On the basis of historical oceanographic data, we investigated the long‐term variations of the intermediate waters in the four regions in the northwestern subarctic Pacific: Oyashio, Okhotsk Sea Mode Water, Upstream Oyashio and East Kamchatka Current. We found bidecadal oscillations in these water properties that are synchronized with the 18.6‐year period nodal cycle. In periods when the diurnal tide is strong, the following characteristics are found: Apparent oxygen utilization and phosphate are low in Oyashio and Okhotsk Sea Mode Water. The thickness of the intermediate layers is large, and thus potential vorticity is correspondingly low, in Oyashio, Okhotsk Sea Mode Water, and Upstream Oyashio. Around the mesothermal (temperature maximum) water, isopycnal potential temperature are low in the areas on the Pacific side, and high in the intermediate layer of Okhotsk Sea Mode Water. The mixing ratio of Okhotsk Sea Mode Water in the Upstream Oyashio water is high. These bidecadal oscillations can be explained by changes in the vertical mixing around the Kuril Straits induced by the diurnal tide whose amplitude is modulated with the 18.6‐year nodal cycle. Higher sea surface salinity water around the Kuril Straits caused by stronger tidal mixing is possibly transported northward along the cyclonic Okhotsk Sea Gyre, and possibly enhances the formation of the dense shelf water. This makes apparent oxygen utilization, phosphate, and potential vorticity lower in Okhotsk Sea Mode Water and Oyashio.

Bi‐decadal climate variation is dominant over the North Pacific on inter‐decadal timescale; however the mechanism has not been fully understood. We here find that the bi‐decadal variations in the North Pacific climate and intermediate waters possibly relate to the 18.6‐year period modulation of diurnal tide. In the period of strong diurnal tide, tide‐induced diapycnal mixing makes surface salinity and density higher and the upper‐layer shallower along the Kuril Islands and the east coast of Japan. Simple model results suggest that the coastal depth adjustment by baroclinic Kelvin waves enhances the thermohaline circulation, the upper‐layer poleward western boundary current and associated heat transport by about 0.05PW. This could also explain the warmer SST in the Kuroshio‐Oyashio Extension regions, where positive feedback with Aleutian Low might amplify the bidecadal variations. The 18.6‐year tidal cycle hence could play a role as a basic forcing for the bi‐decadal ocean and climate variations.

An objective mapping with the data of profiling float array, maintained under the Argo project, revealed eastward propagation of long‐term (>5 years) salinity anomalies in the subsurface and deep neutral density (γ) layers of 27.0–27.6γ along the subarctic front in the North Pacific after 2000. Such propagation was previously inferred from water property variations along a few observation lines and from numerical simulations, mainly for shallow layers. In the western North Pacific, the signs of the anomalies were the same on and below the 27.0γ, whereas in the eastern North Pacific the sign on 27.0γ was opposite to those on 27.4γ. This difference was attributed mainly to slower advection in the deeper layers. These changes were larger than the standard errors inferred from the objective mapping at least. Furthermore, the variation revealed by the float array was similar to decadal changes observed along repeat ship‐based observation lines, and they were also associated with changes in apparent oxygen utilization especially along 165°E. The small salinity changes in the deeper layers inferred from the float array were also detected as decadal differences in highly accurate trans‐basin observations. Furthermore, because the extension of small changes into the subtropical gyre was also captured by the float and ship‐based observations, the influence of the decadal changes on the isopycnal surfaces off the coast of Japan could appear relatively quickly, even in deeper layers (27.0–27.4γ) in the North Pacific.

Diapycnal mixing induced by tide-topography interaction, one of the essential factors maintaining the global ocean circulation and hence the global climate, is modulated by the 18.6-yr period oscillation of the lunar orbital inclination, and has therefore been hypothesized to influence bidecadal climate variability. In this study, the spatial distribution of diapycnal diffusivity together with its 18.6-yr oscillation estimated from a global tide model is incorporated into a state-of-the-art numerical coupled climate model to investigate its effects on climate variability over the North Pacific and to understand the underlying physical mechanism. It is shown that a significant sea surface temperature (SST) anomaly with a period of 18.6 years appears in the Kuroshio-Oyashio Extension region; a positive (negative) SST anomaly tends to occur during strong (weak) tidal mixing. This is first induced by anomalous horizontal circulation localized around the Kuril Straits, where enhanced modulation of tidal mixing exists, and then amplified through a positive feedback due to midlatitude air-sea interactions. The resulting SST and sea level pressure variability patterns are reminiscent of those associated with one of the most prominent modes of climate variability in the North Pacific known as the Pacific decadal oscillation, suggesting the potential for improving climate predictability by taking into account the 18.6-yr modulation of tidal mixing.

A long‐term data synthesis experiment was conducted for the period 1957–2011 using a modified quasi‐global four‐dimensional variational data assimilation system that was originally developed to improve the representation of the deep ocean, including a unique method for anomaly assimilation. The overall characteristics of the resulting ocean state estimate, which is dynamically consistent without any artificial sources or sinks for heat and salt, are evaluated in the Pacific Ocean. It is shown that the data set better represents the comprehensive ocean state: the mean state of the water mass distribution and volume transport and components of temporal variability from the sea surface to the bottom on interannual to multidecadal timescales. This suggests that the data set can be used to examine interactions between temporal variations throughout the entire depth range and is useful for understanding ocean physics and its role in the climate system.