However, it is still more difficult in this area to carry out systematic observations than the other oceans mainly because of stormy weather in winter, which results in the lack of observation data, particularly for deep hydrography, to varify the above hypothesis or knowledge and to pursue new investigations. This is the background which will require us to organize systematic observational network and develop numerical models in the subpolar gyre of the North Pacific, in order to elucidate the circulation and its variability including decadal and/or centennial signals and the dynamics of the intermediate waters as well as the origin of NPIW.
2.1 Circulation of the North Pacific subpolar gyre
It is required to supply the data gap of deep hydrography in order to describe the circulations in the North Pacific subpolar gyre. Intensive observations will provide a data set in this area necessary to understand better the circulation and its variability and property distribution including temperature, saliniy, other chemical items and values related to the behavior of CO2. It includes WHP type observations, surface drifter and subsurface float deployments, satellite altimetry and XBT drops basically in the TRANSPAC region. First we will need to make cross sections of temperature, salinity and other properties with the data of hydrographic observations as the basic figures of the circulation. We will also need to promote the research with inverse modeling for the historical data and WHP data set. As for numerical models, we will need to develop models with higher spatial resolution based on more realistic bottom topography.
2.2 Water masses formation in the North Pacific subpolar gyre
There has been an understanding on the formation process of NPIW which has been realized to be a real water mass that it is formed by mixture of the water originated in the Okhotsk sea and discharged to the Pacific with the waters of subtropical gyre. In order to verify this idea and to make quantitative discussion, it would be much effective to carry out a mapping of NPIW by analysing oceanographic observation data and to draw the patter of subsurface circulation with Lagrangian methods. The detailed process of the origin waters of NPIW, the role of vertical mixing in the channels connecting marginal seas and the Pacific Ocean, interaction and mixing process of the waters with subtopical waters, are also important subjects to be solved.
As some realistic water mass distribution has been reproduced in the oceans simulated in the present numerical models, it is required in the future studies to develop high resolution models which can resolve meso scale eddies. An artificial tracer experiment with the present numerical model has proposed a hypothesis that;
a)most of the lower intermediate water in the North Pacific has its origin in the Okhotsk Sea and diffuses on isopicnal surfaces toward the intermediate layer of the Pacific Ocean,
b)the upper intermediate water has its origin in the sea surface of the Okhotsk, Bering Seas and the transition area between subpolar gyre and the Kuroshio, and diffuses on the
