The Japan Sea is a marginal sea separated from the North Pacific by the Japanese islands. Although the mean depth of the Japan Sea is about 1350 m, the four straits connecting with other seas are shallower than 150 m. Therefore, the deep layer in the Japan Sea is isolated completely from other seas, like a washbowl.

There exists very cold water of 0-1°C in the deep layer of the Japan Sea. Since such cold water is never seen around Japan except for the Japan Sea, the deep water in the Japan Sea is called “the Japan Sea Proper Water (JSPW)”. The JSPW is formed in the Japan Sea, as realized from the bottom topography. That is, the Japan Sea includes a deep water formation mechanism itself.

A change in the JSPW has been reported. Since 1960s, the temperature and dissolved oxygen concentration in the JSPW are gradually increasing and decreasing, respectively. Since the JSPW is considered to be formed in severe winter off the Siberian coast by the deep convection, these observational results indicate an inactiveness of the deep convection in recent decades, suggesting the relationship to global warming.

We are studying the three-dimensional water circulation, including the JSPW, in the Japan Sea via the field observations and data analysis.