WIND RESPONSE CHARACTERISTICS OF LOW-SALINITY SURFACE WATER IN ISAHAY BAY UNDER LARGE-SCALE SALINITY STRATIFICATION

Abstract

 We carried out a 3-D numerical simulation of the Ariake Sea for the heavy rainfall event in northern Kyushu in July 2012, and reproduced the large-scale salinity stratification in Isahaya Bay. In order to investigate the wind response of freshwaters from the Chikugo River and the Kikuchi River, the numerical experiments were conducted under three hypothetical wind conditions, i.e., the conditions of windless, 10 m/s northeasterly wind and 10 m/s southwesterly wind. The residence time and the existence rate of river water in Isahaya Bay were quantified, and the representative residence time was estimated by fitting an empirical function to the existence rate. The northeasterly wind enhances the inflow of the Chikugo River water into Isahaya Bay and also promotes the outflow from the bay, so the residence time is found to become shorter than the windless condition. On the other hand, the southwesterly wind reduces the inflow of freshwaters from the Chikugo River, and makes the residence time very short. The inflow of the Kikuchi River water into Isahaya Bay is restricted by the northeasterly wind, and enhanced by the interaction between the southwesterly wind and the tidal motion.