Internal tides and autumn slack water in Nomi Bay, Japan

Field observation of water quality changes was performed during summer in 2001 and during autumn in 2002 in Nomi Bay in Kochi Prefecture, Japan, where extensive aqua-culture has been carried out. Large-scale internal tides were observed during summer, and DO (Dissolved Oxygen) was sufficiently high even at the bottom. Meanwhile, a phenomenon called "mizu-domari" (slack water) and anoxic water formation were observed during autumn. It is generally well known that anoxic water forms in summer due to strong stratification in vertical direction and it disappears in autumn. Contrary to this general expectation, however, the water quality in Nomi Bay was worse in autumn than that in summer. Thus, the role of the internal tide is quantitatively investigated using a numerical model. It is concluded that internal tides enhance the exchange of seawater, being helpful for maintaining the good water quality of the inner area in Nomi Bay.