We have reviewed previous observational and modeling studies on Pacific interdecadal climate variability and discussed a roadmap for investigating possible relationships between the climate variability and the periodic 18.6-year tidal oscillation in the ocean. Instrumental data in the 20^th century show significant bidecadal climate variability over the mid-latitude North Pacific. Meanwhile, it is recognized that the real climate variability over the North Pacific is under tropical influences via atmospheric teleconnections, which can be occurred intrinsically even without the tidal oscillations. Although proxy data for several hundred years show significant periodic 18.6-yr variations in the Pacific, whether the signal is relevant to basin-scale climate variability or results from local oceanic tidal variations is debatable. For the hypothesis of the tidal control or regulation of the basin-scale climate variability to be accepted by the international communities of climate research, influences of the tidal oscillation on the Pacific climate variability need to be evaluated and mechanisms that link the two phenomena need to be proposed. It is also important that uncertainties in the tidal influences based on modeling approach be estimated.

The 18.6-year modulation of the short-period tides related to the precession of the moonʼs ascending node probably modulates vertical mixing in the ocean through microscale dissipation processes of internal waves. It is hypothesized that the modulation of mixing contributes to the bidecadal variability in physical and biogeochemical variables in the North Pacific and its marginal seas. Previous studies support this hypothesis. However, the quantitative importance of this modulation remains far from clear. Even qualitatively, we do not understand well the mechanism linking the modulation of mixing and the bidecadal variability in the ocean. To verify the hypothesis, it is necessary to evaluate the quantitative importance of ocean variability as a result of the direct ocean response to the modulation of mixing, which requires us to quantitatively discuss the response to both atmospheric forcing variations and mixing modulation. One promising way to do this is via numerical experiments using a reliable ocean model that can reproduce both the mean states and temporal variations through realistic processes. It is also important to obtain more information about bidecadal variability based on observations.

The emission of a large amount of anthropogenic carbon dioxide (CO₂) changes the global carbon cycle, contributing to temperature increase as well as ocean acidification. In addition, the global nitrogen cycle, which is perturbed by industrial fixation, is thought to affect the global carbon cycle. In this review paper, we first introduce how the natural environment changes owing to CO₂ emission and nitrogen fixation. To project future global climate and biogeochemical changes, an Earth system model (ESM) including the global carbon and nitrogen cycles is under development. To embed into the new ESM, we developed a new marine ecosystem model that includes riverine and atmospheric nitrogen inputs as well as iron and phosphate cycles. We briefly introduce this marine ecosystem model. It is well known that the mixing process in the ocean controls the water, carbon, and nitrogen cycles. However, in theparameterization of diapycnal mixing employed in ESMs, the physical processes causing mixing have not been well considered thus far. For further development of an ESM, a new parameterization is required.

The changes related to interdecadal climate variations, such as the Pacific Decadal Oscillation and the 18.6-y tidal cycle, have been discussed in many marine ecosystem studies. The climate regime shift of 1976/77 played an important role in both lower and higher trophic ecosystem change, especially in the North Pacific. By analyzing data from observation of nutrient concentrations, decreasing and increasing trends of nutrients in the surface and subsurface layers, have been reported in many studies. Changes have also been observed in the biomass of phytoplankton and zooplankton. However, quantitative understanding of ecosystems remains a major research challenge because of the complex biodiversity and food web structure. This review focuses on primary producers and zooplankton as key links to higher trophic levels, with the aim of elucidating the mechanism that sustains marine ecosystems.

Various studies have been conducted to elucidate the climate variability impacts on living marine resources. Larval and juvenile stages are critical periods for the recruitment of living marine resources. However, limitations of observation methods for directly investigating the environments that larvae and juveniles experienced have been obstacles to our understanding. We reviewed the previous studies on climate variability impacts on living marine resources and discussed how reconstruction of environmental histories of larvae and juveniles is important for our understanding of climate variability impacts on living marine resources. We proposed a new, integrated method to reconstruct environmental histories of larvae and juveniles using otolith oxygen stable isotope analyses and fish growth–migration models. Together with the growth estimated from otolith daily increments, it is possible to elucidate climate impacts on larval and juvenile growth through environmental histories of larvae and juveniles using their realistic migration routes.