The year-to-year variation in vertical circulation of Ariake Bay was investigated by a box model analysis using monthly observed salinity data from 1990 to 2000. The vertical circulation in the head of the bay has been weakened from 1990 to 2000 due to the decrease of river discharge, while that in the central part of the bay has been intensified due to the decrease of vertical viscosity accompanied by the decreasing of tidal current amplitude in Ariake Bay.
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