Subarctic Front Research Articles

Species structure and year-to-year dynamics of nekton biomass in the upper epipelagic layer of the Pacific waters at Kuril Islands in summer periods of the 2000s

The North-West Pacific waters at Kuril Islands are characterized by high productivity because of high water dynamics due to interaction of the Kamchatka, Oyashio, and Kuroshio Currents and tidal mixing. Abundance, species composition, and spatial density of nekton in the upper epipelagic layer (0-50 m) of this area are considered on the data of trawl surveys conducted in June-July of 2004-2012. In total, 97 species of nekton are found in the trawl catches, including 73 species of fish and 24 species of cephalopods belonged to 15 genera and 8 families. Myctophidae have the highest species richness and biomass among fish (12 species, 33% of the total nekton biomass), and Gonatidae have the highest number of species and biomass among cephalopods (13 species, 11 %). Mesopelagic fishes, being temporary inhabitants of the epipelagic layer, dominate in the nekton community (54 species, 60 % of total biomass), they migrate to the upper layer at night for feeding. Other important ecological groups are salmons migrating to spawning grounds and subtropical species migrating to feeding grounds. Ratio between these groups is very variable. Total abundance of nekton varied from 1.1 t/km² in 2010 to 4.6 t/km² in 2004, on average 2.4 t/km². Range of its spatial variation is even higher: 1.1-6.0 t/km²; it depends on evenness of species composition and abundance of dominant species; generally both total abundance and species richness increase from the north to the south, as boreal species are replaced by low-boreal and subtropical ones. From the other hand, the abundance and species richness increase from the shore toward the ocean: the total number of nekton species increases from 42 to 76, and the number of dominant species - from 4 to 10, so structure of the nekton community becomes more evened and polydominant. The highest abundance and species richness are observed in the Subarctic Front zone. Mean total biomass of nekton in the epipelagic layer of the surveyed area is estimated as 2442 · 10 3 t including 1.6-2.5 million t (37-60 %) of mesopelagic fish that was the major group of nekton in all years except 2012, when the biomass of mesopelagic fish was only 378 · 10 3 t (21 %) while the portion of salmons increased to 40 % because of extremely high biomass of pink salmon (27 % of the total nekton biomass). Year-to-year changes of the nekton abundance in the surveyed area are characterized by negative trend, with its minimal values in 2008, 2010, and 2012, but it is considered as a temporary tendency.

Diatom distributions in northern North Pacific surface sediments and their relationship to modern environmental variables

In order to map the modern distribution of diatoms and to establish a reliable reference data set for paleoenvironmental reconstruction in the northern North Pacific, a new data set including the relative abundance of diatom species preserved in a total of 422 surface sediments was generated, which covers a broad range of environmental variables characteristic of the subarctic North Pacific, the Sea of Okhotsk and the Bering Sea between 30° and 70°N. The biogeographic distribution patterns as well as the preferences in sea surface temperature of 38 diatom species and species groups are documented. A Q-mode factor analysis yields a three-factor model representing assemblages associated with the Arctic, Subarctic and Subtropical water mass, indicating a close relationship between the diatom composition and the sea surface temperatures. The relative abundance pattern of 38 diatom species and species groups was statistically compared with nine environmental variables, i.e. the summer sea surface temperature and salinity, annual surface nutrient concentration (nitrate, phosphate, silicate), summer and winter mixed layer depth and summer and winter sea ice concentrations. Canonical Correspondence Analysis (CCA) and other analyses indicate 32 species and species groups have strong correspondence with the pattern of summer sea surface temperature. In addition, the total diatom flux data compiled from ten sediment traps reveal that the seasonal signals preserved in the surface sediments are mostly from spring through autumn. This close relationship between diatom composition and the summer sea surface temperature will be useful in deriving a transfer function in the subarctic North Pacific for the quantitative paleoceanographic and paleoenvironmental studies.The relative abundance of the seaice indicator diatoms Fragilariopsis cylindrus and F. oceanica of >20% in the diatom composition is used to represent the winter sea ice edge in the Bering Sea. The northern boundary of the distribution of F. doliolus in the open ocean is suggested to be an indicator of the Subarctic Front, while the abundance of Chaetoceros resting spores may indicate iron input from nearby continents and shelves and induced productivity events in the study area.

Variability of eddy kinetic energy in the Sea of Japan from satellite altimetry data

The variability of the geostrophic eddy kinetic energy (EKE) in the Sea of Japan derived from weekly altimetric sea level anomalies spanning from 1992 through 2009 is studied. Nonorthogonal modes of variability are revealed accounting for more than 60% of the total variance. They capture the seasonal variation of the mesoscale energetics in the entire Sea of Japan with the EKE growing in the warm season up to the maximum in October through November and diminishing in the cold season down to the minimum in March through April. In the northern Sea of Japan (northward of the Subarctic Front), where the mean EKE is several times less than in the southern sea, areas of considerable variability are detected. Quasi-biennial EKE oscillations are revealed but not the trends covering the whole record.

New data on the distribution and feeding habits of jellyfish in the Northwest Pacific

In June and July of 2012, the jellyfish catches in the northeastern portion of the surveyed Pacific waters off the Kuril Islands substantially exceeded those in the southwestern portion. This indicates that jellyfish disperse over the studied area predominantly from the southern Bering Sea and from the eastern coast of Kamchatka. Their strobilation probably takes place as well on the shelf and continental slope of eastern Kamchatka. The distribution of jellyfish with medium-sized bells does not show any geographic pattern; the aggregations that are formed are mixed regarding the original locality of individuals. Jellyfish occur within a broad range of surface water temperatures and their catches have declined significantly only in the southeast of the area of surveys near the Subarctic Front. As is seen from the data we compared, not only the abundance of jellyfish, but their feeding activity and, as a consequence, the amount of consumed food decreased by an order of magnitude during the cold season (in the spring 2011). However, irrespective of the season, the largest quantities of food were recorded in the largest and most numerous jellyfish (Phacellophora camtschatica, Chrysaora melanaster). The quantitative results of the studies on the diet of jellyfish may be somewhat underestimated, as fragile jellyfish bodies are easily damaged in trawl nets and evaluating the diet is possible only for intact individuals. Use of specialized catching gear in the future may help us to specify the feeding dynamics in jellyfish, as well they may provide an opportunity to observe their feeding behavior. At the same time, quantitative estimates of the daily-food intake in jellyfish can be obtained only by taking the rates of digestion and prey consumption found under laboratory conditions into account, with their subsequent verification in balance models.

Evaluating the role of fronts in habitat overlaps between cold and warm water species in the western North Pacific: A proof of concept

Cold- and warm-water species' fishing grounds show a spatial synchrony around fronts in the western North Pacific (WNP). However, it is not yet clear whether a front (thermal, salinity or chlorophyll) acts as an absolute barrier to fish migration on either side or its structure allows interaction of species with different physiological requirements. Our objective was to assess potential areas of overlap between cold- and warm-water species using probabilities of presence derived from fishery datasets and remotely sensed environment data in the Kuroshio–Oyashio region in the WNP. Fishery data comprised skipjack tuna (Katsuwonus pelamis) fishing locations and proxy presences (derived from fishing night light images) for neon flying squid (Ommastrephes bartrami) and Pacific saury (Cololabis saira). Monthly (August–November) satellite remotely sensed sea-surface temperature, chlorophyll-a and sea-surface height anomaly images were used as environment data. Maximum entropy (MaxEnt) models were used to determine probabilities of presence (PoP) for each set of fishery and environment data for the area 35–45°N and 140–160°E. Maps of both sets of PoPs were compared and areas of overlap identified using a combined probability map. Results indicated that areas of spatial overlap existed among the species habitats, which gradually widened from September to November. The reasons for these overlaps include the presence of strong thermal/ocean-color gradients between cold Oyashio and warm Kuroshio waters, and also the presence of the sub-arctic front. Due to the high abundance of food along frontal zones, the species use the fronts as foraging grounds while confining within physiologically tolerable waters on either side of the front. The interaction zone around the front points to areas that might be accessible to both species for foraging, which suggests intense prey–predator interaction zones.

An experience of using the turner angle for differentiating water structures in the Northwest Pacific

Using the data of CTD observations in the Kuril region of the Pacific Ocean, vertical structures of water masses are analyzed and differentiated using the Turner angle Tu in the layer between the isopycnic surfaces of 26.5 and 26.8 σθ. The former characterizes the core of the cold intermediate layer in the subarctic water structure and the latter, the intermediate salinity minimum in the subtropical water structure. Vertical variations of temperature and salinity in the specified density range for subarctic and subtropical water structures are opposite to each other, and the Turner angle changes the sign from negative to positive when crossing the boundary between them. The isoline Tu = −45° denotes disappearance of the cold intermediate layer, Tu = 0°, an outcrop of the halocline along with the salinity front formation, and Tu = 45° is the northern boundary of the subtropical water. Obtained are the relationships between the Turner angle and the variations of temperature and salinity within the mentioned layer. Demonstrated are the benefits of the proposed method for localizing the boundaries of the subarctic front as compared with traditional methods.

Reconstruction of the late-Holocene changes in the Sub-Arctic Front position at the Reykjanes Ridge, north Atlantic

Changes in the dynamics of the North Atlantic subpolar gyre are involved in the modulation of the northward salinity and heat transport in the northern North Atlantic via the North Atlantic Current (NAC). Variations in the strength of the East Greenland Current (EGC) can influence the gyre dynamics by impacting deep convection in the Labrador Sea. Oxygen isotope data of three planktonic foraminiferal species (surface water Neogloboquadrina pachyderma dextral coiling and Globigerina bulloides; thermocline recorder Globorotalia inflata) from a site located close to the present Sub-Arctic Front at the Reykjanes Ridge suggest significant strengthening or shifting of the Sub-Arctic Front throughout the late Holocene. The oxygen isotope based inferences are supported by Mg/Ca-derived temperature reconstructions from Neogloboquadrina pachyderma dextral coiling, alkenone-derived sea surface temperature and other paleoclimatic proxy data. The late-Holocene strengthening/shift of the Sub-Arctic Front appears caused by an increasingly more defined and fresher EGC. The proposed subpolar gyre changes may modulate the northward heat transport, and explain the geographically different long-term climatic trends in the North Atlantic during the late Holocene, i.e. a cooling of the EGC-influenced regions and a warming of the NAC-influenced areas from c. 4 to 5 ka. This mechanism cannot, however, explain the simultaneous occurrence of millennial-scale events at c. 5.6, 3.9, 2.7, 1.3 ka and the ‘Little Ice Age’ in both areas. Noteworthy is the steadily increasing amplitude of these cold events at the Reykjanes Ridge, likely induced by drift ice and/or EGC-influence culminating in the ‘Little Ice Age’. A widespread pronounced warming at 2.0 ka seems to represent the ‘Roman Warm Period’ and reflects the warmest period of the late Holocene.

Locally enhanced wintertime air-sea interaction and deep oceanic mixed layer formation associated with the subarctic front in the North Pacific

[1] Satellite-derived high-resolution air-sea flux data and Argo float data were analyzed to reveal the fine-scale spatial structures of air-sea turbulent heat and momentum fluxes and their relationship with the deep oceanic mixed layer around the subarctic front (SAF) in the North Pacific. An important feature inferred from the satellite-derived dataset is the presence of a pair of positive and negative air-sea flux anomaly bands with a width of 100–200 km on both sides of the SAF. Such a fine-scale structure in air-sea fluxes has not been captured in previous datasets. The SAF plays a significant role in determining the fine-scale structure of air-sea fluxes. The oceanic mixed layer is deeper to the south of the SAF, and the maximum mixed layer depth is observed near the positive anomaly band in air-sea fluxes. This study concludes that active air-sea interactions due to the presence of the SAF enhances upward air-sea heat and momentum fluxes to the south of SAF and selectively produces a deep oceanic mixed layer where the formation of Transition Region Mode Water occurs.

A highly productive Subarctic Atlantic during the Last Interglacial and the role of diatoms

The Last Interglacial (LIG), corresponding to Marine Isotope Stage (MIS) 5e, provides a reference of interglacial climate variability in the absence of anthropogenic forcing. Using an expanded section of the LIG gained at Integrated Ocean Drilling Program Site U1304 in the Subarctic Atlantic, we demonstrate that the early MIS 5e was marked by oceanographic conditions conducive for high diatom production and accumulation. The appearance of diatom-dominated laminated oozes ∼3 k.y. after the beginning of MIS 5e at ca. 125 ka coincides with a shift to higher δ 30 Si diat values together with the dominance of Thalassiothrix longissima , indicative of increased nutrient availability and silicic acid utilization in surface waters. Though the Subarctic Front provided the physical conditions for high diatom production and deposition, these processes alone are insufficient to explain the high rates of siliceous productivity and the formation of diatomaceous sediments. Instead, the additional presence of an increased nutrient pool provided by Subantarctic Mode Water played the decisive role in initiating and sustaining diatom production. The high diatom productivity and the occurrence of diatomaceous sediments in the late Quaternary challenge the current hypothesis of a silica-depleted North Atlantic during the LIG.

Diatom phytoplankton response to Holocene climate change in the Subpolar North Atlantic

A detailed investigation of marine core MD99-2251 from the Gardar Drift has been undertaken to examine the extent of Holocene climate variability reflected by changes in diatom floral abundances in the subpolar North Atlantic. The results from this study provide both an overview of climate variability for the entire Holocene and a decadal-scale study focussed around the 8.2 kyr event, where ice rafted debris also has been quantified. The changing composition of diatom assemblages indicates a highly unstable early Holocene from 11.5 to 9 kyr with switches in the dominance of cool Subarctic floras and warmer North Atlantic Current floras. The presence of high productivity events in the diatom floras during this interval suggests that the core location was, at times, in close proximity of the Subarctic Front. An expansion in the importance of cold Arctic/Greenland Current floras occurred from 9 to 7 kyr, followed by a switch to a well developed warm North Atlantic Current flora between 7 and 5 kyr and then more stable conditions during the Late Holocene. Changes in sea surface hydrography, especially the relative strength of the warm North Atlantic Current, are considered to have had the greatest influence on the composition of diatom floral assemblages. The 8.2 kyr event is not recognised as a discrete climate perturbation in the diatom assemblage data, but this event occurred within a broad cooling from 9 to 7 kyr, where the presence of sea-ice and cold water species indicate an increased incursion of the Arctic and Subarctic water masses between 8.8 and 7.8 kyr. The 8.2 kyr event also was not marked by any increase in the delivery of ice rafted debris over the Gardar Drift.

Characteristics of Mesoscale Eddies in the Kuroshio–Oyashio Extension Region Detected from the Distribution of the Sea Surface Height Anomaly

Abstract This study investigates the distribution of the sea surface height anomaly (SSHA) with the aim of quantifying the characteristics of mesoscale eddies in the Kuroshio–Oyashio extension region (KOER), where intense mesoscale eddies are commonly observed during hydrographic surveys. Dense distributions of both anticyclonic eddies (AEs) and cyclonic eddies (CEs) are detected for the first time in KOER with sufficient temporal and spatial coverage, using the Okubo–Weiss parameter without smoothing. Their contribution to the total SSHA variance is estimated to be about 50%. The zones of highest amplitudes are located north and south of the axis of the Kuroshio Extension (KE) for AEs and CEs, which represent warm-core and cold-core rings, respectively; the areas extend poleward along the Japan and Kuril–Kamchatka Trenches, especially for AEs. Eddies of both polarities and with moderate amplitudes are also recognized along the Subarctic Front (SAF). Eddies in areas north and south of the KE generally propagate westward, at a mean rate of 1–5 cm s−1; those along the trenches south of 46°N and along the SAF propagate poleward at mean rates of 1–2 and 0.5–1 cm s−1, respectively. Because of the asymmetric distribution of the AEs and CEs in the areas north and south of the KE, and the asymmetric amplitude of them along the Japan and Kuril–Kamchatka Trenches, there exist significant eddy fluxes of vorticity, heat, and salinity in these areas.

Horizontal transport of the calanoid copepod Neocalanus in the North Pacific: The influences of the current system and the life history

The horizontal transport of calanoid copepod Neocalanus flemingeri, N. plumchrus and N. cristatus in the subarctic North Pacific has been investigated by particle tracking experiments using an ocean circulation model. In our physical numerical model, the current and frontal systems in the subarctic Pacific are reproduced realistically, and fine-resolved (not smoothed) and vertically sheared western boundary currents in the model enable us to assess the differences in horizontal transports among Neocalanus species. In the experiments, seasonal vertical migration and life cycle of Neocalanus species is included, and this attempt is a novel approach in examining the transports of the zooplanktons. The maximal depths of the vertical migrations and the lengths of the surface and mesopelagic dwelling duration are the essential factors in characterizing the trajectory of the horizontal transport. Both small and large forms of N. flemingeri (hereafter NF-SF and NF-LF) whose mesopelagic-inhabited depths are the shallowest among the three Neocalanus species are transported to more distant regions from initial positions compared with the others. The longest transport distance is over 5000 km for NF-SF. On the other hand, the transport distance of the deepest inhabitant N. cristatus (hereafter NC) is the shortest. The differences in the transport distance are due to the current speed of the western boundary currents at the inhabited depth at which they spend most of their lifetime (ca. 3/4 of their lifetime). More than 82% of NF-LF that originated from the Okhotsk Sea completes its life cycle in the Okhotsk Sea, and all the animals out to the Pacific are transported to the south of the subarctic front where their survivals are unexpected due to high temperatures. This is consistent with the observed limited distribution of NF-LF in the Okhotsk Sea and its surrounding waters. Geographical genetic variations of Neocalanus species are reported as very low. This implies frequent genetic exchange all over the Pacific and its adjacent seas. Being consistent with the above, the present study suggests that the trans-Pacific transport of Neocalanus species can be accomplished within a few generations.

Ocean Frontal Effects on the Vertical Development of Clouds over the Western North Pacific: In Situ and Satellite Observations*

Abstract A suite of shipboard and satellite observations are analyzed and synthesized to investigate the three-dimensional structure of clouds and influences from sea surface temperature fronts over the western North Pacific. Sharp transitions are observed across the Kuroshio Extension (KE) front in the marine atmospheric boundary layer (MABL) and its clouds. The ocean’s influence appears to extend beyond the MABL, with higher cloud tops in altitude along the KE front than the surroundings. In winter, intense turbulent heat release from the ocean takes place on the southern flank of the KE front, where the cloud top penetrates above the MABL and reaches the midtroposphere. In this band of high cloud tops, frequent lightning activity is observed. The results of this study suggest a sea level pressure mechanism for which the temperature gradient in the MABL induces strong surface wind convergence on the southern flank of the KE front, deepening the clouds there. In early summer, sea fog frequently occurs on the northern flank of the subtropical KE and subarctic fronts under southerly warm advection that suppresses surface heat flux and stabilizes the surface atmosphere. Sea fog is infrequently observed over the KE front even under southerly conditions, as the warm ocean current weakens atmospheric stratification and promotes vertical mixing. The KE front produces a narrow band of surface wind convergence, helping support a broad band of upward motion at 700 hPa that is associated with the eastward extension of the baiu rainband from Japan in June–July.

Distribution and Modification of North Pacific Intermediate Water around the Subarctic Frontal Zone East of 150°E

Abstract Modification and distribution of North Pacific Intermediate Water (NPIW) in the area north of 30°N and east of 150°E are discussed based on an isopycnal particle tracking experiment with the velocity field estimated from a β-spiral inverse method based on climatological data, annual climatological data analysis, and isopycnal-tracking profiling float data. NPIW emanating from 149.5°E between 35.5° and 37.5°N retains its salinity minimum structure along the eastward flow of the northern subtropical gyre, and these pathways occupy the northern part of the NPIW distribution. On the other hand, NPIW from 149.5°E between 37.5° and 40.5°N loses its salinity minimum as it flows eastward to the “transition domain.” The extinction of the salinity minimum structure is caused by a salinity decrease in the upper layer (26.5–26.6 σθ) in the region at 40°–45°N, 155°–165°E where a winter mixed layer is developed to a depth of 300 m and a density of 26.7 σθ. This extinction was observed by a profiling float tracking a 26.7-σθ isopycnal density surface. Low-salinity subarctic water advected southward across the Subarctic Front by wind-induced Ekman drift from strong westerlies in winter is suggested as the source of the low-salinity mixed layer. After the extinction of the salinity minimum structure, a state without salinity minimum is maintained during transport eastward in the transition domain because the winter mixed layer in the area east of the deep mixed layer region reaches a density surface shallower than 26.5 σθ.

Characteristics of fronts in the Sea of Japan in accordance with vessel and satellite measurements

Multi-year water temperature and salinity observation data collected by sea vessels in 1925–1997 allowed revealing thermal and salinity (halinic) fronts in the Sea of Japan. Their geographic location and intensity are described. A near-shore halinic front is detected at 0–50 m, which separates near-shore and open deep-sea water masses. The satellite observations carried out in 1975–2002 were used to compose a generalized scheme of the thermal fronts at the sea surface and to characterize their temporal variability. Main attention is paid to studying the sub-Arctic front, which presents at all the schemes.

Late quaternary migrations of the subarctic front in the North Atlantic (Based on lithology and nannofossils)

A complex of lithological and micropaleontological methods were used for the study of sediments from the central North Atlantic (50–60°N). The data on the lithology, carbonate content, and occurrence of ash interbeds and ice-rafted material, combined with the nannofossil zonal biostratigraphy, allowed detailed subdivision of five core sections obtained during Cruise 48 of R/V Akademik Mstislav Keldysh. MIS 1–MIS 9 are established in the sections. In some of them, climatic changes related to migrations of the Subarctic Front are established based on the lithology and the characteristic coccolithophorid species: for example, a higher latitudinal position of the front is revealed for the Eemian interglacial (Substage 5e) as compared with that in the Holocene.

Dynamically Induced Anomalies of the Japan/East Sea Surface Temperature

Variability of sea surface temperature (SST) in the Japan/East Sea (JES) was studied using complex empirical orthogonal function (CEOF) analysis. Two daily data sets were analyzed: (1) New Generation 0.05o-gridded SST from Tohoku University, Japan (July 2002-July 2006), and (2) 0.25o-gridded SST from the Japan Meteorological Agency (October 1993-November 2006). Linkages with wind stress curl were revealed using 6-h 1o-gridded surface zonal and meridional winds from ancillary data of the Sea- WiFS Project, a special National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) product (1998-2005). SST anomalies (SSTA) were obtained by removing the seasonal signal, estimated as the leading mode of the CEOF decomposition of the original SST. Leading CEOF modes of residual SSTA obtained from both data sets were consistent with each other and were characterized by annual, semiannual, and quasi-biennial time scales estimated with 95% statistical significance. The Semiannual Mode lagged 2 months behind the increased occurrence of the anticyclonic (AC) wind stress curl over the JES. Links to dynamic processes were investigated by numerical simulations using an oceanic model. The suggested dynamic forcings of SSTA are the inflow of subtropical water into the JES through the Korea Strait, divergence in the surface layer induced by Ekman suction, meridional shifts of the Subarctic Front in the western JES, AC eddy formation, and wind-driven strengthening/weakening of large-scale currents. Events of west-east SSTA movement were identified in July-September. The SSTA moved from the northeastern JES towards the continental coast along the path of the westward branch of the Tsushima Current at a speed consistent with the advective scale.

Circulation features in the Japan/East Sea related to statistically obtained wind patterns in the warm season

A response of the circulation in the Japan/East Sea (JES) to different kinds of wind forcing is studied, with the emphasis on the warm season, using a primitive equation oceanic model. Wind forcing is based on typical patterns obtained from complex empirical orthogonal functions of 1°-gridded NCEP/NCAR 6 h winds for 1998–2005. These patterns are distinguished by a prevailing wind direction. Northwestern wind and strong cyclonic (C) curl prevail in winter, while a variety of patterns occur in the warm season, differing in the wind direction and curl. Three model runs are performed to examine the circulation in response to a prevailing C wind stress curl or an alternating C and anticyclonic (AC) curl or a strong C curl in the warm season. The simulated features are consistent with the observational evidence, in particular with thermal fronts and frequent eddy locations derived from multi-year infrared satellite imagery. The simulated C circulation intensifies and the subarctic region extends southward with the strengthening of a summer C wind stress curl over the JES. Variability of Subarctic Front (SF) in the western JES (between 130°E and 133°E) is strongly affected by summer wind stress curl. Forcing by an AC curl tends to shift SF northward, while SF shifts to the south under the forcing by a C curl, reaching the southern Ulleung Basin in the case of the strong C curl. In the northwestern JES (off Peter the Great Bay, Russia, and North Korea), the SF northwestern branch (NWSF) is simulated. It is a known feature in autumn and early winter and can also occur in the warm season. The simulation results suggest an AC wind stress curl as the forcing of the formation of the NWSF in the warm season. The Siberia Seamount and sharply bending coastline near Peter the Great Bay facilitate partial separation of the Primorye (Liman) Current from the coast. The wind stress curl can be an additional forcing of the Tsushima Warm Current (TWC) branching off the Korea Strait to the East Korea Warm Current (EKWC) and the offshore branch (OB). In the warm season, the simulated TWC bifurcation occurs farther north, the EKWC is strong, and the OB is weak under the forcing of the AC wind stress curl. The EKWC is weak and the OB is strong under the forcing of the strong C wind stress curl.

Variability of sea surface temperature in the Japan Sea and its relationship to the wind-curl field

The variability of sea surface temperature (SST) in the Japan Sea is investigated using the complex EOF analysis of daily data produced at Tohoku University, Japan (New Generation SST; 2002–2006). The relationship with the wind field is investigated from the daily NCEP/NCAR reanalysis data with a 1° spatial resolution. Anomalies in the SST (SSTAs) are calculated by subtracting the basin-average annual variation estimated as a leading mode of temperature. The leading mode of an SSTA represents a adjustment to the annual mean variation, most significant in December in the zone of subtropical waters entering the sea through the Korean Strait and in the northwestern sea, over which a cyclonic wind curl develops in the cold period. The semiannual variability mode is identified, which is characterized by the largest temperature increase (decrease) in the western branch of the subarctic front (in the Tatar Strait), which lags by two months behind the semiannual changes in wind curl over the sea. An episodic SSTA movement is detected in the northern part of the sea, which moves from east to west along the western branch of the Tsushima Warm Current with a speed corresponding in magnitude to an advective scale.

Numerical modeling of the cross-section structure of the subarctic/subtropical boundary in the North Pacific

A simple variational model of the zonal region between waters has been developed with a specified density gradient and a convergence zone. An approximate analytical solution has been constructed. The boundary between waters is shown to divide into two fronts, a convergence front and a density front. The density front should be shifted equatorward relative to the convergence axis. Such a structure corresponds to the subarctic/subtropical boundary in the Pacific Ocean: the subarctic front and the Kuroshio Extension front that goes into the North Pacific Current.