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Although previous studies reported that currents over topographic features, such as seamounts and ridges, cause strong turbulence in close proximity, it has been elusive how far intense turbulence spreads toward the downstream. Here, we conducted a series of intensive in-situ turbulence observations using a state-of-the-art tow-yo microstructure profiler in the Kuroshio flowing over the seamounts of the Tokara Strait, south of Kyusyu Japan, in November 2017, June 2018, and November 2019, and employed a high-resolution numerical model to elucidate the turbulence generation mechanisms. We find that the Kuroshio flowing over seamounts generates streaks of negative potential vorticity and near-inertial waves. With these long-persisting mechanisms in addition to other near-field mixing processes, intense mixing hotspots are formed over a 100-km scale with the elevated energy dissipation by 100- to 1000-fold. The observed turbulence could supply nutrients to sunlit layers, promoting phytoplankton primary production and CO2 uptake. Persistent and intense mixing hotspots are generated where the Kuroshio flows over steep seamounts, with an increase in energy dissipation by two to three orders of magnitude, according to tow-yo microstructure measurements combined with numerical simulations.

While the Kuroshio is known to be a nutrient stream, as these nutrients are in dark subsurface layers, they are not immediately available for photosynthesis unless they are supplied to the sunlit surface layers. Recent microstructure observations have revealed that strong diapycnal mixing caused by the Kuroshio flowing over topographic features and double diffusion in the subsurface layers of the Kuroshio. However, it is still unclear how much nutrient flux can be provided by these microscale mixing processes. In this study, using an autonomous microstructure float and nutrient samplings, nutrient flux caused by the Kuroshio over the Izu Ridge, and that caused by double diffusion in the Kuroshio Extension are quantified. The nitrate diffusive flux is estimated to be $$>1 \,\hbox {mmol} \,\hbox {N}\,\hbox {m}^{-2}\hbox {day}^{-1}$$ > 1 mmol N m - 2 day - 1 over a distance, 20–30 km near the Izu Ridge and $$>0.1 \,\hbox {mmol} \,\hbox {N}\, \hbox {m}^{-2}\hbox {day}^{-1}$$ > 0.1 mmol N m - 2 day - 1 , which persists further downstream direction over 100 km along the Kuroshio, increasing the subsurface chlorophyll-a concentration in the region 200 km downstream. The double-diffusion-induced nitrate flux is estimated to be 1-$$10 \,\hbox {mmol} \,\hbox {N} \,\hbox {m}^{-2}\hbox {day}^{-1}$$ 10 mmol N m - 2 day - 1 in the pycnostad 26–$$26.5\,\hbox {kgm}^{-3}$$ 26.5 kgm - 3 of the Kuroshio Extension, suggesting that whether this double-diffusion-induced nutrient flux in the subsurface layers can ultimately contribute to surface primary production depends on additional eddy up- and northward fluxes.

The Kuroshio Current has been thought to be biologically unproductive because of its oligotrophic conditions and low plankton standing stocks. Even though vulnerable life stages of major foraging fishes risk being entrapped by frontal eddies and meanders and encountering low food availability, they have life cycle strategies that include growing and recruiting around the Kuroshio Current. Here we report that phytoplankton growth and consumption by microzooplankton are stimulated by turbulent nitrate flux amplified by the Kuroshio Current. Oceanographic observations demonstrate that the Kuroshio Current topographically enhances significant turbulent mixing and nitrate influx to the euphotic zone. Graduated nutrient enrichment experiments show that growth rates of phytoplankton and microheterotroph communities were stimulated within the range of the turbulent nitrate flux. Results of dilution experiments imply significant microzooplankton grazing on phytoplankton. We propose that these rapid and systematic trophodynamics enhance biological productivity in the Kuroshio.

To investigate changes in ocean structure during the spring transition and responses of biological activity, two BGC-Argo floats equipped with oxygen, fluorescence (to estimate chlorophyll a concentration – Chl a), backscatter (to estimate particulate organic carbon concentration – [POC]), and nitrate sensors conducted daily vertical profiles of the water column from a depth of 2000 m to the sea surface in the western North Pacific from January to April of 2018. Data for calibrating each sensor were obtained via shipboard sampling that occurred when the floats were deployed and recovered. During the float-deployment periods, repeated meteorological disturbances passed over the study area and caused the mixed layer to deepen. After deep-mixing events, the upper layer restratified and nitrate concentrations decreased while Chl a and POC concentrations increased, suggesting that spring mixing events promote primary productivity through the temporary alleviation of nutrient and light limitation. At the end of March, POC accumulation rates and nitrate decrease rates within the euphotic zone (0–70 m) were the largest of the four events observed, ranging from +84 to +210 mmol C m−2 d−1 and –28 to –49 mmol N m−2 d−1, respectively. The subsurface consumption rate of oxygen (i.e., the degradation rate of organic matter) after the fourth event (the end of March) was estimated to be –0.62 micromol O2 kg−1 d−1. At depths of 300–400 m (below the mixed layer), the POC concentrations increased slightly throughout the observation period. The POC flux at a depth of 300 m was estimated to be 1.1 mmol C m−2 d−1. Our float observation has made it possible to observed biogeochemical parameters, which previously could only be estimated by shipboard observation and experiments, in the field and in real time.

The Kuroshio Large Meander (LM) is known to be highly aperiodic and can last from 1 to 10 years. Since a stationary cold core formed between the Kuroshio and the southern coast of Japan off Enshu-Nada and approaching warm saltier water on the eastern side of the LM changes the local environment drastically, many commercially valuable fish species distribute differently from the non-LM period, impacting local fisheries. Despite this importance of the LM, the influences of the LM on the low trophic levels such as phytoplankton and zooplankton have still been unclear. In this study, satellite daily sea surface chlorophyll data are analyzed in relation to the LM. The results show positive anomalies of the chlorophyll-a concentration along the Kuroshio path during the LM periods, 2004–2005 and 2017–2019, from the upstream off Shikoku to the downstream (140°E). These positive anomalies are started by the triggering meander generated off south of Kyushu, which then slowly propagates to the downstream LM region in both the LM periods. Even though the detailed patterns along the Kuroshio region in the two LM periods were different, similar formations of the positive anomalies on the western side of the LM with shallower mixed layer depth are observed. Furthermore, we found clear relationships between the minimum distance from several stations along the coast to the Kuroshio axis and the mean chlorophyll-a anomaly, with significant correlations with the distance from different stations.

To understand oceanic restratification in the subtropical northwestern Pacific and its influence on biogeochemical (BGC) processes, we examined post-storm restratification events observed from February to April 2018 by BGC-Argo floats, the BGC data from which were stoichiometrically analyzed by Sukigara et al. (2021; this issue). We found that during these events, restratification of the mixed layer (ML) was driven by geostrophic adjustment or ML eddy formation related to surface cooling during February to March. At the end of March, high surface chlorophyll a concentrations were observed within submesoscale eddies and at the edge of a mesoscale cyclonic feature observed from satellite data. Our results indicate that primary production in the subtropical northwestern Pacific is enhanced by the combined effects of mesoscale upwelling, storm-driven formation of a deep ML, subsequent formation of ML eddies, weak cooling, and the length of intervals between storms.

Two Argo floats equipped with oxygen, chlorophyll (Chl), backscatter, and nitrate sensors conducted daily vertical profiles of the water column from a depth of 2000 m to the sea surface in the western North Pacific from January to April of 2018. Data for calibrating each sensor were obtained via shipboard sampling that occurred when the floats were deployed and recovered. Float backscatter observations were converted to particulate organic carbon (POC) concentrations using an empirical relationship derived from contemporaneous float profiles of backscatter and shipboard observations of suspended organic carbon particles. During the float deployment periods, repeated meteorological disturbances (storms) passed over the study area and caused the mixed layer to deepen. During these events, nitrate was entrained from deeper layers into the surface mixed layer, while Chl and POC in the surface mixed layer were redistributed into deeper layers. After the storms, the upper layer gradually restratified, nitrate concentrations in the surface layer decreased, and Chl and POC concentrations increased. When the floats observed the same water mass, the net community production within the euphotic layer (0–70 m), determined from the increases in POC, was 126–664 mg C m−2 d−1 (10.5–55.3 mmol C m−2 d−1) close to the values reported from a nearby area. The C/N ratio of the increase in POC and the decrease in nitrate was closed to the Redfield ratio, which indicates that the sensors were able to observe the net biochemical processes in this area despite the relatively low concentrations of nitrate and POC. To determine the fate of particles transported from the surface ocean to the twilight layer, the ratio of oxygen consumption and nitrate regeneration rates were compared. This O2/N ratio approached the Redfield ratio when the floats followed the same water mass continuously, but the consumption rate of POC was significantly lower than what would be expected based on the oxygen consumption and nitrate release rates. This suggests that dissolved organic carbon was the main substrate for the respiration in the twilight layer.

Recent observational and numerical studies have suggested that the decadal modulation of the Kuroshio Extension system, driven by mesoscale variabilities, profoundly affect the basin scale physical and biogeochemical oceanography. However, it remains unclear how these decadal changes affect distribution and abundance of fish species in this region. In this study, 26,964 swordfish catch data obtained by longliners during 2004 and 2010 in the western North Pacific are analyzed with an eddy-resolving ocean reanalysis by using mesoscale dynamic parameters and an eddy detection technique, to clarify the effects of mesoscale eddies and their variabilities on the swordfish catch and distribution. During this period, the Kuroshio Extension underwent two different dynamic phases: stable path state in 2004, 2005 and 2010; and unstable path state during 2006-2009. Based on our analyses, we show here that swordfish are more concentrated in and near the anticyclonic warm-core eddies in the northern site, 36 -45 • N, of the Kuroshio Extension system, especially during the unstable path phase. This is found to be caused by the interannual modulation of mesoscale eddy activities due to more warm-core rings generated from the unstable Kuroshio Extension, making it easier for fishermen to target swordfish in this region.

Nitrate interleaving structures were observed below the mixed layer during a cruise to the Kuroshio Extension in October 2009. In this paper, we investigate the formation mechanisms for these vertical nitrate anomalies, which may be an important source of nitrate to the oligotrphoc surface waters south of the Kuroshio Extension Front. We found that nitrate concentrations below the main stream of the Kuroshio Extension were elevated compared to the ambient water of the same density (σ 𝜃 = 23.5–25). This appears to be analogous to the “nutrient stream” below the mixed layer, associated with the Gulf Stream. Strong turbulence was observed above the vertical nitrate anomaly, and we found that this can drive a large vertical turbulent nitrate flux $>\mathcal {O}$ (1 mmol N m−2 day−1). A realistic, high-resolution (2 km) numerical simulation reproduces the observed Kuroshio nutrient stream and nitrate interleaving structures, with similar lateral and vertical scales. The model results suggest that the nitrate interleaving structures are first generated at the western side of the meander crest on the south side of the Kuroshio Extension, where the southern tip of the mixed layer front is under frontogenesis. Lagrangian analyses reveal that the vertical shear of geostrophic and subinertial ageostrophic flow below the mixed layer tilts the existing along-isopycnal nitrate gradient of the Kuroshio nutrient stream to form nitrate interleaving structures. This study suggests that the multi-scale combination of (i) the lateral stirring of the Kuroshio nutrient stream by developed mixed layer fronts during fall to winter, (ii) the associated tilting of along-isopycnal nitrate gradient of the nutrient stream by subinertial shear, which forms vertical interleaving structures, and (iii) the strong turbulent diffusion above them, may provide a route to supply nutrients to oligotrophic surface waters on the south side of the Kuroshio Extension.

The Kuroshio Current has been thought to be biologically unproductive due to oligotrophic conditions and low plankton standing stocks. Nevertheless, major foraging fishes are known to grow and recruit around the Kuroshio Current. While mixing and advection supplying nutrients to the euphotic zone are happened by eddies and meanders but limited at the Kuroshio front, there is a risk that survival of vulnerable life stages is encountered under the low food availability. Here we report that phytoplankton productivity is stimulated by turbulent nitrate flux amplified with the Kuroshio Current and rapidly transferred to microzooplankton through their grazing. Oceanographic observations demonstrate that the Kuroshio Current topographically enhances significant turbulent mixing and nitrate influx to the euphotic zone. Gradual nutrient enrichment experiments show growth rates of phytoplankton and microzooplankton communities stimulated within a range of the turbulent nitrate flux. Dilution experiments imply a significant microzooplankton grazing on phytoplankton. We propose that these rapid and systematic trophodynamics enhance invisible biological productivity in the Kuroshio.

The waters of the Kuroshio Current in the northwestern Pacific Ocean transport heat, salt, and organic and inorganic matter from south to north, shaping the ocean ecosystem.

The Argo Program has been implemented and sustained for almost two decades, as a global array of about 4000 profiling floats. Argo provides continuous observations of ocean temperature and salinity versus pressure, from the sea surface to 2000 dbar. The successful installation of the Argo array and its innovative data management system arose opportunistically from the combination of great scientific need and technological innovation. Through the data system, Argo provides fundamental physical observations with broad societally-valuable applications, built on the cost-efficient and robust technologies of autonomous profiling floats. Following recent advances in platform and sensor technologies, even greater opportunity exists now than 20 years ago to (i) improve Argo’s global coverage and value beyond the original design, (ii) extend Argo to span the full ocean depth, (iii) add biogeochemical sensors for improved understanding of oceanic cycles of carbon, nutrients, and ecosystems, and (iv) consider experimental sensors that might be included in the future, for example to document the spatial and temporal patterns of ocean mixing. For Core Argo and each of these enhancements, the past, present, and future progression along a path from experimental deployments to regional pilot arrays to global implementation is described. The objective is to create a fully global, top-to-bottom, dynamically complete, and multidisciplinary Argo Program that will integrate seamlessly with satellite and with other in situ elements of the Global Ocean Observing System (Legler et al., 2015). The integrated system will deliver operational reanalysis and forecasting capability, and assessment of the state and variability of the climate system with respect to physical, biogeochemical, and ecosystems parameters. It will enable basic research of unprecedented breadth and magnitude, and a wealth of ocean-education and outreach opportunities., Sí

[Abstract] / Vertical nitrate fluxes associated with turbulent mixing and upwelling around a small reef in the Kuroshio are quantified by continuously deploying a turbulence microstructure profiler with an attached submersible ultraviolet nitrate analyzer while drifting from the upstream to the downstream of the reef. Flow separations and trains of Kelvin-Helmholtz billows (thickness = 60 m) are identified using a shipboard ADCP and an echo-sounder. The turbulence diffusivity associated with the vigorous turbulent mixing reaches up to O(10^−1 m^2 s^−1), resulting in strong nitrate fluxes of O(1–10^3 mmol m^−2 day^−1). In addition, large differences between the upstream and downstream density profiles suggest a strong upwelling velocity of O(10^−3 m s^−1), as well as an upwelling nitrate flux of O(10^2 mmol m^−2 day^−1) in the entire subsurface layer. / / [Plain Language Summary] / Vertical nitrate fluxes associated with flow-topography interactions around a small reef in the Kuroshio Current are quantified using state-of-the-art oceanographic instruments. When the flow passes over a shallow sill on the flank of the reef, the velocity differences between layers intensify, resulting in a substantial vertical overturning and mixing of the water column. This turbulent mixing causes the observed nitrate flux from deep water to reach a value among the highest observed worldwide. / / [Key Points] / • Flow separations and trains of Kelvin-Helmholtz billows mix the water column around a small reef in the Kuroshio / • Doming of isopycnals/nitraclines suggests strong upwelling in the lee of the reef / • Turbulent nitrate fluxes reach up to O(10^3 mmol m^−2 day^−1)

The annual overturn of lake water (termed holomixis) during winter is essential in maintaining the environment of warm monomictic lakes by transporting heat and organic and inorganic constituents; however, direct observation of wintertime mixing processes is limited. To better understand the detailed physical processes responsible for holomixis, this study investigated a cold low-oxygen water intrusion event on the northeastern slope of weakly stratified Lake Biwa (Japan), observed using mooring systems. The intrusion occurred concurrently with superinertial oscillations of near-bottom currents. The effective Wedderburn number suggests that basin-scale Kelvin waves were excited by north¬eastward winds (~4 m s −1 ) prior to the intrusion event. The results of a modal analysis suggest that the intrusion was caused by a combination of Kelvin and Poincare waves that locally uplifted the cold deep water. The heat budget analyses revealed that a substantial part of the intruded cold water was mixed diapycnally, induced by wind stirring, nighttime convection, and bottom friction. The superinertial currents enhanced the dissipation due to bottom friction by ~30%. The cold water intrusion and subsequent mixing provided local heat flux of 98 W m −2 over 6 days, which was 1.4 times larger than the average surface heat flux over the same period. In addition to previously studied processes, such as surface cooling and gravity currents from the shore, this study indicates that wind-induced baroclinic motions and subsequent diapycnal mixing of a stratified water column contribute to holomixis.

In this study, a Navis-MicroRider microstructure float and an EM-APEX float were deployed along the Kuroshio Extension Front. The observations deeper than 150 m reveal widespread interleaving thermohaline structures for at least 900 km along the front, presumably generated through mesoscale stirring and near-inertial oscillations. In these interleaving structures, microscale thermal dissipation rates χ are very high O( >10−7 K2s−1), while turbulent kinetic energy dissipation rates ϵ are relatively low O( 10−10−10−9 Wkg−1), with effective thermal diffusivity Kθ of O( 10−3 m2s−1) consistent with the previous parameterizations for double-diffusion, and, Kθ is two orders of magnitude larger than the turbulent eddy diffusivity for density Kρ. The average observed dissipation ratio Γ in salt finger and diffusive convection favorable conditions are 1.2 and 4.0, respectively, and are larger than that for turbulence. Our results suggest that mesoscale subduction/obduction and near-inertial motions could catalyze double-diffusive favorable conditions, and thereby enhancing the diapycnal tracer fluxes below the Kuroshio Extension Front.

Observations of the run-up of internal bores in a shallow bay were made with a tow-yo instrument and mooring arrays with high spatial and temporal resolution. Shoreward propagating internal bores have been studied with laboratory experiments and numerical models, but few observational studies have shown the detailed structure of the run-up of internal bores induced by internal tides. Our observations showed that internal bores propagate along the slope, accompanied by strong turbulent mixing and strong sediment resuspension in a shallow bay. The isothermal displacement due to the bores reached 20 m vertically in a water depth of 40 m. Turbidity measurements showed suspended particles transported from the sloping bottom and offshore above the thermocline, forming an intermediate nepheloid layer (INL). At the head of the bore (dense water), a vortex accompanied by strong vertical motion induced strong vertical sediment resuspension and a steep isothermal displacement. The rate of turbulent kinetic energy dissipation reached 10 −6 W kg −1 at the head of the wave. A nonhydrostatic numerical simulation in a two-dimensional domain reproduced fine features associated with the run-up of an internal bore and the vortex motion at its head.

While near-inertial waves are known to be generated by atmospheric storms, recent observations in the Kuroshio Front find intense near-inertial internal-wave shear along sloping isopycnals, even during calm weather. Recent literature suggests that spontaneous generation of near-inertial waves by frontal instabilities could represent a major sink for the subinertial quasigeostrophic circulation. An unforced three-dimensional 1-km-resolution model, initialized with the observed cross-Kuroshio structure, is used to explore this mechanism. After several weeks, the model exhibits growth of 10–100-km-scale frontal meanders, accompanied by O(10) mW m−2 spontaneous generation of near-inertial waves associated with readjustment of submesoscale fronts forced out of balance by mesoscale confluent flows. These waves have properties resembling those in the observations. However, they are reabsorbed into the model Kuroshio Front with no more than 15% dissipating or radiating away. Thus, spontaneous generation of near-inertial waves represents a redistribution of quasigeostrophic energy rather than a significant sink.

The coastal upwelling region of the California Current System (CalCS) is a well-known site of high productivity and lateral export of nutrients and organic matter, yet neither the magnitude nor the governing processes of this offshore transport are well quantified. Here we address this gap using a high-resolution (5 km) coupled physical-biogeochemical numerical simulation (ROMS). The results reveal (i) that the offshore transport is a very substantial component of any material budget in this region, (ii) that it reaches more than 800 km into the offshore domain, and (iii) that this transport is largely controlled by mesoscale processes, involving filaments and westward propagating eddies. The process starts in the nearshore areas, where nutrient and organic matter-rich upwelled waters pushed offshore by Ekman transport are subducted at the sharp lateral density gradients of upwelling fronts and filaments located at ∼25–100 km from the coast. The filaments are very effective in transporting the subducted material further offshore until they form eddies at their tips at about 100–200 km from the shore. The cyclonic eddies tend to trap the cold, nutrient, and organic matter-rich waters of the filaments, whereas the anticyclones formed nearby encapsulate the low nutrient and low organic matter waters around the filament. After their detachment, both types of eddies propagate further in offshore direction, with a speed similar to that of the first baroclinic mode Rossby waves, providing the key mechanism for long-range transport of nitrate and organic matter from the coast deep into the offshore environment.

The upstream Kuroshio flows through Okinawa Trough and the Tokara island chain, the region near the continental shelf of the East China Sea and shallow seamounts, where the Kuroshio can induce strong mixing over the shallow topography. Also, tidal currents over the rough topography may produce internal tides, and associated turbulence. The previous observations show energetic high vertical wavenumber near-inertial wave shear in the Kuroshio thermocline, which implies strong turbulent mixing. However, direct turbulence measurements in this region are very scarce. Using high lateral resolution (1–2 km) direct turbulence measurements, we show here, for the first time, that strong turbulent layers form spatially coherent banded structures with lateral scales of >O(10 km), associated with bands of near-inertial wave/diurnal internal tide shear of high vertical wavenumber in the upstream Kuroshio. The turbulent kinetic energy dissipation rates within these turbulent layers are >O(10−7 W kg−1), and estimated vertical eddy diffusivity shows >O(10−4 m2 s−1) on average. These results suggest that the high vertical wavenumber near-inertial waves propagating in the upstream Kuroshio could have large impacts on the watermass modifications, momentum mixing, nutrient supply, and associated biogeochemical responses in its downstream.

Concurrent physical, chemical and biological observations across the Kuroshio Front collected in October 2009 provide a detailed view of the relationship between the physical environment and the phytoplankton community. Depth profiles were taken at stations 9 km apart along five 70 km transects. With a combination of flow cytometry, microscopy and high-pressure liquid chromatography pigment analysis, we characterized the phytoplankton community structure across the front. The observed phytoplankton community fell into two distinct assemblages, largely separated by the front, but which also reflected patterns in the distribution of Kuroshio and Oyashio water masses shaped by mesoscale lateral mixing. Phytoplankton biomass was elevated where there was a positive vertical flux of nitrate towards the surface, and the frontal circulation drove a lateral transport of nutrients southwards into the subtropical gyre. The observations showed that the phytoplankton respond to forcing on several scales: the phytoplankton community across the front was shaped by a combination of the large scale biogeography of the region, mesoscale mixing of populations and finer scale modification of the light and nutrient environment.

In this study, turbulence and mixing are investigated in a constricted channel at the mouth of Tokyo Bay, using two instruments: a microstructure profiler, TurboMAP-L, and an ADCP. We observed three different stratified conditions, i.e., SSC (Strongly Stratified Condition), MSC (Moderately Stratified Condition) and WSC (Weakly Stratified Condition) over a complete tidal period. The water exchange at the mouth of Tokyo Bay was in the transitional regime between the hydraulic and the diffusive limit with SSC and MSC close to the hydraulic limit and WSC close to the diffusive limit. For SSC and MSC, the enhancement of the stratification and vertical mixing showed asymmetry over a semidiurnal tide cycle. Mixing was stronger during ebb than during flood for SSC. On the other hand, mixing was elevated during flood for MSC. For WSC, weak stratification and strong mixing occurred during both flood and ebb. The SSC asymmetry resembled the strongly stratified condition of Columbia River ( Kay and Jay, 2003a , Kay and Jay, 2003b ), and the MSC asymmetry resembled the Strain-Induced Periodic Stratification condition ( Simpson et al., 1990 ). A principle component analysis applied to the observed velocity profiles showed that both barotropic and baroclinic flow played a role during SSC and MSC, and in turn, barotropic flow dominated during WSC. The probability density function of the turbulent kinetic energy dissipation rate is practically expressed as a single lognormal population for all three conditions.

A 48-h microstructure experiment captured the variation of turbulence in Lake Biwa, Japan, during a strong stratification period, 1 week after a typhoon event. A free-fall microstructure profiler (TurboMAP) and a fine-scale profiler (F-probe) were deployed. An array of five ADCPs positioned close to the experiment site provided current measurements associated with the internal-wave field for a period of one week. Strong winds related to the passage of a typhoon close to the lake generated two low-frequency internal waves: a Kelvin wave and an inertial wave. Both waves were in the first baroclinic mode. This study pictures the superposition of two large-scale internal waves and a stronger current resulting from the two waves being in phase. The synchronization of the waves provoked enhanced shear in the hypolimnion, more than 10 m above the bottom, as well as high dissipation (10−7 W kg−1) and vertical diffusivity reaching 10−4 m2 s−1. The enhanced shear event seems to be related to the current amplitude since it occurred when the current amplitude was increased by the diel wind. Assuming the same turbulence intensity for the enhanced hypolimnetic shear events, this superposition may affect nutrient transfer in the hypolimnion. Also, we witnessed sediment resuspension, consisting of high turbidity and high fluorescence intensity, likely due to a combination of bottom stress and enhanced turbulence.

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Two transects through the Oyashio‐Kuroshio frontal region were made off the coast of Japan using a towed CTD‐fluorometer (Aquashuttle). These show numerous patches of elevated chlorophyll concentration adjacent to the Kuroshio front. In particular, a few patches are found in the main stream of the Kuroshio. Making use of the Temperature and Salinity (T‐S) properties of the identified patches, the origin of the patches is inferred. It is confirmed that the patches appearing in the Kuroshio are not from the adjacent coastal waters. Their T‐S properties are similar to those in the upstream portion of the Kuroshio. These facts suggest that the elevated level of chlorophyll in the patches arise because of mixing that occurs while the Kuroshio passes over the Izu Ridge. The patches found in the main stream of the Kuroshio are transported to the Kuroshio extension, which is the nursery ground for many pelagic larval fish. These patches, if they appear during the spawning season, may provide food for the...