Atlantic Inflow Research Articles

Stable silicon isotopes uncover a mineralogical control on the benthic silicon cycle in the Arctic Barents Sea

Biogeochemical cycling of silicon (Si) in the Barents Sea is under considerable pressure from physical and chemical changes, including dramatic warming and sea ice retreat, together with a decline in dissolved silicic acid (DSi) concentrations of Atlantic inflow waters since 1990. Associated changes in the community composition of phytoplankton blooms will alter the material comprising the depositional flux, which will subsequently influence recycling processes at and within the seafloor. In this study we assess the predominant controls on the early diagenetic cycling of Si, a key nutrient in marine ecosystems, by combining stable isotopic analysis (δ30Si) of pore water DSi and of operationally defined reactive pools of the solid phase. We show that low biogenic silica (BSi) contents (0.26–0.52 wt% or 92–185 μmol g dry wt−1) drive correspondingly low asymptotic concentrations of pore water DSi of ∼100 μM, relative to biosiliceous sediments (>20 wt% BSi) wherein DSi can reach ∼900 μM. While Barents Sea surface sediments appear almost devoid of BSi, we present evidence for the rapid recycling of bloom derived BSi that generates striking transient peaks in sediment pore water [DSi] of up to 300 μM, which is a feature that is subject to future shifts in phytoplankton community compositions. Using a simple isotopic mass balance calculation we show that at two of three stations the pore water DSi pool at 0.5 cm below the seafloor (+0.96 to +1.36 ‰) is sourced from the mixing of core top waters (+1.46 to +1.69 ‰) with the dissolution of BSi (+0.82 to +1.50 ‰), supplemented with a lithogenic Si source (LSi) (−0.89 ±0.16‰). Further, our sediment pore water δ30Si profiles uncover a coupling of the Si cycle with the redox cycling of metal oxides associated with isotopically light Si (−2.88 ±0.17‰). We suggest that a high LSi:BSi ratio and apparent metal oxide influence could lead to a degree of stability in the annual background benthic flux of DSi, despite current pressures on pelagic phytoplankton communities. Coupled with supporting isotopic evidence for the precipitation of authigenic clays in Barents Sea sediment cores, our observations have implications for the regional Si budget.

Weakening and warming of the European Slope Current since the late 1990s attributed to basin-scale density changes

Abstract. Oceanic influences on shelf seas are mediated by flow along and across continental slopes, with consequences for regional hydrography and ecosystems. Here we present evidence for the variable North Atlantic influence on European shelf seas over the last 4 decades using ocean analysis and reanalysis products, as well as an eddy-resolving ocean model hindcast. To first order, flows oriented along isobaths at the continental slope are related to the poleward increase in density in the adjacent deep ocean that supports a geostrophic inflow towards the slope. In the North Atlantic, this density gradient and associated inflow have undergone substantial, sometimes abrupt, changes in recent decades. Inflow in the range 10–15 Sv is identified with eastward transport in temperature classes at 30∘ W in the latitude range 45–60∘ N. Associated with major subpolar warming around 1997, a cool and fresh branch of the Atlantic inflow was substantially reduced, while a warm and more saline inflow branch strengthened, with respective changes of the order of 5 Sv. Total inflow fell from ∼ 15 Sv pre-1997 to ∼ 10 Sv post-1997. In the model hindcast, particle tracking is used to trace the origins of poleward flows along the continental slope to the west of Ireland and Scotland before and after 1997. Backtracking particles up to 4 years, a range of subtropical and subpolar pathways is identified from a statistical perspective. In broad terms, cold, fresh waters of subpolar provenance were replaced by warm, saline waters of subtropical provenance. These changes have major implications for the downstream shelf regions that are strongly influenced by Atlantic inflow, in particular the northern North Sea, where “subtropicalization” of ecosystems has already been observed since the late 1990s.

Long‐Term Moored Current and Temperature Measurements of the Atlantic Inflow Into the Nordic Seas in the Norwegian Atlantic Current; 1995–2020

AbstractInterannual variability of the Atlantic inflow (AI) into the Nordic Seas is studied using moored current and temperature measurements in the Norwegian Atlantic Current during the 25‐year period 1995–2020. We show that the properties of the AI are strongly connected to the dynamics of the Northern Atlantic and demonstrate the robustness of the average winter wind field in driving the variability with 3‐month winter maxima in the range of (40–50) cms−1. In that sense, the zero‐wind‐stress‐curl (ZWSC)‐lines aligned northeastward are crucial. Occasional events with slowdown of winter‐maxima are connected to a meridional and zonal shift of the ZWSC‐lines, and not the strength of the wind. This is manifested in a correlation of 0.3 versus the North Atlantic Oscillation (NAO) winter‐index, demonstrating that the NAO‐index is insufficient to explain the AI‐variability. The records show no long‐term‐trend over the 25‐year period 1995–2020, regarding volume flux, temperature, nor temperature flux.

Impact of Decadal Trends in the Surface Climate of the North Atlantic Subpolar Gyre on the Marine Environment of the Barents Sea

The Barents Sea is a key region in the Earth System and is home to highly productive marine resources. An integrated approach for strategic sustainable management of marine resources in such shelf-sea marine ecosystems requires, among many other aspects, a robust understanding of the impact of climate on local oceanic conditions. Here, using a combined observational and modelling approach, we show that decadal climatic trends associated with the North Atlantic Subpolar Gyre (SPG), within the period 1960–2019, have an impact on oceanic conditions in the Barents Sea. We relate hydrographic conditions in the Barents Sea to the decadal variability of the SPG through its impact on the Atlantic Inflow via the Faroe-Shetland Channel and the Barents Sea Opening. When the SPG warms, an increase in the throughput of subtropical waters across the Greenland-Scotland Ridge is followed by an increase in the volume of Atlantic Water entering the Barents Sea. These changes are reflected in pronounced decadal trends in the sea-ice concentration and primary production in the Barents Sea, which follow the SPG after an advective delay of 4–5 years. This impact of the SPG on sea-ice and primary production provides a dynamical explanation of the recently reported 7-year lagged statistical relationship between SPG and cod (Gadus morhua) biomass in the Barents Sea. Overall, these results highlight a potential for decadal ecosystem predictions in the Barents Sea.

Atlantic inflow is the primary driver of remotely sensed autumn blooms in the Barents Sea

Arctic shelf seas have historically hosted a single spring bloom, contrasting with temperate seas, where additional smaller autumn blooms occur regularly, caused by storm systems mixing nutrient-rich deep waters towards the surface ocean. Post millennium, autumn blooms have increased in frequency in Arctic shelf seas. Delayed sea-ice formation, stronger autumn winds and greater inflows of nutrient-rich temperate waters have all been suggested to support growing annual net primary production and an increasing incidence of autumn blooms. Here, we investigated data sets of remotely sensed September chlorophyll a, sea surface temperature, current and wind speeds. We explored mechanistic drivers of autumn blooms in the Barents Sea, one of the most productive Arctic shelf seas, to better understand the role of strong winds and the ingression of Atlantic waters in the dynamics of autumn blooms. We performed geographically resolved regressions between remotely sensed September chlorophyll a and environmental conditions in the Barents Sea, demonstrating a strong dependency of autumn bloom intensity on Atlantic inflow. This result highlights the importance of increased study of autumn phytoplankton bloom dynamics on Arctic shelf seas, especially the further collection and dissemination of in situ cell count and nutrient data to determine the significance of autumn blooms for wider ecosystem function.

Nordic Seas Heat Loss, Atlantic Inflow, and Arctic Sea Ice Cover Over the Last Century

AbstractPoleward ocean heat transport is a key process in the earth system. We detail and review the northward Atlantic Water (AW) flow, Arctic Ocean heat transport, and heat loss to the atmosphere since 1900 in relation to sea ice cover. Our synthesis is largely based on a sea ice‐ocean model forced by a reanalysis atmosphere (1900–2018) corroborated by a comprehensive hydrographic database (1950–), AW inflow observations (1996–), and other long‐term time series of sea ice extent (1900–), glacier retreat (1984–), and Barents Sea hydrography (1900–). The Arctic Ocean, including the Nordic and Barents Seas, has warmed since the 1970s. This warming is congruent with increased ocean heat transport and sea ice loss and has contributed to the retreat of marine‐terminating glaciers on Greenland. Heat loss to the atmosphere is largest in the Nordic Seas (60% of total) with large variability linked to the frequency of Cold Air Outbreaks and cyclones in the region, but there is no long‐term statistically significant trend. Heat loss from the Barents Sea (∼30%) and Arctic seas farther north (∼10%) is overall smaller, but exhibit large positive trends. The AW inflow, total heat loss to the atmosphere, and dense outflow have all increased since 1900. These are consistently related through theoretical scaling, but the AW inflow increase is also wind‐driven. The Arctic Ocean CO2 uptake has increased by ∼30% over the last century—consistent with Arctic sea ice loss allowing stronger air‐sea interaction and is ∼8% of the global uptake.

Response of Northern North Atlantic and Atlantic Meridional Overturning Circulation to Reduced and Enhanced Wind Stress Forcing

AbstractSurface wind stress strongly influences Atlantic meridional overturning circulation (AMOC) variability on interannual time scales. On longer time scales, however, its role in AMOC variations is less clear. Here, we show a nonlinear AMOC response to globally reduced and enhanced wind stress forcing, based on sensitivity experiments with MPI‐ESM1.2. Under reduced wind stress forcing, the AMOC strength strongly decreases. In contrast, under enhanced wind stress forcing the AMOC strength increases only in the first decades and then decreases, stabilizing at a value similar to the reference simulation. To reveal possible mechanisms underlying this response, we assess the response of the northern North Atlantic circulation and climate to the changed wind stress forcing. Initially, the response is linear: reduced wind stress forcing weakens the gyre circulation and the associated heat and salt transport, leading to larger winter sea ice extent and a shutdown of subpolar deep convection. In the Nordic Seas, the fresher and lighter subsurface state leads to a decrease in the baroclinic pressure and the overflow strength. Under enhanced wind stress forcing, initially, the opposite is happening. However, eventually, subpolar surface density anomalies are determined by warmer temperature rather than increased salinity, leading to a decrease in surface density and a weakening of subpolar deep convection. The resulting AMOC weakening reduces the Atlantic inflow salinity, and subsequently the Nordic Seas baroclinic pressure and overflow strength. The quasi‐equilibrium response of the northern North Atlantic circulation and climate under enhanced wind stress forcing differs from the reference simulation, even though the AMOC strength converges.

Mechanisms of Regional Winter Sea-Ice Variability in a Warming Arctic

AbstractThe 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.

An 18S V4 rRNA metabarcoding dataset of protist diversity in the Atlantic inflow to the Arctic Ocean, through the year and down to 1000 m depth

Abstract. Arctic marine protist communities have been understudied due to challenging sampling conditions, in particular during winter and in deep waters. The aim of this study was to improve our knowledge on Arctic protist diversity through the year, in both the epipelagic (< 200 m depth) and mesopelagic zones (200–1000 m depth). Sampling campaigns were performed in 2014, during five different months, to capture the various phases of the Arctic primary production: January (winter), March (pre-bloom), May (spring bloom), August (post-bloom), and November (early winter). The cruises were undertaken west and north of the Svalbard archipelago, where warmer Atlantic waters from the West Spitsbergen Current meet cold Arctic waters from the Arctic Ocean. From each cruise, station, and depth, 50 L of seawater was collected, and the plankton was size-fractionated by serial filtration into four size fractions between 0.45–200 µm, representing picoplankton (0.45–3 µm), small and large nanoplankton (3–10 and 10–50 µm, respectively), and microplankton (50–200 µm). In addition, vertical net hauls were taken from 50 m depth to the surface at selected stations. The net hauls were fractionated into the large nanoplankton (10–50 µm) and microplankton (50–200 µm) fractions. From the plankton samples DNA was extracted, the V4 region of the 18S rRNA-gene was amplified by polymerase chain reaction (PCR) with universal eukaryote primers, and the amplicons were sequenced by Illumina high-throughput sequencing. Sequences were clustered into amplicon sequence variants (ASVs), representing protist genotypes, with the dada2 pipeline. Taxonomic classification was made against the curated Protist Ribosomal Reference database (PR2). Altogether, 6536 protist ASVs were obtained (including 54 fungal ASVs). Both ASV richness and taxonomic composition varied between size fractions, seasons, and depths. ASV richness was generally higher in the smaller fractions and higher in winter and the mesopelagic samples than in samples from the well-lit epipelagic zone during summer. During spring and summer, the phytoplankton groups diatoms, chlorophytes, and haptophytes dominated in terms of relative read abundance in the epipelagic zone. Parasitic and heterotrophic groups such as Syndiniales and certain dinoflagellates dominated in the mesopelagic zone all year, as well as in the epipelagic zone during the winter. The dataset is available at https://doi.org/10.17882/79823 (Egge et al., 2014).

Oceanographic and biogeochemical drivers cause divergent trends in the nitrogen isoscape in a changing Arctic Ocean

Nitrogen stable isotopes (δ15N) are used to study food web and foraging dynamics due to the step-wise enrichment of tissues with increasing trophic level, but they rely on the isoscape baseline that varies markedly in the Arctic due to the interplay between Atlantic- and Pacific-origin waters. Using a hierarchy of simulations with a state-of-the-art ocean-biogeochemical model, we demonstrate that the canonical isotopic gradient of 2–3‰ between the Pacific and Atlantic sectors of the Arctic Ocean has grown to 3–4‰ and will continue to expand under a high emissions climate change scenario by the end of the twenty-first century. δ15N increases in the Pacific-influenced high Arctic due to increased primary production, while Atlantic sector decreases result from the integrated effects of Atlantic inflow and anthropogenic inputs. While these trends will complicate longitudinal food web studies using δ15N, they may aid those focussed on movement as the Arctic isoscape becomes more regionally distinct.

Meltwater flux from northern ice-sheets to the mediterranean during MIS 12

Planktonic foraminifer oxygen isotopes through MIS 12 were analysed from Ocean Drilling Program Site 977 in the Alboran Sea. After the correction of the sea surface temperature (SST) effect on the δ18O composition of foraminiferal calcite, the resulting seawater δ18O (δ18Ow) was used to reconstruct variations in the δ18Ow of the Atlantic inflow into the Mediterranean. A synchronous record from the KC01B core, in the Ionian Sea, was used to evaluate changes in the oxygen isotope gradient within the Mediterranean due to hydrological variations during MIS 12. Instead of the glacial δ18Ow enrichment expected for the Mediterranean, lower values than today have been observed both in the Alboran and the Ionian seas, especially between 455 ka and the end of MIS 12 (424 ka). These negative oxygen isotope anomalies must have been caused by a flux of freshwater to the Mediterranean during MIS 12. Although the largest fraction of the freshwater anomalies entered the Mediterranean through the Atlantic inflow, especially during Heinrich stadials, the Mediterranean δ18Ow gradient allowed us to identify other sources of freshwater to the eastern basin. One of these sources was probably the meltwater generated at the southern margin of the Fennoscandian ice-sheet that entered via the Caspian and Black seas. However, the proximity of core KC01B to the Adriatic Sea points to meltwater delivered from the Alpine ice-sheet and transported through the Po river into the Mediterranean as the main cause of the Ionian Sea 18O depletions.

Dense Water Formation on the Icelandic Shelf and Its Contribution to the North Icelandic Jet

AbstractThe North Icelandic Jet (NIJ) is the densest component of the Denmark Strait Overflow Water, feeding the abyssal limb of the Atlantic Meridional Overturning Circulation. Here, by using observational and numerical model data, we explore the formation of overflow water on the Icelandic shelf, the mechanisms involved, and its potential contribution to the NIJ. The sparse observational data on the western Icelandic shelf for the month of February shows top‐to‐bottom mixing on the shelf, distinct and well separated from the dense water offshore, with densities larger than 27.8 kg/m3 in some years. Using a 1‐D mixing model and winter heat flux reanalysis, we suggest that waters with densities exceeding 27.8 kg/m3 are likely to be formed on the shelf in most years by the end of winter. High‐resolution numerical model data shows that the transformation of the Atlantic inflow along the northwest Icelandic shelf generates a dense plume whose waters feed into the NIJ. The bulk of the plume cascades down‐slope north of Iceland, funneled through deep cross‐shelf troughs, with some cascading occurring west of Iceland as well. During years of strong cascading events (2008, 2013, and 2016), the modeled dense plume potentially feeds up to 21% of the NIJ transport at the Siglunes and Kögur sections. Back‐tracked Lagrangian particle trajectories confirm that the western Icelandic shelf is a source of the NIJ. The dense plume transport and variability are found to be dependent on the total oceanic heat loss west of Iceland and along Denmark Strait.

Code for "Weakening and warming of the European Slope Current since the late 1990s attributed to basin-scale density changes"

Abstract. Oceanic influences on shelf seas are mediated by flow along and across continental slopes, with consequences for regional hydrography and ecosystems. Here we present evidence for the variable North Atlantic influence on European shelf seas over the last four decades, using ocean analysis and reanalysis products, and an eddy-resolving ocean model hindcast. To first order, flows oriented along isobaths at the continental slope are related to the poleward increase of density in the adjacent deep ocean that supports a geostrophic inflow towards the slope. In the North Atlantic, this density gradient and associated inflow has undergone substantial, sometimes abrupt, changes in recent decades. Inflow in the range 10–15 Sv is identified with eastward transport in temperature classes at 30° W, in the latitude range 45–60° N. Associated with major subpolar warming around 1997, a cool and fresh branch of the Atlantic inflow was substantially reduced, while a warm and more saline inflow branch strengthened, with respective changes of the order 5 Sv. Total inflow fell from ~ 15 Sv pre-1997 to ~ 10 Sv post-1997. In the model hindcast, particle tracking is used to trace the origins of poleward flows along the continental slope to the west of Ireland and Scotland, before and after 1997. Backtracking particles up to 4 years, a range of subtropical and subpolar pathways is identified from a statistical perspective. In broad terms, cold, fresh waters of subpolar provenance were replaced by warm, saline waters, of subtropical provenance. These changes have major implications for the downstream shelf regions that are strongly influenced by Atlantic inflow, the northern North Sea in particular, where “subtropicalization” of ecosystems has already been observed since the late 1990s.

Year-round population dynamics of Limacina spp. early stages in a high-Arctic fjord (Adventfjorden, Svalbard)

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.

Assessing the current and future Arctic Ocean observing system with observing system simulating experiments

AbstractArctic Ocean Observing System Simulation Experiments (OSSEs) were performed with a pan‐Arctic coupled ocean–sea ice data assimilation system to assess the impacts of assimilating available observations on the Arctic ocean–sea ice state. To this end, the adjoint method with a 3‐year assimilation window was used to assimilate synthetic observations sampled from a 4 km model simulation at the spatio‐temporal distribution of the existing observing system. After data assimilation, the sea ice state, including sea ice concentration (SIC), sea ice thickness (SIT), and sea ice volume (SIV), were significantly improved, benefiting from the high spatio‐temporal coverage of SIC observations and the wintertime SIT observations. In contrast, the ocean state is not very well constrained with the existing hydrographic observing system. An additional 1° × 1° ocean profiling array with a 10‐day sampling frequency was seen to substantially improve the estimated ocean temperature and freshwater content. Data from additional moorings deployed in the Fram Strait and continental slope of the Laptev Sea could also improve the pathway of Atlantic inflow to the Arctic Ocean and the temperature of the Atlantic inflow but degrade the mean volume transport through the Fram Strait. Moreover, estimated exchanges between the Arctic Ocean and the Atlantic Ocean through the Fram Strait, Davis Strait and the St. Anna Trough were found to benefit from the high‐density profiling array.

Spatial Distribution of Arctic Bacterioplankton Abundance Is Linked to Distinct Water Masses and Summertime Phytoplankton Bloom Dynamics (Fram Strait, 79°N).

The Arctic is impacted by climate warming faster than any other oceanic region on Earth. Assessing the baseline of microbial communities in this rapidly changing ecosystem is vital for understanding the implications of ocean warming and sea ice retreat on ecosystem functioning. Using CARD-FISH and semi-automated counting, we quantified 14 ecologically relevant taxonomic groups of bacterioplankton (Bacteria and Archaea) from surface (0–30 m) down to deep waters (2,500 m) in summer ice-covered and ice-free regions of the Fram Strait, the main gateway for Atlantic inflow into the Arctic Ocean. Cell abundances of the bacterioplankton communities in surface waters varied from 105 cells mL–1 in ice-covered regions to 106 cells mL–1 in the ice-free regions. Observations suggest that these were overall driven by variations in phytoplankton bloom conditions across the Strait. The bacterial groups Bacteroidetes and Gammaproteobacteria showed several-fold higher cell abundances under late phytoplankton bloom conditions of the ice-free regions. Other taxonomic groups, such as the Rhodobacteraceae, revealed a distinct association of cell abundances with the surface Atlantic waters. With increasing depth (>500 m), the total cell abundances of the bacterioplankton communities decreased by up to two orders of magnitude, while largely unknown taxonomic groups (e.g., SAR324 and SAR202 clades) maintained constant cell abundances throughout the entire water column (ca. 103 cells mL–1). This suggests that these enigmatic groups may occupy a specific ecological niche in the entire water column. Our results provide the first quantitative spatial variations assessment of bacterioplankton in the summer ice-covered and ice-free Arctic water column, and suggest that further shift toward ice-free Arctic summers with longer phytoplankton blooms can lead to major changes in the associated standing stock of the bacterioplankton communities.

Ommastrephid squid spawning in the North Sea: oceanography, climate change and species range expansion

The lesser flying squid (Todaropsis eblanae) and the shortfin squid (Illex coindetii) are two abundant ommastrephids of the northeast Atlantic. Spawning ground existence was inferred from the captures of mature, mated females in summer 2016–2019 and their occurrences were compared with respective oceanographic data from international surveys to gain insight into environmental predictors of their presence throughout the North Sea. Spawning T. eblanae were found in relatively cooler and more saline waters (6–8°C, 34.2–35.1 psu) in the northern North Sea linked to the Fair Isle Current and East Shetland Atlantic Inflow, whilst spawning I. coindetii occurred across the entire North Sea (mostly at 9–10.5°C, 34.1–34.8 psu). We hypothesize that a combination of water salinity and water temperature are key factors in the spatiotemporal distribution of spawning ommastrephid squids as they define water density that is crucial for pelagic egg mass to attain neutral buoyancy.

Overwintering distribution, inflow patterns and sustainability of Calanus finmarchicus in the North Sea

Calanoid copepods are key taxa in the North Sea as they are the main food source for many fish stocks, such as herring, mackerel and cod. In this study we use an individual-based model for Calanus finmarchicus embedded in the NORWegian ECOlogical Model system (NORWECOM) to investigate important population parameters such as biomass and abundance, distribution and interannual variability of the overwintering population, as well as the inflow of C. finmarchicus into the North Sea from adjacent areas for the 2000–2016 period. The modelled spatial–temporal patterns of C. finmarchicus abundance is comparable with the Continuous Plankton Recorder (CPR) Survey data in the northern North Sea. The simulated annual mean biomass of C. finmarchicus amounts to 0.94 million-tonnes of carbon. High overwintering biomass appears in the Norwegian Trench as well as in the north-west shelf region of the North Sea. A decreasing trend in the overwintering biomass has been detected on the path of the East Shetland Atlantic Inflow (ESAI) over the simulated period. The inflow of C. finmarchicus biomass into the North Sea from the north constitutes on average 41% of the annual mean biomass in the North Sea during the simulated 17 years, and thus determines the interannual variability of the biomass. We conclude that the C. finmarchicus population in the North Sea is not self-sustained and is highly dependent on the inflow of C. finmarchicus from the Faroe-Shetland Channel and south of the Norwegian Sea. C. finmarchicus enter the North Sea via three branches of the North Atlantic current with variable depths depending on seasons and topography. Beside the western flank of the Norwegian Trench (carrying 57% of the inflow biomass), we suggest that the ESAI is also an important agent carrying 37% of the total C. finmarchicus inflow biomass through the shelf area into the north-west of the North Sea. The annual mean outflow biomass is larger than the inflow biomass (0.52 versus 0.39 million-tonnes carbon per year), which indicates that the North Sea serves as a feeding ground and growth region for C. finmarchicus. This study is a first step towards a better understanding and quantification of the exchange of C. finmarchicus between the open seas, coastal waters and the fjords.

Strengthening Atlantic Inflow Across the Mid‐Pleistocene Transition

AbstractThe development of larger and longer lasting northern hemisphere ice sheets during the mid‐Pleistocene Transition (MPT) coincided with global cooling. Here, we show that surface waters of the north‐eastern Atlantic actually warmed across this interval (∼1.2–0.8 Ma), which we argue reflects an increase in the north‐eastward transport of heat and moisture via the North Atlantic Current (NAC) into the Nordic Seas (the Atlantic Inflow). We suggest that simultaneous cooling and warming along the north‐western and south‐eastern margins (respectively) of the NAC during Marine Isotope Stage 28 (∼995 ka) reflected the increasing persistence of northern ice sheets as Atlantic Inflow increased. This resulted in a diachronous shift from ∼40 to ∼100 kyr cyclicity across the North East (NE) Atlantic as the growing influence of northern ice sheets spread southwards; to the north‐west of the NAC the first “100 kyr” cycle preceded Termination 12 (∼960 ka), while on the south‐eastern margin of the NAC the transition occurred ∼100 kyr later. Exploratory climate model simulations suggest that increasing Atlantic Inflow at this time could have accelerated ice sheet growth because pre‐existing moderately sized ice sheets allowed the positive effect of increasing precipitation to outpace melting. In addition, we propose that the dependence of post‐MPT ice sheets on moisture transport via the Atlantic Inflow may ultimately contribute to their apparent vulnerability to insolation forcing once a critical size threshold is crossed and high latitude ice sheets become starved of a vital moisture source.

Multicentennial Variability Driven by Salinity Exchanges Between the Atlantic and the Arctic Ocean in a Coupled Climate Model

AbstractThe IPSL‐CM6‐LR atmosphere‐ocean coupled model exhibits a pronounced multicentennial variability of the Atlantic meridional overturning circulation (AMOC), driven by delayed freshwater accumulation and release in the Arctic. The AMOC fluctuations are preceded by salinity‐driven density anomalies in the main deep convection sites in the Labrador and Greenland seas. During a strong AMOC, a combination of reduced sea ice volume and anomalous currents reduces the freshwater export from the Arctic and leads instead to a slow accumulation of freshwater in the central Arctic. Simultaneously, the saltier Atlantic inflow through the Barents Sea results in a positive salinity anomaly in the Eastern Arctic subsurface. When the surface Central Arctic freshwater pool finally reaches the Lincoln Sea, the oceanic currents around Greenland reorganize, leading to the export of the anomalous Arctic freshwater to the North Atlantic, enhancing the stratification in deep convection sites. The AMOC then decreases, positive salinity anomalies appear in the Central Arctic, and the variability switches to the opposite phase. These AMOC and sea ice fluctuations have broader climate impacts, with a strong AMOC leading to a mean warming of about 0.4°C north of 20°N, reaching up to 1°C in the Arctic lower troposphere during winter. In all seasons, a northward displacement of the intertropical convergence zone is also simulated.