The author has mainly studied the mid-depth and abyssal circulation in the ocean. The topics include formation of deep and bottom waters, wind-driven mid-depth zonal fiow, eddy-driven zonal jets, and the problem of western boundary separation. This article classifies them in terms of their temporal and spatial scale, reviewing the effects of watermass formation, wind forcing, and mesoscale eddies on them.

This article investigates water mass modification processes in the Sea of Okhotsk and the Arctic Ocean by using chemical tracers. Oxygen isotope ratio shows the flow of dense shelf water (DSW), forms during sea ice formation, from the northwestern shelf into the Okhotsk Sea Intermediate Water (OSIW). Mean fraction of DSW in OSIW is estimated to be 20%. Distributions of CFCs show that both formation of DSW and tidal mixing in the Kuril straits ventilate the intermediate layer. Tidal mixing is found to contribute more than the DSW to characterize OSIW as cold and low salinity water. In the Arctic Ocean, distributions of sea ice meltwater/brine and other freshwaters are mapped using historical data of oxygen isotope ratio and a newly investigated tracer, alkalinity. Furthermore, recently acquired data of oxygen isotope ratio and nutrients from the Canada Basin are used to estimate freshwater budget of this part of the Arctic Ocean. Finally, a new role for the Arctic throughflow in the global nutrient cycle is proposed; Arctic throughflow transports excess phosphate, produced during denitrification in the North Pacific and the Bering/Chukchi seas, to the Atlantic where it is used by N₂ fixers.

Two-years of continuous observation of wind and current were carried out to investigate the relationship between them in the coastal waters off Tokai-mura, Ibaraki prefecture. Three instruments to measure the current were set in a thin surface layer of 3 m above the strong pycnocline, which is a common feature in coastal waters. Both of the power spectra of wind and currents showed very similar features, an outstanding high peak at 24-hour period and a range of high peaks longer than several-days period. The long term variation of the wind field always contained north-wind component, which contributed to forming the southward current along the shore throughout the year. A high correlation coefficient (0.64) was obtained between the wind and the current at a depth of 0.5 m on the basis of the two-year observation. Harmonic analysis revealed that an outstanding current with 24-hour period was the S₁ component (meteorological tide) , and was driven by land and sea breezes. These breezes also contained solar tidal components such as K₁, P₁ and S₂. These wind components added their own wind driven currents on the original tidal currents. This meant that land and sea breezes generated wind driven currents with solar tidal periods which behaved like astronomical tidal currents. As a result, coastal currents contained pseudo tidal currents which behaved like astronomical tidal currents.

Continuous observations of current and temperature along the coast of the Noto Peninsula, Japan, were carried out in summer 2003 to clarify the characteristics of Kyucho (stormy current). ln this area, a strong current has occurred on the western coast of the peninsula when the southwesterly wind enhanced, accompanied by the passage of Typhoon 0314. The region of strong current turned around near the tip of the Noto Peninsula, and then migrated southward along the eastern coast of the peninsula. When the southwesterly wind was prevailing, the subsurface cold water was upwelled to the surface layer along the eastern coast of the peninsula. After the rapid temperature decrease due to the upwelling, a Kyucho with the maximum velocity of 0.84 m s^-1 was observed. Following the Kyucho occurrence, a sudden increase in the temperature of about 24 °C was observed at a depth of 3-54 m which signifies the intrusion of warm water mass thicker than 50 m. These give an interpretation for the Kyucho that the surface warm water piled up on the western coast of the peninsula due to the Ekman transport migrated from north to south along the eastern coast of the Noto Peninsula as a density current or coastally-trapped waves which caused by the rotating system.