Journal of Oceanography最新文献

Marine prokaryotes are involved in mercury (Hg) speciation in marine environments. However, information regarding the specific microbial lineages contributing to Hg speciation (namely, methylation, demethylation, and reduction) in seawater remains limited. In this study, we investigated the genes involved in methylmercury (MeHg) demethylation (merB), Hg reduction (merA), and Hg methylation (hgcA and hgcB) using a metagenome analysis encompassing free-living (FL; 0.2–3 µm) and particle-attached (PA; > 3 µm) fractions in April and June in Minamata Bay, Japan. We analyzed the total Hg (THg) and MeHg concentrations in dissolved, small particulate (0.2–3 µm), and large particulate (> 3 µm) fractions. Hg analysis showed no significant differences in THg and MeHg concentrations among the three fractions. However, THg and MeHg in the pooled particulate (fractions were higher than those in the dissolved fractions) in June. As for the Hg-related genes, a high merB abundance rather than merA was observed when compared using all samples, suggesting that microbial MeHg demethylation could be prominent rather than Hg reduction in the seawater column. Phylogenetic analyses revealed that Alphaproteobacteria and Actinobacteria were dominant in the total merA and merB-like sequences, respectively. For both merA and merB genes, some lineages appeared in either or both FL and PA fractions. A deltaproteobacteria-like hgcA was also detected in the PA fraction in April. Therefore, the link between distinct phylogenetic lineages within the PA and FL fractions could be significant for Hg speciation in Minamata Bay.

Exploring the variability in design wave heights through extreme value analysis is crucial for understanding extreme oceanographic conditions and is paramount for the safe design and operation of offshore structures. This research delves into a comprehensive analysis of wave height variability in the Indian shelf seas, focusing on wave heights for a 100-year return period at 30 locations using 42 years of reanalysis data. The study employs the Generalized Extreme Value distribution and the Generalized Pareto Distribution (GPD), utilizing the annual maxima approach with three methods: maximum likelihood, probability-weighted moments, and moments of probability scale. The GPD Peaks Over Threshold (POT) approach is explored with thresholds estimated using various (91 to 99) percentiles, alongside a graphical approach utilizing threshold determination through mean residual life (MRL) plots. Results indicate that the GPD POT approach, particularly when employing thresholds derived from MRL plots, outperforms other methods. Thresholds obtained by MRL method closely align with the thresholds obtained through the 99 percentile for all locations in the Arabian Sea. This investigation not only enhances our understanding of the dynamic wave processes in the Indian shelf seas but also underscores the efficacy of specific statistical distributions and approaches in the evaluation of design wave heights.

Tokyo Bay is one of the most productive coastal systems in the world. Surrounded by a growing urban metropolis, the bay has experienced large fluctuations in water quality over the past decades due to eutrophication and regulatory requirements for wastewater treatment. However, the spatial and temporal variability of chlorophyll a (Chl a) and associated environmental factors remains unclear. In this study, water temperature, salinity, light condition, Chl a and nutrient concentrations was investigated monthly at three stations in the inner area from November 2016 to October 2020. In the surface water, the Chl a and nutrient concentrations largely fluctuated seasonally. In the summer season (May–September), with sufficient nutrient input from rivers and wastewater treatment plants, Chl a concentration in the northwestern part of the bay (St. AO) was positively correlated with water temperature with extremely high concentration (mean, 90.5 µg L⁻¹). On the other hand, those in the northern (St. CB) and central (St. F3) areas were negatively correlated with nutrient concentrations, especially DIP and DSi which sometimes decreased to below the detection limit (BDL). In the winter–spring season (January–April), the Chl a concentration was relatively high at St. CB (mean, 25.9 µg L⁻¹), which could be attributed to light availability, sufficient light penetration to the bottom, and vertical mixing of the entire water column. These results indicated that the factors controlling Chl a differed among areas and seasons in the bay.

The Oyashio region east of northern Japan has experienced frequent marine heatwaves (MHWs) since 2010, and in the summer and fall of 2022, sea surface temperature hit a record high as of that year. This study examined the impact of the 2022/23 MHW on dissolved oxygen (DO) by analyzing observations from a vessel and biogeochemical Argo floats. It was found that warm saline water from the Kuroshio Current intruded at ~ 42°N in July. DO anomalies from the climatology above a depth of 200 m were negatively correlated with the temperature anomalies at the same depth, while the opposite was true for deeper depths. In the density coordinate, DO and temperature anomalies exhibited a strong negative correlation when the potential density (σ_θ) was less than ~ 27.0 kg m⁻³. Thus, it was demonstrated that subsurface DO anomalies could be statistically predicted from temperature and salinity fields using this relationship. Notably, DO anomalies could be divided into components related to isopycnal mixing and density-surface heaving. This decomposition revealed a dynamical process, whereby the intrusion of the Kuroshio water, which is lighter than the Oyashio water, pushed down the density surfaces, causing oxygenation. Meanwhile, isopycnal mixing tended to mitigate the increase of DO concentration since DO concentration was smaller in the south than in the north on an isopycnal surface of σ_θ < 27.0 kg m⁻³. This study clarified that, during the 2022/23 MHW, deoxygenation occurred near the surface owing the warming, whereas the DO concentration increased in the subsurface layer.

The Oyashio, a southern part of the western boundary current in the North Pacific subarctic gyre, carries cold and fresh seawater with abundant nutrients southward from the high-latitude, influencing regional climate in the East Asia and marine environment in the western mid-latitude North Pacific. Previously, a distribution of the Oyashio water has been evaluated by empirical temperature thresholds; for example, in spring (March–May) when the Oyashio intrudes southward into the east of Japan, the Oyashio water is defined at 100-m depth as ≤ 5 °C. However, this method is not necessarily adequate under the changing climate because upper ocean temperature may change over time due to some causes unrelated to cold water transport by the Oyashio (e.g., surface heat fluxes). In this study, we developed an objective method to evaluate the Oyashio water distribution applicable under the changing climate with a focus on a thermohaline front located at the warm- and salty-side boundary of the Oyashio water. We identified isohalines at 100-m depth best corresponding to the thermohaline front in each month and used them as the Oyashio water threshold. Using the developed method, we further investigated the springtime Oyashio water distribution east of Japan (in the North Pacific south of 43°N, 141–148°E). The area of the Oyashio water shows inter-annual variation and significant long-term decrease. It was suggested that these temporal variation and change reflect changes in a distribution of anti-cyclonic meso-scale eddies off Hokkaido, which block the southward Oyashio intrusion into the east of Japan.

A surface layer (upper 20 m depth) heat budget analysis, derived from a hindcast regional-scale ocean modeling experiment, was employed to examine the underlying mechanisms behind the emergence of sea surface temperature anomalies (SSTA) in the Indonesian seas during El Niño events over the 1995–2019 course. Prior to the emergence of warm SSTA, which typically appeared following the mature phase of El Niño and lasted for almost a year, apparent anomalous heat accumulation occurred for at least 2–4 months and peaked in conjunction with the climatic event. Further examination revealed possible east–west distinct dynamics in the heat budget variations within the region during El Niño years. The anomalous heat accumulation in the western part of Indonesian seas (Java Sea) was predominantly caused by modulation in the surface net heat flux. Whereas in the eastern part (Banda Sea), the ocean circulation also exerted important influence in addition to the surface net heat flux. The ocean circulation in the eastern Indonesian seas notably contributed to moderate the effect of surface net heat flux during El Niño growth. Moreover, the same ocean circulation was responsible for prolonging the anomalous heat accumulation in the eastern Indonesian seas from mature to decay phase of the El Niño, ultimately resulted in warmer SSTA than that in the western part. The study conducted here provides additional insights on how the Indonesian seas responded to the El Niño and further reaffirms the idea that the climatic event results in anomalous warming across the Indonesian seas.

In numerical models that adopt a no-slip boundary condition, a jet initially separates from the shore and flows eastward south of the inter-gyre boundary. This premature separation is caused by positive vorticity created by friction on the western boundary, which is paired with negative vorticity created by the conservation of the potential vorticity advected from the south. In this study, we propose a new model that includes vortex pair forcing in the western boundary region, where a vorticity structure causes separation and extension of the western boundary current (WBC) jet. Our model can separately treat flow that is locally driven by large-scale wind and the WBC extension jet driven by a vortex pair. We confirmed that our model could successfully replicate an extension jet prematurely separated from the western boundary for a wind-driven, two-layer quasi-geostrophic model. The dominant physical structure of the jet in our vortex pair forcing model differed in the upstream and downstream regions. In the upstream region, the intrusion of vortex pairs due to forcing is the dominant factor, while in the downstream region, the effect of eddies begins to manifest. We also examined the effects of a large-scale wind-driven background flow on the jet. When the background flow only contains an eastward component, the jet extends eastward. When the background flow contains a meridional component, this strongly affects the extended jet and can terminate the eastward jet structure.

The North Pacific Central Mode Water (CMW) is examined from the viewpoint of its volume variations. The volume of the CMW layers thicker than (150 m) is used as an index of CMW variations, which successfully represents the year-to-year and decadal variations in the CMW volume. The CMW index shows the variation close to that of the Pacific Decadal Oscillation (PDO). The CMW variation is strongly tied with large-scale, dominant variations in sea surface temperature (SST), surface dynamic height (SDH), and sea surface height (SSH) anomalies in the North Pacific, with a significant correlation with the PDO. Year-to-year and decadal variations of CMW volume in May to July are significantly correlated with the wintertime Aleutian Low and 500(hPa) geopotential height variations, which indicates that the Aleutian Low induces the eastward extension/retreat of the strong winter westerlies and resultant net surface heat flux anomaly over the CMW distribution region. Thus, the CMW volume variation can be regarded as a significant manifestation of the PDO. The SST, SDH, and SSH anomalies are associated with the surface cooling in the northern sector of the CMW distribution region. On the other hand, the SDH and SSH anomalies throughout a year and the SST anomaly in the cold season in the southeastern sector of the CMW region are formed due to the heaving of isopycnal surfaces in the subsurface layer above the CMW in response to its volume variations.

This study provides the first estimation of sea ice-melt amount in the Sea of Okhotsk based on spring hydrographic data accumulated for nearly a hundred years. Just after sea ice melts completely, a low-salinity layer appears on the ocean surface, overlying the layer of Winter Water at the freezing point. The integration of the salinity decrease from Winter Water should correspond to the total ice-melt amount. We developed an algorithm to extract the profiles that clearly show the salinity deficit and converted the salinity deficit to the ice-melt amount from all available data. The climatological map shows that ice-melt amount decreases toward the ice edge and exhibits large values around the northern Sakhalin Island, reflecting the ice thickness distribution. In the southern area (south of 48°N), where sea ice is transported from the north, the average ice-melt amount is estimated to be ~ 71 cm in thickness. It is clearly shown that the ice-melt amount has decreased by ~ 30% in the southern area since the 1990s. These changes possibly affect the regional climate through the decreased latent heat of sea ice and potentially affect biological production through weakened stratification caused by decreased ice melt. We also suggested that ice-melt amount did not show a significant trend during the 1930s–1970s, implying that our methodology could extract information on sea ice before the era of satellite observations.

During August 2017, the Kuroshio began to follow its large meander (LM) path, and this was 12 years after the last LM event. Such LM events lead to the formation of an inshore cyclonic eddy (ICE) within the Kuroshio inshore region between the Kuroshio and the southern coast of Japan. We analyzed data from repeated seasonal surveys along a transect that followed 138°E, and from additional surveys, satellite observations, Argo floats, and a high-resolution reanalysis dataset to describe the temporal variation of the LM and ICE between 2017 and 2021. Cross sections from the repeated surveys highlighted the variations in the water mass structures. The time series of the indicators of the ICE intensity such as the Kuroshio volume transport and the planetary contribution of potential vorticity (Q) in the ICE revealed a maintenance trend in their intensity during the observation period. High-Q water was distributed in the ICE during the LM period compared to the non-LM period, and showed a clear seasonality within the shallower layers, suggesting the injection of Q via the advection from the upstream region in summer. Additional surveys captured an eddy that became detached from the ICE during summer 2020. This detached eddy had a Q value greater than the ICE and velocity similar to the Kuroshio, suggesting that the main stream had become temporarily separated. The structures and volume transport of warm water intrusions into the inshore region were also examined, and centrifugal instability was considered to be one of their generation mechanism.