This article reviews recent achievements in the study of mid-latitude atmosphere-ocean-marine ecosystem interactions from two Japanese research programs: the Climatic Hotspot2 project (FY2019–2023) and the first two years of the Habitable Japan project (FY2024–2028). Understanding of the mid-latitude ocean-atmosphere interactions, particularly the ocean’s influence on the atmosphere established in the Climatic Hotspot1 project (FY2010–2014), has advanced through the two projects addressing changing climate and oceanic/atmospheric extreme events, such as the longest observed Kuroshio large meander from 2017 to 2025, northward meandering of the upstream Kuroshio Extension in 2023–2024, marine/atmospheric heatwaves, heavy rainfall and snowfall, and tropical cyclones. Relations between weather and its extremes over East Asia and global climate variations have also been clarified. Both projects have conducted and are conducting in-situ observation campaigns to clarify various processes in the ocean, atmosphere, and their interactions and improve numerical models. By incorporating atmospheric/marine chemistry, marine ecosystems, and fisheries as new research domains and putting more emphasis on prediction, the Habitable Japan project aims to answer the main question: Will mild climate and abundant water/fisheries resources persist as a basis of survival for those of us living in Japan in the future?
Two Argo profiling floats equipped with biogeochemical sensors for pH and oxygen were deployed in winter 2021 in the northwestern region of the North Pacific subtropical gyre near Japan. They were used to observe the seasonal evolution of total dissolved inorganic carbon and dissolved oxygen, and to diagnostically assess their budgets in its upper layers. For the period from April to September 2021, net community production in the upper 100 dbar, as evaluated by converting pH to total dissolved inorganic carbon, was 3.3 ± 0.3 mol C m− 2 at 24° N−26° N and 3.5 ± 0.3 mol C m− 2 at 31°N−36° N, and the net sea-to-air oxygen flux was 5.1 ± 0.5 and 6.3 ± 0.5 mol O2 m− 2, respectively, in these regions when integrated over the period. The large O2 release during these months is attributed to the decrease in oxygen solubility due to rising water temperature and is reinforced by biological oxygen production associated with net community production that occurs despite inorganic nutrient limitation in the euphotic layer. In the Shallow Oxygen Maximum, which is broadly observed below the surface density cap during the warmer months in the western North Pacific subtropical gyre, oxygen release to the atmosphere was in close balance with biological oxygen production. The larger net oxygen release into the atmosphere above and net oxygen consumption by community respiration below are responsible for the emergence of the Shallow Oxygen Maximum.
The deep-sea environment before and after the 2024 Noto Peninsula Earthquake was investigated using a series of hydrographic and mooring observations in the Toyama Deep-Sea Channel (TDSC), Toyama Bay. Transient oxygen depletion was observed after the earthquake, and low-oxygen conditions in the TDSC persisted for at least 152 days after the mainshock. The concurrent increase in nutrient concentrations suggests organic matter decomposition in the TDSC. In addition, the turbidity (light transmittance anomaly) temporarily increased after the mainshock, suggesting advection or resuspension of particulate matters. Two turbidity current events were observed in March 30–31 and May 17–18 in the post-earthquake mooring observations at the thalweg, characterized by strong (> 16 cm s− 1), bottom-intensified downslope currents accompanied by transient temperature and turbidity increases. The observed turbidity currents were considered to be weaker and smaller than those at the mainshock. Although these turbidity current events correlated with meteorological disturbances, comparison of pre- and post-earthquake current conditions suggests that small-scale turbidity currents became more likely to occur after the mainshock. This study suggests that the mainshock on January 1, 2024 induced large-scale turbidity currents in the TDSC, transporting large volumes of organic-rich sediment to the deep sea from a shallow upstream region. As a result of the subsequent organic matter decomposition, transient oxygen depletion and nutrient increases were induced. In addition, small-scale turbidity currents frequently occurred after the mainshock. However, oxygen and turbidity returned to pre-earthquake conditions in June 2025 with decrease in small-scale turbidity current frequency.
Assessing local impacts of sea-level rise under global warming is increasingly important for developing climate adaptation strategies. However, current climate models lack the horizontal resolution required to resolve key regional processes and tend to exhibit substantial climate biases. Here, we perform dynamical downscaling of climate-model warming projections using a global eddy-resolving ocean model. When forced by observation-based atmospheric fields, the model successfully reproduces the oceanic climatology, including sharp frontal structures east of Japan. In the 2 K and 4 K global-warming experiments, the model reveals pronounced dynamic sea-level (DSL) rises along these fronts, as well as a broad rise over the subtropical gyre. The frontal DSL rise is primarily caused by changes in surface wind stress, whereas the subtropical rise is associated with the warming of Subtropical Mode Water (STMW). In the 4 K (2 K) experiments, coastal DSL around Japan increases by 5.2 ± 1.2 cm (2.9 ± 1.1 cm) relative to the basin mean, mainly due to a wind-driven northward shift of the Kuroshio Extension (KE). In contrast, in island regions such as Okinawa, thermal effects dominate and wind-stress changes suppress the rise. The Kuroshio path south of Japan is also modified in response to global warming, with a reduced frequency of the large-meander states. These changes in the Kuroshio path may contribute to regional differences in coastal DSL rise along the southern coast of Japan. Overall, the results highlight the importance of the realistic reproduction of climatological ocean states for reliable regional DSL projections around Japan.
We performed an intercomparison of three eddy-resolving ocean reanalyses—FORA-WNP30, GLORYS12V1, and BRAN2020—for the western North Pacific over the period 1993–2016. While all three products exhibit high consistency in large-scale surface mean fields due to data assimilation, notable discrepancies are found in mesoscale variability and subsurface structures, reflecting differences in model configuration and assimilation schemes. Furthermore, a vertical section analysis along 137°E reveals discernible discrepancies in the representation of subsurface water masses among the products. FORA-WNP30, employing a four-dimensional variational (4D-VAR) scheme, exhibits high spatiotemporal variability in sea surface height (SSH). However, its sea surface salinity (SSS) variability is strongly dampened by a short restoring timescale. GLORYS12V1 retains intense coastal SSS variability due to the absence of strong surface restoring. However, it exhibits lower eddy kinetic energy levels compared to the others, likely due to dissipative viscosity parameterization. BRAN2020 shows systematic differences east of Japan, including reduced SSH variability and lower temporal correlations. These are attributed to its simplified bathymetry (capped at 5000 m), which eliminates topographic steering effects and alters eddy propagation speeds. Additionally, BRAN2020 exhibits a weaker vertical salinity contrast around the North Pacific Intermediate Water (NPIW), suggesting the influence of excessive vertical mixing. Our results highlight that horizontal resolution is not the sole determinant of performance; bathymetry, subgrid-scale parameterizations, and boundary conditions play equally critical roles in reproducing regional ocean dynamics.
Nutrients distributed in the surface layer in winter are an important source of phytoplankton bloom in spring. To understand nutrient dynamics in winter, nutrient distributions were examined based on observations from eight survey cruises (2004–2016) in the northern East China Sea (NECS). By analyzing data from these cruises along with data from seven winter cruises (1988–1996) by the Japan Meteorological Agency, interannual variabilities and trends of nutrient distributions were assessed. Within the studied area, a water mass with high temperature and high salinity was distributed in the east and another water mass with low temperature and low salinity was in the west. Nitrate + nitrite (NOx) and dissolved inorganic phosphorus (DIP) concentrations were negatively correlated with temperature, showing that sea surface cooling in winter plays an important role in the supply of nutrients to the surface layer in the eastern part. Both of these concentrations fluctuated with interannual variations in sea surface temperature. In the western part, although NOx concentrations showed an increasing trend over time, DIP concentrations did not, suggesting nitrogen input from anthropogenic sources in this region. Consequently, the NOx/DIP ratio increased up to 21 in the western part. Data from 15 winter cruises showed that the mechanisms of nutrient supply to the surface layer are clearly different between the east and west of the NECS.
Submesoscale eddies play a crucial role in the transport of energy and plankton in the upper ocean despite their small size and short life cycles. However, their dynamics in the coastal waters of the Kamchatka Peninsula and the northern Kuril Islands remain poorly understood. Based on an analysis of over 4500 satellite radar and optical images collected from 2015 to 2024, more than 1100 eddies with diameters up to 24 km were identified. Cyclonic eddies with diameters of 2–4 km dominated, comprising approximately 80
Long-term mooring observations conducted in the western tropical North Pacific (12°50′N, 137°E) for 8 years (1989–1997) at four depths (500, 700, 2500, and 4500 m) revealed interannual variability in the zonal flow component (U) of the subsurface layer (500 and 700 m) with a period of 4 years. Spectral analysis indicated that coherent variability extended to 2500 m. This variability showed that the phase at 2500 m preceded that at 700 m by 7 months, which suggested upward phase propagation. High-resolution simulation results suggested that the observed interannual variability in U may be due to the movement of zonal jets appearing in the model. The 5-year pressure (P) harmonic, which represents the simulated interannual variabilities, was somewhat longer than the observed period and showed anticlockwise phase propagation in the surface layer along a circuit in the equatorial and tropical North Pacific. The mooring site was located in the latitude band of the circuit with the off-equatorial Rossby waves, where the westward–downward energy translation and westward–upward phase propagation of the 5-year P and U harmonics were seen. The observed upward phase propagation of U may be a manifestation of such vertically propagating waves, as their ray paths and corresponding equi-phase lines were consistent with the amplitude and phase distributions of the 5-year P harmonic. These results suggest that the observed interannual variability may represent an aspect of the ENSO cycle, providing a new insight for the linkage between the ENSO and deeper ocean variability.
Four-dimensional variational ocean reanalysis for the seas around Japan over 60 years (FORA-JPN60) is unprecedented reanalysis data for the seas around Japan, featuring high-resolution (2 km) and long-term (1960–2020). It is the successor to FORA-WNP30 covering the western North Pacific with 10 km resolution and aims at reproducing coastal circulation variability in addition to mesoscale and larger-scale variability in the open ocean. For this purpose, FORA-JPN60 incorporates various improvements from FORA-WNP30. The horizontal resolution of the ocean model is much increased, and the model incorporates tidal forcing and river discharge. As for data assimilation, an assimilation method for satellite altimeter data is improved to consider non-steric sea-level signals. Furthermore, observational data from fishery agencies and local governments are utilized in the reanalysis experiment to enhance in situ observations around Japan. After evaluating the behavior of the cost function and innovations in the variational assimilation method of FORA-JPN60, sea surface temperature and sea surface height fields are validated with focus on long-term changes, variability and reproducibility in coastal waters. This is part I of a two-part series and a comprehensive evaluation of FORA-JPN60 will be given in part II.
This study investigates interannual variations in temperature and salinity in the southern Okhotsk Sea, including the Soya Warm Current (SWC) region, based on observational data over the past 40 years (1984—2023). A key challenge is resolving the strong temperature and salinity front between the SWC and offshore areas to accurately assess interannual variability. To address this, we applied a mapping method (Mensah and Ohshima, J Atmos Oceanic Technol, 2023) and updated climatological fields of temperature and salinity. Analysis of anomalies from the climatologies reveal distinct warming trend confined to the SWC region. The strongest warming occurs at depth of 30‒50 m with a rise of 1.36°C over 40 years. These results suggest that the observed coastal warming is driven primarily by inflow from the Japan Sea, rather than by local atmospheric forcing. This is consistent with warming trends in sea surface temperature and 2 m air temperature over the southern Japan Sea. In contrast, the absence of a warming signal offshore of the SWC can be explained by the annual thermal reset, whereby the surface is cooled to the freezing point each winter during sea-ice cover, erasing accumulated thermal anomalies. Regarding salinity, a pronounced freshening trend is observed in autumn in the offshore region of the SWC, mainly attributed to increased Amur River discharge after 2010. Time series of the anomalies indicate that temperature, salinity, and volume transport in the SWC vary synchronously, despite considerable interannual variability.
Marine heatwaves (MHWs) are characterized by extremely high water temperatures persisting for at least several consecutive days, and their severe ecological and economic impacts have been increasing. In this study, the impacts of interannual-to-decadal sea surface temperature (SST) variability on MHWs in 10 areas around Japan were evaluated using daily satellite-based SST data from 1983 to 2022. To evaluate these impacts, MHWs were detected using SST without the interannual-to-decadal variability, while keeping the threshold for the MHW detection unchanged. As a result, the annual MHW days averaged for the 10 areas were reduced by 53
A new procedure to automated error detection for Argo procedure is introduced to produce intermediate-quality dataset quickly. The procedure is one of the improve methods based on the previous study with a path-signature, in which we incorporate the new machine learning methods. The performances trained with the dataset produced in 2016 were similar to the ones based on 2022 dataset and robust from 2017 to 2021. This suggests that the method in this study was successfully learn the general features of QCs and can discriminate error profiles relatively close to the one in Argo data centers.
We present a vertical and horizontal multiple plankton sampler (VHMPS) for meso- and microzooplankton. This sampler is an upgraded version of the vertical multiple opening and closing plankton sampler (VMPS) and is designed for oblique and vertical tows. The VHMPS can perform depth monitoring and close plankton nets at specific depths. The standard system of the VHMPS includes a mouth opening area of 0.25 m2, six nets, and a maximum operating depth of 3,000 m. Moreover, the VHMPS can filter large volumes of water by employing oblique tows, making it effective for collecting plankton in environments with low zooplankton abundance.
This study examines wintertime wind-induced near-inertial internal waves (NIWs) in the Sea of Japan. Energy source of NIWs from the atmosphere is the greatest in winter, but energy propagation into the ocean and contribution to turbulent mixing are unclear. We analyzed horizontal current records from a mooring system covering the top 1200 m at the full depth of 1770 m. During the winter, noticeable NIW events were caused by the cold outbreaks from the Eurasian continent. Although downward-propagating NIWs were dominant, upward-propagating NIW was identified around the main pycnocline in March 2020, when FATO mooring site located the north of an anti-cyclonic eddy (ACE) center. A detailed examination of wave properties near the interface revealed that the intrinsic frequencies of the downward- and upward-propagating waves were nearly coincident with 1.1f. This result implies that upward-propagating NIW is the reflected wave of downward-propagating NIW generated by the same atmospheric disturbances. Turbulent mixing due to Kelvin–Helmholtz instability was implied during the reflection event. The reflection layer was likely the lower pycnocline, splitting the Tsushima Warm Current water from the Japan Sea Proper Water. We discussed the following reflection mechanisms: reflection at the seafloor, partial reflection at the internal boundary, and internal reflection around the edge of ACE. One interpretation would be the internal reflection. The ray-tracing calculations using an idealized ACE structure revealed that the temporal vertical reflection of NIW and kinetic energy amplification around the rim of the ACE.
Noctiluca scintillans is a globally distributed harmful algal bloom (HAB) species known for potentially causing fish mortality and economic losses to fisheries. N. scintillans tends to accumulate near the sea surface, making it particularly susceptible to transport by ocean currents, however, direct evidence of long-distance dispersal has remained limited. Year-round monitoring in Kumamoto revealed that the Indonesian (Jakarta-type, K2) genotype occurs predominantly during the autumn high-abundance period, coinciding with smaller cell sizes that match Jakarta population. To evaluate the plausibility of long-distance transport, we conducted Lagrangian particle-tracking simulation using OSCAR surface currents. The results showed a plausible physical ocean connectivity between Indonesia and Japan within 600 days, with consistent patterns across different particle-release numbers indicating that arrival probabilities remained low but spatially robust. Recognizing that OSCAR provides a 0.25° satellite-derived representation of basin-scale surface circulation that does not explicitly resolve mesoscale eddies, we interpret these trajectories as possible connectivity pathways rather than literal particle tracks. Together, our genetic, morphological, and particle-tracking simulation results indicate that N. scintillans populations in Yatsushiro Bay likely consist of both regional and foreign genetic contributors, highlighting the potential for long-range connectivity under contemporary circulation patterns.
Heavy rainfall events can severely reduce salinity in enclosed seas and cause physiological stress to marine organisms such as bivalves, and their frequency and intensity have been in an increasing trend in recent years. In the study area, Nanao Bay, located on Noto Peninsula, Japan, several flood disasters associated with heavy rainfall occurred in recent years, and in some cases, extremely low salinity water was observed in the aquaculture ground. There is no river whose flow rate is monitored in the watershed, and the impact of heavy rainfall events on the environment of this bay remains to be fully understood. In this study, a coastal ocean model with a rainfall-runoff model was developed and optimized using the results of the field observations, and the influences of heavy rainfall events and winds on low salinity water dynamics and seawater exchange were numerically investigated. The results of the field observations showed the influence of two prevailing winds in this area on low salinity water dynamics; northeasterly winds caused low salinity water to stagnate at the bay head, while southwesterly winds quickly dissipated low salinity water. The results of the sensitivity experiments showed that estuary circulation was enhanced by southwesterly winds common in July–August while suppressed by northeasterly winds frequent in late summer. The coastal ocean model with the rainfall-runoff model well reproduced the low salinity water dynamics, suggesting that it would be useful for analyzing the impact of increased heavy rainfall on the coastal area.
Coupled Model Intercomparison Project (CMIP) and Ocean Model Intercomparison Project phase 2 (OMIP2) models from the CMIP6 group were used in the current study to represent the annual mean biases of hydrographic features. OMIP2 models are ocean-only simulations, while CMIP models are coupled ocean-atmosphere-land-sea ice simulations. These models are assessed against observations in the Tropical Indian Ocean (TIO). This study found that many models from both CMIP and OMIP2 exhibited cold temperature biases at the surface and warm biases in the subsurface on an annual scale, respectively. Overall, the CMIP models were observed to have larger biases than the OMIP2 models. Also, stronger saltier biases were identified in the south-eastern Arabian Sea (AS) and western Bay of Bengal. In addition, a deeper thermocline was identified in CMIP models compared to OMIP2 and observations in the northern AS and Seychelles-Chagos Thermocline Ridge. This deeper thermocline is associated with subsurface warm biases. Brunt-Väisälä frequency revealed weaker stratification from surface to 100 m with a peak at 80 m. Further, vertical shear of horizontal currents revealed strong shear bias at the top 40 m, that can result in vertical mixing, which is chiefly accountable for the biases of temperatures and salinities. Heat and salt transport analyses across different straits in the TIO indicated positive transport to the north (east) and negative transport to the south (west). Positive transport occurred during post-monsoon season, while negative transport occurred during other seasons. SST-based upwelling index analysis revealed stronger upwelling signals in CMIP models than in OMIP2 during the summer months across all regions, primarily driven by stronger winds. A strong negative correlation has been identified between surface temperature and wind speed in CMIP models across most of the TIO, suggesting that strong surface wind speeds drive vertical mixing, which in turn leads to further surface cooling.
In this study, we analyzed decreasing trends in the amplitude (− 0.185 m century− 1, or − 12.1
In this study, the scattering and dissipation processes of diurnal coastal trapped waves (CTWs) are investigated using mooring observations and direct microstructure measurements in the Suruga Trough, Japan. First, we perform spectral and harmonic analyses of the observed data, confirming the presence of diurnal CTWs characterized by velocity and temperature perturbations that are most pronounced near the seafloor. The interaction of bottom-intensified CTW currents with a small-scale bathymetric bump generates high-frequency waves, exhibiting spectral peaks at the harmonics of the diurnal tidal constituents. These high-frequency waves are identified as internal lee waves based on the dispersion relationship and are responsible for strong mixing on the lee side of the bathymetric bump, as evidenced by microstructure measurements. Using moored velocity records, we further examine the parameter dependence of internal lee wave generation by CTWs propagating over the bathymetric bump. The results indicate that internal lee wave activity intensifies when the tidal excursion parameter T_e falls within an intermediate range between 1 and the stratification parameter N^* . In contrast, when T_e exceeds N^* , the energy of internal lee waves decreases markedly due to the formation of “evanescent waves” resulting from strong interactions between CTWs and the bathymetric bump.
