The deep and bottom waters of the Japan Sea (depth ≳ 1000 m) are characterized by low temperature and weak stratification. In the Japan Basin, especially, the bottom water (depth ≳ 2500 m), called the Japan Sea bottom water (JSBW), with a thickness of over 1000 m, is mostly homogeneous. Several studies in recent decades have suggested that the potential temperature of the JSBW is gradually increasing, even though the thick homogeneous vertical structure is being maintained. Although downward mixing of heat from the upper layer was believed to be responsible for this characteristic feature, our microstructure measurements found that the intensity of turbulence in the JSBW is too weak to explain the formation of the gradually warming thick homogeneous layer. By use of a one-dimensional numerical model we demonstrated that this characteristic feature of the JSBW can be reasonably explained on the basis of a combination of geothermal heating from the seabed and an appropriate horizontal supply of cold water to the bottom layer.
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