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Distributions of diapycnal, isopycnal, and thickness diffusivity coefficients were estimated using long-term (2001–2015) mean salinity distributions on neutral density ( γ ) surfaces in seven oceanic basins (except the equatorial region). Diffusivity amplitudes and spatial differences were consistent with distributions previously estimated by various parameterizations of fine-scale structures. This result suggests that these parameterizations are consistent with water mass modifications shown by salinity distributions. Although the estimated diapycnal diffusivity coefficients were the lowest among those used by general circulation models to reproduce tracer distributions, they were closer to directly observed values. Because diapycnal diffusivity was generally small, meridional transports in the upper layers (above about 1600 dbar) associated with diapycnal mixing were relatively small. This result may indicate that diapycnal mixing in coastal regions has relatively large effects on meridional transport, along with water mass formation and mixing and wind forcing at the sea surface. The major water mass transports in subtropical gyres estimated in this study were consistent with previous estimates and corresponded to subduction rates determined using mixed layer thickness budgets and transient tracer inventories. These results, based on recent observation network data, show that understanding of water mass formation and modifications can be improved by taking into account diffusive effects, but more detailed assessments of estimation errors are needed to clarify the role of mixing in water mass modification.

A recently developed technique for microstructure measurement based on a fast-response thermistor mounted on a conductivity-temperature-depth equipment was used on eight cruises to obtain 438 profiles. Thus, the spatial distribution of turbulent dissipation rates across the North Pacific sea floor was illustrated, and was found out to be related to results obtained using tide-induced energy dissipation and density stratification. The observed turbulence distribution was then compared with the dissipation rate based on a high-resolution numerical ocean model with tidal forcing, and discrepancies and similarities between the observed and modelled distributions were described. The turbulence intensity from observation showed that the numerical model was overestimated, and could be refined by comparing it with the observed basin-scale dissipation rate. This new method makes turbulence observations much easier and wider, significantly improving our knowledge regarding ocean mixing.

This study investigates an isopycnal temperature/salinity T / S , or spiciness, anomaly in the upper south Indian Ocean for the period from 2004 to 2015 using observations and reanalyses. Spiciness anomalies at about 15°S on 24–26 σ θ are focused on, whose standard deviation is about 0.1 psu in salinity and 0.25°C in temperature, and they have a contribution to isobaric temperature variability comparable to thermocline heave. A plausible generation region of these anomalies is the southeastern Indian Ocean, where the 25 σ θ surface outcrops in southern winter, and the anticyclonic subtropical gyre advects subducted water equatorward. Unlike the Pacific and Atlantic, spiciness anomalies in the upper south Indian Ocean are not T / S changes in mode water, and meridional variations in SST and sea surface salinity in their generation region are not density compensating. It is possible that this peculiarity is owing to freshwater originating from the Indonesian Seas. The production of spiciness anomalies is estimated from surface heat and freshwater fluxes and the surface T / S relationship in the outcrop region, based on several assumptions including the dominance of surface fluxes in the surface T / S budget and effective mixed layer depth proposed by Deser et al. The result agrees well with isopycnal salinity anomalies at the outcrop line, which indicates that spiciness anomalies are generated by local surface fluxes. It is suggested that the Ningaloo Niño and El Niño–Southern Oscillation lead to interannual variability in surface heat flux in the southeastern Indian Ocean and contribute to the generation of spiciness anomalies.

The spatial distribution and seasonality of halocline structures in the subarctic North Pacific (SNP) were investigated using Argo profiling float data and various surface flux data collected in 2003–17. The permanent halocline (PH) showed zonal patterns in the spatial distributions of its depth and intensity and tended to be shallow and strong in the eastern SNP but deep and weak in the west. Mean distributions of PH depth and intensity corresponded to the winter mixed layer depth and sea surface salinity, respectively, indicating that it forms in association with the development of the winter mixed layer. In the Western Subarctic Gyre and Alaskan Gyre, where a relatively strong PH formed, PH intensity and depth showed clear seasonal variations, and deepening of the mixed layer compressed the underlying PH during the cooling period, resulting in intensification and development of the PH in late winter. In both regions, upwelling of high-salinity water also contributed to PH intensification. The summer seasonal halocline (SH) showed distinct zonal differences in frequency and intensity, which were opposite to the PH distribution. While an SH formed in the western and central SNP and coastal regions, it was seldom present in the eastern area. This zonal contrast of SH corresponded to freshening of the mixed layer during the warming period, primarily reflecting freshwater flux. Geostrophic and Ekman advection play important roles in spatial differences in SH intensity and depth. SH development contributed to PH intensification in the following winter, by decreasing salinity above the PH through entrainment.

The accident of the Fukushima Dai-ichi nuclear power plant in March 2011 released a large amount of radiocesium into the North Pacific Ocean. Vertical distributions of Fukushima-derived radiocesium were measured at stations along the 149°E meridian in the western North Pacific during the winter of 2012. In the subtropical region, to the south of the Kuroshio Extension, we found a subsurface radiocesium maximum at a depth of about 300 m. It is concluded that atmospheric-deposited radiocesium south of the Kuroshio Extension just after the accident had been transported not only eastward along with surface currents but also southward due to formation/subduction of subtropical mode waters within about 10 months after the accident. The total amount of decay-corrected 134 Cs in the mode water was an estimated about 6 PBq corresponding to 10–60% of the total inventory of Fukushima-derived 134 Cs in the North Pacific Ocean.

Five high-resolution hydrographic sections were conducted at the periphery of an anticyclonic mesoscale eddy north of the Kuroshio Extension (KE) to explore the evolution of submesoscale structure in the subsurface layer associated with mesoscale eddies. The five sections continuously captured patches of low temperature and salinity water originated from the subarctic region with a horizontal scale of 10–20 km and a vertical scale of 100 m in the strain field at the periphery and in the interior of the eddy detected mainly by the surface Okubo-Weiss parameter. Each patch on the same isopycnal constituted a submesoscale filament. A cold and fresh filament in the intermediate layer in and around the eddy was recently ventilated compared with ambient water, suggesting that submesoscale filaments contribute to a rapid water mass transport from the outside to the inside of the eddy, as well as from the subarctic to the KE region. Filaments at the periphery of the eddy were forced by the convergence in the cross-frontal direction. The horizontal tracer gradient of the filaments evolved in the downstream direction, and 10–50% of the evolution was explained by geostrophic forcing. Moreover, the analysis implied that the evolution of the vertical tracer gradient might have been caused by the vertical shear of the horizontal velocity. The resulting patches were expected to contribute to an effective mixing, suggesting the influence of submesoscale filaments on not only the water mass transport but also the transformation in and around mesoscale eddies in the KE region.

Energy dissipation rates are an important characteristic of turbulence; however, their magnitude in observational profiles can be incorrectly determined owing to their irregular appearance during vertical evolution. By analysing the data obtained from oceanic turbulence measurements, we demonstrate that the vertical sequences of energy dissipation rates exhibit a scaling property. Utilising this property, we propose a method to estimate the population mean for a profile. For scaling in the observed profiles, we demonstrate that our data exhibit a statistical property consistent with that exhibited by the universal multifractal model. Meanwhile, the population mean and its uncertainty can be estimated by inverting the probability distribution obtained by Monte Carlo simulations of a cascade model; to this end, observational constraints from several moments are imposed over each vertical sequence. This approach enables us to determine, to some extent, whether a profile shows an occasionally large mean or whether the population mean itself is large. Thus, it will contribute to the refinement of the regional estimation of the ocean energy budget, where only a small amount of turbulence observation data is available.

One mechanism by which the ocean uptakes carbon dioxide is through the biological carbon fixation and its subsequent transport to the deep ocean, a process known as the biological carbon pump. Although the importance of the biological pump in the global carbon cycle has long been recognized, its actual contribution remains uncertain. Here, we quantify the carbon export from the upper ocean via the biological carbon pump by revealing the upper ocean dissolved oxygen balance. Calculations of dissolved oxygen budget quantified net oxygen removals from the upper ocean by physical processes (air–sea exchange, advection, and diffusion) and indicated net biological oxygen production that compensated for those removals. The derived oxygen production is converted to carbon units using the photosynthetic ratio, and inferred an estimated global annual carbon export through the biological pump of 7.36 ± 2.12 Pg C year −1 with providing insights into the overall ocean carbon cycle. The biological carbon pump exports about 7.36 Pg of carbon globally per year from the upper ocean, according to an estimation of the dissolved oxygen budget that accounts for air–sea exchange, advection, and diffusion.

An observation operator in data assimilation was formalized based on the signatures extracted from the integral quantities contained within observed vertical profiles in the ocean. A four-dimensional variational global ocean data assimilation system, founded on this observation operator, was developed and utilized to conduct preliminary data assimilation experiments over a ten-year assimilation window, comparing the proposed method, namely profile-by-profile matching, with the traditional method, namely point-by-point matching. The proposed method not only demonstrated a point-by-point skill comparable to the traditional method but also provided superior analysis fields in terms of profile shapes on the temperature-salinity plane. This is an indication of a well-balanced analysis field, in contrast to the traditional method, which can produce extremely poor relative errors for certain metrics. Additionally, signatures were shown to successfully represent properties of the water column, such as steric height, and serve as an effective new diagnostic tool. The top-down, or macro–micro, viewpoint in this method is fundamental to the extent that it can offer an alternative view of how we comprehend ocean observations, holding significant implications for the advancement of data assimilation.

Two zonal high-density hydrographic sections along 41° N and 37.5° N east of Japan were occupied in April 2013 and June 2016 to examine the formation of Central Mode Water (CMW) and Transition Region Mode Water (TRMW) in relation to fronts and eddies. In the 41° N section traversing the meandering subarctic front, the denser variety of CMW (D-CMW) and TRMW was formed continuously on both sides of the front, except for the part of the section located south of the Kuroshio bifurcation front where the lighter variety of CMW (L-CMW) and D-CMW was formed instead. L-CMW and D-CMW were also formed in the eastern part of the 37.5° N section between the Kuroshio Extension front and the Kuroshio bifurcation front, but were hardly formed in the western part of the section west of the bifurcation point of the two fronts. D-CMW and TRMW pycnostads in the western part of the 41° N section observed in April 2013 tended to exhibit more than one core (vertical minimum of potential vorticity), which might be formed by destruction of deep winter mixed layers. Such multiple-core structure was also observed in L-CMW and D-CMW pycnostads in the eastern part of both the sections south of the Kuroshio bifurcation front in June 2016, being particularly abundant in three anticyclonic eddies. It was likely to be formed by the exchange of low-potential vorticity water among the eddies and the ambient region in association with eddy-to-eddy interaction, suggesting a new mechanism of mode water subduction.