Fortnightly Variation in the Bottom Thermal Front and Associated Circulation in a Semienclosed Sea

Summer hydrographic data from 1971 to 2000 demonstrate the presence of bottom cold water (BCW) and a bottom thermal front around the BCW in a semienclosed sea in the Seto Inland Sea of Japan. The horizontal gradient of water temperature across the bottom front was larger in neap tide than in spring tide, which is the opposite the pattern observed in the fortnightly variations in other areas (e.g., the Irish Sea). A numerical model for the Inland Sea reproduced the presence of BCW and a bottom front as well as the same fortnightly variation in the horizontal gradient of water temperature across the bottom front as in observational data. Being the same BCW as those in other areas, the presence of BCW in the Inland Sea is also caused by spatial variation in tidal mixing. The intensification of the bottom front in neap tide in the Inland Sea results from a combination of tidal mixing and horizontal advection of warm water to the BCW. The presence of a bottom residual current in the direction across the bottom front results in the horizontal advection of warm water to the BCW, and its fortnightly variation is responsible for the fortnightly variation of the bottom front in the Inland Sea. The presence of the BCW induces a surface cyclonic circulation in the direction approximately along the bottom front; the intensification and weakening of the bottom front with the spring–neap tidal cycle, combined with the fortnightly variation in the bottom boundary layer thickness, produce a slightly stronger surface cyclonic circulation in neap tide than in spring tide.