Influence of Ambient Current on the Temperature Increasing Rate of Bottom Cold Water in a Shelf Sea: a Case Study of the Seto Inland Sea, Japan

Based on hydrographic data collected monthly from 1971 to 2000 in the Seto Inland Sea, a semi-enclosed shelf sea in the western Japan, the presence of bottom cold water (BCW) in summer was confirmed in several wide basins (called “Nada” in Japanese) inside the sea. The water temperature inside the BCW shows two types of increasing rates: a constant one from spring to summer in most wide basins but an acceleration from spring to summer in a specific wide basin named Iyo Nada. The causes for different increasing rate in the wide basins were analyzed with a numerical model that is able to reproduce the aforementioned phenomenon. Using model results, we calculated the advection and diffusion of heat for the BCWs in both horizontal and vertical directions. The intensification of horizontal advection of heat in the bottom layer was found to be the major cause for the acceleration of increasing rate of bottom water temperature in the Iyo Nada. Since the horizontal gradient of water temperature inside and outside the BCW does not change greatly from spring to summer, the intensification of horizontal advection of heat is therefore attributed to the intensification of subtidal current in the bottom layer. Numerical experiments show that the gradually intensified estuarine circulation from spring to summer that flows across the BCW in the Iyo Nada is the cause for the acceleration of increasing rate of water temperature inside the BCW. In other wide basins, the bottom estuarine circulation does not encounter the BCW, and therefore does not affect the increasing rate of water temperature inside the BCW. As the presence of BCW is a common phenomenon in stratified shallow waters，the influence of ambient subtidal current on it is likely an important process to understand its seasonal and interannual variations.