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 (σθ) 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 σθ < 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.

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.

In this article the author's name Naoto Iwasaka was incorrectly written as Iwasaka Naoto.

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.

The North Pacific Central Mode Water (CMW) is examined from the viewpoint of its volume variations. The volume of the CMW layers thicker than 150m\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$150 m$$\\end{document} 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 500hPa\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$hPa$$\\end{document} 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.

The Japan Sea shows a much stronger warming of long-term sea surface temperature (SST) than surrounding oceans. The warming trend possesses a meridionally alternating zonal band pattern, with weak trends along the paths of the Japan Sea Throughflow and strong trends in the remaining interior region. Using idealized models of the Japan Sea Throughflow and atmospheric heating, this study examines the process behind the formation of such spatial patterns in the SST trend. We find that zonal band structures form in a flat rectangular coastline model, and heat budget analysis shows that horizontal heat transport, due to throughflow, reduces the warming effect created by the surface heat flux. A weak SST trend appears around the jet, while a strong SST trend appears elsewhere. Bathymetric effects are also examined using a model with realistic coastline settings. The location of the western boundary current stabilizes, and the coastal branch begins to disconnect from the Japanese coastline toward the north, allowing a more stable SST warming region to form in the southern interior region. Lagrangian particle tracking experiments confirm that a weak (strong) SST trend corresponds to a short (long) residence time, and eddies in the Japan Sea prolong the residence time in interior regions. The model results suggest that the accumulation time of surface heating is essential to the spatial distribution of the long-term SST warming trend.

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.