Subarctic Front Research Articles

Current-controlled sedimentation in a megatransform system: a case study of the Charlie-Gibbs Fracture Zone

The Charlie-Gibbs Fracture Zone (CGFZ) is the main transform system in the North Atlantic Ocean. It serves as a primary deep-water gateway for the Iceland-Scotland Overflow Water (ISOW) current. Using high-resolution seismo-acoustic profiling, three types of contourite drift were identified in the valleys of the inactive fractures and active transform of the CGFZ: (1) channel-related drifts, (2) confined drift, and (3) plastered drift. The ISOW current is the main agent controlling drift formation in the northern valley and eastern part of the southern valley, while the Denmark Strait Overflow Water or Low Deep Water is suggested for the western part of the southern valley. Examination of two sediment cores shows that the contourite drifts at least at their upper parts consist of alternated muddy and silty contourites and pelagic/hemipelagic sediments. The contourite deposition corresponds to the present interglacial interval. During glacial interval (MIS 3-2) and the last glacial/interglacial transition, pelagic/hemipelagic sedimentation prevailed corresponding with high IRD input. The position of the highly productive Subarctic Front over the study area induced the formation of diatom ooze beds at the beginning of MIS 1 and high-carbonate pelagites during some intervals of MIS 3.

Steady nutrient upwelling around a biological hotspot of the confluence between the quasi-stationary jet and the Oyashio in the western North Pacific

The quasi-stationary jet, a branch of the Kuroshio Extension, transports warm saline water in the mixed water region of the western North Pacific. Around the subarctic front between the quasi-stationary jet and Oyashio and its downstream area is a biologically productive area including small pelagic fishes. However, how nutrient is supplied to the euphotic zone in this region remains elusive, especially into the quasi-stationary jet. Using high-resolution hydrography sections across the jet, we showed that Oyashio water isopycnally intrudes under the jet around 26.5–26.8 σθ and forms nutrient-rich intermediate water. Upwelling associated with ageostrophic secondary circulation across the front, caused by confluence, uplifts the intermediate water. A local nitrate maximum was also identified inside the jet by the hydrographic observation. Upwelling has been suggested as a precondition for nutrient supply from nutrient-rich intermediate water to the jet through water mixing which potentially sustains high biological production in the downstream.

Feature-oriented reconstruction of vertical temperature profile: A feasibility study in the Northwest Pacific Ocean

The Northwest Pacific is an area rich in oceanic frontal system that influences regional circulation patterns, heat transport, and biogeochemical processes. Understanding the three-dimensional thermal structure of the western subarctic front is essential for studying its dynamics and climate implications. In this study, we developed an empirical feature model to reconstruct the thermal structure of ocean fronts using satellite observations and ARMOR3D product which assimilates Argo data. The model established an empirical relationship between temperature profiles and the front centerline, as well as the temperature difference across the centerline. The reconstructed temperature profiles agree well relative to the validation dataset with a mean bias smaller than 0.5◦C from the sea surface down to 1000 m, demonstrating the model's reliability. This research offers a methodology for reconstructing the three-dimensional thermal structure of oceanic fronts, providing insights into further studies related to the intrinsic features of ocean fronts.

Climate-induced surface water variability at Monte San Nicola type-section (Sicily, southern Italy): New data across the Gelasian GSSP

We present the first high-resolution results on planktonic foraminiferal stable oxygen isotopes and calcareous plankton assemblages at the Monte San Nicola GSSP section (near Gela, Sicily), the type-section for the Lower Pleistocene Gelasian Stage. The oxygen isotope profile is remarkably similar to the Eastern Mediterranean oxygen isotope record and indicates that the studied section, extending from marine isotope stage (MIS) G4 to MIS 103, clearly records glacial-interglacial variability. Cyclic changes of calcareous plankton assemblages (calcareous nannofossils and foraminifera) indicate that warmer and stratified surface waters occurred during interglacials, while cooler and unstable conditions developed during glacial phases. Signatures of further increase in surface water stratification are also captured by our surface water proxies and are coeval with enhanced monsoon run-off, developed during precession minima/insolation maxima. The surface water changes recorded at the Monte San Nicola section are in phase with North Atlantic climate variability, even at suborbital scale, and reveal evidence of the first significant southward migration of the Subarctic Front in the mid-latitudes during MIS 104, slightly below the GSSP. The overall dataset provides precise alignment of the Gelasian GSSP within MIS 103 as well as new climatostratigraphic constraints, close to the GSSP, thus improving its correlation potential outside the type-area.

Quantitative Analysis on the Influence of the Oceanic Front on Underwater Acoustic Detection with Investigated Marine Data

At present, some shortcomings of the research on coupling modeling of the oceanic front–sound field may need attention: (1) Most of the acoustic propagation simulation is based on ideal front models, but the application of investigated marine data is lacking. (2) Most studies focus on the acoustic field characteristics, with the influence of fronts on acoustic propagation, but few studies aim at the direct quantitative analysis of the performance of underwater acoustic detection in oceanic fronts. To deal with the above problems, based on the measured data in the northwest Pacific Ocean, here, we first design different sound source layout schemes and calculate sound field characteristics in the sub-Arctic front using the ray theory. Then, the cumulative detection probability model is built based on the active sonar equation to evaluate the efficiency of underwater detection. Finally, the detection probability is calculated and expressed by regionalization, and the influence of the sub-Arctic front on underwater detection is quantitatively analyzed. The results show that the sub-Arctic front can significantly affect the underwater acoustic detection. The sound source located in the front in the cold-water mass has a better detection performance, especially detecting towards cold water (horizontal detection range > 60 km). In contrast, the sound source located in the warm-water mass has a poor detection performance (horizontal detection range in shallow sea < 10 km).

Dynamics of the subpolar gyre and transition zone of the North Atlantic during the last glacial cycle

Past studies of marine sediments of the North Atlantic documented millennial-scale sea-surface cooling related to the Dansgaard-Oeschger events that influenced the oceanic fronts and surface hydrology during the last glacial cycle. Most studies have focused on using ice-rafting, single or multiple species of planktonic foraminifers' abundances, and geochemistry to assess environmental changes. However, detailed planktonic foraminiferal assemblage data in the North Atlantic Transition Zone (NATZ) and subpolar gyre (SPG) for the full last glacial cycle are scarce. Here we report planktonic foraminiferal abundances, oxygen isotopes, and ice-rafted detritus (IRD) counts from two sediment cores, Hu9007-08 and Hu71022-377, collected from the Milne seamount of southwestern SPG (SW SPG) and NATZ, respectively. Further, we have used the published planktonic foraminiferal assemblage, IRD, and isotopic data from the eastern and western SPG to provide an integrated reconstruction of changes in the surface water masses and dynamics of the Polar, Arctic, and subarctic fronts (PF, AF, and SAF, respectively). Despite the overall warmth of the SW SPG (near the Newfoundland Basin) compared to the eastern SPG during marine isotope stage (MIS) 5, the North Atlantic-wide cooling events C24 and C21 were still identified in the SW SPG. Further, the planktonic foraminiferal abundances in cores EW9303-37JPC (i.e., scarce Neogloboquadrina pachyderma and abundant Globigerinoides inflata) and Hu9007-08 reveal a relatively warm MIS 5e compared to the Holocene. However, the decrease in the abundance of G. inflata suggests that the SW SPG was influenced by the seasonal presence of subpolar waters and possibly a southward shift in the oceanic frontal systems between ∼126 and ∼120 ka. During the Heinrich ice-rafting events, when meltwater flooded the surface North Atlantic, the PF, AF, and SAF shifted southward, and the polar/subpolar water masses briefly invaded the western and eastern SPG and NATZ. In the early part of MIS 3 (60–40 ka), the oceanic frontal system shifted northward from its position during MIS 4. The SPG and NATZ gradually warmed, and the subpolar species Globigerina bulloides in cores Hu9007-08, Hu71022-377, and CH69-K09 increased in the middle of MIS 3 (i.e., 38–30 ka), suggesting continuous sea surface warming while the oceanic frontal systems continued to move northward. This hydrological environment rapidly cooled in the late MIS 3-MIS 2 (30–19 ka), with the PF shifted southward to the mid-latitude (∼45°N) during the Last Glacial Maximum. During the last deglaciation-Holocene, the regions of cores Hu9007-08 and Hu71022-377 were mainly influenced by the warm North Atlantic Current, and the SAF was located north of both cores. This study presents the first large-scale water mass movements and frontal changes in the SPG and NATZ to better understand the dynamics of the surface circulation in the North Atlantic during the last glacial cycle.

Detection and characteristics analysis of the western subarctic front using the high-resolution SST product

Detection and characteristics analysis of the western subarctic front using the high-resolution SST product

ENSO vs glacial-interglacial-induced changes in the Kuroshio-Oyashio transition zone during the Pleistocene

The subarctic front (SAF) in the pelagic North Pacific is the northernmost front of the Kuroshio-Oyashio transition zone separating the subpolar and subtropical gyres and is marked by a strong sea surface temperature gradient. A complex interplay of e.g. variations of currents, the wind system and other forcing mechanisms causes shifts of the SAF’s position on timescales from orbital to interannual. In this study, we present proxy data from the Emperor Seamount chain, which reveal a link between long-term ENSO (El Niño/Southern Oscillation) dynamics in the tropics and shifts of the SAF. Based on sediment core SO264-45-2 from Jimmu Seamount (46°33.792’N, 169°36.072’E) located close to the modern position of the SAF, we reconstruct changes in (sub)surface temperature ((sub)SSTMg/Ca) and δ18Osw-ivc (approximating salinities) via combined Mg/Ca and δ18O analyses of the shallow-dwelling foraminifera Globigerina bulloides and the near-thermocline-dwelling Neogloboquadrina pachyderma, biological productivity (XRF-based Ba/Ti ratios), and terrigenous input via dust (XRF-based Fe). From ~600 to ~280 ka BP we observe significantly higher SSTMg/Ca than after an abrupt change at 280 ka BP. We assume that during this time warmer water from the Kuroshio-Oyashio transition zone reached the core site, reflecting a shift of the SAF from a position at or even north of our study site prior to 280 ka BP to a position south of our study site after 280 ka BP. We propose that such a northward displacement of the SAF between 600-280 ka BP was induced by sustained La Niña-like conditions, which led to increased transport of tropical ocean heat into the Kuroshio-Oyashio transition zone via the Kuroshio Current. After ~280 ka BP, the change to more El Niño-like conditions led to less heat transfer via the Kuroshio Current with the SAF remaining south of the core location. In contrast, our productivity record shows a clear glacial-interglacial pattern that is common in the North Pacific. We assume that this pattern is connected to changes in nutrient supply or utilization, which are not primarily driven by changes of the Kuroshio and Oyashio Currents or the SAF.

Lagrangian Overturning Pathways in the Eastern Subpolar North Atlantic

Abstract The strength of the Atlantic meridional overturning circulation (AMOC) at subpolar latitudes is dominated by water mass transformation in the eastern subpolar North Atlantic Ocean (SPNA). However, the distribution of this overturning across the individual circulation pathways of both the Subpolar Gyre (SPG) and the Nordic seas overflows remains poorly understood. Here, we introduce a novel Lagrangian measure of the density-space overturning to quantify the principal pathways of the time-mean overturning circulation within an eddy-permitting ocean model hindcast. By tracing the trajectories of water parcels initialized from the northward inflows across the Overturning in the Subpolar North Atlantic Program (OSNAP) East section, we show that water mass transformation along the pathways of the eastern SPG accounts for 55% of the mean strength of the eastern subpolar AMOC. Water parcels following the dominant SPG pathway, sourced from the Subarctic Front, form upper North Atlantic Deep Water by circulating horizontally across sloping isopycnals in less than 2 years. A slower SPG route, entrained by overflow waters south of the Iceland–Faroes Ridge, is a crucial conduit for subtropical-origin water masses to penetrate the deep ocean on subdecadal time scales. On reproducing our findings using time-averaged velocity and hydrographic fields, we further show that the Nordic seas overflow pathways integrate multiple decades of water mass transformation before returning across the Greenland–Scotland Ridge. We propose that the strong disparity between the overturning time scales of the SPG (interannual) and the Nordic seas overflows (multidecadal) has important implications for the propagation of density anomalies within the eastern SPNA and hence the sources of AMOC variability.

Decadal changes in the basin-wide heat budget of the mid-latitude North Pacific Ocean

Research has indicated that the meridional heat transport (MHT) in the North Pacific Ocean (NP) across 24°N increased in the 1980s and 1990s, resulting in different heat distributions: the ocean heat content (OHC) increased in the 1980s, while the net surface heat release was strengthened in the 1990s; however, the reasons for these differences remained unclear. The authors revisited the investigation of the heat budget in the NP mainly using hydrographic observations to understand why the heat distribution was different between these 2 decades and extend the analysis to the 2010s. The OHC in the upper 700 m north of 24°N and east of 137°E exhibited sharp increases around 1990 as well as in the 2010s, while it was nearly stable in between. The northward retreat of the subarctic gyre boundary coincided with the spin-up of the subtropical gyre in the late 1980s, thereby allowing warm anomaly from the subtropics to propagate northeastward. Meanwhile, the concurrent weakening of the wintertime westerlies resulted in the suppression of the surface heat loss in the western NP. In contrast, the southward shift of the subarctic front suppressed the OHC rise, despite the MHT increase in the late 1990s. In the 2010s, unprecedented warming occurred in the eastern NP. The MHT estimation based on hydrographic observations indicates that the net surface release must have been suppressed since the MHT did not increase; however, the latest atmospheric reanalysis datasets failed to reproduce this.

Weakened pacific overturning circulation, winter monsoon dominance and tectonism re-organized Japan Sea paleoceanography during the Late Miocene global cooling

The Late Miocene global cooling (LMGC; approximately 7.9–5.8 Ma) was associated with remarkable changes in monsoon dynamics, biogenic bloom in the global oceans, and the rise of modern ecosystems at the expense of old biota. However, the possible linkage between the environmental changes and ecosystem shifts during the LMGC is still debated. In this paper, we show the high-resolution changes in the fluxes of selected radiolarian species, suggesting a drastic reorganization in the paleoceanography and ecosystem in the Japan Sea during the LMGC. The endemic radiolarian Cycladophora nakasekoi dominated the Japan Sea until 7.4 Ma when the Japan Sea sediment changed from dark radiolarian-rich sediment to organic-poor diatom ooze. Changes in the fluxes of C. nakasekoi and Tricolocapsa papillosa, the latter related to changes in the Pacific central water (PCW), show 100, 200, and ~ 500 ka cycles with their high flux mostly within the darker sediment intervals during the low-eccentricity period until 7.4 Ma, suggesting that orbitally paced PCW inflow might have been the major nutrient source into the Japan Sea. At about 7.4 Ma, these species decreased at the expense of increased Larcopyle weddellium, a radiolarian related to the North Pacific intermediate water (NPIW), and Cycladophora sphaeris, a subarctic radiolarian species, implying a decrease in PCW inflow and an increase in the inflow of NPIW and subarctic shallow water. Such a change would have been related to the LMGC-induced weakening in the Pacific Meridional overturning circulation and the southward shift of the subarctic front due to intensified East Asian winter monsoon. Such a drastic reorganization in the hydrography in the Japan Sea probably caused changes in nutrient provenance from the PCW to the NPIW and resulted in faunal turnover, marked by the disappearance of the old regional and endemic faunal components, such as C. nakasekoi.

Early Pleistocene (Marine Isotope Stage 40–36) paleoceanography in the northwestern Pacific: Evidence from faunal and oxygen isotope analyses of planktonic foraminifera

The Mid-Pleistocene Transition (MPT) is characterized by a shift from a 41-kyr to a 100-kyr climate cycle of approximately 1.25–0.7 Ma. However, proxy records before the MPT are sparse in the northwestern Pacific region. To investigate the marine environment around the oceanographic convergence zone of the northwestern Pacific prior to the MPT, we conducted biostratigraphic and oxygen isotope analyses of planktonic foraminifera derived from the upper part of the Kiwada Formation of the Kazusa Group, which is distributed in the Boso Peninsula, central Honshu, Japan. The interval investigated was deposited from Marine Isotope Stages (MIS) 41 to 36 (ca. 1.31–1.20 Ma). The last occurrence of Globigerinoides obliquus is emplaced 30 m above the tephra layer Kd1 during MIS 37. The first occurrence of Neogloboquadrina inglei is placed somewhere below the tephra layer Kd8 and is older than 1.310 Ma. Time-series environmental proxy changes in the planktonic foraminiferal assemblages and oxygen isotopes suggest that the marine environment off the Boso Peninsula was influenced by the Kuroshio Current and the transitional water close to the Kuroshio Front, while the latitudinal shifts of the Kuroshio Front were associated with the glacial-interglacial periods prior to the MPT. Our results suggest that obliquity variability played a significant role in the mid-latitude Pacific Ocean before the MPT, and the hydrographic change in the North Pacific Subtropical Gyre seems to be connected to high-latitude climatic changes. Moreover, the temporary southward migration of the Kuroshio and Subarctic fronts occurred during the late MIS 38, which may be associated with the stronger East Asian winter monsoon. Thus, mid-latitude Pacific oceanographic changes that occurred before the MPT may be linked not only to global remote connections but also to local atmospheric circulation. • We conducted the biostratigraphic and oxygen isotope analyses of the planktonic foraminifera in the Kiwada Formation, central Japan. • The last occurrence of Globigerinoides obliquus was correlated with the late MIS 37. • Palaeoceanographic changes in the mid-latitudes seems to be connected to high-latitude climatic changes.

The essential bioactive role of nickel in the oceans: Evidence from nickel isotopes

The role of nickel (Ni) in ocean biogeochemical cycles is both under-studied and controversial. Strong correlations between Ni and organic carbon in modern and ancient marine sediments suggest a prominent biogeochemical role over a substantial portion of Earth history. Addition of Ni to culturing and seawater incubation experiments produces strong responses in terms of cell growth, particularly of nitrogen-fixing organisms. But the implied limiting role for phytoplankton growth is inconsistent with observations in the real ocean, specifically that photic zone Ni concentrations never descend to the very low values that characterise other bioactive, and often bio-limiting, metals like iron. These two observations can be reconciled if a large portion of the total dissolved Ni present in open-ocean surface waters is not bio-available on short timescales. Here we present new Ni concentration and stable isotope data from the GEOVIDE transect in the North Atlantic. We interpret these new data in the light of the growing database for Ni stable isotopes in the modern ocean, with implications for the biogeochemical importance of Ni.In the new North Atlantic dataset, the lowest Ni concentrations (1.8-2.6 nmol/L) and highest δ60Ni (up to +1.67‰) are associated with low nitrate, south of the subarctic front (SAF). By contrast, stations at latitudes north of the SAF, with higher surface nitrate, show very subdued variation in Ni concentrations throughout the entire depth of the water column (3.6±0.3 nmol/L, mean and 2SD), and no variation in δ60Ni beyond the narrow global deep-ocean range (+1.33±0.13 ‰). These North Atlantic Ni isotope data also show relationships with nitrogen isotope effects, observed in the same samples, that are suggestive of a link between Ni utilisation, isotope fractionation and nitrogen fixation.The global dataset, including the new data presented here, reveals a biogeochemical divide with Ni isotope fractionation only occurring in low latitude surface waters. A simple observationally constrained three-dimensional model of Ni cycling suggests that the creation of this isotopically heavy, Ni-poor, end-member, together with the physical circulation and remineralisation at depth, can explain the global Ni-δ60Ni systematics. Taken together, these findings hint at Ni-N co-limitation in the modern ocean. We advocate for more extensive and detailed culturing/incubation studies of this neglected metal in order to elucidate its potentially crucial biogeochemical role.

Phytoplankton‐Nitrifier Interactions Control the Geographic Distribution of Nitrite in the Upper Ocean

AbstractAs a key intermediate in the nitrogen cycle, nitrite is involved in multiple biological pathways that regulate the distribution and availability of nitrogen in the ocean. In the oligotrophic gyres, nitrite accumulates near the base of the euphotic zone, demonstrating a subsurface maximum, termed the primary nitrite maximum; while in subpolar regions, nitrite concentrations are elevated in the near‐surface ocean. As yet, the mechanisms controlling this meridional pattern remain unclear. Here, we present vertically resolved profiles of rates of nitrite production and consumption extending from the Subtropical Gyre to the Subarctic Front in the North Pacific Ocean. Our results indicate that the latitudinal distributions of nitrite across this basin are influenced by variations in phytoplankton‐nitrifier interactions. In the well‐lit oligotrophic surface, phytoplankton dominates rapid nitrite cycling via coupled release and re‐assimilation; below the euphotic zone, diminished light stress on nitrite oxidizers results in rapid turnover and limits nitrite. By contrast, in subpolar regions where nitrate concentrations are elevated in the euphotic zone, nitrite is released during assimilative nitrate reduction and competition between phytoplankton and nitrifiers for ammonium is relaxed, facilitating ammonia oxidation. These processes, together with differential light sensitivities of ammonia and nitrite oxidizers, allow net accumulation of nitrite. Furthermore, we demonstrate a substantial contribution of urea oxidation in forming the primary nitrite maximum and balancing the two steps of marine nitrification. Our findings reveal physical‐biological interactive controls on nitrite cycling and distributions in the ocean and help disentangle the complex effect of phytoplankton‐microbe interactions on marine nitrogen biogeochemistry.

Two-Tiered Transition of the North Atlantic Surface Hydrology during the Past 1.6 Ma: Multiproxy Evidence from Planktic Foraminifera

Analyses of planktic foraminiferal assemblage data, test morphology, and stable oxygen isotopes from the Integrated Ocean Drilling Program (IODP) Site U1304 in the North Atlantic reveal a stepwise regional migration of the oceanic fronts around 0.6 Ma and 0.4 Ma, i.e., during Marine Isotope Stages (MISs) 15 and 11, respectively. Both changes of planktic foraminiferal assemblages and shell carbonate isotopes indicate that the cold Arctic waters in general persisted at IODP Site U1304 from 1.6 to 0.6 Ma (MIS 15), even though the warmer waters originating from the Atlantic waters episodically bathed Site U1304 during the interglacial periods. During the time-interval from ca. 0.6 to 0.4 Ma (MISs 15–11), an alternating dominance of Artic and Atlantic waters at the Site U1304 has been suggested from isotopic evidence. In MIS 11, the dominant planktic foraminiferal species Neogloboquadrina pachyderma experienced a short-term but significant decrease in test size. The test-size change may have been caused by accelerated reproduction in more favorite feeding conditions over the long-lasting interglacial period around the Subarctic Front. This finding is supported by the presence of massive diatoms oozes in the same time-interval. The modern-type glacial/interglacial change of the surface water system established since ca. 0.4 Ma (MIS 11) followed the Mid-Brunhes Event.

Evidence of Deep DOC Enrichment via Particle Export Beneath Subarctic and Northern Subtropical Fronts in the North Pacific

Here we provide compelling evidence that deep particle export enhanced dissolved organic carbon (DOC) concentrations beneath the Pacific’s Subarctic Front (SA, ∼42°N) and Northern Subtropical Front (NST, ∼34°N). We report three main findings: First, deep export of subjectively small particles (128–512 μm) was apparent throughout the frontal zone in which the SA resides. However, export of large particles was specifically associated with the SA, rather than the entire frontal zone, and appeared to exclusively transfer DOC into the bathypelagic water column. Second, a similar DOC enrichment existed beneath the NST, though this signal was curiously not accompanied by observable particles (>128 μm). We conclude that export occurring previously in winter left this DOC behind as a residue, though the associated particles were no longer present by spring. Third, the presence of strong hydrographic fronts was not the only control on export that resulted in these unique DOC distributions. Deep export and DOC enrichment was also controlled by latitude-specific biogeochemical and hydrographic conditions, such as depth of the nutricline and seasonal mixed layer shoaling. Given these observations, the fronts within the transitional region of the North Pacific are clearly special locations for deep carbon sequestration and for providing uncommon DOC enrichment that ultimately supports the deep microbial community.

Mesoplankton across the North Atlantic: Composition and environmental drivers at 59.5th latitude

North Atlantic zooplankton communities vary in terms of abundance, biomass and species composition depending on various environmental factors. In this study we analyzed the influence of hydrological factors (water temperature, salinity and density) on mesoplankton and retrieve boundaries between mesoplankton assemblages across the Subpolar North Atlantic. The material was collected in July 2013 with the use of Juday plankton net vertical hauls (0–300 m layer) at 55 stations arranged along a longitudinal transect along 59.5° N during the 41-th expedition of R/V “Akademik Ioffe”. Both total abundance and biomass were higher in the eastern part of the transect and greatly decreased in the western part except the Greenland shelf and slope where the values were high. Four mesoplankton assemblages were retrieved by cluster analysis: two of them (above the Rockall Plateau and the Island Basin) were associated with warm Atlantic waters, the other two assemblages (in the Irminger Basin and above the Greenland slope and shelf) inhabited cold Arctic waters. The major biogeographic boundary was linked to the Subarctic Front above the Reykjanes Ridge. The best correlations of zooplankton characteristics with the surface chlorophyll a amount were obtained when averaging the surface chlorophyll data over May and 3 latitudinal and longitudinal degrees around each station. This can be connected to the main spring phytoplankton bloom, occurring during May in this area. The exception was the western-most part of the transect above the Greenland shelf and slope that harbored separate mesoplankton assemblages with high abundance and biomass, dominated by early mesoplankton stages (e.g. young copepodites and nauplii). These assemblages were likely supported by summer subsurface phytoplankton bloom caused by a water discharge from the melting Greenland Ice Shield.

ХАРАКТЕРИСТИКИ АНТИЦИКЛОНИЧЕСКОГО ВИХРЯ, СФОРМИРОВАВШЕГОСЯ У ТИХООКЕАНСКОГО ПОБЕРЕЖЬЯ ЮЖНЫХ КУРИЛ ВЕСНОЙ 1999 г.

Based on the analysis of oceanographic survey materials and sea surface temperature satellite observations, the conditions of formation and characteristics of the anticyclonic eddy located in the region of the South Kuril Islands in the spring of 1999 were studied. It was shown that it separated from the subarctic front as result of its interaction with the Oyashio current stream in the second half of March and ceased to exist in mid-June, which is also confirmed by observations of the concentration of chlorophyll-a. In contrast the eddies formed as a result of meandering of the warm Kuroshio and Tsugaru currents, the eddy core, located at depths of 150–300 m, was characterized by lower values of temperature and salinity (4°–4.6°С and 33.55–33.65 е.п.с.) and low current velocities ( about 10 cm/s). The eddy was rather weakly distinguished in the density field both on horizontal and vertical distributions, as well as according to satellite altimetry data.

Abundance and ecology of Myctophidae fishes in the Gulf of Alaska in winter season

Distribution and abundance of myctophid fish species in the upper epipelagic layer are analyzed based on results of surveys in the Gulf of Alaska in winters of 2019 and 2020. A common myctophid community driven by blue lanternfish Tarletonbeania crenularis was revealed that occupied likely the entire eastern part of the Subarctic Front zone eastward from 150ºW. Abundance and biomass of four mass myctophid species were generally comparable between the northeastern and northwestern Pacific. Small-sized juveniles prevailed among northern lampfish Stenobrachius leucopsarus, which were likely transported by the Subarctic Current and could be recruited to the Bering Sea stock. Patterns of daily vertical migrations are discussed for the mass myctophid species. New observations confirm general understandings on the myctophid species input to functioning of fish community.

A Case Study of the Thermohaline Fine-scale Structure of the Sub-Arctic Front

ABSTRACT This paper analyzes the characteristics of the thermohaline fine-scale structure of the sub-Arctic front (SAF) in the Kuroshio–Oyashio confluence region using temperature, salinity, and current data obtained from surveys in June 2016. Thermohaline intrusion is found to be prevalent in the sea area of the SAF, occurring predominantly in frontal zones with depths less than 200 m and in North Pacific intermediate water (NPIW) layers with depths greater than 350 m. The intrusion in the SAF zones with depths less than 200 m has a higher intensity but smaller dimensions in the NPIW. According to our analysis, the thermohaline intrusion in the SAF sea zone is mainly attributable to the joint effect of double diffusion and flow instability. At depths of 0–350 m, the thermohaline intrusion is vulnerable to the influences of salt fingering, diffusive convection, and symmetric instability. At depths deeper than 350 m, diffusive convection is significant. In addition, shear instability might be a contributor. Considering that the effect of turbulent mixing is primarily governed by symmetric instability in the SAF sea areas, the flow instability characterized by a dominant contribution from symmetric instabilities might exert a more significant influence on thermohaline intrusion. The more distinct thermohaline staircases at layers deeper than 350 m, which are mainly produced by diffusive convection, have larger vertical scales than the staircases in other layers.