Search

Enhanced warm, salty subarctic inflows drive high-latitude atlantification, which weakens oceanic stratification, amplifies heat fluxes, and reduces sea ice. In this work, we show that the atmospheric Arctic Dipole (AD) associated with anticyclonic winds over North America and cyclonic winds over Eurasia modulates inflows from the North Atlantic across the Nordic Seas. The alternating AD phases create a "switchgear mechanism." From 2007 to 2021, this switchgear mechanism weakened northward inflows and enhanced sea-ice export across Fram Strait and increased inflows throughout the Barents Sea. By favoring stronger Arctic Ocean circulation, transferring freshwater into the Amerasian Basin, boosting stratification, and lowering oceanic heat fluxes there after 2007, AD+ contributed to slowing sea-ice loss. A transition to an AD- phase may accelerate the Arctic sea-ice decline, which would further change the Arctic climate system.

The Norwegian Sea gyre (NSG) is a large body of Arctic intermediate water and deep dense overflow waters, which circulate counterclockwise within the Norwegian Sea. Argo float trajectories presented in this study suggest that the NSG attains its strongest and most focused flow downstream of a confluence of subarctic waters from the Iceland Sea and the Jan Mayen Ridge at steep bathymetry north of the Faroe slope. Based on hydrographic data from a meridional standard section across this flow (1988 to present), the first baroclinic estimate of the NSG circulation strength is provided. We, furthermore, show that the NSG circulation regulates key aspects of both the poleward Atlantic Water (AW) currents and the equatorward near-bottom and mid-depth flows in the Norwegian Sea – the main arteries of the Meridional Overturning Circulation. More specifically, we demonstrate close links between the NSG circulation and (i) the observed Faroe Bank Channel Overflow (FBCO) transport, (ii) variable depth of the main thermocline separating AW from the underlying colder and denser subarctic water masses, and (iii) satellite-derived sea-surface heights (SSHs) in the southern Nordic Seas. In general, a strong NSG and weak FBCO transport are associated with an uplifted thermocline and depressed SSH. Along a narrow band near the Norwegian and Shetland slopes, a strong NSG – oppositely – links to a depressed interface. Daily records of the FBCO transport, and satellite altimetry in a sensitive region north of the Iceland-Faroe Ridge, complement our hydrographic monitoring of the NSG strength. Together these records constitute valuable indicators for aspects of the Norwegian Sea physical oceanography, which likely have an impact on regional climate, ecology and biological productivity.

The gradual anthropogenic‐driven retreat of Arctic sea ice is overlaid by large natural (internal) year‐to‐year variability. In winter, sea‐ice loss and variability are currently most pronounced in the Barents Sea. As the loss of winter sea ice continues in a warming world, other regions will experience increased sea‐ice variability. In this study, we investigate to what extent this increased winter sea‐ice variability in the future is connected to ocean heat transport (OHT). We analyze and contrast the present and future link between Pacific and Atlantic OHT and the winter Arctic sea‐ice cover using simulations from seven single‐model large ensembles. We find strong model agreement for a poleward expanding impact of OHT through the Bering Strait and the Barents Sea under continued sea‐ice retreat. Model differences on the Atlantic side can be explained by the differences in the simulated variance of the Atlantic inflows. Model differences on the Pacific side can be explained by differences in the simulated strength of Pacific Water inflows, and upper‐ocean stratification and vertical mixing on the Chukchi shelf. Our work highlights the increasing importance of the Pacific and Atlantic water inflows to the Arctic Ocean and highlights which factors are important to correctly simulate in order to capture the changing impact of OHT in the warming Arctic. Plain Language Summary: The winter sea‐ice cover in the Arctic is retreating with global warming, but with a lot of variability from year to year. Some of this variability is determined by how much oceanic heat is transported into the Arctic Ocean via the Fram Strait, Barents Sea, and Bering Strait. We explore how this link between oceanic heat transport and sea ice will change in the future when the sea ice retreats further into the Arctic Ocean. We compare several climate models and find that most of them show a northward expanding footprint of heat transport through the Barents Sea and the Bering Strait. How much these oceanic transports still affect the future sea ice depends on far the sea ice retreats, changes in the inflowing waters, and the vertical stability of the upper layer in the Arctic Ocean. Key Points: Future climate model projections show a poleward shifted impact of Atlantic and Pacific Ocean heat transport on winter sea‐ice variabilityModels with a larger variance of Atlantic inflows simulate a larger influence of Atlantic heat transport on sea iceModels with a stronger volume transport and downstream stratification simulate a larger influence of Pacific heat transport on sea ice [ABSTRACT FROM AUTHOR]

Recent changes in the Arctic sea‐ice are strongly influenced by the recent increase in heat transport from vigorous Atlantic inflows, so‐called Atlantification. This Atlantification can induce physical and ecological changes near the Atlantic gateway. Here, we used the observational data sets and 26 Earth system models to estimate Atlantic water intrusion, and firstly suggest the impact of Atlantification on marine productivity in the Barents Sea in a warming climate, especially on boreal spring. In a warming climate, the heat transport across the Barents Sea Opening (BSO) is projected to be enhanced (45.5 ± 34.9 TW) by the end of the 21st century compared to the present climate. This poleward intrusion of the Atlantic water is likely to increase productivity with the largest increase in spring (70%). In a warming climate, the productivity is enhanced by Atlantification‐induced changes in physical states—ocean temperature, circulations, stratification, and sea‐ice. Based on inter‐model analyses, we estimated that the Atlantification can explain approximately 26% of the productivity changes in the Barents Sea. Thus, Atlantification is critical for future changes in biological productivity and physical states over the Arctic Ocean. Plain Language Summary: Human‐induced greenhouse gases are causing the sea‐ice in the Arctic Ocean to decrease. This is making the edges of the sea‐ice retreat poleward to the central Arctic. The Atlantic water, which is warm, salty, and nutrient‐rich, is also expanding northwards. This is causing the Arctic water to become more like Atlantic water, which is called "Atlantification." Atlantification‐induced physical manifestations and their future changes have been relatively well understood, while influence of the marine productivity still has large uncertainties. In this study, we analyzed the 26 state‐of‐the‐art Earth system models (ESMs) and estimated the future changes caused by the Atlantification, and related productivity changes based on diverse responses of model projections in the Barents Sea where the largest productivity change exists over the Arctic Ocean. We find that the Atlantification‐induced changes in temperature, sea‐ice, and vertical mixing, can enhance the level of productivity in the Barents Sea, especially in boreal spring. Our results suggest that understanding the interactions between the Atlantic water and Arctic systems such as ocean, cryosphere, and biology is critical to projecting future Arctic productivity. Key Points: The extents of Atlantic water expand to the central Arctic basin due to increased heat transport through the Atlantic gateways in a warming climateThe intensified Atlantification drives the enhancement of spring productivity in the Barents SeaNotable divergence in physical manifestations of Atlantification among CMIP models engenders significant challenges in projecting future Arctic productivity [ABSTRACT FROM AUTHOR]

The increasing influence of Atlantic inflows in the Arctic Ocean in recent decades has had a potential impact on regional biogeochemical cycles of major and trace elements. The warm and salty Atlantic water, entering the Eurasian Basin through the Norwegian Sea margin and the Barents Sea, affects particle transport, sink, phyto-, and zooplankton community structure and could have far-reaching consequences for the marine ecosystems. This study discusses the elemental composition of suspended particulate matter and fluffy-layer suspended matter derived from samples collected in the Barents Sea and northern Norwegian Sea in August 2017. The mosaic distribution of SPM elemental composition is mainly determined by two factors: (i) The essential spatial variability of biological processes (primary production, abundance, and phytoplankton composition) and (ii) differences in the input of terrigenous sedimentary matter to the sea area from drainage sources (weak river runoff, melting of archipelago glaciers, etc.). The distribution of lithogenic, bioessential, and redox-sensitive groups of elements in the particulate matter was studied at full-depth profiles. Marine cycling of strontium in the Barents Sea is shown to be significantly affected by increasing coccolithophorid bloom, which is associated with Atlantic water. Mn, Cu, Cd, and Ba significantly enrich the suspended particulate matter of the benthic nepheloid layer relative to the fluffy layer particulate matter within the benthic boundary layer. [ABSTRACT FROM AUTHOR]

The Arctic winter sea ice cover is in retreat overlaid by large internal variability. Changes to sea ice are driven by exchange of heat, momentum, and freshwater within and between the ocean and the atmosphere. Using a combination of observations and output from the Community Earth System Model Large Ensemble, we analyze and contrast present and future drivers of the regional winter sea ice cover. Consistent with observations and previous studies, we find that for the recent decades ocean heat transport though the Barents Sea and Bering Strait is a major source of sea ice variability in the Atlantic and Pacific sectors of the Arctic, respectively. Future projections show a gradually expanding footprint of Pacific and Atlantic inflows highlighting the importance of future Atlantification and Pacification of the Arctic Ocean. While the dominant hemispheric modes of winter atmospheric circulation are only weakly connected to the sea ice, we find distinct local atmospheric circulation patterns associated with present and future regional sea ice variability in the Atlantic and Pacific sectors, consistent with heat and moisture transport from lower latitudes. Even if the total freshwater input from rivers is projected to increase substantially, its influence on simulated sea ice is small in the context of internal variability. Significance Statement: The winter sea ice cover in the Arctic is declining due to global warming, but the decline is quite variable because of the chaotic nature of the climate system. We want to understand what causes this variability, both for the present and the next decades, and for different regions of the Arctic. We find that now and in the future, variability in the winter sea ice is influenced by transport of oceanic heat and atmospheric heat and moisture from the Pacific and Atlantic side of the Arctic. These findings improve our understanding of what influences changes in the winter sea ice cover, and could help to improve predictions of sea ice in the Arctic. [ABSTRACT FROM AUTHOR]

The thecosome pteropods Limacina helicina and L. retroversa are important contributors to the zooplankton community in high-latitude environments but little is known about their distribution and life cycle under polar conditions. We collected the early life stages (< 1 mm) of the thecosome population in 2012 and 2013 at a bi-weekly to monthly resolution in fjord highly influenced by Arctic waters as well as Atlantic inflows (Adventfjorden, Svalbard, 78°N), together with environmental parameters. L. retroversa only occurred episodically, in association with the inflow of Atlantic water, with low numbers and random size distributions. This suggests that this boreal species does not fulfill its life cycle in Adventfjorden. In contrast, young specimens of L. helicina were present during the entire study. Veligers hatched in late summer/autumn and measured 0.14 mm on average. They grew with rates of 0.0006 mm day−1 over the 10–11 months of development. Only thereafter, growth accelerated by one order of magnitude and maximal rates were reached in autumn (0.0077 mm day−1). Our results indicate that L. helicina reaches a size of 1 mm after approximately 1.5 years in Adventfjorden. We therefore suggest that L. helicina overwinters the first year as a small juvenile and that it needs at least 2 years to reach an adult size of 5 mm in Adventfjorden. This reveals an complex and delicate aspect of the life-cycle of L. helicina and further research is needed to determine if it makes the population especially vulnerable towards climate changes. [ABSTRACT FROM AUTHOR]

The Calanus finmarchicus population of the North Sea has collapsed since the late 1950s, while abundance of temperate Atlantic and neritic species groups has risen. These changes are explored in relation to the changing environment of the North Sea. Non-parametric regression methods are used throughout the study, in order to compare the spatial, long-term and seasonal dynamics of the changes in both biotic (e.g. C. finmarchicus) and physical variables (e.g. temperature, salinity, and stratification). The fall in the population of C. finmarchicus has coincided with a long-term freshening and warming of the eastern North Sea and a long-term increase in the salinity of the western North Sea. At the same time the prevalence of temperate Atlantic and neritic zooplankton species has risen. The changes may be explained by differing origins of Atlantic water entering the North Sea since the late 1950s. [Copyright &y& Elsevier]

Winter Arctic sea ice is a crucial climate indicator, declining at an accelerated rate compared to the past and playing a significant role in Arctic amplification over recent decades. The sea-ice concentration (SIC) in the Greenland–Barents Sea (GBS) shows considerable interannual variability, yet the link between this variability and the El Niño–Southern Oscillation (ENSO) remains uncertain. Here, we identify a reversed relationship between the autumn Central Pacific (CP)-type ENSO and the winter GBS SIC around the mid-1980s. Observational and model experiments demonstrate that, before the mid-1980s, CP ENSO triggered a double wave pattern propagating toward the Arctic, generating a positive geopotential height anomaly in the Arctic. Such an anomaly, along with a northerly anomaly, favored cold-air advection and intrusion into the GBS, resulting in an increased SIC. After the mid-1980s, however, CP ENSO only induced a single wave train towards the Arctic, favoring a positive geopotential height anomaly over Iceland. As a result, the southerly anomaly transported abundant moisture into the GBS and consequently reduced the SIC. The variation in wave patterns can largely be attributed to the sea surface temperature anomaly in the tropical Atlantic induced by CP ENSO. Our findings highlight the unstable connection between tropical and polar regions, which provides a basis for better understanding the mechanisms of Arctic sea-ice changes. [ABSTRACT FROM AUTHOR]

We analyzed the temporal trends (1998–2022) of surface phytoplankton Chlorophyll (Chl) concentration in the Arctic at the local, regional, and pan‐Arctic scales. We used four empirically derived Chl satellite ocean color products: two global merged products and two MODIS products, one calibrated to the Arctic. At the local level, between 10% and 40% of the area with valid pixels showed statistically significant Chl trends, with ∼2/3 ${\sim} 2/3$ of those pixels showing increases, and the other third indicating a decrease. At the regional level, only the Barents and Chukchi Seas had consistent Chl increases across products. At the pan‐Arctic level, most products showed Chl increases in the months of July and September (0.3%–0.9% Chl year−1 ${\text{year}}^{-1}$), even after removing the effect of new open water pixels. Overall, Chl is changing in the Arctic, although trends vary threefold depending on the product and spatial‐averaging assumptions used. Plain Language Summary: The Arctic is undergoing critical physical changes that can affect marine ecosystems. Here we analyzed how the concentration of phytoplankton (microorganisms at the base of the marine food‐web) has changed since 1998. To do so, we investigated the temporal trends of chlorophyll (a signature of phytoplankton) as derived from satellites. We found that about 10%–40% of the area with valid satellite pixels had statistically significant phytoplankton trends, with some regions increasing and others decreasing. Over the entire Arctic, Chl has been increasing since 1998, however, the magnitude and statistical significance of the trends varied depending on the satellite product used. Key Points: Depending on the month and product, 10%–40% of the area with valid pixels showed statistically significant Chl trendsChl trends in the Arctic are heterogeneous, with some regions increasing and other decreasingMagnitude and significance of Chl trends varied depending on the satellite product used [ABSTRACT FROM AUTHOR]

Sea-ice loss and increasing unpredictability have disturbed and harmed Arctic peoples and ecosystems. In addition, studies demonstrate that sea ice plays a key role in climate variability and air–sea CO2 exchanges. Sea-ice data sets provide environmental baselines, validate proxies and models, and serve in regional and temporal comparisons. Accordingly, sea-ice and sea-ice–related variables are particularly valuable in climate modeling, paleoclimatology, and ecology to document past and present environmental changes and predict future outcomes. This article provides an overview of modern, historical, and long-term proxy sea-ice data sets that cover the last millennium. We describe available Arctic sea-ice data sources, discuss each data set's strengths and limitations, and compare multisourced Arctic sea-ice histories in different regions. We conclude with remarks on the impacts of internal forcing from natural feedbacks and oscillations versus anthropogenic impacts on sea-ice variability. We draw upon remaining uncertainties regarding causes of past sea-ice variability and advocate for continued use of multiple data sources in sea-ice reconstruction–related studies and further development of a multisourced past sea-ice data network. [ABSTRACT FROM AUTHOR]

Simple Summary: The observed warming trend in the Arctic has prompted investigations into the potential impacts on local communities, particularly in regard to the loss of sea ice and altered primary production. This study looked at Macoma calcarea bivalves, a common species found in near-bottom environments around Svalbard. The mollusks could be split into two groups. The first group was mostly made up of individuals from cold-water stations in Storfjorden. The second group was from warmer-water stations in Grønfjorden and Coles Bay. The two groups had different numbers, sizes, and growth patterns. In colder waters, the mollusks were smaller. Some grew faster than others. Most of the population parameters were different from those in the Pechora, Kara, and Greenland seas. Water temperature was the main reason for the differences in abundance and biomass. Pebbles also had an effect on biomass. Our findings show that Macoma calcarea can be used to monitor the environment in the Arctic. Ongoing warming in the Arctic has led to significant sea-ice loss and alterations in primary production, affecting all components of the marine food web. The considerable spatial variability of near-bottom environments around the Svalbard Archipelago renders the local fjords promising sites for revealing responses of benthic organisms to different environmental conditions. We investigated spatial variations in abundance, biomass, and growth parameters of the common bivalve Macoma calcarea in waters off western Spitsbergen and identified two distinct groups of this species: one composed mainly of cold-water stations from Storfjorden (Group I) and the other comprising warmer-water stations from Grønfjorden and Coles Bay (Group II). Within these groups, the mean abundance, biomass, production, and mortality accounted for 0.2 and 429 ind. m−2, 20 and 179 g m−2, 18.5 and 314 g m−2 year−1, and 0.22 and 0.10 year−1 respectively. The size–frequency and age–frequency distributions were biased towards smaller and younger specimens in Group I, while Group II displayed more even distributions. The maximum ages were 11 and 21 years, respectively. The mollusks from cold water were significantly smaller than their same-aged counterparts from warmer water. Two groups of Macoma were identified: slow-growing individuals with a rate of 1.4 mm and fast-growing individuals with a growth rate of 1.8 mm. Most population parameters were higher than those observed in the Pechora, Kara, and Greenland Seas. Redundancy analysis indicated water temperature as the main driving factor of abundance and biomass, while the latter was also influenced by the presence of pebbles. Our findings provide new insights into the growth patterns and spatial distribution of Macoma at high latitudes and confirm that this species can serve as a reliable indicator of environmental conditions. [ABSTRACT FROM AUTHOR]

The Arctic Ocean has experienced significant sea ice loss over recent decades, shifting towards a thinner and more mobile seasonal ice regime. However, the impacts of these transformations on the upper ocean dynamics of the biologically productive Pacific Arctic continental shelves remain underexplored. Here, we quantified the summer upper mixed layer depth and analyzed its interannual to decadal evolution with sea ice and atmospheric forcing, using hydrographic observations and model reanalysis from 1996 to 2021. Before 2006, a shoaling summer mixed layer was associated with sea ice loss and surface warming. After 2007, however, the upper mixed layer reversed to a generally deepening trend due to markedly lengthened open water duration, enhanced wind-induced mixing, and reduced ice meltwater input. Our findings reveal a shift in the primary drivers of upper ocean dynamics, with surface buoyancy flux dominant initially, followed by a shift to wind forcing despite continued sea ice decline. These changes in upper ocean structure and forcing mechanisms may have substantial implications for the marine ecosystem, potentially contributing to unusual fall phytoplankton blooms and intensified ocean acidification observed in the past decade. This study shows that sea ice loss and changing atmospheric conditions have reshaped the upper layers of the Arctic shelf seas, with wind mixing becoming more important since 2007. [ABSTRACT FROM AUTHOR]

Climate change is rapidly modifying biodiversity across the Arctic, driving a shift from Arctic to more boreal ecosystem characteristics. This phenomenon, known as borealization, is mainly described for certain functional groups along sub-Arctic inflow shelves (Barents and Chukchi Seas). In this review, we evaluate the spatial extent of such alterations across the Arctic, as well as their effects on ecosystem-level processes and risks. Along the inflow shelves, borealization is driven by long-term strengthened inflow of increasingly warm waters from the south and punctuated by advection and low sea ice extreme events. A growing body of literature also points to an emerging borealization of the other Arctic shelf ecosystems, through a "spillover" effect, as local changes in environmental conditions enable movement or transport of new species from inflow shelves. These modifications are leading to changes across functional groups, although many uncertainties remain regarding under-sampled groups, such as microbes, and technical challenges of consistent, regular monitoring across regions. There is also clear consensus that borealization is affecting phenology, species composition, community traits, population structure and essential habitats, species interactions, and ecosystem resilience. Non-dynamic environmental factors, such as depth and photoperiod, are thought to limit the complete borealization of the system, and may lead to intermediate, "hybrid" ecosystems in the future. We expect current borders of Arctic and boreal ecosystems to progress further northward and ultimately reach an equilibrium state with seasonal borealization. Risks to the system are difficult to estimate, as adaptive capacities of species are poorly understood. However, ice-associated species are clearly most at risk, although some might find temporary refuge in areas with a slower rate of change. We discuss the likely character of future Arctic ecosystems and highlight the uncertainties. Those changes have implications for local communities and the potential to support Blue Growth in the Arctic. Addressing these issues is necessary to assess the full scale of Arctic climate impacts and support human mitigation and adaptation strategies. [ABSTRACT FROM AUTHOR]

Climate change is impacting marine ecosystems throughout the circumpolar Arctic, altering seasonal habitats and the food bases for fishes, seabirds, and marine mammals. Arctic and Subarctic regions provide resources for resident species and for species that migrate to the north from more southerly regions. Changes in northerly latitudes thus impact endemic as well as non-endemic animals. Herein, we review what is known about climate-driven changes in the migration patterns of Arctic and Subarctic marine vertebrates, including: 1) Arctic residents with seasonal movements – those fishes, seabirds, and marine mammals that complete their entire life cycle within the Arctic but exhibit seasonal movements; 2) Breeding migrants – many seabirds enter the Arctic to breed and subsequently migrate south in the fall; and 3) Summer visitors for feeding – certain species of boreal fishes, seabirds and marine mammals arrive during the northern summer to feed on abundant prey though they breed elsewhere. Migratory movements are often driven by the timing and extent of sea ice, which defines suitable habitat for some animals and limits access to open water and prey for others. Longer open-water seasons, warmer ocean temperatures, and stronger winds have resulted in earlier production blooms in spring and often, extended open-ocean plankton blooms into late summer, resulting in altered prey types and distributions. A common thread among taxa is that shifts in distribution and timing of migrating animals indicate they are traveling farther north, or shifting longitudinally, and migrations are occurring over longer seasonal time frames. Species performing multiple lifetime migrations or long-distance migrants may need to adjust migration timing or routing iteratively to match changes in marine productivity. Altered animal distributions or phenology, and reduced sea ice, affects access to animals that are critical nutritional, economical, and cultural components of Indigenous people's lives in the Arctic. Ongoing changes challenge the resilience and adaptability of Arctic people and ecosystems, and will require adaptive research and management approaches. [ABSTRACT FROM AUTHOR]

We examine daily surface air temperatures (SAT) in the Arctic under global warming, synthesizing changes in mean temperature, variability, seasonality, and extremes based on five Earth system model large ensembles from the Coupled Model Intercomparison Project Phase 6. Our analysis shows that the distribution of daily Arctic SAT changes substantially, with Arctic mean temperatures being distinguishable from pre‐industrial levels on 84% and 97% of days at 1.5 and 2°C of global warming, respectively, and on virtually every day at 3°C of global warming. This shift is primarily due to the rapid rise in average temperature resulting from Arctic amplification and is exacerbated by a decrease in the variability of daily Arctic SAT of approximately 8.5% per degree of global warming. The changes in mean temperature and variability are more pronounced in the cold seasons than in summer, resulting in a weakened and shifted seasonal cycle of Arctic SAT. Moreover, the intensity and frequency of warm and cold extreme events change to varying degrees. The hottest days warm slightly more, while the coldest days warm 4–5 times more than the global average temperature, making extreme cold events rare. Changes in local SAT vary regionally across the Arctic and are most significant in areas of sea‐ice loss. Our findings underscore the Arctic's amplified sensitivity to global warming and emphasize the urgent need to limit global warming to mitigate impacts on human and natural systems. Plain Language Summary: The Arctic warms about three times faster than the globe on average, impacting ecosystems and human societies profoundly. While such average warming is scientifically interesting, it is the changes in local temperatures on a daily basis that humans will be experiencing. Based on climate model simulations, we therefore examine how these daily temperatures in the Arctic change with global warming. We find that the distribution of daily Arctic temperatures shifts dramatically under global warming, barely overlapping with pre‐industrial temperatures. This shift is mainly due to the amplified mean warming of the Arctic and is exacerbated by the lower fluctuations in daily temperatures with global warming. The changes in mean temperature and variability are more pronounced in winter, leading to shifting seasons and a decreasing temperature contrast between Arctic summers and winters. The intensity and frequency of warm and cold extreme events are changing. In particular, the coldest days warm 4–5 times more than the global average temperature, making extreme cold events rare. These changes vary across the Arctic, with areas losing sea ice experiencing the most significant shifts. This study emphasizes the urgent need to reduce global warming to protect Arctic ecosystems and communities. Key Points: The distribution of daily Arctic temperatures shifts drastically under global warming, barely overlapping with pre‐industrial temperaturesThis is due to amplified Arctic warming, along with a reduction in the variability of daily temperaturesThe hottest days warm slightly more, while the coldest days warm 4–5 times more than the global mean temperature [ABSTRACT FROM AUTHOR]

Simple Summary: The ecosystem of the Barents Sea region is facing alterations due to the increased inflow of warm Atlantic water, necessitating an understanding of the structure of seafloor communities to predict changes in the local food web. Our investigation of polychaete communities along the Kola Transect revealed a taxonomically diverse fauna, with a predominantly boreo-Arctic species composition. The study found considerable variability in polychaete abundance and biomass, with distinct community structures corresponding to the distribution and properties of water masses within the study area. Depth was identified as the primary driver of diversity indices, while salinity and water temperature collectively explained a significant portion of the variation in abundance. The increased water temperatures due to warming were unfavorable for Arctic species. The findings of this study have important implications for understanding the impact of environmental changes on benthic communities in the Barents Sea region. The identification of polychaetes as potential biological indicators highlights their importance in monitoring ecosystem changes. The Barents Sea region is influenced by an increased inflow of warm Atlantic water, which impacts all components of the local ecosystem. Information on the state of benthic communities is required to predict alterations in the food web's structure and functioning. The spatial distribution of polychaete communities was investigated in relation to environmental conditions at nine stations along the Kola Transect (70°00′–74°00′ N, 33°30′ E) in April 2019. A taxonomically diverse fauna containing 114 taxa was found, with 95 identified at the species level. The fauna was composed predominantly of boreo-Arctic species (63%), followed by boreal (22%) and Arctic species (13%). The polychaete abundance and biomass exhibited considerable variability, ranging from 910 to 3546 ind. m−2 and from 3.4 to 72.7 g m−2, with average values of 1900 ind. m−2 and 18.7 g m−2, respectively. Cluster analysis revealed three distinct polychaete communities differing in dominant species composition, abundance, and biomass. The southern region featured the most abundant community, the middle part exhibited the highest diversity, and the northern area presented the community with the highest biomass. These spatial variations in community structure corresponded closely to the distribution and properties of water masses within the study area. Multivariate analysis identified depth as the primary driver of diversity indices, with higher values observed at shallow water sites. Salinity and water temperature together explained 46% of the variation in abundance, reflecting warming effects and showing positive or negative effects, depending on the taxa. Furthermore, an increase in water temperature had a positive impact on the contribution of boreal species to the total material, while exerting a strong negative effect on the overall community biomass, underscoring the potential of polychaetes in biological indication. [ABSTRACT FROM AUTHOR]

Abstract: The so‐called gyre index appears to be related to core aspects of the North Atlantic subpolar gyre, meridional overturning circulation, hydrographic properties in the Atlantic inflows toward the Arctic, and in marine ecosystems in the northeast Atlantic Ocean. Recent publications, however, present a more linear version of this index with less of the key interannual‐to‐decadal variability. This has introduced uncertainty about the meaning and usefulness of the gyre index. We claim that these concerns are primarily caused by the fact that the recently produced “gyre index” is not the same as the original gyre index and discuss possible reasons. [ABSTRACT FROM AUTHOR]

The future of Arctic marine ecosystems has received increasing attention in recent years as the extent of the sea ice cover is dwindling. Although the Pacific and Atlantic inflows both import huge quantities of nutrients and plankton, they feed into the Arctic Ocean in quite diverse regions. The strongly stratified Pacific sector has a historically heavy ice cover, a shallow shelf and dominant upwelling-favourable winds, while the Atlantic sector is weakly stratified, with a dynamic ice edge and a complex bathymetry. We argue that shelf break upwelling is likely not a universal but rather a regional, albeit recurring, feature of "the new Arctic". It is the regional oceanography that decides its importance through a range of diverse factors such as stratification, bathymetry and wind forcing. Teasing apart their individual contributions in different regions can only be achieved by spatially resolved time series and dedicated modelling efforts. The Northern Barents Sea shelf is an example of a region where shelf break upwelling likely does not play a dominant role, in contrast to the shallower shelves north of Alaska where ample evidence for its importance has already accumulated. Still, other factors can contribute to marked future increases in biological productivity along the Arctic shelf break. A warming inflow of nutrient-rich Atlantic Water feeds plankton at the same time as it melts the sea ice, permitting increased photosynthesis. Concurrent changes in sea ice cover and zooplankton communities advected with the boundary currents make for a complex mosaic of regulating factors that do not allow for Arctic-wide generalizations. [ABSTRACT FROM AUTHOR]

The Indo-Pacific Warm Pool (IPWP) has been warming due largely to increasing greenhouse gas emissions, but its impact on Arctic sea ice remains unclear. Our study finds a significant negative correlation between the IPWP index and sea ice concentration in northeastern Canada during boreal autumn (October-December). Our results suggest that IPWP warming statistically accounts for 45% of sea ice loss observed in this region. We introduce the "Arctic capacitor effect of the IPWP", a novel concept that expounds upon the distant connection between greenhouse gas emissions and Arctic sea ice loss. Specifically, as greenhouse gases elevate temperatures in the IPWP, increasing temperature gradient and tropical convection, a planetary wavetrain is initiated. This wavetrain, along with transit eddy feedback, traverses towards the Arctic and thereby influences the strength of the Arctic vortex and its associated effects on Arctic sea ice. Our findings highlight the crucial role of tropical oceans in the broader context of global climate change, emphasizing the necessity of accounting for their impact on polar climate. [ABSTRACT FROM AUTHOR]

Most climate proxies of sea surface temperatures suffer from severe limitations when applied to cold temperatures that characterize Arctic environments. These limitations prevent us from constraining uncertainties for some of the most sensitive climate tipping points that can trigger rapid and dramatic global climate change such as Arctic/Polar Amplification, the disruption of the Atlantic Meridional Overturning Circulation, sea ice loss, and permafrost melting. Here, we present an approach to reconstructing sea surface temperatures globally using paired Mg/Ca - δ18Oc recorded in tests of the polar to subpolar planktonic foraminifera Neogloboquadrina pachyderma. We show that the fidelity of Mg/Ca-based paleoclimate reconstructions is compromised by variations in seawater carbonate chemistry which can be successfully quantified and isolated from paleotemperature reconstructions using a multiproxy approach. By applying the calibration to the last glacial maximum, we show that marine polar amplification has been underestimated by up to 3.0 ± 1.0 °C in model-based estimates. Standard climate proxies cannot quantify sea surface temperatures below 4 °C. Here, temperature signals recorded in shells (Mg/Ca) of polar foraminifera are isolated to resolve past marine polar amplification and climate change accurately. [ABSTRACT FROM AUTHOR]

Arctic sea-ice extent has strongly decreased since the beginning of satellite observations in the late 1970s. While several drivers are known to be implicated, their respective contribution is not fully understood. Here, we apply the Liang-Kleeman information flow method to five different large ensembles from the Coupled Model Intercomparison Project Phase 6 (CMIP6) over the 1970-2060 period to investigate the extent to which fluctuations in winter sea-ice volume, air temperature and ocean heat transport drive changes in subsequent summer Arctic sea-ice extent. This allows us to go beyond classical correlation analyses. Results show that air temperature is the most important controlling factor of summer sea-ice extent at interannual time scale, and that winter sea-ice volume and Atlantic Ocean heat transport play a secondary role. If we replace air temperature by net shortwave and downward longwave radiations, we find that the sum of influences from both radiations is almost similar to the air temperature influence, with the longwave radiation being dominant in driving changes in summer sea-ice extent. Finally, we find that the influence of air temperature is more prominent during periods of large sea-ice reduction and that this temperature influence has overall increased since 1970. [ABSTRACT FROM AUTHOR]

Repeated measurements of benthic and pelagic parameters in the rapidly changing Arctic Ocean provide a unique insight into spatial and interannual trends and changes in the ecosystem. Here, we compiled biogenic and biogeochemical measurements collected from sediment cores at the Long-Term Ecological Research Observatory HAUSGARTEN located in the Fram Strait. A total of 21 stations were visited yearly over a period of 18 years (2002–2019). The time series highlighted an increase in bacterial numbers for samples collected 50 days after the peak phytoplankton bloom. Although bacterial abundances were not bathymetric depth-dependent when viewed across all years, we observed a seasonal trend in benthic microbial abundance closely related to the timing of the phytoplankton bloom with a time-lag of 100 days between the surface phytoplankton peak and the peak in bacterial abundance in the sediment. Considering the residence time of phytoplankton in the upper ocean and the water depth, we estimated an average settling velocity for phytodetritus of 30 m.d−1, which is similar to previous observations from Fram Strait. This suggests that settling organic matter promotes vertical microbial connectivity and benthic bacterial abundance in the deep ocean, shaping the microbial biogeography, diversity, and biogeochemical processes. [ABSTRACT FROM AUTHOR]

The Canada Basin is the largest basin in the Arctic Ocean. Its unique physical features have the highest concentration of nutrients being found in the subsurface layer, referred to as the subsurface nutrient maximum layer (SNM). Under climate change in the Arctic, the SNM is an essential material base for primary productivity. However, long-term trends of nutrient variations and dominant factors related to nutrient levels in the SNM are still unclear. In this study, we analyzed the SNM variations and main influencing factors of the Canada Basin based on the Global Ocean Data Analysis Project Version 2 between 1990 and 2015 and the Chinese Arctic Research Expedition between 2010 and 2016. We found that the nutrient concentrations in the SNM were relatively stable for decades [average concentrations of nitrate, phosphate, and silicate were (13.6 ± 2.4) µmol/L, (1.8 ± 0.2) µmol/L, and (31.5 ± 5.7) µmol/L, respectively]. Nutrient reservoirs were dominated by physical processes. Inflow and outflow water of the SNM contributed about 60.4% and −50.2% to the nutrient stocks, respectively, while particle deposition and remineralization in the Canada Basin contributed approximately one-third to the nutrient stocks. Nitrogen fixation and denitrification in the Canada Basin had no substantial impact on nutrient stocks. The overall stabilization of the SNM over the past few decades implied that the SNM would not substantially affect short term primary productivity. Understanding the long-term trends and dominant factors of reservoirs in the SNM will provide useful insights into the changing Canada Basin ecosystem. [ABSTRACT FROM AUTHOR]

The existence of multiple equilibria (ice-covered and ice-free states) is explored using a set of coupled, nondimensional equations that describe the heat and salt balances in basins, such as the Arctic Ocean, that are subject to atmospheric forcing and two distinct water mass sources. Six nondimensional numbers describe the influences of atmospheric cooling, evaporation minus precipitation, solar radiation, atmospheric temperature, diapycnal mixing, and the temperature contrast between the two water masses. It is shown that multiple equilibria resulting from the dependence of albedo on ice cover exist over a wide range of parameter space, especially so in the weak mixing limit. Multiple equilibria can also occur if diapycnal mixing increases to O(10−4) m2 s−1 or larger under ice-free conditions due to enhanced upward mixing of warm, salty water from below. Sensitivities to various forcing parameters are discussed. Significance Statement: The purpose of this study is to better understand under what circumstances high-latitude seas, such as the Arctic Ocean, can exist in either an ice-covered or an ice-free state. The temperature and salinity of the ocean, as well as the heat exchange with the atmosphere, are drastically different depending on which state the ocean is in. The theory presented here identifies how forcing from the atmosphere and ocean dynamics determines whether the ocean is ice covered, ice free, or possibly either one. [ABSTRACT FROM AUTHOR]

The Northeast Greenland Shelf is a region currently considered to be an annual net sink of carbon dioxide (CO2) from the atmosphere. Water from the Northeast Greenland Shelf is advected to the formation regions of North Atlantic Deep Water; therefore, any carbon stored in the region may be retained in the global oceans on the timescales of the thermohaline circulation. We present the most extensive study of carbon chemistry on the Northeast Greenland Shelf to date, made possible by opportunistic sampling due to a sudden decrease in the sea ice concentration in late-August and September 2017. These are the first full-depth measurements of total alkalinity and dissolved inorganic carbon at latitudes between 75 and 79° N, with additional data collected in the region of the Northeast Water Polynya and outside of Young Sound. We find that surface mixed-layer concentrations are variable and (for many stations) higher than the interpolated atmospheric concentration for the region during the sampling period. Below the surface mixed layer, CO2 concentrations increase linearly with decreasing apparent oxygen utilisation. The mixed layer deepens during the study period; this is associated with apparent changes in CO2 uptake. The Northeast Greenland Shelf is a hydrologically complex region with many processes influencing the carbonate system at smaller scales than our sampling density. The scatter in the dataset represents more than mere outliers, and the lack of relationship between the outliers and any measured variable indicates a strong influence of a currently undescribed (set of) variable(s) and/or process(es) at the sampled scales. These data were collected during a time of radically low sea ice concentrations for the region and may be an indication of future conditions. As they indicate the potential of the region to act as a seasonal source of CO2 to the atmosphere, this may modify our current estimate of the region as a strong annual net sink that is relatively protected from the immediate influence of atmospheric warming and climate change. [ABSTRACT FROM AUTHOR]

The modern Arctic is characterized by a decreased ice cover and significant interannual variability. However, the reaction of the High Arctic ecosystem to such changes is still being determined. This study tested the hypothesis that the key drivers of changes in phytoplankton are the position and intensity of Atlantic water (AW) flow. The research was conducted in August 2017 in the northern part of the Barents Sea and in August 2020 in the Nansen Basin. In 2017, the Nansen Basin was ice covered; in 2020, the Nansen Basin had open water up to 83° N. A comparative analysis of phytoplankton composition, dominant species, abundance, and biomass at the boundary of the ice and open water in the marginal ice zone (MIZ) as well as in the open water was carried out. The total biomass of the phytoplankton in the photic layer of MIZ is one and a half orders of magnitude greater than in open water. In 2017, the maximum abundance and biomass of phytoplankton in the MIZ were formed by cold-water diatoms Thalassiosira spp. (T. gravida, T. rotula, T. hyalina, T. nordenskioeldii), associated with first-year ice. They were confined to the northern shelf of the Barents Sea. The large diatom Porosira glacialis grew intensively in the MIZ of the Nansen Basin under the influence of Atlantic waters. A seasonal thermocline, above which the concentrations of silicon and nitrogen were close to zero, and deep maxima of phytoplankton abundance and biomass were recorded in the open water. Atlantic species—haptophyte Phaeocystis pouchettii and large diatom Eucampia groenlandica—formed these maxima. P. pouchettii were observed in the Nansen Basin in the Atlantic water (AW) flow (2020); E. groenlandica demonstrated a high biomass (4848 mg m−3, 179.5 mg C m−3) in the Franz Victoria trench (2017). Such high biomass of this species in the northern Barents Sea shelf has not been observed before. The variability of the phytoplankton composition and biomass in the Franz Victoria trench and in the Nansen Basin is related to the intensity of the AW, which comes from the Frame Strait as the Atlantic Water Boundary Current. [ABSTRACT FROM AUTHOR]

Deep-water formation in the eastern Subpolar North Atlantic Ocean (eSPNA) and Nordic Seas is crucial for maintaining the lower limb of the Atlantic Meridional Overturning Circulation (AMOC), of consequence for global climate. However, it is still uncertain which processes determine the deep-water formation and how much Atlantic and Arctic waters respectively contribute to the lower limb. To address this, here we used Lagrangian trajectories to diagnose a global eddy-resolving ocean model that agrees well with recent observations highlighting the eSPNA as a primary source of the AMOC lower limb. Comprised of 72% Atlantic waters and 28% Arctic waters, the density and depth of the AMOC lower limb is critically dependent on Atlantic-Arctic mixing, primarily in the vicinity of Denmark Strait. In contrast, Atlantic waters gaining density through air-sea interaction along the eastern periphery of Nordic Seas and not entering the Arctic Ocean make a negligible contribution to the lower limb. The authors use a global eddy-resolving ocean model and show that the Atlantic-Arctic mixing is necessary for determining the density and depth of the Atlantic Meridional Overturning Circulation return flow. [ABSTRACT FROM AUTHOR]

Arctic sea ice mediates atmosphere-ocean momentum transfer, which drives upper ocean circulation. How Arctic Ocean surface stress and velocity respond to sea ice decline and changing winds under global warming is unclear. Here we show that state-of-the-art climate models consistently predict an increase in future (2015–2100) ocean surface stress in response to increased surface wind speed, declining sea ice area, and a weaker ice pack. While wind speeds increase most during fall (+2.2% per decade), surface stress rises most in winter (+5.1% per decade) being amplified by reduced internal ice stress. This is because, as sea ice concentration decreases in a warming climate, less energy is dissipated by the weaker ice pack, resulting in more momentum transfer to the ocean. The increased momentum transfer accelerates Arctic Ocean surface velocity (+31–47% by 2100), leading to elevated ocean kinetic energy and enhanced vertical mixing. The enhanced surface stress also increases the Beaufort Gyre Ekman convergence and freshwater content, impacting Arctic marine ecosystems and the downstream ocean circulation. The impacts of projected changes are profound, but different and simplified model formulations of atmosphere-ice-ocean momentum transfer introduce considerable uncertainty, highlighting the need for improved coupling in climate models. The authors use climate models and show that projected declining and weakening Arctic sea ice, combined with stronger winds, will enhance ocean surface stress. This increased momentum transfer will spin up surface currents, leading to a more energetic Arctic Ocean in the future. [ABSTRACT FROM AUTHOR]

How microzooplanktonic ciliate adaptative strategies differ across diatom bloom and non-diatom bloom areas in the Arctic Ocean remains poorly documented. To address this gap, two different situations were categorized in the Arctic Ocean at summer 2023: diatom bloom stations (DBS) (genus Thalassiosira , chain-like) and non-diatom bloom stations (nDBS). Total abundance of ciliate at 3 m and 25 m in DBS was 2.8 and 1.8 folds higher than in nDBS, respectively. Aloricate ciliates were singled out in both DBS and nDBS, whilst their average abundance and biomass of large size-fraction (>50 μm) in former were 4.5–5.6 folds higher than in latter. Regarding tintinnids, high abundance of Ptychocylis acuta (Bering Strait species) mainly occurred at DBS, coupled with distribution of co-occurring Pacific-origin species Salpingella sp.1, collectively suggested a strong intrusion of Pacific Inflow during summer 2023. Additionally, presence of high abundance of Acanthostomella norvegica and genus Parafavella in nDBS might indicate the trajectory of the Transpolar Drift. Alternatively, tintinnids can serve as credible bioindicators for either monitoring currents or evaluating microzooplankton Borealization. Average abundance of total ciliate within 15–135 μm body-size spectrum in DBS was higher than nDBS. Moreover, spearman's rank correlation between biotic and abiotic analysis revealed that temperature and dissolved oxygen at DBS determined tintinnid species richness and ciliate total abundance, respectively. The results clearly demonstrate that remarkable divergences in large size-fraction of ciliate abundance between DBS and nDBS validate their irreplaceable role in controlling phytoplankton outbreak and associated biological processes in polar seas. [Display omitted] • Diatom bloom in the Arctic triggered notable divergences in ciliate community. • Tintinnids act as compelling bellwether for indicating Pacific and Atlantic Inflows. • Ciliate adaptative strategies in DBS were increase abundance/large size-fraction. • Large ciliate play an irreplaceable role in controlling diatom bloom in polar seas. [ABSTRACT FROM AUTHOR]

The Barents Sea, an important component of the Arctic Ocean, is experiencing changes in its ocean currents, stratification, sea ice variability, and marine ecosystems. Inflowing Atlantic Water (AW) is a key driver of these changes. As AW predominantly enters the Barents Sea via the Barents Sea Opening, other pathways remain relatively unexplored. Comparisons of summer climatology fields of temperature from the last century with those from 2000–2019 indicate warming in the Storfjordrenna trough and along two shallow banks, Hopenbanken and Storfjordbanken, within the Svalbard Archipelago. Additionally, they indicate shoaling of AW that extends further into the "channel" between the islands of Edgeøya and Hopen. This region emerges as a pathway enabling AW to enter the northwestern Barents Sea. Moreover, 1-year-long records from a mooring deployed between September 2018 and November 2019 at the saddle of this channel show the flow of Atlantic-origin waters into the Arctic domain of the northwestern Barents Sea. The average current is directed eastwards into the Barents Sea and exhibits significant variability throughout the year. Here, we investigate this variability on timescales ranging from hours to months. Wind forcing mediates currents, water exchange, and heat exchange through the channel by driving geostrophic adjustment to Ekman transport. The main drivers of the warm-water inflow and across-saddle transport of positive temperature anomalies include persistently strong semidiurnal tidal currents, intermittent wind-forced events, and wintertime warm-water intrusions forced by upstream conditions. We propose that similar topographic constraints near AW pathways may become more important in the future. Ongoing warming and shoaling of AW, coupled with changes in large-scale weather patterns, are likely to increase warm-water inflow and heat transport through the processes identified in this study. [ABSTRACT FROM AUTHOR]

The sea ice spring bloom is crucial for sustaining Arctic marine food webs, with sea ice algae serving as primary carbon sources for higher trophic levels. Despite the prevailing dominance of diatom species in sea ice spring blooms, our study highlights a notable deviation, showcasing a bloom driven by dinoflagellates. Through field sampling of first-year sea ice cores and subsequent analysis of physical and biogeochemical parameters, combined with amplicon sequencing of the 18S rRNA gene, we investigated the occurrence and implications of this significant dinoflagellate bloom, with a particular focus on Polarella glacialis. Our findings reveal that high irradiances at the top of the ice core, coupled with elevated nutrient availability and warm ice conditions, are key drivers of this phenomenon, as elucidated by redundancy analysis. Moreover, our results suggest a potential climate-driven decline in snow cover on sea ice, increased open leads, and thinner sea ice, which may favor the proliferation of dinoflagellates over diatoms. This alternative dinoflagellate-dominated bloom could have profound ecological consequences, given the enriched omega-3 fatty acid content of dinoflagellates, thereby influencing energy transfer within the Arctic marine food web. Furthermore, our study identifies the presence of not only Polarella glacialis but also Chytridinium, an ectoparasite on copepod eggs, and the green algae Ulothrix in relatively high abundances within the sea ice. These findings shed light on the intricate interplay between environmental factors and microbial community dynamics within Arctic sea ice ecosystems. [ABSTRACT FROM AUTHOR]

The wave-affected marginal ice zone (MIZ) is an essential part of the sea ice cover and crucial to the atmosphere–ice–ocean interaction in the polar region. While we primarily rely on in situ campaigns for studying MIZs, significant challenges exist for the remote sensing of MIZs by satellites. This study develops a novel retrieval algorithm for wave-affected MIZs based on the delay-Doppler radar altimeter on board CryoSat-2 (CS2). CS2 waveform power and waveform stack statistics are used to determine the part of the sea ice cover affected by waves. Based on the CS2 data since 2010, we generate a climate record of wave-affected MIZs in the Atlantic Arctic, spanning 12 winters between 2010 and 2022 (10.5281/zenodo.8176585,). The MIZ record indicates no significant change in the mean MIZ width or the extreme width, although large temporal and spatial variability is present. In particular, extremely wide MIZ events (over 300 km) are observed in the Barents Sea, whereas in other parts of the Atlantic Arctic, MIZ events are typically narrower. We also compare the CS2-based retrieval with the retrievals based on the laser altimeter of ICESat2 and the synthetic aperture radar images from Sentinel-1. Under spatial and temporal collocation, we attain good agreement among the MIZ retrievals based on the three different types of satellite payloads. Moreover, the traditional sea-ice-concentration-based definition of MIZ yields systematically narrower MIZs than CS2, and no statistically significant correlation exists between the two. Beyond its application to CS2, the proposed retrieval algorithm can be adapted to historical and future radar altimetry campaigns. The synergy of multiple satellites can improve the spatial and temporal representation of the altimeters' observation of the MIZs. [ABSTRACT FROM AUTHOR]

In the era of climate change-related restructuring of planktonic protist communities, it is especially important to identify possible shifts in their taxonomic composition. While traditional microscopy-based morphological classification is time-consuming and requires experienced taxonomists, metabarcoding seems to substantially accelerate the determination of taxonomic composition. In this study, based on samples collected in summer 2019 from the West Spitsbergen Current, we analysed planktonic protists using both methods. Metabarcoding, based on high-throughput sequencing of the V4 region of the 18S rRNA gene, resulted in a much higher number of operational taxonomic units (OTUs) and sample diversity than microscopy, although the resolution of taxonomic identification ranged from species to phyla. Most morphology-based identification was performed at the species or genus level, additionally allowing us to include information about dominants and size fractions. The highest proportion of 45% shared taxa by both methods was recorded at the class level. The composition of dominant protists differed between the approaches, with most similarities being observed in Bacillariophyceae, for which two genera, Thalassiosira and Eucampia, were found to be the most abundant with both methods. For Dinophyceae, the most abundant representatives identified by microscopy were Gymnodinium spp., Prorocentrum minimum and Gonyaulax gracilis, while in the metabarcoding approach, most dinoflagellates were identified to the class level only. Given the different levels of accuracy of taxonomic determinations and possible biases in results connected to the chosen methodology, we advocate using an integrative taxonomic approach for the classification of planktonic protists based on the combination of microscopy and molecular methods. [ABSTRACT FROM AUTHOR]

The Arctic rapidly transforms due to global warming and increased human activities, triggering complex changes at unprecedented speeds that challenge conventional institutional responses. We analyse these changes through the lenses of social, political, and environmental boundaries and investigate their impacts on both inhabitants' livelihoods and the region's political framework. Employing an interdisciplinary approach, we highlight the complexities of understanding the interplay among global, regional, and local dynamics in an era where human and non-human aspects are entwined. Our analysis concentrates on three areas: definition of the Arctic; legal disputes concerning the waters around the Svalbard Archipelago; evolving natural hazards and societal risk perceptions in Longyearbyen. Through these examples, we underscore the intricate nature of social, political, and ecological changes and how they challenge current boundary-making processes. By combining knowledge from different systems and scales, our research reveals the interplay between policy-driven science, science-influenced policy, and performative behaviors in reshaping today's Arctic borders and boundaries. We particularly emphasize how climate change is challenging borders and advocate for a departure from static definitions, towards the formulation of environmentally conscious, socially just, and politically viable policies, acknowledging the new biophysical realities of the Anthropocene. [ABSTRACT FROM AUTHOR]

The Arctic sea ice cover has decreased rapidly over the last few decades both in extent and thickness. Here we present multi‐year (2013–2022) observations of sea ice thickness in the northwestern Barents Sea based on Upward Looking Sonar measurements and show that the winter sea ice has become thicker over the last decade. Sea ice thickness from the Pan‐Arctic Ice Ocean Modeling and Assimilation System (PIOMAS) reproduces both the observed variability and recent 10‐year trend and shows that this thickening (0.24 m decade−1) has not been seen since the 1990s. Using PIOMAS we find that the recent increase in sea ice thickness can be explained by increased sea ice freezing as a result of lower temperatures in the ocean and in the atmosphere. The recent thickening is set in the context of a long‐term thinning trend, with PIOMAS showing much thinner ice now than in the 1980s. Plain Language Summary: The Arctic sea ice cover is becoming smaller and thinner due to global warming. We have measured sea ice thickness in the Barents Sea since 2013, and find that the ice thickness has increased since the measurements were initiated, contrary to what we would expect in a warming world. The Arctic sea ice cover can, however, increase for periods typically up to a decade due to natural climate variability. We find that increased sea ice formation due to lower ocean and air temperatures has caused the recent thickening of the ice cover. We also find that the Barents Sea ice thickness has decreased since the 1980s, and despite the recent thickening, the ice cover is much thinner now than it used to be. Key Points: Upward Looking Sonar measurements in the Barents Sea show increased sea ice thickness during the last decadeIn an ice‐ocean reanalysis (PIOMAS) the recent thickening is due to increased ice freezing, associated with lower ocean and air temperaturesThe long‐term thickness trend is negative, meaning that despite recent thickening, ice is now much thinner than in the 1980s [ABSTRACT FROM AUTHOR]

A multidisciplinary survey was carried out in the Pacific Arctic and sub-Arctic regions of the North Pacific Ocean on the Korean icebreaking research vessel Araon. During this survey, ichthyoplankton fishes in the Pacific Arctic and sub-Arctic region ranged from the Bering Sea to the northern Chukchi Shelf in summer. The most dominant species was Gadus chalcogrammus, followed by Pleuronectes quadrituberculatus and Boreogadus saida. Gadus chalcogrammus and P. quadrituberculatus were particularly abundant near the Bering Sea and Bering Strait, whereas B. saida was dominant in the Chukchi Sea. Hierarchical cluster analysis revealed four distinct ichthyoplankton communities in Pacific Arctic and sub-Arctic regions based on geographical regions. However, Eleginus gracilis, which was previously known to be seen between latitudes 66.5 °N and 69.5 °N, was found above 70 °N, suggesting that its distribution extends further north. Furthermore, we noticed that Benthosema glaciale, which is usually found in the Atlantic sector of Arctic Ocean, was observed in the northern Chukchi Sea. In addition to these unusual species distributions, several species that are mainly observed in coastal areas are observed in the Chukchi Sea region. The observed influx of various uncommon fish species into the Chukchi Sea can be attributed to multiple factors, including freshwater inflow from the East Siberian Sea and the intrusion of warm Atlantic and Pacific waters, which are strongly affected by global warming. Consequently, it is imperative to conduct rigorous monitoring of the Pacific Arctic region, with a particular focus on the Chukchi Sea, to better understand the implications of global warming. [ABSTRACT FROM AUTHOR]

Van Vliet-Lanoë, B.; Andrieu, V.; Cliquet, D.; Authemayou, C.; Le Roy, P., and Renouf, J.C., 2024. Sea ice and the Middle to Recent Quaternary: Marine highstands in Western Europe. Journal of Coastal Research, 40(3), 445–473. Charlotte (North Carolina), ISSN 0749-0208. This study analyzes the raised beaches of less than 700 kilo annum (ka) above present sea level along the English Channel, which are associated with negative anomalies in δ18O and evidence for anchor ice. The dated coastal events were compared in terms of recent storminess, sea-ice extents, and permafrost with regional paleoclimates. In western France, late Eemian and early interstadial highstands (HSs) indicate an extended annual sea-ice duration during early transgressions, with orbitally forced cold winters and a still-cool Southern Ocean. During regressions, storminess developed with a still-warm intertropical ocean but Arctic cooling. The Northern Atlantic ice sheets starved during early stadials, with aridity because of the seasonal sea-ice extent. (1) A delayed sea-level drop resulted from some ice storage in permafrost and sea ice, which caused depleted δ18O seawater (δ18Osw) values and limited ice caps. (2) The large ice sheets along the Northern Atlantic spread progressively and were delayed until marine isotope stage 4 (MIS 4). Their development resulted from summer precipitation rise caused by the cyclic restoration of thermohaline circulation during Dansgaard–Oechger (DO) events alternating with multiyear sea-ice cover. This led to a more unstable North Atlantic Current, which was repeatedly destabilized. (3) Early glacial storage partly or fully vanished during early DO or interstadial HSs, with climate conditions close to those of the present day. [ABSTRACT FROM AUTHOR]

By transporting warm and salty water poleward, the Gulf Stream system maintains a mild climate in northwestern Europe while also facilitating the dense water formation that feeds the deep ocean. The sensitivity of North Atlantic circulation to future greenhouse gas emissions seen in climate models has prompted an increasing effort to monitor the various ocean circulation components in recent decades. Here, we synthesize available ocean transport measurements from several observational programs in the North Atlantic and Nordic Seas, as well as an ocean state estimate (ECCOv4-r4), for an enhanced understanding of the Gulf Stream and its poleward extensions as an interconnected circulation system. We see limited coherent variability between the records on interannual timescales, highlighting the local oceanic response to atmospheric circulation patterns and variable recirculation timescales within the gyres. On decadal timescales, we find a weakening subtropical circulation between the mid-2000s and mid-2010s, while the inflow and circulation in the Nordic Seas remained stable. Differing decadal trends in the subtropics, subpolar North Atlantic, and Nordic Seas warrant caution in using observational records at a single latitude to infer large-scale circulation change. [ABSTRACT FROM AUTHOR]

In recent decades, the Arctic Ocean has undergone changes associated with enhanced poleward inflow of Atlantic and Pacific waters and increased heat flux exchange with the atmosphere in seasonally ice-free regions. The associated changes in upper-ocean heat content can alter the exchange of energy at the ocean–ice interface. Yet, the role of ocean heat content in modulating Arctic sea ice variability at sub-seasonal timescales is still poorly documented. We analyze ocean heat transports and surface heat fluxes between 1980–2021 using two eddy-permitting global ocean reanalyses, C-GLORSv5 and ORAS5, to assess the surface energy budget of the Arctic Ocean and its regional seas. We then assess the role of upper-ocean heat content, computed in the surface mixed layer (Qml) and in the 0–300 m layer (Q300), as a sub-seasonal precursor of sea ice variability by means of lag correlations. Our results reveal that in the Pacific Arctic regions, sea ice variability in autumn is linked with Qml anomalies leading by 1 to 3 months, and this relationship has strengthened in the Laptev and East Siberian seas during 2001–2021 relative to 1980–2000, primarily due to reduced surface heat loss since the mid-2000s. Q300 anomalies act as a precursor for wintertime sea ice variability in the Barents and Kara seas, with considerable strengthening and expansion of this link from 1980–2000 and 2001–2021 in both reanalyses. Our results highlight the role played by upper-ocean heat content in modulating the interannual variability of Arctic sea ice at sub-seasonal timescales. Heat stored in the ocean has important implications for the predictability of sea ice, calling for improvements in forecast initialization and a focus upon regional predictions in the Arctic region. [ABSTRACT FROM AUTHOR]

Environmental DNA (eDNA) preserved in marine sediments is increasingly being used to study past ecosystems. However, little is known about how accurately marine biodiversity is recorded in sediment eDNA archives, especially planktonic taxa. Here, we address this question by comparing eukaryotic diversity in 273 eDNA samples from three water depths and the surface sediments of 24 stations in the Nordic Seas. Analysis of 18S-V9 metabarcoding data reveals distinct eukaryotic assemblages between water and sediment eDNA. Only 40% of Amplicon Sequence Variants (ASVs) detected in water were also found in sediment eDNA. Remarkably, the ASVs shared between water and sediment accounted for 80% of total sequence reads suggesting that a large amount of plankton DNA is transported to the seafloor, predominantly from abundant phytoplankton taxa. However, not all plankton taxa were equally archived on the seafloor. The plankton DNA deposited in the sediments was dominated by diatoms and showed an underrepresentation of certain nano- and picoplankton taxa (Picozoa or Prymnesiophyceae). Our study offers the first insights into the patterns of plankton diversity recorded in sediment in relation to seasonality and spatial variability of environmental conditions in the Nordic Seas. Our results suggest that the genetic composition and structure of the plankton community vary considerably throughout the water column and differ from what accumulates in the sediment. Hence, the interpretation of sedimentary eDNA archives should take into account potential taxonomic and abundance biases when reconstructing past changes in marine biodiversity., (© 2024 John Wiley & Sons Ltd.)

The sea‐ice cover in the Barents and Kara Seas (BKS) displays pronounced interannual variability. Both atmospheric and oceanic drivers have been found to influence sea‐ice variability, but their relative strength and regional importance remain under debate. Here, we use the Liang‐Kleeman information flow method to quantify the causal influence of oceanic and atmospheric drivers on the annual sea‐ice cover in the BKS in the Community Earth System Model large ensemble and reanalysis. We find that atmospheric drivers dominate in the northern part, ocean heat transport dominates in the central and northeastern part, and local sea‐surface temperature dominates in the southern part. Furthermore, the large‐scale atmospheric circulation over the Nordic Seas drives ocean heat transport into the Barents Sea, which then influences sea ice. Under future sea‐ice retreat, the atmospheric drivers are expected to become more important. Plain Language Summary: The sea ice in the Barents and Kara Seas (BKS) is melting due to Arctic warming, but this is overlaid by large natural variability. This variability is caused by variations in the ocean and the atmosphere, but it is not clear which is more important in which parts of the region. We use a relatively new method that allows us to quantify cause‐effect relationships between sea ice and atmospheric and oceanic drivers. We find that in the north of the BKS, the atmosphere has the biggest impact, in the central and northeastern parts, it is the heat from the ocean, and in the south, it is the local sea temperature. We also find that wind patterns over the Nordic Seas affect how much oceanic heat comes into the Barents Sea, and that, in turn, affects the sea ice. Looking ahead, as the ice is expected to melt more in the future, the atmosphere is likely to become more important in driving sea ice variability in the BKS. This study helps us better understand how the ocean and atmosphere work together to influence the yearly changes in sea ice in this region. Key Points: Ocean heat transport drives sea‐ice variability in the central and northeastern Barents SeaAtmospheric temperature drives sea‐ice variability in the northern Barents‐Kara SeasAtmospheric circulation over the Nordic Seas drives ocean heat transport, which then influences sea‐ice variability [ABSTRACT FROM AUTHOR]