Structure and Transport of Atlantic Water North of Svalbard From Observations in Summer and Fall 2018

The transport of warm Atlantic Waters north of Svalbard is one of the major heat and salt sources to the Arctic Ocean. The circulation pathways and the associated heat transport influence the variability in the Arctic sea ice extent, the onset of freezing, and marine ecosystems. We present observations obtained from research cruises and an autonomous underwater glider mission in summer and fall 2018, to describe the hydrographic structure, volume transport, and circulation patterns of the warm Atlantic Water Boundary Current between 12°E and 24°E north of Svalbard. The Atlantic Water volume transport reaches a maximum of 3.0  ± 0.2 Sv in October, with an intraseasonal variability of 1 Sv (1 Sv = 106 m3 s−1). During summer and late fall, we observed an Atlantic Water recirculation flowing westward (0.1–0.2 Sv) in the outer part of the section away from the shelf break. This counter current appears to be a part of an anticyclonic circulation in the Sofia Deep. The strength of the Atlantic Water recirculation and the Atlantic Water boundary current is very sensitive to the wind stress curl: The boundary current volume transport doubled in less than a week, corresponding to a transition from strongly negative (−10−6 N m−3) to strongly positive (10−6 N m−3) wind stress curl over the Sofia Deep. 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.publishedVersio

Observations of Turbulence at a Near-Surface Temperature Front in the Arctic Ocean

High-resolution ocean temperature, salinity, current, and turbulence data were collected at an Arctic thermohaline front in the Nansen Basin. The front was close to the sea ice edge and separated the cold and fresh surface melt water from the warm and saline mixed layer. Measurements were made on 18 September 2018, in the upper 100 m, from a research vessel and an autonomous underwater vehicle. Destabilizing surface buoyancy fluxes from a combination of heat loss to the atmosphere and cross-front Ekman transport by down-front winds reduced the potential vorticity in the upper ocean. Turbulence structure in the mixed layer was generally consistent with turbulence production through convection by heat loss to atmosphere and mechanical forcing by moderate winds. Conditions at the front were favorable for forced symmetric instability, a mechanism drawing energy from the frontal geostrophic current. A clear signature of increased dissipation from symmetric instability could not be identified; however, this instability could potentially account for the increased dissipation rates at the front location down to 40 m depth that could not be explained by the atmospheric forcing. This turbulence was associated with turbulent heat fluxes of up to 10 W m−2, eroding the warm and cold intrusions observed between 30 and 60 m depth. A Seaglider sampled across a similar frontal structure in the same region 10 days after our survey. The submesoscale-to-turbulence-scale transitions and resulting mixing can be widespread and important in the Atlantic sector of the Arctic Ocean.publishedVersio

Propriétés et circulation des Eaux Atlantiques au nord du Svalbard dans un Arctique en mutation

The Atlantic Water (AW) inflow is crucial for the heat and salt budget of the Arctic. This PhD thesis brings new insights to the inflow of AW in the area north of Svalbard.The IAOOS (Ice Atmosphere Ocean Observing System) platforms were deployed during the N-ICE2015 expedition which gathered the first winter hydrographic data in the area. They document shallow warm water over the Svalbard continental slope that melts sea ice with ice-ocean heat fluxes reaching up to 400W.m-2. Heat is brought from the AW layer up to the surface through near-inertial waves generated by winter storms, large barotropic tides over steep topography and/or geostrophic adjustments. Sea ice extent largely differs between winters 2015 and 2016. 1/12° operational model outputs from Mercator-Ocean suggest that convection-induced upward heat fluxes explain the differences. Model outputs are also used to examine the AW inflow pathways : besides the Svalbard Branch and the Yermak Branch, the model shows an AW winter pathway not much documented before : the Yermak Pass Branch across the Yermak Plateau. Finally, the model suggests an important mesoscale activity throughout the AW flow. The Yermak Pass Branch properties are examined using one-year (2007-2008) of moored ADCP data in the Yermak Pass. The flow is largely dominated by tides. In winter, baroclinic eddies of AW with a periodicity of 5 to 10 days and pulses of AW monthly/bimonthly are found, carrying AW eastward through the Pass. Model outputs suggest that the Yermak Pass Branch is a robust winter pattern over the last 10 years, carrying on average 31% of the volume transport of the West Spitsbergen Current.Les Eaux Atlantiques (AW) sont cruciales pour le budget de sel et de chaleur de l'Arctique. Ce doctorat apporte de nouvelles informations sur l'entrée des AW dans la région du nord Svalbard. Les plateformes IAOOS ont collecté pendant la campagne N-ICE2015 les premières données hydrographiques d'hiver de la région. Elles ont documentées des eaux chaudes peu profondes sur le talus continental du Svalbard qui ont généré des flux de chaleur océan-glace atteignant 400 W/m2 et faisant fondre la glace. Cette chaleur est amenée des AW vers la surface par des ondes quasi-inertielles causées par des tempêtes hivernales, de grandes marées barotropes sur des pentes raides et/ou des ajustements géostrophiques. Les extensions de glace sont très différentes entre 2015 et 2016. Les sorties du modèle opérationnel de Mercator Ocean (1/12°) suggèrent que les flux de chaleur orientés vers la surface et induits par la convection expliquent ces différences. En plus de la Svalbard Branch et de la Yermak Branch, le modèle présente un chemin robuste l'hiver à travers le plateau du Yermak: la Yermak Pass Branch. Enfin, le modèle suggère une activité méso-échelle importante le long du courant des AW. Les propriétés de la Yermak Pass sont examinées avec un an de données ADCP (2007-2008) dans la Yermak Pass. Le courant est dominé par la marée. En hiver, des tourbillons baroclines d'AW avec une périodicité de 5-10 jours et des entrées sporadiques d'AW tous les un/deux mois sont observés, transportant les AW vers l'Est. Le modèle suggère que la Yermak Pass Branch est une structure robuste d'hiver les 10 dernières années et transporte en moyenne 31% du transport volumique du West Spitsbergen Current

Expendable Conductive-Temperature-Depth (xCTD) data from MOSAiC leg 5 north of Greenland

Xctd (Expendable Conductive-Temperature-Depth) data were collected north of Greenland during MOSAiC leg5, when R/V Polarstern was relocated north between leg4 and leg5. The data were collected mid-August 2020. The data contained temperature, salinity and pressure from the surface to 500 m depth. The data are despiked and corrected from a salinity offest of 0.03. The data were collected by team OCEAN during leg 5 ( Zoe Koenig, Mario Hoppmann, Salar Karan and Jacob Allerholt) at the aft of the ship

Upper-Ocean Turbulence Structure and Ocean-Ice Drag Coefficient Estimates Using an Ascending Microstructure Profiler During the MOSAiC Drift

Sea ice mediates the transfer of momentum, heat, and gas between the atmosphere and ocean. However, the under-ice boundary layer is not sufficiently constrained by observations. During the Multidisciplinary drifting Observatory for the Study of the Arctic Climate (MOSAiC), we collected profiles in the upper 50-80 m using a new ascending vertical microstructure profiler, resolving the turbulent structure within 1 m to the ice. We analyzed 167 dissipation rate profiles collected between February and mid-September 2020, from 89°N to 79°30′N through the Amundsen Basin, Nansen Basin, Yermak Plateau, and Fram Strait. Measurements covered a broad range of forcing (0–15 m s−1 wind and 0–0.4 m s−1 drift speeds) and sea ice conditions (pack ice, thin ice, and leads). Dissipation rates varied by over 4 orders of magnitude from 10−9 W kg−1 below 40 m to above 10−5 W kg−1 at 1 m. Following wind events, layers with dissipation urn:x-wiley:21699275:media:jgrc25172:jgrc25172-math-0001 W kg−1 extended down to 20 m depth under pack ice. In leads in the central Arctic, turbulence was enhanced 2 to 10 times relative to thin ice profiles. Under-ice dissipation profiles allowed us to estimate the boundary layer thickness (4±2 m), and the friction velocity (1–15 mm s−1, 4.7 mm s−1 on average). A representative range of drag coefficient for the MOSAiC sampling site was estimated to (4–6) × 10−3, which is a typical value for Arctic floe observations. The average ratio of drift speed to wind speed was close to the free-drift ratio of 2% with no clear seasonal or regional variability.publishedVersio

Atlantic Water Inflow Through the Yermak Pass Branch: Evolution Since 2007

International audienceThirty-four months (2017–2020) of mooring data were recently obtained at 80.6°N, 7.26°E in the main branch of Atlantic Water inflow to the Arctic, the Yermak Pass Branch. The Yermak Pass Branch was sampled at that same location during 14 months a decade ago (2007–2008) when sea ice was abundant (mean sea-ice concentration of 74% vs. 39% during the recent deployment). We focus on time scales larger than 50 hr. The new mooring data set shows an increase in the velocity variations of 40% compared to the 2007–2008 period. Year 2018 was exceptional with ice-free conditions over the entire year and an intensified mesoscale activity compared to other years. Temperature and salinity time series at 340 m showed significant trends over 3 years (freshening of −0.07 g/kg and cooling of about −0.9°C in 3 years). The performance of 1/12° Mercator-Ocean operational model at the mooring location was precisely assessed. The modeled Atlantic Water transport was on average larger during 2017–2020 (40% larger) than during 2007–2008. The synoptic transport time series ranged between −1 and 5 Sv over 2007–2020 and showed large seasonal and interannual variations. The transport was larger in winter than summer. However, occasionally negative transport (<−0.7 Sv) through the Yermak Pass Branch occurred during winters (“Blocking events”). These blocking events are associated with recirculations and eddy activity and were more common over the last years from 2016 onward. The model suggested that a Northern Branch crossing the Yermak Plateau further north (81.6°N) intermittently developed

Tidally-forced lee waves drive turbulent mixing along the Arctic Ocean margins

In the Arctic Ocean, limited measurements indicate that the strongest mixing below the atmospherically forced surface mixed layer occurs where tidal currents are strong. However, mechanisms of energy conversion from tides to turbulence and the overall contribution of tidally driven mixing to Arctic Ocean state are poorly understood. We present measurements from the shelf north of Svalbard that show abrupt isopycnal vertical displacements of 10–50 m and intense dissipation associated with cross‐isobath diurnal tidal currents of ∼0.15 m s−1. Energy from the barotropic tide accumulated in a trapped baroclinic lee wave during maximum downslope flow and was released around slack water. During a 6‐hr turbulent event, high‐frequency internal waves were present, the full 300‐m depth water column became turbulent, dissipation rates increased by a factor of 100, and turbulent heat flux averaged 15 W m−2 compared with the background rate of 1 W m−2.publishedVersio

Structure and Transport of Atlantic Water North of Svalbard From Observations in Summer and Fall 2018

The transport of warm Atlantic Waters north of Svalbard is one of the major heat and salt sources to the Arctic Ocean. The circulation pathways and the associated heat transport influence the variability in the Arctic sea ice extent, the onset of freezing, and marine ecosystems. We present observations obtained from research cruises and an autonomous underwater glider mission in summer and fall 2018, to describe the hydrographic structure, volume transport, and circulation patterns of the warm Atlantic Water Boundary Current between 12°E and 24°E north of Svalbard. The Atlantic Water volume transport reaches a maximum of 3.0  ± 0.2 Sv in October, with an intraseasonal variability of 1 Sv (1 Sv = 106 m3 s−1). During summer and late fall, we observed an Atlantic Water recirculation flowing westward (0.1–0.2 Sv) in the outer part of the section away from the shelf break. This counter current appears to be a part of an anticyclonic circulation in the Sofia Deep. The strength of the Atlantic Water recirculation and the Atlantic Water boundary current is very sensitive to the wind stress curl: The boundary current volume transport doubled in less than a week, corresponding to a transition from strongly negative (−10−6 N m−3) to strongly positive (10−6 N m−3) wind stress curl over the Sofia Deep. 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

Volume transport of the Antarctic Circumpolar Current: Production and validation of a 20 year long time series obtained from in situ and satellite observations

International audienceA 20 year long volume transport time series of the Antarctic Circumpolar Current across the Drake Passage is estimated from the combination of information from in situ current meter data (2006–2009) and satellite altimetry data (1992–2012). A new method for transport estimates had to be designed. It accounts for the dependence of the vertical velocity structure on surface velocity and latitude. Yet unpublished velocity profile time series from Acoustic Doppler Current Profilers are used to provide accurate vertical structure estimates in the upper 350 m. The mean cross-track surface geostrophic velocities are estimated using an iterative error/correction scheme to the mean velocities deduced from two recent mean dynamic topographies. The internal consistency and the robustness of the method are carefully assessed.Comparisons with independent data demonstrate the accuracy of the method. The full-depth volume transport has a mean of 141 Sv (standard error of the mean 2.7 Sv), a standard deviation (std) of 13 Sv, and a range of 110 Sv. Yearly means vary from 133.6 Sv in 2011 to 150 Sv in 1993 and standard deviations from 8.8 Sv in 2009 to 17.9 Sv in 1995. The canonical ISOS values (mean 133.8 Sv, std 11.2 Sv) obtained from a year-long record in 1979 are very similar to those found here for year 2011 (133.6 Sv and 12 Sv). Full-depth transports and transports over 3000 m barely differ as in that particular region of Drake Passage the deep recirculations in two semiclosed basins have a close to zero net transport

Winter ocean-ice interactions under thin sea ice observed by IAOOS platforms during N-ICE2015: Salty surface mixed layer and active basal melt

International audienc