 (warm season), and on 29, 30 in January, 2010 (cold season). The Tsugaru warm current widely spreads near the Sanriku coast, other warm water masses are over the offshore sea of the coast, and the sea surface is warmed by the warm atmosphere on 18, 19 in July, 2007. The cold Oyashio water system lies under these warm water masses. The large water temperature difference makes it easy to form shadow zones in the warm water masses near the sea surface. If a sound source is settled in the cold Oyashio water system under the warm water masses, the sound is trapped in the sound channel formed near the Oyashio water system. Moreover, water temperature of 30m depth near the coast, though it is in the Tsugaru warm current domain, is relatively cold to make shadow zone in surrounding warmer domain of the Tsugaru warm current and in the other warmer water masses near the sea surface if a sound source is settled near the coast. The Tsugaru warm current was only 40 km wide near the Sanriku coast on 29, 30 in January, 2010. The Oyashio water system widely spread over the sea surface and the Coastal Oyashio water system occupied a part of the sea surface to form a surface duct.