Department of Chemical Oceanography, Atmosphere and Ocean Research Institute, The University of Tokyo
Graduate School of Science and Engineering, University of Toyama
National Institute of Advanced Industrial Science and Technology
抄録
Chemical tracers in seawater, as well as physical parameters such as temperature and salinity, have been measured to better characterize the dynamics of water convection and its spatiotemporal changes in the Sea of Japan (also called the Japan Sea), a semi-closed, hyperoxic marginal sea (maximum depth: ∼3,800 m) in the northwestern corner of the Pacific Ocean. Repeated conductivity, temperature, and depth (CTD) observations and measurements of dissolved oxygen, for more than 30 years, have confirmed that the bottom layer of the Japan Sea, with a thickness of ∼1 km below the boundary at a depth of ∼2,500 m, is characterized by vertical homogeneity with fluctuations of potential temperature and dissolved oxygen of <0.001◦C and <0.5 μmol kg−1, respectively. The timescale of the abyssal circulation in the Japan Sea has been estimated to be 100–300 years, using 14C and other chemical tracers. Stable isotope analyses for dissolved He, O2 and CH4 have given us information on their unique geochemical cycles in the Japan Sea. Profiles of the short-lived radioisotope 222Rn just above the sea bottom have brought new insights into the short-term lateral water movement with a timescale of several days in the Japan Sea bottom water. It is of special concern that the gradual deoxygenation and warming of the bottom water over the last 30 years have resulted in an ∼10% decrease in dissolved oxygen and ∼0.04◦C increase in potential temperature, suggesting a change of the deep convection system in the Japan Sea. The temporal changes in the vertical profiles of tritium from 1984 to 1998 have suggested a shift of the abyssal circulation pattern from a “total (overall) convection mode” to a “shallow (partial) convection mode”. It is likely that the global warming since the last century has hindered the formation of dense surface seawater and its ability to sink down to the bottom, isolating the bottom layer from the deep convection loop that is indispensable as the source of cold and oxygen-rich water. However, the decreasing trend of bottom dissolved oxygen between 1977 and 2010 was not monotonous; rather, it was interrupted by an occasional break in the winter of 2000–2001, when severely cold weather may have resulted in especially dense surface water to sink down to the bottom layer for its ventilation.
The Sea of Japan and Its Unique Chemistry Revealed by Time-Series Observations over the Last 30 Years
Gamo_2014_MEEP.pdf
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