Atlantic water inflow into the Arctic Ocean: studies of pathways, transport and mixing processes using observations from ships and autonomous underwater vehicles

Nordishavet spiller en viktig rolle i det globale klimasystemet. Produksjon av kaldt vann med høy tetthet, samt frysing av sjøis i Arktis, bidrar til å drive den Atlantiske meridionale omveltningssirkulasjonen og kan påvirke atmosfærens sirkulasjonsmønstre så langt sør som til midlere breddegrader. Atlanterhavsvann som strømmer gjennom Framstredet og Barentshavet, inn i Arktis, regulerer både produksjonen av vann med høy tetthet og utstrekningen av sjøis. Dermed har Atlanterhavsvann en nøkkelrolle i Arktis og det globale klimasystemet som helhet. Denne studien beskriver innstrømningsveiene til Atlanterhavsvann, samt de underliggende mekanismene som styrer innstrømningen, prosessene for varmetap og blanding med de Arktiske vannmassene nord for Svalbard og i det nordvestlige Barentshavet. Data samlet inn ved hjelp av autonome undervannsfartøy (AUV-er) er sentrale i denne studien. Mer enn 15 000 hydrografiske profiler er samlet inn i løpet av fem tokt og elleve AUV-oppdrag mellom 2018 og 2022, og er presentert i form av fire artikler. Data er samlet inn som en del av prosjektet Arven etter Nansen. Ved bruk av observasjoner langs kontinentalskråningen nord for Svalbard, mellom 12°Ø og 24°Ø, beskriver vi hydrografisk struktur, volumtransport og sirkulasjonsmønstre til Atlanterhavsgrensestrømmen. Volumtransporten til grensestrømmen når et maksimum på 3.0 ± 0.2 Sv i oktober, men styrken til grensestrømmen er følsom for vindstress, og dobler volumtransporten sin på mindre enn en uke når gjennomsnittlig vindstress over regionen endrer seg. En tidligere ukjent bunnintensivert vannstrøm observeres å strømme parallelt med grensestrømmen mellom 1500 og 2000 meter dybdekonturene. Historiske data i regionen støtter tilstedeværelsen av den bunnintensiverte vannstrømmen. Oppfølgingsstudiene konsentrerte seg om det nordvestlige Barentshavet. Målrettede målinger ved bruk av AUV-er gir en stor forbedring i romlig og tidsmessig dekning av observasjoner. For å utnytte teknologien, spesielt ved målinger av turbulens, utforsket vi potensialet til en propelldrevet AUV. Vi utstyrte AUV-en med en turbulenspakke og rapporterer datakvaliteten, samt diskuterer begrensningene av dissipasjonsestimater fra skjærsensorene. Det propelldrevet AUV-oppdraget i Barentshavet, vinteren 2021, varte i 5 timer, og AUV-en hadde en typisk horisontal hastighet på 1.1 m/s. AUV-en ble programmert for å finne og krysse maksimal temperaturgradient på 10, 20 og 30 m dyp langs 4 km strekk. Selv om AUV-vibrasjonene, på grunn av propellen, forstyrrer målingene med skjærsensorene, filtreres støyen bort ved å fjerne vibrasjonsinduserte komponenter fra skjær-spekter ved bruk av akselerometersignal. Dissipasjonsestimatene fra AUV-en viser god overensstemmelse med nærliggende vertikale mikrostrukturprofiler fra skip, noe som indikerer at turbulensmålingene fra oppsettet er pålitelige for dette relativt turbulente miljøet. Sirkulasjonsveier, hydrografi og volumtransport av Atlanterhavsvann og Arktiske vann i det nordvestlige Barentshavet utforskes ved hjelp av data fra tre tokt og ni glideroppdrag gjennomført mellom 2019 og 2022, samt historiske data samlet mellom 1950 og 2009. Vi setter søkelys på utveksling og dynamikk på tvers av Polarfronten og nærliggende område. Observasjonene våre viser at 0.9 ± 0.1 Sv av Atlanterhavvann når Polarfront-regionen før vannet sprer seg langs flere forgreninger og til slutt dykker under Arktiske vannmasser. Mengden Atlanterhavsvann som lagres nord for Polarfronten kontrolleres av tetthetsforskjellen mellom Atlanterhavsvannet og det Arktiske vannet, og nådde et maksimum på 90-tallet da det Arktiske vannet nord for fronten var spesielt ferskt. I nyere tid (2019 til 2022) ble Atlanterhavsvannet som strømmer inn i Barentshavet opptil 0.1 g/kg ferskere sammenlignet med tidligere tiår. Dette førte til en økt temperaturgradient på tvers av Polarfronten og en redusert transport av varmt vann nordover på tvers av fronten. Ved bruk av data fra to tokt og fire glider-oppdrag, spesifikt samlet for å undersøke dynamikken og variabiliteten til Polarfronten, beskriver vi strukturen til fronten, dens variasjon og forekomsten av blanding av vannmasser. Vi observerer at varmt og salt Atlanterhavsvann trenger inn under kaldere og ferskere Arktisk vann, noe som setter opp en baroklin front og en geostrofisk strøm med hastigheter opp mot 25 cm/s. Den estimerte østlige transporten fra den geostrofiske strømmen er 0.3 ± 0.2 Sv. Korttidsvariasjoner i dypet, under øvre grenselag, skyldes tidevannsstrømmer og mesoskala virvler. Effektene av tidevannsstrømmer er hovedsakelig begrenset til bunnsjiktet, mens virvlene betydelig påvirker posisjonen til fronten og endrer helningen til tetthetslinjene og følgelig den tilgjengelige potensielle energien i fronten. Betydelig transformasjon av vannmasser observeres på tvers av fronten, noe som sannsynligvis skyldes virvelindusert blanding langs tetthetskonturene. Til tross for sesongendringer i de øvre grenselag på tvers av fronten (0-100 m), forble posisjonen til fronten under 100 m dybde relativt uforstyrret. Samlet sett har artiklene i denne avhandlingen bidratt til å øke vår kunnskap om innstrømmingen av Atlanterhavsvann i Nordishavet, dens veier og mekanismer som kontrollerer blandingen og distribusjonen av varme fra Atlanterhavsvannet til de omkringliggende Arktiske vannmassene. Dette arbeidet representerer et viktig skritt i retning av å forstå Atlanterhavsvannet sin innflytelse på Nordishavet, noe som er avgjørende for bærekraftig forvaltning og for å forutsi fremtiden til de Arktiske økosystemene.The Arctic Ocean plays an important role in the global climate system. Dense water production and sea ice freezing in the Arctic contribute to the functioning of the Atlantic Meridional Overturning Circulation and can affect atmospheric circulation patterns as far south as mid-latitudes. The Atlantic Water (AW) inflow through Fram Strait and Barents Sea into the Arctic Ocean regulates both the dense water production and the sea-ice extent, thus has a key role in the Arctic Ocean and the global climate system. The transport of AW into the Arctic is the major heat and salt source to the Arctic Ocean and influences the onset of freezing and the functioning of marine ecosystems. This research contributes to the understanding of the inflow of AW into the Arctic Ocean through Fram Strait and the Barents Sea. The study describes the pathways of the AW inflow, as well as the underlying mechanisms controlling the inflow, processes of heat loss, and mixing with the surrounding waters north of Svalbard and in the northwestern Barents Sea. Essential to this study is data collected by means of autonomous underwater vehicles (AUVs) in challenging Arctic conditions. Data collected from five scientific cruises and 11 AUV missions between 2018 and 2022, resulting in more than 15,000 hydrographic profiles, are collated and presented in the form of four research papers. All data are collected as part of the Nansen Legacy project. Using detailed observations in the region along the continental slope north of Svalbard between 12°E and 24°E, we describe the hydrographic structure, volume transport, and circulation patterns of the warm AW boundary current. The AW volume transport reaches a maximum of 3.0 ± 0.2 Sv in October, with an intraseasonal variability of 1 Sv. The strength of the AW boundary current is sensitive to the wind stress curl, doubling its volume transport in less than a week when the wind stress curl averaged over the region transitioned from strongly negative to strongly positive values. A previously unknown, deep bottom-intensified current is observed to flow parallel to the boundary current, between the 1,500 and 2,000 m isobaths. Historical data in the region support the presence of the bottom-intensified current. The follow-up studies concentrated on the northwestern Barents Sea. Targeted measurements from AUVs offer a step change in the spatial and temporal coverage of observations. To exploit the technology, particularly for turbulence measurements, we explored the potential of a thruster-propelled AUV. We instrumented the AUV with a turbulence sensor package, and using this novel setup, we report on the data quality and discuss the limitations of turbulence dissipation rate estimates from shear probes. The AUV mission in the Barents Sea in winter lasted for 5 h, operating at a typical horizontal speed of 1.1 m/s. The AUV was programmed to find and cross the maximum along-path thermal gradient at 10, 20 and 30 m depths along 4 km transects. Although the AUV vibrations contaminate the shear probe records, the noise is mitigated by removing vibration-induced components from shear spectra using the accelerometer signal. Dissipation estimates from the AUV show good agreement with nearby vertical microstructure profiles obtained from ship, indicating that the turbulence measurements from the AUV are reliable for this relatively turbulent environment. Circulation pathways, hydrography and volume transports of Atlantic- and Arctic-origin waters on the northwestern Barents Sea are explored using data from three cruises and nine glider missions conducted between 2019 and 2022, as well as historical data collected between 1950 and 2009. In particular, we focus on the exchange and dynamics across the thermohaline polar front (PF) region. Our observations show that 0.9 ± 0.1 Sv of Atlantic-origin water reaches the PF region before splitting into several branches and eventually subducting beneath Polar Water (PW). The amount of Atlantic-origin water stored in the basin north of the PF is controlled by the density difference between AW and PW, and reached a maximum in the 90s when PW was particularly fresh. In the recent period from 2019 to 2022, the inflow of AW into the Barents Sea freshened by up to 0.1 g/kg compared to previous decades. This led to an increased temperature gradient across the PF and a reduced poleward transport of warm water. Using data targeted to resolve the dynamics and variability of the PF, we describe the structure of the front, its variability and associated mixing. Ocean stratification, currents, and turbulence data were obtained during seven ship transects across the PF near 77°N, 30°E in fall and winter conditions. These transects are complemented by nine glider missions using ocean gliders, one of which was equipped with microstructure sensors to measure turbulence. Across the front, we observe warm and salty AW intruding below the colder and fresher PW, setting up a baroclinic front and geostrophic currents reaching 25 cm/s, with estimated eastward transport of 0.3 ± 0.2 Sv. Short-term variability below the surface mixed layer arises from tidal currents and mesoscale eddies. While the effects of tidal currents are mainly confined to the bottom boundary layer, eddies induce significant shifts in the position of the front, and alter the isopycnal slopes and the available potential energy of the front. Substantial water mass transformation is observed across the front, likely a result of eddy-driven isopycnal mixing. Despite the seasonal changes in the upper layers of the front (0–100 m) influenced by atmospheric forcing, sea ice formation, and brine rejection, the position of the front beneath 100 m depth remained relatively unperturbed. Collectively, the papers in this thesis have advanced our knowledge about the AW inflow into the Arctic Ocean, its pathways and mechanisms controlling the mixing and distribution of heat from AW to the surrounding Arctic waters. This work represents an important step towards comprehending the influence of AW on the Arctic Ocean, essential for sustainable management and predicting the future of Arctic marine ecosystems.Doktorgradsavhandlin

Structure and drivers of ocean mixing north of Svalbard in summer and fall 2018

The Arctic Ocean is a major sink for heat and salt for the global ocean. Ocean mixing contributes to this sink by mixing the Atlantic- and Pacific-origin waters with surrounding waters. We investigate the drivers of ocean mixing north of Svalbard, in the Atlantic sector of the Arctic, based on observations collected during two research cruises in summer and fall 2018. Estimates of vertical turbulent heat flux from the Atlantic Water layer up to the mixed layer reach 30 W m−2 in the core of the boundary current, and average to 8 W m−2, accounting for ∼1 % of the total heat loss of the Atlantic layer in the region. In the mixed layer, there is a nonlinear relation between the layer-integrated dissipation and wind energy input; convection was active at a few stations and was responsible for enhanced turbulence compared to what was expected from the wind stress alone. Summer melting of sea ice reduces the temperature, salinity and depth of the mixed layer and increases salt and buoyancy fluxes at the base of the mixed layer. Deeper in the water column and near the seabed, tidal forcing is a major source of turbulence: diapycnal diffusivity in the bottom 250 m of the water column is enhanced during strong tidal currents, reaching on average 10−3 m2 s−1. The average profile of diffusivity decays with distance from the seabed with an e-folding scale of 22 m compared to 18 m in conditions with weaker tidal currents. A nonlinear relation is inferred between the depth-integrated dissipation in the bottom 250 m of the water column and the tidally driven bottom drag and is used to estimate the bottom dissipation along the continental slope of the Eurasian Basin. Computation of an inverse Froude number suggests that nonlinear internal waves forced by the diurnal tidal currents (K1 constituent) can develop north of Svalbard and in the Laptev and Kara seas, with the potential to mix the entire water column vertically. Understanding the drivers of turbulence and the nonlinear pathways for the energy to turbulence in the Arctic Ocean will help improve the description and representation of the rapidly changing Arctic climate system.publishedVersio

Atlantic water properties, transport and heat loss from mooring observations north of Svalbard

The Atlantic Water inflow to the Arctic Ocean is transformed and modified in the area north of Svalbard, which influences the Arctic Ocean heat and salt budget. Year-round observations are relatively sparse in this region partially covered by sea ice. We took advantage of one-year-long records of ocean currents and hydrography from seven moorings north of Svalbard. The moorings are organized in two arrays separated by 94 km along the path of the Atlantic Water inflow to investigate the properties, transport and heat loss of the Atlantic Water in 2018/2019. The Atlantic Water volume transport varies from 0.5 Sv (1 Sv = 106 m3s−1) in spring to 2 Sv in fall. The first mode of variation of the Atlantic Water inflow temperature is a warm/cold mode with a seasonal cycle. The second mode corresponds to a shorter time scale (6–7 days) variability in the onshore/offshore displacement of the temperature core linked to the mesoscale variability. Heat loss from the Atlantic Water in this region is estimated, for the first time using two mooring arrays and conserving the volume transport. The heat loss varies between 302 W m−2 in winter to 60 W m−2 in spring. The onshore moorings show a westward countercurrent driven by Ekman setup in spring, carrying transformed-Atlantic Water. The offshore moorings show a bottom-intensified current that covaries with the wind stress curl. These two mooring arrays allowed for a better comprehension of the structure and transformation of the slope currents north of Svalbard.publishedVersio

Diffusive and advective cross-frontal fluxes of inorganic nutrients and dissolved inorganic carbon in the Barents Sea in autumn

The Atlantic Water, entering the Arctic through the Barents Sea and Fram Strait, is the main source of nutrients in the Arctic Ocean. The Barents Sea is divided by the Polar Front into an Atlantic-dominated domain in the south, and an Arctic-dominated domain in the north. The Polar Front is a thermohaline structure, which is topographically-steered at sub-surface, and influenced by the seasonal sea ice edge near the surface. Exchanges of nutrients between the inflowing Atlantic Water and the surrounding waters are key for the primary production in the Barents Sea. In October 2020, we measured nutrients (nitrate, phosphate and silicic acid), dissolved inorganic carbon (DIC), ocean stratification, currents and turbulence in the vicinity of the Polar Front in the Barents Sea within the framework of the Nansen Legacy project, allowing estimates of horizontal and vertical advective fluxes and turbulent fluxes of nitrate and DIC. We studied the autumn situation when primary production was declining. We found a substantial transfer of nitrate and DIC across the Polar Front from the Atlantic domain to the Arctic domain. Up to one quarter of the replenishment of the nitrate in the mixed layer during winter could be attributed to vertical mixing during wind events, shared approximately equally between advective and turbulent fluxes. The vertical turbulent fluxes bring nutrients from the subsurface Atlantic Water to the surface. We also identified an export of nitrate and DIC from the Barents Sea to the Nordic Seas occurring along the eastern shelf of Svalbard. Our study shows the role of vertical fluxes in fall and winter to precondition for the following spring bloom

Structure and drivers of ocean mixing north of Svalbard in summer and fall 2018

The Arctic Ocean is a major sink for heat and salt for the global ocean. Ocean mixing contributes to this sink by mixing the Atlantic- and Pacific-origin waters with surrounding waters. We investigate the drivers of ocean mixing north of Svalbard, in the Atlantic sector of the Arctic, based on observations collected during two research cruises in summer and fall 2018. Estimates of vertical turbulent heat flux from the Atlantic Water layer up to the mixed layer reach 30 W m−2 in the core of the boundary current, and average to 8 W m−2, accounting for ∼1 % of the total heat loss of the Atlantic layer in the region. In the mixed layer, there is a nonlinear relation between the layer-integrated dissipation and wind energy input; convection was active at a few stations and was responsible for enhanced turbulence compared to what was expected from the wind stress alone. Summer melting of sea ice reduces the temperature, salinity and depth of the mixed layer and increases salt and buoyancy fluxes at the base of the mixed layer. Deeper in the water column and near the seabed, tidal forcing is a major source of turbulence: diapycnal diffusivity in the bottom 250 m of the water column is enhanced during strong tidal currents, reaching on average 10−3 m2 s−1. The average profile of diffusivity decays with distance from the seabed with an e-folding scale of 22 m compared to 18 m in conditions with weaker tidal currents. A nonlinear relation is inferred between the depth-integrated dissipation in the bottom 250 m of the water column and the tidally driven bottom drag and is used to estimate the bottom dissipation along the continental slope of the Eurasian Basin. Computation of an inverse Froude number suggests that nonlinear internal waves forced by the diurnal tidal currents (K1 constituent) can develop north of Svalbard and in the Laptev and Kara seas, with the potential to mix the entire water column vertically. Understanding the drivers of turbulence and the nonlinear pathways for the energy to turbulence in the Arctic Ocean will help improve the description and representation of the rapidly changing Arctic climate system

Technical note: Turbulence measurements from a light autonomous underwater vehicle

A self-contained turbulence instrument from Rockland Scientific was installed on a light autonomous underwater vehicle (AUV) from OceanScan Marine Systems and Technology Lda. We report on the data quality and discuss limitations of dissipation estimated from two shear probes during a deployment in the Barents Sea in February 2021. The AUV mission lasted for 5 h, operating at a typical horizontal speed of 1.1 m s−1 . The AUV was programmed to find and cross the maximum along-path thermal gradient at 10, 20 and 30 m depths along 4 km transects. Although the AUV vibrations contaminate the shear probe records, the noise is mitigated by removing vibrationinduced components from shear spectra using the accelerometer signal measured in multiple directions. Dissipation rate estimates in the observed transects varied in the range 1 × 10−8 and 6×10−6 W kg−1 , with the values from the two orthogonal probes typically in agreement to within a factor of 2. Dissipation estimates from the AUV show good agreement with nearby vertical microstructure profiles obtained from the ship during the transects, indicating that the turbulence measurements from the AUV are reliable for this relatively turbulent environment. However, the lowest reliable dissipation rates are limited to 5 × 10−8 W kg−1 , making this setup unfit for use in quiescent environments.publishedVersio