Coastal Oyashio Water Research Articles

Community structure of demersal fish over the continental shelf off the southeastern coast of Hokkaido Island: The effect of eurythermy in dominant species

The seasonal dynamics of a boreal demersal fish community and the factor governing its structure have been analyzed, using a total of 169 Danish seine samples collected off the southeastern coast of Hokkaido Island where is an important nursery for Japan Pacific population of walleye pollock Gadus chalcogrammus. The sea floor of the area was covered by cold (<2 °C) Coastal Oyashio Water during spring which was replaced by Oyashio Water and Modified Soya Current Water by autumn. Antlered sculpin Enophrys dicerus was the dominant species accounting for 23.2% of total biomass followed by plain sculpin Myoxocephalus jaok (19.8%), Kamchatka flounder Atheresthes evermanni (8.2%) and walleye pollock (7.4%). Based on species composition and biomass, the samples were classified into the four faunal groups: i) upper-shelf assemblage in which antlered and plain sculpins in combined accounted for 61.4%, ii) mid-shelf assemblage dominated by antlered sculpin (27.0%) and various fish taxa iii) lower shelf assemblage dominated by gadids (32.8%) and iv) less abundant lower-shelf assemblage dominated by antlered sculpin (33.3%) and other cottoid species (30.4%). Depth was the most important factor determining the structure of assemblages accounting for 66.2% of the variance explained by environmental variables, outweighing the effects of season (19.4%) and temperature (14.4%). This result was ascribed to the eurythermy of the dominant species which persisted at relatively narrow depth ranges under the seasonally fluctuating thermal environments.

Species‐specific patterns in spatio‐temporal dynamics of juvenile chum salmon and their zooplankton prey in Otsuchi Bay, Japan, revealed by simultaneous eDNA quantification of diverse taxa from the same water samples

AbstractTo understand the spatio‐temporal dynamics of juvenile chum salmon Oncorhynchus keta and their zooplankton prey, the neritic Pseudocalanus newmani, the oceanic Eucalanus bungii, and Themisto japonica, we developed species‐specific environmental DNA (eDNA) quantification methods. First, we confirmed that juvenile chum salmon collected in Otsuchi Bay preyed on the target zooplankton by the stomach contents analyses using the developed assays. Size‐fraction analyses of the water collected in the bay showed that P. newmani and E. bungii DNA were derived mostly from nauplii and early copepodids. We then investigated the distribution and abundance of zooplankton and juvenile chum salmon in Otsuchi Bay using eDNA analyses of the same water samples collected from January to June in 2018 and 2019. Chum salmon DNA was found in the bay from the end of January to mid‐June. P. newmani DNA appeared across the bay throughout the season and were most abundant from February to May, while the other two species DNA were only found in spots, in lower quantities, from March to May. The timings that the zooplankton DNA quantities became greater were corresponded well to that of the Oyashio water intrusion into the bay. In addition, the environmental data indicates that P. newmani was possibly transported also by the Coastal Oyashio water in 2019. The present study successfully revealed species‐specific patterns in the dynamics of zooplankton and chum salmon, reflecting their ecological characteristics and oceanographic conditions, and showed the effectiveness of simultaneous eDNA quantifications for diverse taxa from the same water samples.

Intrusion of Coastal Oyashio water to Funka Bay and Tsugaru Strait occasionally disturbed by Kuroshio-originating warm core ring

Intrusion of Coastal Oyashio water to Funka Bay and Tsugaru Strait occasionally disturbed by Kuroshio-originating warm core ring

Spatial variation in diatom abundance and composition in Biwase Bay and Hamanaka Bay (Eastern Hokkaido, Japan), with reference to environmental features.

This study aims to examine the spatial variation of diatom abundance and composition along the nearshore areas of Biwase Bay and Hamanaka Bay, eastern Hokkaido. Terrestrial input via Kiritappu Wetland is expected to affect variation and composition differently depending on the position of the two bays. We conducted an oceanographic survey in June 2014 to measure seawater temperature, salinity, colored dissolved organic matter (CDOM) absorption, nutrient concentrations, and total and size-fractionated chlorophyll (Chl) a concentration at 11 stations of the shallowest (<5 m) parts of the bays. These were grouped into four areas (Areas 1 and 2 in Biwase Bay, and Areas 3 and 4 in Hamanaka Bay) based on the distance of the location from the wetland outlet (nearest in Area 1 to the farthest in Area 4). Diatoms are the major primary producers in the water column. Therefore, we also determined genus level cell abundance and diversity of diatoms to compare similarity among areas. Sea surface temperature was the lowest at Area 4, whereas sea surface salinity was the lowest at Area 1. The contribution of CDOM absorption, an indicator of wetland-influenced river discharge, and silica concentration was highest at Area 1. Total amount of nitrite and nitrate concentrations was the highest at Area 4. Total amount of Chl a concentration was also lowest in Area 1. Our size-fractionated Chl a results revealed that while the size composition of phytoplankton varied among areas, micro-sized (>10 µm) phytoplankton were predominant in Area 4. Finally, diatom composition at the genus level differed greatly among areas. Pennate diatoms were predominant in Areas 1 and 2, but centric diatoms dominated in Areas 3 and 4. Our results suggested great spatial variability in oceanographic conditions among areas, with less influence of wetland and more influence of Coastal Oyashio Water based on distance from the wetland outlet. Diatom composition showed geographical division between Biwase and Hamanaka Bays.

Distribution of Harmful Algae (Karenia spp.) in October 2021 Off Southeast Hokkaido, Japan

An unprecedented large-scale outbreak of harmful algae, including Karenia selliformis and Karenia mikimotoi, was reported in mid-September 2021 in the northwest Pacific Ocean off southeastern Hokkaido, Japan. It inflicted catastrophic damage on coastal fisheries in the ensuing months. To understand the spatiotemporal distribution of Karenia spp. abundance, we conducted extensive ship-based surveys across several water masses during 4–14 October, 2021 and analyzed in-situ data in combination with Sentinel-3-derived ocean color imagery with a horizontal resolution of 300 m. High chlorophyll-a concentrations (exceeding 10 mg m–3) were identified mainly in coastal shelf–slope waters of &amp;lt;1,000-m water depth occupied by Surface Coastal Oyashio Water or Modified Soya Warm Current Water. Karenia spp. abundance was strongly correlated with chlorophyll-a concentration, which typically had a shallow vertical maximum within the surface mixed layer. Large- and small-scale distributions of Karenia spp. abundance at the ocean surface were estimated from two satellite-imagery products: maximum line height and red-band difference. Maps generated of Karenia spp. abundance revealed snapshots of dynamic Karenia bloom distributions. Specifically, the cores of Karenia blooms were located on continental shelves, sometimes locally exceeded 104 cells mL–1, and seemed to be connected intermittently to very nearshore waters. Relatively high-abundance areas (&amp;gt;103 cells mL–1) of Karenia spp. on the shelf were characterized by submesoscale (i.e., 1–10 km) patch- or streak-like distributions, or both. Within a roughly 24-h period from 12 to 13 October, Karenia-spp. abundances averaged over the shelf abruptly increased more than doubled; these abundance spikes were associated with the combined effects of physical advection and algal growth. The obtained maps and features of Karenia spp. abundance will provide basic estimates needed to understand the processes and mechanisms by which algal blooms can inflict damage on regional fisheries.

Annual and seasonal changes in the assemblage of planktonic copepods and appendicularians in Funka Bay before and after intrusion of Coastal Oyashio Water

Annual and seasonal changes in the abundance and the assemblage of planktonic copepods and appendicularians were clarified in Funka Bay. Zooplankton samples were obtained by vertical hauls with a NORPAC net (0.10-mm mesh aperture) from December to May over a 4-year period (2014–2017). Temporal fluctuations of mean abundance of copepods were small from December 2013 to May 2014 under the low water temperature condition (maximum 1.55 times difference between adjacent survey periods), while those were large in other 3 years (1.60–1.93 times). Maximum mean abundance of appendicularians in 2014 (6.0 × 104 ind./m2) was lower than those in 2015–2017 (8.6 × 104–1.4 × 105 ind./m2). Less saline (≤ 33.3) and cold Coastal Oyashio Water (COW) intruded in early February 2014, late February 2015, and late March 2016 and 2017. Phytoplankton blooms occurred in March of all 4 years. Copepods and appendicularians increased nearer periods of phytoplankton bloom than after COW intrusion. In the results of canonical correspondence analysis, the assemblage of copepods and appendicularians gradually changed from small-sized and warm-water species to large-sized and cold-water ones. Northwesterly wind speed had an effect equal to COW intrusion (82% of COW intrusion) to the change in taxonomic compositions of copepods and appendicularians.

Influence of Coastal Oyashio water on massive spring diatom blooms in the Oyashio area of the North Pacific Ocean

We hypothesized that the characteristics of Coastal Oyashio water in the North Pacific Ocean during winter–spring have a major influence on the generation of massive spring diatom blooms throughout the Oyashio area. To test this hypothesis, we analyzed in situ data from May during 1990–2016 along a transect known as the “A-line” to monitor conditions in the Oyashio off the southeast coast of Hokkaido, Japan. Chlorophyll-a concentrations were high (≥5 mg m−3) on the shelf and in the offshore Oyashio areas around the First and Second Oyashio Intrusions. About 80% of the high chlorophyll-a concentrations in the Oyashio area were linked with low-salinity (≤33.0) water that was associated with the Coastal Oyashio water and its modified waters. For the remaining Oyashio area, the so-called “Oyashio water” with a high salinity of 33.0–33.7 was near the sea surface and associated with low chlorophyll-a concentrations with a frequency distribution mode at 1–2 mg m−3. Diatom communities with high abundance and diversity were also connected (disconnected) between the shelf and the offshore Oyashio areas with low (high) salinity and high (low) chlorophyll-a concentrations. Analysis of historical temperature–salinity data near the sea surface revealed that the Coastal Oyashio water and its modified waters appear most frequently around the shelf along Hokkaido and the Kuril Islands, and that the frequency of appearance decreases with increasing distance offshore. We suggest that the transport and pathway around the islands of the Coastal Oyashio water and its modified waters, which contain high nutrient concentrations and the seeds for massive spring blooms, are the primary factors regulating the generation and spatial extent of the massive spring blooms over the Oyashio area.

Responses of phytoplankton assemblages to iron availability and mixing water masses during the spring bloom in the Oyashio region, NW Pacific

AbstractSpring phytoplankton blooms play a major role in the carbon biogeochemical cycle of the Oyashio region, western subarctic Pacific, where the seasonal biological drawdown effect on seawater pCO2 is one of the greatest among the world's oceans. However, the bloom often terminates before depleting macronutrients, and the initiation and magnitude of the bloom is heterogeneous. We conducted a high resolution taxonomic and physiological assessment of phytoplankton in relation to the different physicochemical water masses of Coastal Oyashio Water (COW), Oyashio water (OYW), and modified Kuroshio water (MKW) in the Oyashio region from April to June 2007. Massive diatom blooms were found in April. Then, chlorophyll a concentration, cell abundance of diatom taxa, and the maximum photosystem II photochemical efficiency (Fv/Fm) were positively correlated with the mixing ratios of COW, suggesting that the spring bloom in April was strongly affected by the intrusion of COW. In the OYW, intensive blooms occurred from the middle of May under low dissolved iron (DFe) concentration (&lt; 0.26 nM). Redundancy analysis showed that while diatom blooms accompanied by COW were related to DFe concentration, this was not the case in the OYW. These results indicated that diatoms in the OYW possess different iron adaptation strategies compared with diatoms in the water masses affected by COW. This led to the spatial heterogeneity of the Oyashio spring bloom. The results presented here demonstrate that water mass characterization with detailed assessments of phytoplankton taxonomy and physiological status can improve our understanding of marine ecosystems.

Parmales abundance and species composition in the waters surrounding Hokkaido, North Japan

The abundance and species composition of Parmales (Bolidophyceae), a siliceous marine phytoplankton, were investigated in the waters surrounding Hokkaido, North Japan. In the Pacific Ocean, high Parmales abundances were observed in the Oyashio region (84.9–95.2 cells mL−1), in the surrounding area of the cold Oyashio Current, and in the Coastal Oyashio Water (35.1–41.4 cells mL−1) being low temperature and salinity in spring. In the Okhotsk Sea, Parmales abundance was moderate (>5 cells mL−1) in the Intermediate Cold Water, which is formed by convective mixing under the pack ice. On the other hand, their abundance was quite low in warm water masses, such as the Tsushima Warm Current in the Sea of Japan and the Soya Warm Current in the Okhotsk Sea. Parmales cells were also found in the sea ice in the Notoro-ko Lagoon connected to the Okhotsk Sea, suggesting that they have an ice algal life cycle. Triparma spp. were almost dominant in the study areas with the occasional contribution of Tetraparma spp. in the Okhotsk Sea. These results indicate that Parmales abundance and species composition are significantly influenced by water currents.

Determination of the freshwater origin of Coastal Oyashio Water using humic-like fluorescence in dissolved organic matter

Coastal Oyashio Water (COW), defined as a water mass with a temperature lower than 2 °C and a salinity lower than 33.0, is distributed in the North Pacific Ocean off southeastern Hokkaido, Japan, from winter to spring. COW is rich in macronutrients and dissolved iron and is thus considered to affect the spring phytoplankton blooms in the Oyashio region. Although river water and sea-ice melt water have been considered freshwater end-members of COW, the contributions of these freshwater sources to COW have not been well described. In this study, the humic-like components in dissolved organic matter were first applied as a parameter to evaluate the freshwater end-members of COW in March 2015. Linear regressions with negative slopes were determined between the humic-like components and the salinity of COW. The intercepts of the regressions against the humic-like components were within the ranges of those observed for the local rivers of Hokkaido but were very different from those of sea ice. These findings suggest that river water contributed to the COW observed here as a freshwater end-member, although the contribution of sea-ice melt water to COW could not be evaluated. This novel approach also highlighted two different less-saline water masses in COW. The first was characterized by a lower temperature and relatively high levels of humic-like components, while the second was higher in temperature and had higher levels of humic-like components. It is suggested that these different characteristics are due to the contributions of water from different rivers and/or different effects of sea-ice melt water.

Monthly variations of hydrographic structures and water mass distribution off the Doto area, Japan

Monthly variations of hydrographic structures and water mass distributions were investigated over a nearly 30-year period (January 1982–March 2011) off the Doto area, Japan, to examine temporal evolutions and devolutions of representative water masses in this area. In the continental shelf area, the Coastal Oyashio Water (COW) was distributed along the coast during January–May, when COW has been modified by relatively higher salinity water, which may have originated from the Oyashio Water (OW) off the Kuril Islands. On the other hand, the Surface COW (S-COW) may have formed with COW heated by solar radiation, simultaneously mixing with the Tokachi River freshwater and OW in the continental shelf area, and the area of this S-COW spread offshoreward during June and July, and stayed in the offshore area during June and October. Although coastal density current structures, probably due to the Modified Soya Warm Current Water (M-SWCW) inflows, were conspicuous in the continental shelf area during August–October, those structures were weak after November. These weakening structures may be due to developed surface mixed layer caused by surface cooling, and decay of volume transport of the Soya Warm Current in the Okhotsk Sea, and such weakening accordingly may lead to weakening of higher salinity water inflows from the upstream regions. M-SWCW was radically replaced by COW during December and January with rapid salinity decreases, which suggest extremely lower salinity water inflows, probably due to influences of the East Sakhalin Current Water.

VERTICAL DISTRIBUTION OF WALLEYE POLLOCK JUVENILES BEFORE AND AFTER THE PERIOD OF TRANSITION FOR FEEDING IN FUNKA BAY, HOKKAIDO, JAPAN

We examined the vertical distributions of walleye pollock Theragra chalcogramma juveniles for two size groups (smaller and larger than 30 mm) during the period of transition for feeding (PTF) and after this period in Funka Bay, Hokkaido, Japan. Samplings were conducted in May and June in 2006 and 2007, and water temperature and salinity were measured. The correlation between juvenile sizes and distributed depth was observed using net sampling. From this result, size groups were divided by depth. In addition, the distribution and abundance of juveniles were examined using acoustic data. In the PTF (May), juvenile size increased with increasing depth, and after the PTF (June), most juveniles descended. In the PTF, juveniles smaller than 30 mm in TL dominated above 43 m depth in 2006, while juveniles larger than 30 mm in TL dominated below 18 m depth in 2007. After the PTF, juvenile sizes were larger in 2006 than in 2007. The relationships between juvenile distributions and water masses in the PTF were analyzed, and it was found that surface water of coastal Oyashio water (S-CO) dominated in the layers where small juveniles were distributed, and that Oyashio water (OW) dominated in the layers where large juveniles were distributed.

Dynamics of the Coastal Oyashio and Its Seasonal Variation in a High-Resolution Western North Pacific Ocean Model

AbstractThe Coastal Oyashio (CO) carries the cold, fresh, and relatively light water mass called the Coastal Oyashio Water (COW) westward along the southeastern coast of Hokkaido in winter and spring. To investigate dynamics of the CO and its seasonal variation, model experiments are executed using a western North Pacific general circulation model with horizontal resolutions of approximately 2 and 6 km. The 2-km resolution model reproduces the properties of COW with temperature of 0°–2°C and salinity of 32.2–32.6 and reproduces its distribution. COW is less dense than offshore water by 0.2 kg m−3, and it forms a surface-to-bottom density front with a width of 10 km near the shelf break. The CO appears as a baroclinic jet current along the front with a maximum velocity of approximately 40 cm s−1. The velocity and density structures and the front location relative to bathymetry indicate that the CO can be understood in terms of a simplified dynamical model developed for the shelfbreak front in the Middle Atlantic Bight. In contrast to the 2-km resolution model, the 6-km model cannot realistically reproduce the COW distribution. This is because only the 2-km model can represent the sharp density structure of the shelfbreak front and the accompanying CO. The CO exists during the limited period from January to April. This is directly connected with seasonal variation of the COW inflow from the Okhotsk Sea to the North Pacific Ocean through the Nemuro and Kunashiri Straits, indicating that the seasonal variation of the CO is ultimately controlled by the variation of the circulation in the Okhotsk Sea induced by the monsoon.

A mixing analysis of surface water in the Oyashio region: Its implications and application to variations of the spring bloom

Abstract We analyzed the near-daily hydrographic data collected from ships at a station southeast of Hokkaido Japan during April 2007. We calculated mixing ratios of Coastal Oyashio Water (COW), Oyashio Water (OYW), and modified Kuroshio Water (MKW). The COW mixing ratio tended to be high in depths shallower than 30 m during April, while the OYW ratio was high at the deeper depths. Diapycnal mixing due to the strong wind was suggested between these layers. It is suggested that stratification of low-salinity COW initiates the spring bloom in early-April, but that it is not important for temporal changes of chlorophyll at each depth during and after the bloom. Despite the intense water exchange at the station, the COW ratio correlated significantly with chlorophyll for the overall data set during April and for the data in 2–4-day blocks. Silicic acid, nitrate, and phosphate were also significantly correlated with the COW ratio for each 2–4-day block. This implies that each of these variables changed simultaneously at a constant COW ratio within the temporal and spatial scales of the phytoplankton bloom, were it not for the occurrence of vigorous vertical mixing and large-scale advective events. Using the linear regression equations for each 2–4-day block, we calculated time-series of chlorophyll and nutrients at the mean COW ratio (46%). Their rates of change in these series reveal the spring bloom dynamics in 2007 as follows: The spring bloom ended due to silicate depletion and grazing by large zooplankton in mid-April. In late-April, the nutrients increased, but chlorophyll decreased, indicating little nutrient utilization.

Temporal variability and bioavailability of iron and other nutrients during the spring phytoplankton bloom in the Oyashio region

Abstract Iron [dissolved Fe (D-Fe) and total dissolvable Fe (T-Fe)] and nutrient concentrations in the surface water of the Oyashio region (northwestern North Pacific) were measured before and during spring phytoplankton bloom (March to May in 2007). During the pre-bloom period (middle of March), we observed vertically uniform concentrations of iron (0.3–0.5 nM [D-Fe] and 3–5 nM [T-Fe]), macronutrients (10–15 μM NO3+NO2, 1.0–1.5 μM PO4 and 20–30 μM Si(OH)4) and chlorophyll a (Chl-a, 0.3–0.4 μg l−1) throughout the upper 125 to 150 m due to vertical mixing during winter. Water temperature and salinity before the bloom were also vertically uniform with >5 °C and S>33.5, the conditions of a modified Kuroshio warm-water (MKW) ring, higher than those of the Coastal Oyashio Water (COW). The cold COW (

Interannual variability in the magnitude and timing of the spring bloom in the Oyashio region

Abstract Inter-annual variability in the magnitude and timing of the spring bloom was investigated for the Oyashio region (40 °-48 °N, 143 °E-152 °E) using 10 years (from 1998 to 2007) of satellite ocean-color data. Geostrophic currents were examined using satellite altimeter data. Early spring blooms (>1.5 mg m−3) occurred in early April 2001 and late March 2002. The 2001 bloom continued for one month. Late blooms occurred from mid-May 1999, early June 2004 and late April 2006, continuing for about 1 month, 8 days and 16 days, respectively. A strong bloom (4.7 mg m−3) also occurred in mid-April 1998; however, it terminated in early May. We classified the Oyashio region based on the pattern of temporal variation of Chl-a concentration from March to June. The spatio-temporal variability in Chl-a concentration during spring was different among years. The area where Chl-a concentration was highest in April was more extensive in 2001, 2002 and 2006 than usual. In 1999, the area where Chl-a concentration was highest in May was the widest among the 10 years. Mesoscale eddies and currents with high velocity were frequently observed in the area of high Chl-a concentration east of Hokkaido, propagating Coastal Oyashio Water of low salinity and low density into the oceanic region. That strengthened stratification in the surface layer. We suggest that this seaward transfer of coastal water could be one of the important factors for phytoplankton distribution in two ways: (1) horizontal advection of water with high Chl-a concentration and (2) enhancement of stratification in the oceanic region.

Coastal Oyashio South of Hokkaido, Japan

Abstract Coastal Oyashio Water (COW), the cold low-salinity water lying along the southeast coast of Hokkaido, sometimes flows into Funka Bay on the southwest coast in winter and spring. Because this water is low in density, a density current called the “Coastal Oyashio” was assumed to carry COW along these coasts. However, water property maps for April 1989, February 1992, February 1994, and February 1989 show that only for the latter month did COW extend as far west as Cape Erimo on the southwest coast. Steady-state flows were calculated using a baroclinic model and the density data collected in April 1989. Model results show that the Oyashio carries COW southwestward along the continental slope southeast of Hokkaido. It bifurcates south of Cape Erimo with a coastal portion turning around the cape but not always reaching the southwest coast because of the density structure along that coast. When the Oyashio crosses isobaths, the joint effect of baroclinicity and bottom relief (JEBAR) causes a barotropic...

An Effect of Coastal Oyashio water and current on scallop farming

An Effect of Coastal Oyashio water and current on scallop farming

Effects of the anticyclonic eddies on water masses, chemical parameters and chlorophyll distributions in the Oyashio current region

Data from the R/V Mirai cruise (May‐June 2000) have been examined to discover how mesoscale processes associated with eddy dynamics directly affect the water masses, the distributions and the vertical fluxes of the dissolved oxygen, nutrients and dissolved inorganic carbon in the western subarctic Pacific. Using maps of the temperature, salinity, dissolved oxygen, nutrients, chlorophyll and sea-air pCO 2 difference we show that the boundaries of the anticyclone eddies in the study region were composed of high productivity coastal Oyashio water. The coastal waters were wrapped around the anticyclone eddies (thus creating a high productivity belt) and intruded inside of them. Using SeaWifs data we demonstrate that temporal variations in the position and the strength of anticyclone eddies advected the Kuril island coastal high productivity waters to the pelagic part, resulting in temporal variations of the chlorophyll in the Oyashio region. Computed vertical fluxes of the dissolved oxygen (DO), inorganic carbon (DIC) and silicate show that the anticyclonic eddies in the Kuroshio-Oyashio Zone are characterized by enhanced vertical fluxes of the DO and DIC between the upper (σ θ = 26.7‐27.0) and lower (σ θ = 27.1‐27.5) intermediate layer, probably due to the intrusions of the Oyashio waters into the eddies.

A late winter hydrography in Hidaka Bay, south of Hokkaido, Japan

Hydrographic observations in Hidaka Bay, south of Hokkaido, Japan were carried out in late winter 1996 and 1997 to examine the spatial distributions and circulation features of two different water masses, i.e., Coastal Oyashio Water (COW) and Tsugaru Warm Water (TWW), and their modifications. It is known that COW is mostly composed of cold and low-salinity water of the melted drift ice coming from the Okhotsk Sea and flows into Hidaka Bay from winter to spring and TWW with high-salinity continuously supplies from the Tsugaru Strait to the North Pacific. Cold surface mixed layers (<26.2σθ, 0–100 m depth) were found mainly over the shelf slope, confirming that anti-clockwise flow of COW was formed. TWW was relatively high in salinity and low in potential vorticity, and had some patch-like water masses with a temperature and salinity maximum in the limited area in the further offshore at the deeper density levels of 26.6–26.8σθ. The fine structure of vertical temperature and salinity profiles appeared between TWW and COW is an indication of enhanced vertical mixing (double-diffusive mixing), as inferred from the estimated Turner angles. At a mouth of the Tsugaru Strait in late winter 1997, a significant thermohaline front between TWW and the modified COW was formed and a main path of TWW spreaded south along the Sanriku coast, probably as the bottom controlled flow. Hence, the patch-like TWW observed in late winter is isolated from the Tsugaru Warm Current and then rapidly modified due to a diapycnal mixing.