Satoshi Osafune

Effects of the 18.6-yr Modulation of Tidal Mixing on the North Pacific Bidecadal Climate Variability in a Coupled Climate Model

AbstractDiapycnal 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 tida...

Possible mechanisms of decadal‐scale variation in PO4 concentration in the western North Pacific

[1] We suggest possible mechanisms of decadal-scale variation in PO 4 concentration in the Oyashio and Kuroshio-Oyashio transition waters, western North Pacific. Significant decreasing and increasing trends in PO 4 have been observed in the surface and mid-layers of the ocean in this region, respectively. Synchronous bidecadal-scale oscillations in PO 4 were also found between the two layers. Differences in the relationship of the trends and the oscillation in these layers suggest that they are driven by separate processes. The trend component may be induced by attenuation of water exchange between the two layers. In contrast, the influence of the 18.6-yr period nodal tidal cycle on the formation rate of intermediate water in the Okhotsk Sea may cause the bidecadal-scale oscillation. The PO 4 concentration showed a significant positive correlation with Neocalanus plumchrus biomass, the dominant mesozooplankton species in the Oyashio and Kuroshio― Oyashio Transition waters. These relationships suggest that the variation in PO 4 affected production of N. plumchrus due to changes in primary production.

Evaluation of the applicability of the Estimated State of the Global Ocean for Climate Research (ESTOC) data set

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.

Role of the oceanic bridge in linking the 18.6 year modulation of tidal mixing and long‐term SST change in the North Pacific

The impact of the 18.6 year modulation of tidal mixing on sea surface temperature (SST) in the North Pacific is investigated in a comparative study using an ocean data synthesis system. We show that remote impact through a slow ocean response can make a significant contribution to the observed bidecadal variation in wintertime SST near the center of action of the Pacific Decadal Oscillation in the eastern Pacific. A comparative data synthesis experiment showed that the modified SST variation is amplified by bidecadal variation in the westerly wind. This relationship between SST and wind variations is consistent with an observed air-sea coupled mode in the extratropics, which suggests that a midlatitude air-sea interaction plays an important role in enhancing the climate signal of the 18.6 year modulation. This result supports the hypothesis that the 18.6 year tidal cycle influences long-term variability in climate; thus, knowledge of this cycle could contribute toward improving decadal predictions of climate.

Multidecadal change in the dissolved inorganic carbon in a long‐term ocean state estimation

By using a 4-dimensional variational data assimilation system capable of estimating physical and biogeochemical variables for the global ocean, we investigated multi-decadal changes in the dissolved inorganic carbon (DIC) in the ocean. The system was newly constructed with a pelagic ecosystem model and an oceanic general circulation model to integrate available ocean observations obtained with a wide range of observation tools. The distribution of estimated DIC was by and large consistent with previous reports. We validated the changes in DIC along the World Ocean Circulation Experiment (WOCE) Hydrographic Program sections. The correlation coefficients of the modeled versus observed decadal difference patterns of DIC ranged from 0.25 to 0.51 in the Pacific Ocean, from 0.36 to 0.62 in the Atlantic Ocean, and from 0.23 to 0.57 in the Indian Ocean, and were significant at the 95% confidence level. Thus, at basin scale, the reproducibility of long-term climate change was similar. Estimation of vertical DIC fluxes in each basin showed that the fluxes changed on a multi-decadal time scale in our system. These changes were possibly due to changes in the dynamical state of CO2 absorption and to changes in ocean circulation. Our integrated dataset on the basis of a dynamically self-consistent ocean state is a promising tool for examining long-term changes in the ocean carbon cycle. This article is protected by copyright. All rights reserved.

Wind-Induced Mixing in the North Pacific

AbstractTemporal variability of the winter input of wind energy flux (wind power) and its relationship to internal wave fields were examined in the North Pacific. The dominant long-term variability of the wind power input, estimated from a mixed layer slab model, was inferred from an empirical orthogonal function analysis, and it was found that variability partly corresponded to the strength and movement of the Aleutian low. Responses of the internal wave field to the input of wind power were examined for two winters with a meridional float array along 170°W at a sampling interval of 2 dbar. Time series of the vertical diffusivities inferred from density profiles were enhanced during autumn and winter. After comparing diffusivities inferred from densities sampled at 2- and 20-dbar intervals, Argo floats with a vertical resolution of 20 dbar were used to detect spatial and temporal variability of storm-related mixing between 700 and 1000 dbar in the North Pacific over a period of 10 years. Horizontal maps ...

Improvement of Ocean State Estimation by Assimilating Mapped Argo Drift Data

We investigated the impact of assimilating a mapped dataset of subsurface ocean currents into an ocean state estimation. We carried out two global ocean state estimations from 2000 to 2007 using the K7 four-dimensional variational data synthesis system, one of which included an additional map of climatological geostrophic currents estimated from the global set of Argo floats. We assessed the representativeness of the volume transport in the two exercises. The assimilation of Argo ocean current data at only one level, 1000 dbar depth, had subtle impacts on the estimated volume transports, which were strongest in the subtropical North Pacific. The corrections at 10°N, where the impact was most notable, arose through the nearly complete offset of wind stress curl by the data synthesis system in conjunction with the first mode baroclinic Rossby wave adjustment. Our results imply that subsurface current data can be effective for improving the estimation of global oceanic circulation by a data synthesis.