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This chapter reviews interdecadal variations of the Oyashio from the 1960s to the 2010s and the impacts of the Oyashio on coastal waters with a focus on the Extreme Cold Water Events (ECWEs) that occur during winter–spring near the Japanese coast adjacent to the Oyashio. The southernmost latitude of the First Oyashio Intrusion migrated southward in response to the basin-scale wind stress from the mid-1960s to the mid-1980s, while it changed less interdecadally from the mid-1990 to the mid-2010s. For the latter period, the Oyashio transport/velocity, including the intensity of the Western Subarctic Gyre, decreased interdecadally. Representative ECWEs occurred during winter–spring in 1963, 1974, 1981, 1984, 2006, and 2014. These ECWEs occurred under different conditions during the two periods; the ECWEs in 1963, 1974, 1981, and 1984 were accompanied by extreme southward intrusions of the Oyashio, whereas the ECWEs in 2006 and 2014 were not. The ECWEs in 2006, 2014, 1974, and 1984 occurred concomitantly with an extreme intensification of wind-induced circulation in the Okhotsk Sea that lasted a few months. Hence, the ECWEs could be caused by either or both of extreme southward intrusion of the Oyashio and extreme intensification of wind-induced circulation in the Okhotsk Sea.

To investigate the influence of a river discharging outside a bay on circulation in the bay, we carried out current and salinity measurements from mooring systems and hydrographic observations in Matsushima Bay, Japan, and off the Naruse River, which discharges outside the bay. Previously, enhancement of horizontal circulation in the bay induced by increased freshwater input from the Naruse River was reported to have degraded the seedling yield of wild Pacific oysters in the bay, but the freshwater inflow from the river was not directly measured. Our hydrographic observations in Katsugigaura Strait, approximately 3 km southwest of the Naruse River mouth, detected freshwater derived from the river. The mooring data revealed that freshwater discharged by the river flowed into Matsushima Bay via the strait and that the freshwater transport increased when the river was in flood. The inflow through straits other than Katsugigaura was estimated by a box model analysis to be 26–145 m3 s−1 under normal river discharge conditions, and it decreased to 6 m3 s−1 during flood conditions. During flood events, the salt and water budgets in the bay were maintained by the horizontal circulation: inflow occurred mainly via Katsugigaura Strait, and outflow was mainly via other straits.

Matsushima Bay is one of the famous cultivation area of oysters. However, it is reported that the seedling collection decline in 2013. It is necessary to clarify this cause in order to take a countermeasure. There is some possibility that strong tidal currents through narrow straits cause oyster larvae in the bay to flow out. It is pointed out that increase of river discharge leads to a large flow of them. This research aims to clarify the influence on the movement of larvae in Matsushima Bay, especially focused on currents and river discharge. In this paper, the numerical model for tidal currents around Matsushima Bay is developed as the first step of the study to clarify their influence on them. The model is based on finite volume method with unstructured grid system and generalized terrain-following coordinate system (FVCOM). The model reproduces the influence on tidal currents by heat flux, precipitation, evaporation, surface wind stress and river discharge. The results are compared with some observation data to verify the validity of the model. The characteristics of currents around Matsushima Bay are also investigated from the result reproduced by the model, focused on the influence of fresh water from rivers.

Oyster and scallop aquaculture has flourished in Ofunato Bay, on the Sanriku Coast of Japan. Between 1967 and 2011, this bay was a highly enclosed area by a bay-mouth breakwater, and while the breakwater provided calm conditions suitable for aquaculture, there was concern about water quality deterioration. The Tohoku Earthquake off the Pacific coast and the subsequent major tsunami on 11 March 2011 caused extensive damage to the Sanriku Coast, resulting in the collapse of the breakwater of Ofunato Bay. We analyzed the monthly hydrographic observational data that were collected between 1996 and 2015, supplemented with hydrographic observations and direct current measurements that were carried out after the earthquake. The historical hydrographic observational data showed the lowest dissolved oxygen (DO) concentrations were observed near the bottom in the bay. The average and standard deviation of the lowest DO concentrations of each monthly observation during the period before and after the earthquake were 5.81 ± 3.72 and 7.22 ± 1.49 mg L−1, respectively, and are significantly different. The oceanic water inflow and water exchange rate estimated by a box model based on the salt budget under the steady-state estuarine circulation were also significantly different before and after the earthquake. An increase in the inflow of oceanic water and the more rapid water exchange in the bay after the earthquake induced marked improvements in the bottom DO concentration. These changes are considered to be attributable to the improved circulation of oceanic water in the bay that resulted from the collapse of the bay-mouth breakwater.

Sendai Bay in northern Japan suffered serious damage from massive tsunamis generated by the 2011 off the Pacific coast of Tohoku earthquake. The physical disturbance caused by a tsunami may affect the coastal ecosystem, including the planktonic diatom community. We investigated seasonal changes in the diatom community structure at a coastal and an offshore station in Sendai Bay, from June 2011 (3 months after the tsunami) to April 2014. Diatom abundance increased at both stations during the spring. Sporadic increases were also recorded at the coastal station during the summer because of silicate input from river discharge. Seasonal succession of the diatom communities was similar at both the coastal and offshore stations. The onset of the spring bloom consisted mainly of Chaetoceros spp. when water temperatures were low. Subsequently, species such as Skeletonema costatum s.l. became dominant as salinity and nutrient concentrations decreased. Cell density decreased from summer into early winter. Leptocylindrus danicus became dominant in the summer, but was replaced by Thalassiosira cf. mala from autumn into winter. Redundancy analysis (RDA) showed that most of the variation in the diatom community could be explained by temperature, salinity, NO3 −, NO2 −, PO4 3−, and SiO2. In addition, the occurrence of diatom species before the tsunami showed a similar pattern to that after the tsunami, suggesting that the tsunami did not have a serious impact on the diatom community in Sendai Bay.

The Western Isoguchi Jet, a quasi-stationary baroclinic jet that separates from the Kuroshio Extension, has been detected for a decade. Here we report the first observations of the vertical structure and temporal variations of the jet via moored current meters (September 2011–September 2012) and intensive hydrographic measurements (September 2010, 2011, and 2012). The jet flows northeastward and supplies subtropical water (northeastward velocity was ∼ 0.16 m s−1 and temperature was ∼ 4.4 ° C at 300 m depth, and they were ∼ 0.05 m s−1 and ∼ 2.9 ° C at 1000 m depth). We newly estimated the volume, heat, and salt transports of the jet to be 13.6–26.5 Sv, 0.0745–0.173 PW (reference potential temperature of 4.0 ° C ), and 5.58–9.91 × 10 6 kg s−1 (reference salinity of 33.5), respectively. Year-to-year changes of the jet axis complicated its horizontal distribution. The jet flowed stably northeastward throughout the year with weak seasonal variability. The vertical structure of the jet is quite different from that of the surrounding flow. These observations are consistent with the mechanism proposed by Mitsudera et al. (2018) and Miyama et al. (2018) to be responsible for the formation and maintenance of the jet.

We investigated the main controlling factors and predictability of extreme sea surface temperature changes along the Sanriku coast (the east coast of the northern part of Japan's main island). We analyzed distributions of water properties and flow fields via intensive observations using a conductivity–temperature–depth profiler and a coastal water-temperature monitoring system from January 1998 to December 2012. Satellite altimetry and tide gauge data were also analyzed to investigate more widespread horizontal and temporal variation of the sea surface flow field. Anomalous temperature events (2 °C lower and higher than climatological monthly values) were observed in winter 2006 and fall 2010 and 2012 along the Sanriku coast. In winter (fall) 2006 (2010, 2012), we observed both unusually thick and wide cold/fresh (warm/saline) waters, corresponding to the Oyashio (Tsugaru Warm Current) waters. At that time, sea surface velocities of the Oyashio (Tsugaru Warm Current) along the Hokkaido coast (Tsugaru Strait) were also high. We propose new methods for predicting extreme temperature changes a few months in advance, based on current observations.

We investigated the interdecadal trend of Oyashio velocity and transport during 1993–2011 based mainly on linear trend analysis of altimetry and in situ temperature-salinity data from a monitoring line (“A-line”) off the southeastern Hokkaido coast. Significant trends of increasing sea level were detected on the continental slope, north of the Kuril-Kamchatka Trench. Sea level anomaly data revealed a localized clockwise circulation centered near the trench, the suggestion being that the strength of the Oyashio on the slope and the offshore return flow had decreased. The Oyashio mainstream seemed to have shifted from a nearshore to an offshore path. Steric heights estimated from the A-line data exhibited an increasing trend north of the trench, where 50–80% of the increase was determined by halosteric components attributable to a trend of decreasing salinity in the subsurface. The trend of decreasing salinity was related to downward displacement of isohaline/isopycnal surfaces. The largest displacement was above the trench. Horizontal pressure gradients associated with southwestward flows on the slope were weakened. The Oyashio transport decreased by 8.9 Sv (106 m3 s−1) in 19 years. A mesoscale eddy analysis revealed that clockwise eddies appeared more frequently in recent years near the trench around the A-line and could decrease the Oyashio transport. A baroclinic, long Rossby-wave model also predicted that a large-scale baroclinic response to the wind stress could weaken the Oyashio velocity in the upper layer. Dynamical linkage between the localized eddies and large-scale response remains to be clarified in future work.

We investigated the elevated chlorophyll-a (Chl-a) zone found along the coast in winter in Sendai Bay, Japan, using hydrographic observations and a one-dimensional ecosystem model. Chlorophyll-a distribution was vertically homogeneous with a horizontal gradient and could be approximated as a power function of bottom depth; Chl-a concentration drastically increased with decreasing bottom depth, despite temperature and salinity being almost vertically and horizontally homogeneous. The observed results revealed significant correlations among Chl-a and nutrients concentrations proportional to the Redfield ratio. Diatoms accounted for more than 99% of the detected total cells, indicating the occurrence of a diato m bloom. A one-dimensional ecosystem model, which incorporated vertical mixing and the self-shading effect of phytoplankton, revealed that bottom depth was responsible for the occurrence of the bloom during the convective season in coastal area where vertical mixing reached the bottom and that there existed the critical bottom depth where the integrated Chl-a in the water column remained constant. A bloom could occur where the bottom depth is shallower than the critical bottom depth, not when the depth of the mixed layer is shallower than the classical critical depth and the stratification is established. From the observational and model results, it is suggested that the diatom bloom was induced by oceanic water intrusion, which transported nutrients to the bay and the elevated Chl-a zone was formed within a month after the intrusion.

The hydrographic structure of a quasi-stationary jet separated from the Kuroshio Extension, a phenomenon that possibly leads to deepening of the winter mixed layer, is revealed via intensive observations using a conductivity–temperature–depth profiler and a ship-mounted acoustic Doppler current profiler (ADCP) carried out in September 2009. Data collected from profiling floats set to drift isopycnal layers and time series of sea surface buoys and satellite altimeters were also analyzed to understand the continuity, water characteristics, and time variation of the jet. Although the flow field was complex due to disturbances such as energetic eddies, the jet transported subtropical water to the downstream (northeastern) observation region in a layer shallower than 400 dbar and its highest velocity was concentrated in the mid- and deep layers (≤0.30 m s−1 at the sea surface and ≥0.05 m s−1 at 800-m depth). The velocity axes of the jet detected from the satellite data corresponded to those detected through analysis of the ADCP data, and the intensity of the jet at the sea surface varies over interannual-to-decadal time scales. Part of the interannual-to-decadal variation in the velocity field of the jet is controlled by the dynamic state of the Kuroshio Extension and is correlated with that of the Kuroshio Extension latitude (linear correlation coefficient r ~ 0.67). The relationship between these variations can be interpreted qualitatively as being responsible for the inertial streamers that separate from the crests of meanders of the Kuroshio Extension to the jet region due to the large steering effect.

Regeneration of a warm anticyclonic ring as a result of interaction with cold water masses was observed within the western subarctic gyre of the North Pacific. Satellite, profiling float, and shipboard observations revealed that a warm-core ring originated from the Kuroshio Extension, propagating northeastwards, entrained cold and fresh water masses from the coastal area of Hokkaido, which are typically recognized within the ring as water that is colder than 2.5 °C. The potential temperature and planetary contribution of potential vorticity of the cold water in the coastal area of Hokkaido were

Intensive hydrographic observations obtained with an underwater glider revealed the vertical structure with high horizontal resolution, as well as the temporal variation, of the Subpolar Front and the Tsushima Warm Current in the Japan Sea. Even when sea surface current velocities were large (>0.50 m s−1), the glider was able to completed the observations along the planned straight line. The glider detected changes in the frontal structures and the water mass distributions on spatial scales smaller than O(10 km) and on timescales smaller than O(1 month). We showed that the assimilation of glider data by an ocean dynamic model increased velocity magnitudes in the simulated flow field.

The Oyashio region in the western North Pacific supports high biological productivity and has been well monitored. We applied the NEMURO (North Pacific Ecosystem Model for Understanding Regional Oceanography) model to simulate the nutrients, phytoplankton, and zooplankton dynamics. Determination of parameters values is very important, yet ad hoc calibration methods are often used. We used the automatic calibration software PEST (model-independent Parameter ESTimation), which has been used previously with NEMURO but in a system without ontogenetic vertical migration of the large zooplankton functional group. Determining the performance of PEST with vertical migration, and obtaining a set of realistic parameter values for the Oyashio, will likely be useful in future applications of NEMURO. Five identical twin simulation experiments were performed with the one-box version of NEMURO. The experiments differed in whether monthly snapshot or averaged state variables were used, in whether state variables were model functional groups or were aggregated (total phytoplankton, small plus large zooplankton), and in whether vertical migration of large zooplankton was included or not. We then applied NEMURO to monthly climatological field data covering 1 year for the Oyashio, and compared model fits and parameter values between PEST-determined estimates and values used in previous applications to the Oyashio region that relied on ad hoc calibration. We substituted the PEST and ad hoc calibrated parameter values into a 3-D version of NEMURO for the western North Pacific, and compared the two sets of spatial maps of chlorophyll-a with satellite-derived data. The identical twin experiments demonstrated that PEST could recover the known model parameter values when vertical migration was included, and that over-fitting can occur as a result of slight differences in the values of the state variables. PEST recovered known parameter values when using monthly snapshots of aggregated state variables, but estimated a different set of parameters with monthly averaged values. Both sets of parameters resulted in good fits of the model to the simulated data. Disaggregating the variables provided to PEST into functional groups did not solve the over-fitting problem, and including vertical migration seemed to amplify the problem. When we used the climatological field data, simulated values with PEST-estimated parameters were closer to these field data than with the previously determined ad hoc set of parameter values. When these same PEST and ad hoc sets of parameter values were substituted into 3-D-NEMURO (without vertical migration), the PEST-estimated parameter values generated spatial maps that were similar to the satellite data for the Kuroshio Extension during January and March and for the subarctic ocean from May to November. With non-linear problems, such as vertical migration, PEST should be used with caution because parameter estimates can be sensitive to how the data are prepared and to the values used for the searching parameters of PEST. We recommend the usage of PEST, or other

The lateral transport of organic carbon in large grazing copepods (Neocalanus cristatus, Neocalanus flemingeri, Neocalanus plumchrus and Eucalanus bungii) from the Oyashio area to the mixed water region (MWR) by the coastal Oyashio intrusion was estimated using the data of VMPS (vertical multiple plankton sampler) and 1500 dbar-referred geostrophic transport from the CTD (conductivity temperature depth sensor) data of five cruises during June 2001 to April 2002 on a repeat observation section OICE (Oyashio Intensive observation line off Cape Erimo), which extends southeastward from Hokkaido Island, Japan. The transport to MWR by the coastal Oyashio intrusion was estimated to be 5.3 × 1011 g C for the four species. Data from profiling floats also indicated that the copepods were advected from OICE to MWR by the coastal Oyashio intrusion within about 2 months. This transport is considered to be one of the significant sources of organic carbon in MWR as it is larger than the amount of large zooplankton consumed by Pacific saury (Cololabis saira) in MWR, one of the dominant copepod predators in this region.

Large quantities of radioactive materials were released into the air and the ocean as a result of the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident, caused by the 2011 Tohoku earthquake and the subsequent major tsunami off the Pacific coast. There is much concern about radioactive contamination in both the watershed of the Abukuma River, which flows through Fukushima Prefecture, and its estuary, where it discharges into the sea in Miyagi Prefecture. We investigated radioactive cesium dynamics using mixing diagrams obtained from hydrographic observations of the Abukuma River Estuary. Particulate radioactive cesium dominates the cesium load in the river, whereas the dissolved form dominates in the sea. As the salinity increased from

間欠的な外海水進入を伊勢湾におけるCTD・ADCP・係留観測によって研究した。外海水は15cm s-1にも達する速度で海底に沿って進入し,湾内底層を低温,高塩化させた。外海水の進入は約2日間にわたって続き,もともと湾内下層にあった水塊を中居に押し上げた。残差流によるボリュームフラツクスから,湾内での湧昇速度を見積もると,底層での湧昇速度は5.9m d-1に達し,エスチュアリー循環流による湧昇速度の10倍以上となった。このような外海水進入は,ほぼ毎年発生している可能性が示唆された。外海水進入に伴う湧昇は,夏季の湾内庭層にある高濃度の栄養塩を有光層にすみやかに輸送し,高濃度の亜表層クロロフィル極大の形成に寄与している。

Recent surveys in Ise Bay, which is a major gulf-type region of freshwater influence in Japan, reveal that a cold dome is often observed in the stratified season. To elucidate the formation mechanism of the cold dome, detailed hydrographic and ADCP surveys were conducted in August 1995. The results show that a cold (T