Kuril Straits Research Articles

Modeling the barotropic tidal dynamics of the Kuril region

A 3D boundary-value problem for computation barotropic tidal dynamics is solved at Kuril region, including southern part of the Okhotsk Sea, the Kuril Straits and its continental slope. The model is realised in statement for the contravariant components of velocity vector of the Programmed Complex “Cardinal”. Modeling the tidal dynamics of the straits of the Kuril Сhain and its continental slope is of particular importance due to the high geostrategic prestige of the region. The exceptional complexity of the relief of the area containing dozens of underwater volcanoes requires solving the problem in a complete non-hydrostatic formulation; this necessitates a multiprocessor implementation of the model with a high grid resolution for model representativity. An efficient methods for tidal dynamic reproduction at the 2D vertical cross-cuts domain is described. A statement of boundary problem at vertical cross-cuts is considered both in non-hydrostatic formulation and hydrostatic approximation. The results of computation contain the field of level tidal currents generated by the dominant wave K1 and summary wave in the synodic period; results of comparison the bottom vertical velocity at submarine mountain and on sharp depth changes in the hydrostatic and non-hydrostatic statements, result related to structure vertical velocity at cross-cut, the frequency spectrum and energy of tidal currents.

Energy Conversion Rate from Subinertial Surface Tides to Internal Tides

Abstract Subinertial, topographically trapped diurnal internal tides are an important energy source for turbulent mixing in the subarctic oceans. However, their generation may not be estimated by the conventional barotropic-to-baroclinic conversion because their vertical structure is sometimes barotropic, unlike superinertial internal tides that are always baroclinic. Here, a new energy diagram is presented, in which the barotropic mode is decomposed into the surface and topographic modes, with the latter being classified as part of the internal modes together with the baroclinic mode. The energy equation for the newly defined topographic mode is then derived, providing an appropriate formulation of the energy conversion rate from the subinertial surface tides to the topographically trapped internal tides. A series of numerical experiments confirm that the formulation successfully predicts the energy conversion rate for various cases, with the relative contribution of the baroclinic and topographic modes varying significantly depending on the bottom topography and stratification. Furthermore, this surface-to-internal conversion is demonstrated to give a significantly larger estimate than the barotropic-to-baroclinic conversion for subinertial tides. Applying the formulation to the results of a realistic numerical simulation in the Kuril Straits, an area with the strongest mixing due to subinertial diurnal tides, shows that the surface mode is converted into the baroclinic and topographic modes with comparable magnitudes, responsible for most of the energy dissipation in this area. These results indicate the need to reestimate the global distribution of the generation rate of the subinertial internal tides using our new formulation and to clarify their dissipation mechanisms. Significance Statement Diurnal internal tides in mid- to high-latitude oceans are very different from semidiurnal internal tides, in that they are trapped by topographic features and characterized by uniform rotation throughout the water column rather than by vertical oscillations within the water column. Focusing on this character, we formulate for the first time the generation rate of diurnal internal tides trapped over variable bottom topography. A series of idealized numerical experiments and application to the Kuril Straits show the validity and usefulness of this formulation, which provides a significantly larger generation estimate than previous studies. The results of this study are important for accurate global mapping of turbulent mixing induced by the breaking of internal tides.

Water Circulation, Temperature, Salinity, and pCO2 Distribution in the Surface Layer of the East Kamchatka Current

The ship-borne observations of the temperature, salinity, pCO2 (1995–2020) and satellite geostrophic velocity fields, SST, and chlorophyll concentration are used to identify the factors that determine the spatio-temporal variability of seawater parameters on the western boundary of the subarctic North Pacific. In winter, the surface layer of the East Kamchatka Current (EKC) was characterized by two types of water: the waters with a negative temperature (−1.0–−0.5 °C) and salinity of 32.4–32.9 and waters with a positive temperature (0.4–1.7 °C) and salinity of 33.0–33.1. The source of water with negative (positive) temperature and decreased (increased) salinity for the EKC zone is the Bering Sea shelf (Aleutian Basin). The surface waters in the eastern Kamchatka area in winter were close to gas equilibrium with the atmosphere or supersaturated with carbon dioxide (pCO2 = 380–460 µatm). In summer, extremely low pCO2 values (140–220 µatm) in the surface layer of the eastern Kamchatka and the northern Kuril Islands regions have been associated with the decreased salinity (32.1–32.6) of the waters. The distributions of the temperature, salinity, and pCO2 in the surface layer of the central Kuril Islands are determined by the location and intensity of the Kuril eddies and the EKC stream jets. The water mixing in the central Kuril Straits and the Kruzenshterna Bank area leads to increased salinity (33.2–33.4) and high values of pCO2 (480–670 µatm) in the surface layer of the EKC. The comparison of the pCO2 data collected in winter demonstrates an increase in pCO2 between 1998/2001 and 2018/2020 at about 50 µatm in the surface waters with a salinity of 33.0–33.1, which is in agreement with an increase in pCO2 in the atmosphere at 46 µatm (from 368 to 414 µatm) during this period.

Trace element composition of the Southwestern Sakhalin Chum Salmon

The study is devoted to the microelement composition of organs and tissues of chum salmon (Oncorhynchus keta), which came for spawning in September 2018 in the r. Lovetskaya on the southwestern coast of Sakhalin. Samples of organs and tissues of fish, prepared in Sakhalin, were frozen and delivered to Vladivostok for chemical analysis. Trace elements were determined from acid mineralizates by atomic absorption on a Shimadzu AA 6800. It was found that the content of microelements indicating anthropogenic impact on the environment (Zn, Cu and Ni) in the Sea of Japan chum was significantly higher than the content of those in the Okhotsk Sea fish. So, for example, the amount of zinc in the muscles of the Sea of Japan chum salmon was 2.5 times higher than in those of the Sea of Okhotsk fish. In the liver, the effect was even greater - the difference between the content of this element in the Sea of Japan and the Sea of Okhotsk reached 19 times. At the same time, in the organs and tissues of the Okhotsk chum salmon, which crossed the impact geochemical zone in the Kuril-Kamchatka region at least twice during its life cycle, the content of lead and cadmium, witnesses of underwater and surface volcanism in the Kuril straits, prevailed. The content of lead in the muscles of chum salmon from r. Raidovaya was 4 times more than in the muscles of chum salmon from the river. Lovetskaya; in the liver, the difference was 5 times.

Systematic identification of microplastics in abyssal and hadal sediments of the Kuril Kamchatka trench.

The occurrence of microplastics throughout marine environments worldwide, from pelagic to benthic habitats, has become serious cause for concern. Hadal zones were recently described as the "trash bins of the oceans" and ultimate sink for marine plastic debris. The Kuril region covers a substantial area of the North Pacific Ocean and is characterised by high biological productivity, intense marine traffic through the Kuril straits, and anthropogenic activity. Moreover, strong tidal currents and eddy activity, as well as the influence of Pacific currents, have the potential for long distance transport and retention of microplastics in this area. To verify the hypothesis that the underlying Kuril Kamchatka Trench might accumulate microplastics from the surrounding environments and act as the final sink for high quantities of microplastics, we analysed eight sediment samples collected in the Kuril Kamchatka Trench at a depth range of 5143-8250m during the Kuril Kamchatka Biodiversity Studies II (KuramBio II) expedition in summer 2016. Microplastics were characterised via Micro Fourier Transform Infrared spectroscopy. All samples were analysed in their entirety to avoid inaccuracies due to extrapolations of microplastic concentrations and polymer diversities, which would otherwise be based on commonly applied representative aliquots. The number of microplastic particles detected ranged from 14 to 209kg-1 sediment (dry weight) with a total of 15 different plastic polymers detected. Polypropylene accounted for the largest proportion (33.2%), followed by acrylates/polyurethane/varnish (19%) and oxidized polypropylene (17.4%). By comparing extrapolated sample aliquots with in toto results, it was shown that aliquot-based extrapolations lead to severe under- or overestimations of microplastic concentrations, and an underestimation of polymer diversity.

Variabilities of the Sea Surface Height in the Kuril Basin of the Sea of Okhotsk: Coherent Shelf-Trapped Mode and Rossby Normal Modes

AbstractA time series analysis of the sea surface height anomaly (SSHa) was conducted in the Kuril Basin of the Sea of Okhotsk. The mapping of the satellite-derived SSHa data was optimized to mitigate the effects of sea ice on the SSHa field during winter and early spring. Complex empirical orthogonal functions (CEOFs) were then used to analyze the SSHa field, revealing that the first three modes account for 55% of the signal variance. Mode 1 mainly represents the coherent variability trapped over the shelves all along the coastal regions and the Kuril Islands. Both seasonal and interannual variations are strongly correlated with the alongshore wind stress and can be well explained by an arrested topographic wave. Mode 3 is a propagating mode that exhibits trains of southwestward-propagating, high-amplitude anomalies. One possible mechanism for this is first-mode baroclinic Rossby waves, whose energy propagates from the Kuril Straits toward the Kuril Basin. However, mode 3 can be better interpreted as barotropic Rossby normal modes generated in the deep Kuril Basin. Mode 2 is a standing mode that may encompass the baroclinic variability in the basin. The monthly mean of the SSHa in the Kuril Basin is primarily governed by variability in mode 1, with mode 2 contributing to a lesser extent, and mode 3 being insignificant.

Seasonal changes of water mass, circulation and dynamic response in the Kuril Basin of the Sea of Okhotsk

The seasonal variability of the hydrography and anticyclonic circulation in the Kuril Basin of the Sea of Okhotsk, from which the North Pacific ventilation originates, is studied from bi-monthly climatologies of temperature, salinity, dynamic height and current velocity. The subsurface hydrography of the Kuril Basin is characterized by the dominant role of fresh and cold waters transported by the East Sakhalin Current during winter and spring, and relatively warm and saline waters flowing with the Soya Warm Current. At the intermediate layer, the influence of dense shelf water is maximum from May to August, and this water mixes with surrounding waters in around half a year to form a well homogenized Okhotsk Sea Intermediate Water, the source of North Pacific ventilation. The anticyclonic circulation typical of the Kuril Basin exhibits a strong seasonality, being absent in winter and showing a maximum amplitude in summer. Its formation is mainly related to the dynamic response of isopycnals to wind forcing. A dynamic height maximum appears along the coastal regions from Sakhalin Island to the southern Kuril Straits in winter, likely migrates towards the Kuril Basin in March–April and forms the anticyclonic system between May and October. The dynamic height – and thus the flow field – is mainly determined by isopycnal displacement in and around the Kuril Basin, whereas advection of water masses has a negligible effect. Surface intensification and decay of the anticyclonic circulation seems to be related to other processes such as heat exchange with the atmosphere.

Intra-annual variability of the mixed layer parameters in the Okhotsk Sea

All available oceanographic data for the deep-water part of the Okhotsk Sea, in total 111,944 stations collected in 1931–2014, are analyzed after removing the duplicate and lowquality ones. The mixed layer depth is determined on the water temperature profle for each station and monthly mean values of temperature and salinity in the mixed layer are calculated for the 0.5о-size grid. In May-October, the mixed layer depth varies within the range 5–25 m, with the highest values in the areas with active water dynamics. On the contrary, the mixed layer is much deeper in December-April when its thickness exceeds 40–60 meters and even reaches 100–120 m in the central Kuril Straits. Two types of temperature distribution change annually within the mixed layer in the Okhotsk Sea: the winter pattern with higher (positive) temperature at Kuril Islands occurs in December-April and the summer pattern is formed in June-September and is distinguished by spots of lower temperature in the dynamically active areas with strong tidal and non-tidal currents (Kuril Straits, Kashevarov Bank, entrance to the Shelikhov Bay, Shantar Islands vicinity, northeastern shelf of Sakhalin Island). Transition patterns of the temperature distribution are observed in May, October, and November. Largescale patterns of salinity distribution within the mixed layer are permanent throughout a year and indicate prevailing currents and other processes in the upper layer of the Okhotsk Sea. The maximum salinity is observed in the southern Okhotsk Sea, whereas the lower salinity values are usual for the coastal waters (except the coasts of Kuril Islands) affected to the river runoff.

Diversity and distribution of peracarid crustaceans (Malacostraca) from the abyss adjacent to the Kuril-Kamchatka Trench

During the KuramBio expedition in 2012, macrobenthos samples were collected with an epibenthic sledge from 12 sites on the abyssal plain of the Northwest Pacific Basin adjacent to the Kuril-Kamchatka Trench (KKT) and on the western slope of the KKT, at depths between 4830 and 5780 m. In terms of the peracarid fauna, the NW Pacific abyss was found to be one of the richest and densely populated deep-sea regions of the World Ocean. With 482 species, 168 genera, 61 families and 5 orders, the superorder Peracarida was found to be the most diverse and one of the most abundant macrobenthic taxa in the collection. Of the taxa found, 3 genera and 386 (80%) species were apparently new to science, 30 genera and the family Dendrotionidae (Isopoda) being recorded for the first time in the NW Pacific. The peracarid fauna composition at the abyssal depths of the KKT region was typical of the open, non-isolated abyssal plain. The Isopoda proved the most diverse and abundant peracarid order (245 species, 61.3% of the individuals), followed by the Amphipoda (79 species, 19.6%), Tanaidacea (77 species, 9.1%), the Cumacea (70 species, 9.7%), and the Mysida (11 species, 0.23%). The highest peracarid abundance (up to 1047 ind./1000 m2) was recorded on the western slope of the trench, abundances at the abyssal plain being lower (125–639 ind./1000 m2). The peracarid distribution in the area studied was affected by a complex of various factors, the food availability reflected in the Particular Organic Matter (POM) flux belonging to the most important ones. Specifically, increases in the peracarid abundance and species richness corresponded in most cases with a high sediment organic carbon content or the high surface productivity level in the area. As indicated by the sediment composition, the food input to the deep-sea system apparently differed between the sites sampled. On the trench slope, accumulation of organic debris, transported down the slope from shallower regions, played an apparently major role. The abyss in the northern and in the southern parts of the area sampled is influenced by the high surface productivity, resulting from the regional hydrography. The area east of the Middle Kuril Islands is likely influenced by the organic material transported from the Sea of Okhotsk through the Kuril Straits.

RECENT CHANGES IN PRODUCING OF THE INTERMEDIATE WATER IN THE OKHOTSK SEA AND ITS PROPERTIES

Data of long-term observations on the cold intrusions in the warm intermediate layer of the Okhotsk Sea are overviewed, considering these intrusions as fresh portions of the Intermediate water of local origin. They are formed in the process of the high-density Bottom Shelf water cascading from the northwestern shelf, mainly in the submarine canyon at northwestern Sakhalin. Transport of these new-formed portions is traced in the flows of cyclonic circulation of the Okhotsk Sea waters from the area of their origin to the southern Kuril Straits, that takes from 2 to 6 years. Similar intrusions are found in the Pacific, mainly at southern Kuril Islands, where they penetrate presumably through the Vries and Bussol Straits. Year-to-year variability of the Bottom Shelf water and the new-formed Intermediate water properties are analyzed in dependence on external factors. There is determined that the volume and density of the Bottom Shelf water depend on winter severity that could be quantitatively explained by the ice cover; all these parameters decrease recently. Salinity of the new-formed Intermediate water has similar changes. Besides, tidal mixing influences on its salinity, in particularly at the Kuril Straits, so the tidal 18.6-year cycle is presented in the salinity variations. On the contrary, variations of the minimal temperature in the cold-water lenses of the fresh Intermediate water of local origin do not correlate with the changes in their production because of auto-compensatory effect for temperature in the process of its cascading to different depths. The same effect is obviously applied to dissolved oxygen, that’s why the oxygen content in the upper intermediate layer at the area of cascading is rather stable and doesn’t correlate with the ice cover changes. However, ventilation of the medium and deeper parts of the intermediate layer decreases on the background of recent lowering of the Intermediate water producing, in particularly the production of the water with σq ≥ 26.8. On a distance from the area of cascading, the tendency to deoxygenizing spreads over the whole intermediate layer of the Okhotsk Sea.

Distribution of diatoms and geochemical features of holocene sediments from the Kuril Basin (Sea of Okhotsk)

Sediments from the Kuril Basin were collected during the Russian-German deep-sea expedition SokhoBio (Sea of Okhotsk Biodiversity Studies). Concentrations of biogenic and trace elements, quantitative content, and species composition of diatom assemblages in surface sediments and core samples were analyzed. Distribution in the surface sediments depending on different environmental conditions as well as changes in the diatom frustule content, the ratio of dominant diatom species, and the ecological structure of diatom assemblages were studied.The collected sediments were silt and clayey silt, with SiO2am and Corg contents ranging from 15.85% to 36.61% and 0.84–1.92%, respectively. The spatial distribution patterns of the elements studied and the diatoms, as well as vertical profiles of the sediment cores, revealed that the contents of biogenic components and trace elements, such as amorphous silica, organic carbon, barium and phosphorus, changed in parallel with the change in the number of diatoms and depended mainly on the biological productivity of the sea. The change in water mass parameters and in the hydrological regime also affected the chemical composition of the bottom sediments over time.A predominance of oceanic species is observed in the sediments at the bottom of the basin, and the mixed oceanic-neritic composition of diatoms is characteristic of slope deposits. Moreover, the assemblages reflect the influence of Pacific Ocean water masses at stations close to the Kuril Straits, as well as in the warm waters of the Sea of Japan at the station in the Soya Current zone.A nearly synchronous increase in the impact of Okhotsk water masses and the entry of the Sakhalin Current waters into the Kuril Basin as one moved deeper into the core at stations with different climatic conditions was observed.The productivity and species diversity of diatoms and the chemical composition of sediments are reflections of the regional features of climatic processes.

Migrations of juvenile chum salmon in the Okhotsk Sea; comparative analysis of efficiency for fish hatchery complexes in Far East of Russia and in Japan

Abundance and structure of feeding aggregations of chum salmon are analyzed on the results of the otoliths processing sampled in different areas of the Okhotsk Sea in 2011-2014 and scheme of their autumn migrations in dependence on the sea surface temperature (SST) patterns, using the data on catching the fish with the otoliths marked in hatcheries. The migrations were similar for all chum groupings from the Okhotsk Sea and adjacent waters: their juveniles moved to Kamchatka coast until SST decreasing to 11 о С in October, where they formed mixed aggregations, then density of the aggregations increased until SST 9-10 о С, after that the aggregations moved to the central part of the sea and further southward passing the Kuril Straits in November under SST 7-8 о С. The portion of marked fish from hatcheries in mixed catches was 2.1-7.8 % (on average 3.9 %). Taking into account these values and the ratio between marked and non-marked fish for certain hatcheries, the ratio between wild stocks and hatchery release of different origin is determined. The portion of chum salmon from Japanese hatcheries is estimated as 9.6-41.6 % (on average 27.3 %), the portion of chum from Russian hatcheries is considerably lower - 0.5-2.6 % (on average 1.2 %), and chum salmons of natural origin dominate in all years with the portion 55.8-89.9 %. For the fish of artificial origin, the ratio between the number of juveniles feeding in the sea and their release from hatcheries is estimated as 3.0-4.2 % (on average 3.6 %) for Japanese hatcheries and 0.3-0.7 % (on average 0.5 %) for Russian hatcheries. So, survival of chum from Japanese hatcheries is higher in 8 times on average, and total abundance of the feeding artificial chum salmons of Japanese origin is higher than those of Russian origin in more than 20 times. However, the hatcheries in Sakhalin and Kuril Islands has similar efficiency as Japanese ones, in opposite to other areas. The abnormally low estimations of the efficiency for some Russian hatcheries allow us to suppose that the otolith marking program is not realized correctly in some regions of Russian Far East, possibly the marks don’t correspond exactly to the schemes of marking.

Lagrangian analysis of formation, structure, evolution and splitting of anticyclonic Kuril eddies

We studied in detail a mesoscale anticylonic eddy that has been sampled in the R/V Professor Gagarinskiy cruise (June–July 2012) in the area east of the Kuril Islands in the northwestern subarctic Pacific. Lagrangian approach was applied to study formation, structure and evolution of this feature called the eddy A and of its parent eddy B using a simulation with synthetic tracers advected by the AVISO velocity field. We used different Lagrangian methods and techniques to identify those eddies and their boundaries, to know their structure and to document their deformation, metamorphoses and splitting. It has been found that the eddy A was born as a result of splitting of the eddy B with the core water to be borrowed from the eddy B which, in turn, was influenced by the Okhotsk Sea water flowing into the ocean through the Kuril straits. The periphery of the eddy A was formed mainly by East Kamchatka Current water in the process of its winding onto the eddy A core by portions. All these processes have been documented in detail with the help of drift and tracking Lagrangian maps computed forward and backward in time with a large number of synthetic tracers distributed over the studied area. We have found a Lagrangian structure of those eddies and the ways how they have gained and released water. Simulated and measured locations of the center of the eddy A and its boundary have been be estimated to coincide with the accuracy of ≈7–10 and ≈15–20km, respectively. Our simulations were validated in part by tracks of available surface drifters and Argo floats. We presented CTD hydrographic observations of the Kuril eddy A from the surface to deep waters and compared observed and simulated results in order to establish origin and properties of water masses constituting that eddy.

Spatial-temporal patterns of euphausiids ecology in the Okhotsk Sea

The most common regularities in spatial distribution of mass Euphausia species and their size classes and ontogenetic stages are determined for the Okhotsk Sea using the data averaging by biostatistical areas and applying the average results to the centers of the areas that allows to smooth small-scale patterns as patchiness. Thysanoessa raschii and Th. inermis occupy mostly shelf areas and Th. longipes and Euphausia pacifica - deep-water areas; among them Th. raschii and Th. longipes are more abundant and Th. inermis and E. pacifica are supposedly allochtonous species which penetrate to the Okhotsk Sea through the Kuril Straits, so they are distributed mainly in the southern part of the Sea. Mean biomass and abundance of all species in the upper (0-50 m) and deeper (50-200 m) layers are calculated for each stage and size class in the daytime and nighttime, separately: most of them, including both adults and furcilias, concentrate mainly in the deeper layer and their night biomass is higher in both layers though about a half of animals raise to the sea surface at night. Only eggs, nauplii and calyptopes of euphausiids don’t change their abundance considerably between day and night, and most of them occupy the upper layer round the clock. Obviously, the increasing of late furcilias and adults in the deeper layer at night is caused by their diurnal migration from the layers below 200 m. Seasons of the highest abundance for early stages come in natural order that is a sign of their short duration.

Effects of temporal variation in tide-induced vertical mixing in the Kuril Straits on the thermohaline circulation originating in the Okhotsk Sea

Tidally induced vertical mixing is important for thermohaline circulation. Previous estimations of tidal mixing have aimed to obtain time-averaged values, and ocean general circulation models (OGCMs) typically parameterize such mixing using a temporally constant strength. However, tidal mixing is known to vary temporally during tidal or spring–neap cycles. Here, we investigate the effects of temporal change in tidally induced vertical diffusivity (κt) in the Kuril Straits using an OGCM. The results demonstrate that variations of vertical mixing on diurnal, 2-week, and 1/2-year timescales induce significant differences in the net effect of mixing and, therefore, in the thermohaline circulation originating in the Okhotsk Sea. For diurnal and 2-week variations, the strength of the tidal mixing effect depends on (1) the period and length of the duration over which κt is larger than the temporal average and (2) the amplitude of the temporal variation of κt, even if the time-averaged values are the same. This is explained by the relative importance of two states. In a quasi-equilibrium state, a larger κt results in weaker stratification and vice versa, and thus the net tidal mixing effect is weaker when κt is variable than when it remains constant. Conversely, in an adjustment stage just after an increase in κt, a larger κt acts on stronger stratification and vice versa, resulting in a stronger mixing effect. For a 1/2-year variation, the strength of the tidal mixing effect also depends on the phase relationship with seasonal variation in stratification. These results imply the necessity of considering temporal change when estimating tidal mixing from observations, specifying it in OGCMs, and understanding its effects.

Remotely propagating salinity anomaly varies the source of North Pacific ventilation

The dense shelf water (DSW) produced in the Sea of Okhotsk causes the deepest ventilation in the North Pacific and plays a key role in overturning to the depth of intermediate layers. Salinity determines the density of DSW in cold seas such as this. However, the variability of DSW salinity has yet to be quantified and its causes remain unknown because of the paucity of available observation data. Here, we describe the variability of DSW salinity in the period from 1950 to 2005 through the analysis of a newly compiled hydrographic dataset, expanded with Russian measurements in the Sea of Okhotsk and the Bering Sea. DSW salinity exhibits a decreasing trend of −0.0024±0.0015y−1 over 1950–2005, in addition to decadal-scale variability with a typical magnitude of ∼0.1. We have found that DSW variability is controlled largely by surface salinity anomalies that propagate along pathways associated with ocean currents from the Bering Sea to the Sea of Okhotsk. These salt pathways can be traced farther upstream to the Alaskan Stream and western subarctic gyre. We suggest that this is a possible pathway which consists of a previously undefined upper branch of the North Pacific overturning cell. Along this pathway, the effects of the nodal tide that has an 18.6-year period may also propagate from the Kuril Straits and the Aleutian Passes. Our results indicate that long-term atmospheric variations and enhanced hydrological cycles over the North Pacific, which are associated with global climate changes, are conducted to the intermediate layer through surface salt pathways and subsequent DSW ventilation and lead to variability in material circulation and biogeochemical cycles.

Direct breaking of large-amplitude internal waves in the Urup Strait

Observations and model data have been analyzed for the Urup Strait, one of the Kuril Straits, focusing on processes driving the extremely strong vertical mixing. Observations over 1day on the Pacific side of the sill of the Urup Strait show the generation of a large-amplitude internal wave (LAIW), characterized by the depression of potential density surfaces caused by a strong off-sill flow. When the LAIW developed, the potential density surface of 26.7 σθ was depressed downward by more than 300m, and vertical mixing was markedly elevated within the 400-m-thick layer 26.6–26.7 σθ, with a layer-mean (maximum) energy dissipation rate and vertical diffusivity of 1.2×10−6 (7.9×10−6Wkg−1) and 4.0×10−1 (2.8m2s−1), respectively. A three-dimensional model simulation of the Kuril Straits driven by the diurnal barotropic tide reproduces well the observed features relevant to the generation of LAIWs in and around the Urup Strait. The structure and generation pattern of LAIWs in the Urup Strait are consistent with those of the arrested lee waves observed in the Hawaiian ridge and Luzon Strait, although the depth at the top of the sill is far shallower in the Urup Strait. It is suggested that barotropic tidal energy with diurnal frequency is converted into LAIWs through the excitation of topographically trapped waves with hydraulically supercritical velocity near the sill top, and it is these waves that are responsible for the extremely strong mixing on the sill-flanks of the shallow Kuril Straits.

Quantitative evaluation of iron transport processes in the Sea of Okhotsk

Comprehensive observations of the iron (Fe) distribution in the western Sea of Okhotsk were conducted and revealed the existence of two Fe transport processes in the sub-polar marginal sea. One transport process is Fe loading from the Amur River and transport by the East Sakhalin Current (ESC), and the other is Fe transport by the intermediate water (part of which was reported by Nishioka et al., 2007). Here, we report on quantitative evaluations of these two Fe transport processes. The surface dissolved Fe (Diss-Fe) and low salinity water distribution clearly indicate the influence of Fe discharge from the Amur River and the Fe that is transported by the East Sakhalin Current. The amounts of total dissolvable Fe (TD-Fe) and Diss-Fe that cross the surface of the northeast Sakhalin coastal area are estimated at 9.0×108∼1.3×109gyr−1 and 1.0×108∼1.5×108gyr−1, respectively. Although the ESC surface transport system along the Sakhalin coast is effective, the length-scale estimation of TD-Fe transport indicated that only 1.5% of the Fe at the mouth of the Amur River reached 52°N, which may be due to scavenging by biological particulates. High Fe anomalies were observed at the bottom of the continental shelf and the shelf break along the Sakhalin coast. The extremely low temperature and low N* water indicate that Fe resuspension due to the reducing properties of sediment occurred on the shelf and that the Fe was introduced to Dense Shelf Water (DSW) by tidal mixing. We estimate that the amounts of TD-Fe and Diss-Fe involved in the DSW on the continental shelf are 8.8×1010∼2.5×1011gyr−1 and 2.3×109∼6.6×109gyr−1, respectively. Length-scale estimates of TD-Fe transport indicate that 20% of the TD-Fe on the continental shelf remained in the Kuril Basin; thus, the TD-Fe in the intermediate water was efficiently transported to the Kuril Basin. These results indicate that two orders of magnitude more Fe is derived from the continental shelf by the intermediate water than by surface water and that Fe is transported a greater distance by intermediate water than by the surface layer. Additionally, the Fe that reached the Kuril Straits was mixed by intensive tidal mixing and influenced the vertical profiles of the water columns on both sides of the Kuril Straits in the Kuril Basin and the Oyashio region. Our quantitative evaluation also indicates other Fe inputs around the Kuril Strait.

Internal tides and turbulent mixing observed in the Bussol Strait

Repeated observations with a period of about 24h of hydrography, current velocity, and microstructures were performed at three stations surrounding a seamount in the middle of the Bussol Strait, the deepest and widest one of the Kuril Straits, to reveal spatial and temporal variability of internal tides and associated turbulent mixing. It is found that isopycnal displacements are dominated by diurnal tidal components, which show phase differences (namely, time lags) between the three stations that can be explained by a first mode topographically trapped wave (TTW) propagating clockwise around the seamount. Furthermore, at the station located near the center of the strait where energy dissipation rates are largest, diurnal variations of isopycnals and velocities are amplified toward the ocean bottom, consistent with the vertical structure of the first mode TTW. At that station, vigorous turbulent mixing with the energy dissipation rate exceeding 10-6m2s-3 and diapycnal diffusivity exceeding 10-1m2s-1 was observed in deep layers when the diurnal tidal current consisting of the first mode TTW flows from the Okhotsk Sea to the North Pacific, enhancing the mean current. These spatial and temporal variation patterns are confirmed to be reproduced by a previous numerical model successfully for the isopycnals and velocities, and partially for the turbulent mixing. The total energy dissipation rate is, however, by up to a factor of 3–10 smaller than predicted by the numerical model although the observations were performed during spring tides, suggesting that the actual diapycnal mixing is overall weaker than the previous model estimate and/or that extremely strong mixing occurs within highly localized areas.

Distribution of vertical diffusivity in the Bussol' Strait: A mixing hot spot in the North Pacific

Strong vertical mixing in the Kuril Straits is believed to be an important contribution to the ventilation of the intermediate layer and water mass transformation in the North Pacific, especially for density greater than 27.1σθ. Furthermore, a recent modeling study has suggested that the vertical profile of vertical diffusivity is a key factor in the determination of thermohaline circulation in the North Pacific. Here we report the distribution of vertical diffusivity in the Bussol' Strait, the main conduit of water exchange and a possible central site of strong mixing in the Kuril Straits. Our analysis is based on a set of highly densed CTD observations, with a total of 127 casts across the strait in 2001. Vigorous density inversions occurred in the strait with the largest vertical displacement being over 250m. We estimated the vertical diffusivity coefficient Kρ from the Thorpe scale for all the CTD data. The vertical average of Kρ estimated from all the casts is 60×10−4m2s−1. Overall, Kρ is relatively small in the upper 300m (density range approximately 26.5–26.7σθ), whereas it is relatively large below a depth of 500m (density range of >26.8σθ), with a maximum at the depths of 1100–1700m. The distributions of Kρ and the amplitude of the diurnal tidal current are similar, suggesting that the mixing is caused by the strong diurnal tidal current. The amplification of the diurnal (tidal) current over slopes near the bottom causes the Kρ maximum at depths of ∼1100–1700m. We also introduce an empirical relationship between Kρ and the amplitude of the diurnal tidal current. The vertical diffusivity is one order of magnitude larger at the spring tide than at the neap tide, suggesting that there is extremely large variability of tidal mixing with the fortnightly modulation. In the intermediate layer at densities of 27.3–27.6σθ, large Kρ values (>60×10−4m2s−1) corresponds well to the colder and less-saline water mass characterized in the Bussol' Strait, confirming that water mass transformation occurs locally in the strait through strong diapycnal mixing.