Spacial and temporal variations of water characteristics in the Japan Sea bottom layer

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Abstract
The Japan Sea is an almost landlocked marginal sea. We measured profiles of CTD potential temperature at 9 stations in the southeastern Japan Sea to clarify the characteristics of the abyssal circulation in the Sea. It was shown that the results were closely correlated with the topography of the Japan Sea which is characterized by a rise in the center of the Sea (Yamato Rise) and three basins (Japan, Yamato and Tsushima Basins) around the Yamato Rise. At each station in the Basins (6 stations), we observed "the bottom layer", a layer of constant potential temperature (within .+-. 0.0005.degree. C) below 2,000-2,500 m depth to the bottom. Dissolved oxygen is also constant (within .+-. 1 .mu.mol kg-1) in the bottom layer. the bottom layer of the Japan basin (northern basin) is shown to have lower potential temperature by 0.012.degree. C and higher dissolved oxyben content by 5 .mu.mol kg-1 than that of the Yamato Basin (southern basin). There is no bottom layer at the remaining 3 stations located in the passage between the Japan Basin and the Yamato Basin, possibly due to the topographic effect of the Yamato Rise which restricts the exchange and mixing of the two Basin bottom waters. By comparing the vertical profiles of dissovled oxygen in 1969, 1977, 1979 and 1984 in the Japan and the Yamato Basins, it was found that in both Basins the thickness of the bottom layer decreased by 400 m between 1969 and 1984, and the bottom oxygen concentration was also decreased by 5-7 .mu.mol kg-1 between 1977 and 1984. These temporal variations were interpreted to be transient, probably caused by the recent reduction or cessation of new bottom water formation in the northern Japan Sea.

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