Seasonal and spring-neap tidal changes in ocean structure of the lower Chikugo river estuary based on monitoring data from oceanographic observation tower

Ito Yuji, Hayami Yuichi, Katano Toshiya, Koriyama Masumi

doi:10.5928/kaiyou.21.1_1

<p>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.</p>

Seasonal and spring-neap tidal changes in ocean structure of the lower Chikugo river estuary based on monitoring data from oceanographic observation tower

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