VARIABILITY OF STRATIFICATION IN SHALLOW TIDAL ESTUARY

Abstract

A long-duration observation of stratification has been conducted in the Ohta Estuary. During the spring tide, the data demonstrate a semi-diurnal stratification cycle driven by tidal straining of freshwater-induced horizontal density gradient. The wind-driven current induced by a land and sea breeze brings about diurnal stratification cycle during the neap tide. During the neap tide, the change of stratification is mainly caused by the variation of surface density Tn addition to the short cycle of stratification, the spring-neaps mixing cycle and the variation of river discharge bring about the longer period of stability. The current profiler mooring observation shows that the velocity gradient during the ebb is larger than the flood velocity gradient. This results in the stronger tidal straining during the ebb tide. The well-mixed condition around the low water slack is associated with the small depth and strong tidal stirring.