A tidal front influenced by river discharge

Abstract

The detailed structure of an oceanic front found in Osaka Bay, Japan, was observed on board R/V Toyoshio-Maru in May, 1987. The front separates the stratified nearshore region from the well-mixed region of the central part of the bay. The differences of water temperature, salinity and density (σ t) across the front are ∼ 1.5°C, 4.0‰ and 3.0, respectively. The strength of the tidal mixing may be represented by a reciprocal of the parameter H / U 3 (where H is the water depth in m and U is the amplitude of the tidal current in m s −1 ). The front runs in the domain where the value of log( H / U 3 ) ranges from 2.5 to 3.0, showing that tidal stirring plays an important role in its formation. However, the critical value for Osaka Bay is considerably smaller than that for other tidal fronts. The influence of river discharge is taken into account, and it is shown that the front found in Osaka Bay has the nature of a tidal front highly influenced by river discharge. Furthermore, temporal observations of the current, temperature and salinity at the station near the front revealed its vivid and changeable structure.