Gravitational circulation and its response to the variation in river discharge in the Seto Inland Sea, Japan

Abstract

In this study gravitational circulation and its response to the variation in river discharge have been investigated in the Seto Inland Sea, Japan. Monthly hydrographic surveys have been conducted from 2002 to 2006 in Hiuchi‐Nada, a basin located between two straits. Horizontal salinity gradients along the channel varied as the 9/10 power of the river discharge into one end of the basin. Significant volume fluxes which were proportional to the horizontal salinity gradients were found. The relationship between river discharge (R m3 s−1) and the volume flux (Q m3 s−1) was represented as Q = 13.9R0.88. When river discharge increased, vertical density gradients were intensified and then along‐channel horizontal density gradients were gradually enhanced due to vertical mixing at the western strait. These gradients drove gravitational circulations along the channel. The gravitational circulations were not uniform across the channel and accompanied by horizontal circulations. In the northern part of the basin, low‐salinity water at the eastern end of the basin was directed toward the west and high‐salinity water was directed toward the east. The eastward flows arising from the intrusion of relatively heavy water at the western strait extended from the bottom to the surface. In the southern part, low‐salinity water was directed toward the west. The westward flows arising from the intrusion of relatively light water at the eastern strait concentrated in the middle layer and formed a plume‐like structure, accompanied by the eastward flow under the plume. These situations were responsible for the development of the bulk exchange flux.