Global thermohaline and material circulations originate from dense water formation through sea-ice production in the polar and ice-covered oceans. Due to difficulties associated with in situ observations, sea-ice production and deep/intermediate water formation were not well understood until recently. Methods for estimating sea-ice production on large scales have been developed using the thin-ice algorithm of satellite microwave radiometer data and heat flux calculation. Mapping of sea-ice production in the Southern Ocean has shown that the Cape Darnley polynya is the second highest ice production area after the Ross Ice Shelf polynya. Furthermore, direct observations have revealed that this polynya is the missing (fourth) source of Antarctic Bottom Water. The Okhotsk Northwestern polynya exhibits the highest ice production in the Northern Hemisphere, and the resultant dense water formation leads to overturning in the North Pacific, that extends to the intermediate layer. The dense water is transported southward by the East Sakhalin Current, which is the western boundary current of the Okhotsk Sea. The polynya ice production has shown a significant decrease over the past 30-50 years, likely causing the weakening of the North Pacific overturning. These studies demonstrate the strong linkage between variabilities of sea-ice production and bottom/intermediate water formation.

A long-term, 82-year numerical simulation is conducted under time-independent wind forcing using an ocean general circulation model (Modular Ocean Model Ver. 3) to elucidate the mechanism of selt-organized 7~8-year oscillation in the North Pacific subtropical gyre. The Kurosio recirculation (KR) region, the Kuroshio-Extension recirculation (KER) region, and the North Pacific subtropical gyre (ST) region are defined, which the long-term sea-surface height data in those regions are analized statistically. Analysies of the empirical orthogonal function (EOF) in the ST region indicates that the first EOF mode is coinsident with the meridional migration of the northward Kuroshio-Extension jet and that the second-third EOF mode corresponds to the first baroclinic basin mode with a 7~8-year period in the subtropical gyre. An investigating into the correlation of the latter mode with the strength of the quasi-harmonic oscillation for KR, KER, and ST regions, reveals the the existence of spectral peaks in their 7~8-year period oscillations and that they demonstrate lagged correlations with each other. Mesoscale eddy disturbances generated by the Kuroshio-Extension jet and the Shatsky Rise play an important role and propagate southwestward as basin-mode baroclinic Rossby waves. This 7~8-year long-term variability in the North Pacific subtropical gyre is a self-sustained oceanic oscillation, a phenomenon that is capable of occurring in oceanic areas.