Intensification of water temperature increase inside the bottom cold water by horizontal heat transport

Bottom cold water (BCW) is widely reported to occur in many coastal areas in summer. Some BCW masses have an increase in temperature of only 1 °C from early spring to summer (e.g., North Sea, Yellow Sea), while the others have a 4-10 °C increase (e.g., Irish Sea, Seto Inland Sea). A model of heat transport with vertical diffusion alone gave good approximation of the temperature increase in the North Sea type BCW masses, while additional heat input from other processes was required to simulate the large temperature increases in Irish Sea type BCWs. BCWs in the Yellow Sea and the Seto Inland Sea are then taken as examples to compare heat transport into the two types of BCWs through three-dimensional numerical modeling. The small warming range of the BCW in the Yellow Sea results mainly from downward heat input by vertical diffusion. In contrast, the large temperature range of warming in the BCW in the Seto Inland Sea is a result of both advection of heat from the surrounding water and vertical diffusion. We therefore infer that advection of heat into the BCW is a necessary condition for the high temperature increase range in Irish Sea type BCWs. Moreover, advection of heat into one BCW in the Seto Inland Sea intensifies from April to July, which is induced by gradually intensified bottom currents. This example supports the importance of heat transport by advection for BCWs where the temporal change in advection of heat is capable of altering the temperature increase rate inside the BCW.