OS1.3

North of Svalbard is a key region for the Arctic Ocean heat and salt budget as it is the gateway for one of the main branches of Atlantic Water to the Arctic Ocean. As the Atlantic Water layer advances into the Arctic, its core deepens from about 250 m depth around the Yermak Plateau to 350 m in the Laptev Sea, and gets colder and less saline due to mixing with surrounding waters. The complex topography in the region facilitates vertical and horizontal exchanges between the water masses and, together with strong shear and tidal forcing driving increased mixing rates, impacts the heat and salt content of the Atlantic Water layer that will circulate around the Arctic Ocean.

In September 2018, 6 moorings organized in 2 arrays were deployed across the Atlantic Water Boundary current for more than one year (until November 2019), within the framework of the Nansen Legacy project to investigate the seasonal variations of this current and the transformation of the Atlantic Water North of Svalbard. The Atlantic Water inflow exhibits a large seasonal signal, with maxima in core temperature and along-isobath velocities in fall and minima in spring. Volume transport of the Atlantic Water inflow varies from 0.7 Sv in spring to 3 Sv in fall. An empirical orthogonal function analysis of the daily cross-isobath temperature sections reveals that the first mode of variation (explained variance ~80%) is the seasonal cycle with an on/off mode in the temperature core. The second mode (explained variance ~ 15%) corresponds to a short time scale (less than 2 weeks) variability in the onshore/offshore displacement of the temperature core. On the shelf, a counter-current flowing westward is observed in spring, which transports colder (~ 1°C) and fresher (~ 34.85 g kg^-1) water than Atlantic Water (θ > 2°C and S_A > 34.9 g kg^-1). The processes driving the dynamic of the counter-current are under investigation. At greater depth (~1000 m) on the offshore part of the slope, a bottom-intensified current is noticed that seems to covary with the wind stress curl. Heat loss of the Atlantic Water between the two mooring arrays is maximum in winter reaching 250 W m^-2 when the current is the largest and the net radiative flux from the atmosphere to the ocean is the smallest (only 50 W m^-2 compared to about 400 W m^-2 in summer).

How to cite: Koenig, Z., Kalhagen, K., Kolås, E., Fer, I., Nilsen, F., and Cottier, F.: Atlantic Water properties, transport, and water mass transformation north of Svalbard from one-year-long mooring observations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2505, https://doi.org/10.5194/egusphere-egu21-2505, 2021.

Kongsfjorden is an Arctic fjord situated in the Svalbard archipelago. The fjord hydrography is influenced by the warm and saline Atlantic water from West Spitsbergen Current (WSC) flowing northward on the shelf slope and the cold and fresh polar waters circulating on the shelf. Once several conditions are satisfied, Atlantic waters from the WSC can extensively flood the fjord. These intrusions are majorly confined to the summer season although some strong events have been identified also in winter. In this study, a decade of continuous observations is used to examine changes in water properties and water masses variability in inner Kongsfjorden, with a special focus on Atlantic water intrusions. Data have been gathered by the National Research Council of Italy (CNR) through the Mooring Dirigibile Italia (MDI) in addition to summer CTD surveys. MDI was deployed in September 2010 at 100m depth and comprises various temperature and salinity sensors placed at different depths. Analysis of the longest temperature series reveals a positive linear trend since 2010. However, both temperature and salinity present a peak at the beginning of 2017 and decreasing values toward the end of the series. No significant trends were found when considering the monthly water column temperature as average of few sensors’ measurements. Yet, differentiating the seasonal contributions reveals that summer temperatures feature a fast warming (0.26 °C/yr) whereas winters do not show a statistically significant linear trend. Temperature and salinity observations gathered at 25 and 85m depth are used to depict water masses variability accorfing to previous water masses classifications. Some evidences are noted: first, events of Atlantic water intrusions are always confined near the bottom and they are never seen at 25m, whilst summer freshwater is found only in the near surface. Second, the timing of occurrence of these two water types seems to be related: the presence of large freshwater volumes close to the surface are preceded by the arrival of warm and saline waters. This evidence is interpreted as the melting signal of Kronebreen, the largest tidewater glacier in Kongsfjorden, triggered by the intrusion of Atlantic water. As a result, the large freshwater input manages to dilute the Atlantic water settled near the bottom. Third, the temperature and salinity peaks at the beginning of 2017 are associated to a massive Atlantic water flooding in the inner fjord lasting several months in summer/autumn 2016 and 2017. After this period, Atlantic water is seen only for few months in summer 2019. CTD measurements are used to depict the summer hydrography of Kongsfjorden and the focus is drawn on the characterisation of the seasonal cycle for each available year of measurements. Finally, the drivers of Atlantic water intrusion events are examined, as the presence of low pressure systems and the wind patterns in the region.

How to cite: De Rovere, F., Chiggiato, J., Langone, L., Schröeder, K., Miserocchi, S., and Giglio, F.: Water masses variability in inner Kongsfjorden during 2010-2020, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2121, https://doi.org/10.5194/egusphere-egu21-2121, 2021.

We analyzed hydrographic data from several autonomous oceanographic buoys within the MOSAiC Distributed Network together with regular CTD casts from the MOSAiC Central Observatory during the 2019/20 winter in the Amundsen Basin. These drifting platforms can yield as small as ~300 m (or 10 min) horizontal resolution, providing unprecedented perspectives for the (sub)mesoscale dynamics. Full-depth CTD profiles yielded the first baroclinic Rossby radii (R₁) of ~7.5 km, which is consistent with previous studies based on climatology. Near-surface layers shallower than the halocline were not always mixed. Restratification was commonly observed, suggesting the onset of baroclinic instabilities and/or eddies emanating from the lateral fronts. A surface-layer eddy with estimated radius of ~5 km was fortuitously observed and coincident with the surrounding mixed geostrophic shear in the vertical, that is, oppositely-sloping isopycnals within the depth range of ~20 – 200 m. This structure is reminiscent of the Charney-type baroclinic instability, resulting from a difference in the sign of the vertical gradient of the interior quasigeostrophic potential vorticity and that of the surface buoyancy forcing. A reconstructed surface dynamic height field supports this argument, showing that submesoscale to mesoscale surface lateral buoyancy gradients are ubiquitous. This result implies that the study domain could be inherently unstable and prone to generate baroclinic eddies. We also observed that the slopes of the density horizontal wavenumber spectra changed at the halocline depths (~40 – 75 m) after a ~3-day storm event with peak speeds ~ 20 m s^-1. We hypothesize that such change could be related to the Ekman pumping due to large ice drift (~50 cm s^-1) and its resultant stress curl during the storm. Our analyses underline that thinning Arctic sea ice and increasing ice drift could together trigger more oceanic heat flux into the cold halocline by storms, further deteriorating winter ice growth in the Amundsen Basin.

How to cite: Fang, Y.-C., Rabe, B., Kuznetsov, I., Hoppmann, M., Tippenhauer, S., Schulz, K., Regnery, J., Janout, M., Rohde, J., Belter, J., Krumpen, T., and Nicolaus, M. and the MOSAiC Ocean Team: Winter upper-ocean thermohaline variability observed from drifting ice platforms in the Amundsen Basin, Arctic Ocean, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3723, https://doi.org/10.5194/egusphere-egu21-3723, 2021.

Atlantic water flows over the Greenland-Iceland-Scotland Ridge into the Norwegian Sea. Along its path towards the Arctic, the Atlantic water is cooled by strong air-sea fluxes. Deep winter mixed layers modify the stratification and properties of the Atlantic water and precondition its flow into the Arctic, thus influencing Arctic sea ice and climate. Atlantic water also recirculates in the Greenland sea where deep water formation contributes to the dense limb of the Atlantic Meridional Overturning Circulation. It is thus of paramount importance to represent mixed layer deepening and lateral heat exchanges processes in the Nordic Seas in climate models.

Heat exchanges in the Nordic Seas are influenced by narrow current branches, instabilities and eddies, which are not accurately represented in low resolution climate model (with grid ~ 50-100km).  Here we examine the mixed layer dynamics and heat exchanges using the latest generation of European high resolution global coupled models in the framework of HighResMip (5-15km grids in the Nordic Seas). We investigate in detail the effect of model resolution on the mixed layer depth and water mass formation in relation with the Atlantic water circulation and modification between the Norwegian and the Greenland Sea. First results show an increased northward ocean heat transport, a more realistic representation of the ocean current system in the Nordic Seas, and consequently an improved spatial distribution of the turbulent surface heat flux compared to standard resolution CMIP6 models. The mixed layer depth itself however varies strongly between different HighResMIP models. Summarizing, our assessment of the high resolution coupled simulations of the historical period demonstrates that future climate projections at high resolution have a huge potential, but also limitations.

How to cite: Tréguier, A. M., Koenigk, T., Doroteaciro, I., Camille, L., and Docquier, D.: Mixed layers along the Atlantic water path in the Nordic seas in HighResMIP models, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1983, https://doi.org/10.5194/egusphere-egu21-1983, 2021.

In recent decades, the retreat of the Arctic sea ice has modified vertical momentum fluxes from the atmosphere to the ice and the ocean, in turn affecting the surface circulation. Satellite altimetry has contributed in the past ten years to understand these changes. Most oceanographic datasets are however to date limited either to open ocean and ice-covered regions, given that different techniques are required to track sea surface height over these two surfaces. Hence, efforts to generate unified Arctic-wide datasets are still required to further basin-wide studies of the Arctic Ocean surface circulation.

We present here the assessment of a new Arctic-wide gridded dataset of the Sea Level Anomaly (SLA) and SLA-derived geostrophic velocities. This dataset is based on Cryosat-2 observations over ice-covered and open ocean areas in the Arctic during 2011 to 2018.

We compare the SLA and geostrophic currents derived hereof to in situ observations of ocean bottom pressure, steric height and near-surface ocean velocity, in three regions: the Fram Strait, the shelf break north of the Arctic Cape and the Laptev Sea continental slope. Good agreement in SLA is shown at seasonal time scales, with the dominant component of SLA variability being steric height both in Fram Strait and at the Arctic Cape. On the other hand, ocean bottom pressure dominates SLA changes at the Laptev Sea site. The comparison of velocity at two mooring transects, one in Fram Strait and the other at the Laptev Sea continental slope, reveals that the correlation is highest at the moorings closest to the shelf break, where currents are faster and the seasonal cycle is enhanced.

The seasonal cycle of SLA and geostrophic currents as derived from the altimetric product is in favourable agreement with previous results. A quasi-simultaneous occurrence of the SLA maximum happens between October and January; similar phase has been found in steric height seasonal cycle by studies using hydrographic profiles in several regions of the Arctic Ocean. We thereby find the highest SLA amplitude over the shelves, which other studies point to be possibly related to winter-enhanced shoreward water mass transport. Seasonal variability in the geostrophic currents is most pronounced along the shelf edges, representing a basin wide, coherent seasonal acceleration of the Arctic slope currents in winter and a deceleration in summer. This is consistent with the shelf-amplified SLA seasonal cycle described above. Density driven coastal currents near Alaska and Siberia have variable cycle, consistent with the cycle of river runoff and local wind forcing. Enhanced south-western limb of the Beaufort Gyre in early winter is in agreement with a combination between the Beaufort High buildup and relatively thin sea ice.

In summary, we provide evidence that the altimetric data set has skills to reproduce the seasonal cycle of SLA and geostrophic currents consistently with in situ data and findings from other studies. We suggest that this dataset could be used not only for large scale studies but also to study Arctic boundary currents.  

How to cite: Doglioni, F., Rabe, B., Ricker, R., and Kanzow, T.: Seasonal cycle of Arctic Ocean circulation inferred from satellite altimetry, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4743, https://doi.org/10.5194/egusphere-egu21-4743, 2021.

¹²⁹I measurements on samples collected during GEOTRACES oceanographic missions in the Arctic Ocean in 2015 have provided the first detailed, synoptic ¹²⁹I sections across the Eurasian, Canada and Makarov Basins. ¹²⁹I is discharged from European nuclear fuel reprocessing plants since several decades and is carried north into the Arctic Ocean with waters of Atlantic origin. Here the measurements of its passage can be used to identify the ocean circulation at different depth horizons. Elevated ¹²⁹I levels measured over the Lomonosov and Alpha-Mendeleyev Ridges in 2015 were associated with tracer labeled, Atlantic-origin water bathymetrically steered by the ridge systems through the central Arctic while lower ¹²⁹I levels were evident in the more poorly ventilated basin interiors. ¹²⁹I levels of 200-400 x 10⁷ at/l measured in intermediate waters had increased by a factor of 10 compared to results from the same locations in 1994-1996 owing to the arrival of a strong increase in the discharges from La Hague, that occurred during the 1990s. Comparisons of the patterns of ¹²⁹I between the mid-1990s and 2015 delineate large scale circulation changes that occurred during the shift from a positive Arctic Oscillation and a cyclonic circulation regime in the mid-1990s to anticyclonic circulation in 2015. These are characterized by a broadened Beaufort Gyre in the upper ocean, a weakened boundary current and partial AW flow reversal in the southern Canada Basin at mid-depth. Tracer ¹²⁹I simulations using the coupled ocean-sea ice model NAOSIM agree with both, the historical ¹²⁹I results and recent GEOTRACES data sets, thereby lending context and credibility to the interpretation of large-scale changes in Arctic circulation and their relationship to shifts in climate indices revealed by the tracer ¹²⁹I distributions. We will present measurements and simulation results of ¹²⁹I for the 1990s and 2015 and put them into the context of ocean circulation responses to changing atmospheric forcing regimes.

How to cite: Karcher, M. J., Smith, J. N., Casacuberta, N., Williams, W. J., Kenna, T., and Smethie Jr., W. M.: A changing Arctic Ocean: How measured and modeled 129I distributions indicate fundamental shifts in circulation between 1994 and 2015, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7198, https://doi.org/10.5194/egusphere-egu21-7198, 2021.

From September 2019 to September 2020, the sea-level atmospheric pressure over the Beaufort Gyre region (BGR) was reduced relative to climatology and a well pronounced cyclonic circulation forcing of sea ice and ocean lasted more than eight months. This resulted in the following: increased sea ice area in 2020 relative to 2019; periodic reversals of sea ice drift from anticyclonic to cyclonic; the formation of an unusual donut-shaped sea ice cover pattern (in August-September 2020); upwelling in the central BGR with a reduction of freshwater content by ~1000 km³; downwelling along the periphery of the BGR; changes in the intensity and trajectories of freshwater fluxes from the Mackenzie river and Bering Strait and fresh water contributions to the BGR freshwater content; unusual warming of the Pacific water layer in the northern BGR; and biogeochemical changes driven by ocean circulation and water mass redistribution. Numerical modeling is used to better understand the causes and consequences of the observed changes. Sea-level atmospheric pressure from NCAR/NCEP reanalysis, sea ice concentration and ice motion from NSIDC, altimetry based sea surface heights from Technical University of Denmark, and hydrographic data from the Beaufort Gyre project and USCGC Healy expeditions are used in the study.

How to cite: Proshutinsky, A., Krishfield, R., Timmermans, M.-L., Le Bras, I., Toole, J., Pickart, R., Williams, B., Zimmermann, S., Platov, G., Golubeva, E., Dukhovskoy, D., Rose, S., and Andersen, O.: 2019-2020 mechanisms of fresh water release from the Beaufort Gyre region of the Arctic Ocean, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5775, https://doi.org/10.5194/egusphere-egu21-5775, 2021.

Anthropogenic chemical tracers are powerful tools to study pathways, water mass provenance and mixing processes in the ocean. Releases of the long-lived anthropogenic radionuclides ¹²⁹I and ²³⁶U from European nuclear reprocessing plants label Atlantic Water entering the Arctic Ocean with a distinct signal that can be used to track pathways and timescales of Atlantic Water circulation in the Arctic Ocean and Fram Strait. Apart from their application as transient tracers, the difference in anthropogenic radionuclide concentrations between Atlantic- and Pacific-origin water provides an instrument to distinguish the interface between both water masses. In contrast to classically used water mass tracers such as nitrate-phosphate (N:P) ratios, the two radionuclides are considered to behave conservatively in seawater and are not affected by biogeochemical processes occurring in particular in the broad shelf regions of the Arctic Ocean.

Here we present a time-series of ¹²⁹I and ²³⁶U data across the Fram Strait, collected in 2016 (as part of the GEOTRACES program) and in 2018 and 2019 (by the Norwegian Polar Institute). While the overall spatial distribution of both radionuclides was similar among the three sampling years, significant differences were observed in the upper water column of the EGC, especially between 2016 and 2018. This study is the first attempt to investigate the potential of ¹²⁹I and ²³⁶U as water mass composition tracers in the East Greenland Current (EGC). We discuss how the ¹²⁹I - ²³⁶U tracer pair can be applied to estimate fractions of Atlantic and Pacific Water, especially considering their time-dependent input into the Arctic Ocean.

How to cite: Wefing, A.-M., Casacuberta, N., Christl, M., Karcher, M., and Dodd, P. A.: Annual variability of the long-lived anthropogenic radionuclides 129I and 236U in the Fram Strait and their use as water mass composition tracers, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8662, https://doi.org/10.5194/egusphere-egu21-8662, 2021.

Atlantic Water (AW) enters the Arctic through Fram Strait as the West Spitsbergen Current (WSC). When reaching the south of Yermak Plateau, the WSC splits into the Svalbard, Yermak Pass and Yermak Branches. Downstream of Yermak Plateau, AW pathways remain unclear and uncertainties persist on how AW branches eventually merge and contribute to the boundary current along the continental slope. We took advantage of the good performance of the 1/12° Mercator Ocean model in the Western Nansen Basin (WNB) to examine the AW circulation and volume transports in the area. The model showed that the circulation changed in 2008-2020. The Yermak Branch strengthened over the northern Yermak Plateau, feeding the Return Yermak Branch along the eastern flank of the Plateau. West of Yermak Plateau, the Transpolar Drift likely shifted westward while AW recirculations progressed further north. Downstream of the Yermak Plateau, an offshore current developed above the 3800 m isobath, fed by waters from the Yermak Plateau tip. East of 18°E, enhanced mesoscale activity from the boundary current injected additional AW basin-ward, further contributing to the offshore circulation. A recurrent anticyclonic circulation in Sofia Deep developed, which also occasionally fed the western part of the offshore flow. The intensification of the circulation coincided with an overall warming in the upper WNB (0-1000 m), consistent with the progression of AW. This regional description of the changing circulation provides a background for the interpretation of upcoming observations.

How to cite: Athanase, M., Provost, C., Artana, C., Pérez-Hernández, M. D., Sennéchael, N., Bertosio, C., Garric, G., Lellouche, J.-M., and Prandi, P.: Changes in Atlantic Water circulation patterns and volume transports North of Svalbard over the last 12 years (2008-2020), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9750, https://doi.org/10.5194/egusphere-egu21-9750, 2021.

Atlantic Water, which is transported northward by the West Spitsbergen Current, partly recirculates (i.e. turns westward) in Fram Strait. This determines how much heat and salt reaches the Arctic Ocean, and how much joins the East Greenland Current on its southward path. We describe the Atlantic Water recirculation's location, seasonality, and mesoscale variability by analyzing the first observations from moored instruments at five latitudes in central Fram Strait, spanning a period from August 2016 to July 2018. We observe recirculation on the prime meridian at 78°50'N and 80°10'N, respectively south and north of the Molly Hole, and no recirculation further south at 78°10'N and further north at 80°50'N. At a fifth mooring location at 79°30'N, we observe some influence of the two recirculation branches. The southern recirculation is observed as a continuous westward flow that carries Atlantic Water throughout the year, though it may be subject to broadening and narrowing. It is affected by eddies in spring, likely due to the seasonality of mesoscale instability in the West Spitsbergen Current. The northern recirculation is observed solely as passing eddies on the prime meridian, which are strongest during late autumn and winter, and absent during summer. This seasonality is likely affected both by the conditions set by the West Spitsbergen Current and by the sea ice. Open ocean eddies originating from the West Spitsbergen Current interact with the sea ice edge when they subduct below the fresher, colder water. Additionally the stratification set up by sea ice presence may inhibit recirculation.

How to cite: Hofmann, Z., von Appen, W.-J., and Wekerle, C.: Seasonal and Mesoscale Variability of the Two Atlantic Water Recirculation Pathways in Fram Strait, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-175, https://doi.org/10.5194/egusphere-egu21-175, 2021.

How to cite: Provost, C., Athanase, M., Pérez-Hernández, M.-D., Sennéchael, N., Bertosio, C., Artana, C., Garric, G., and Lellouche, J.-M.: Atlantic Water Modification North of Svalbard in the Mercator Physical System From 2007 to 2020, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9797, https://doi.org/10.5194/egusphere-egu21-9797, 2021.

Understanding variable properties and dynamics of the Atlantic water (AW) inflow into the Arctic Ocean, and their impacts on ocean heat content, ocean-atmosphere-sea ice exchanges, changing sea ice cover and propagation of anomalies are key prerequisites to elucidate drivers and mechanisms behind the new, warmer regime of the Arctic Ocean. As the AW progress northwards, its properties are modified by ocean-atmosphere interactions, mixing and lateral exchanges. Warm anomalies reaching the Arctic Ocean can result from smaller heat loss during the AW northward passage through Fram Strait, and/or from an increased oceanic advection. Vertical structure of the Atlantic water layer implies the depth of winter convection and access to oceanic heat carried northward by the inflow.

During the last two decades warming of the Atlantic inflow has been reported to progress into the Arctic Ocean, however with strong interannual variations and quasi-periodic pulses of water with extraordinary high temperature. Here we present results from 20 years of annual hydrographic surveys, covering the Atlantic water inflow in the eastern Norwegian and Greenland seas, Fram Strait up to the southern Nansen Basin. Interannual changes in the AW properties and transport are analyzed with a focus on the en route modifications of AW inflow in the Fram Strait Branch and changes in the integrated ocean heat content.

After leaving Fram Strait, the part of AW continues eastward and enters the Arctic Ocean boundary current along different pathways north of Svalbard. The strongest ocean-atmosphere-sea ice interactions and lateral oceanic exchanges in this region lead to substantial local modification of the Atlantic inflow before it continues farther eastward around the rim of the Arctic Ocean. Observations from year-round moorings deployed since 2013 north of Svalbard are used to describe changes in the Atlantic water properties, vertical structure, and dynamics on monthly to seasonal and interannual time scales and their links to the upstream conditions and local and regional atmospheric forcing. Vertical heat fluxes from the Atlantic layer are derived to evaluate the ocean-air and ocean-sea ice exchanges in the only region of the Arctic Ocean where Atlantic-origin water has still contact with sea ice cover.

How to cite: Beszczynska-Möller, A., Walczowski, W., and Grynczel, A.: Modifications of Atlantic inflow along the Fram Strait Branch to the Arctic Ocean and its variability north of Svalbard from ship-borne and moored observations in the last two decades., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13553, https://doi.org/10.5194/egusphere-egu21-13553, 2021.

On the basis of various data sets we traced formation of a ‘dome’- shaped density structure over the Central Bank - an important morphological element of the Barents Sea bottom topography. The major conclusion, which follows from our analysis, based on direct winter measurements in 2019, is that under reduced ice cover, transformation of thermohaline structure during the cold season principally differs from that under the ‘normal’ climate conditions in the 20th century. Transition from the stratified vertical structure (in summer) to the homogeneous one (in winter) is governed by thermal convection. Additional input of warm and salty water with inflowing AW is crucial to allow reaching the seabed vertical mixing before the temperature drops to the freezing point. Cascading of dense water from the bank commences as soon as convection has spread to the seabed. The influence of cascading on the Barents Sea hydrographic structure extends far away from the bank. In the absence of advective influx of salt and warm water vertical convection can also reach the seabed. However, under this condition formation of sea ice and haline convection is required. In this case water temperature in the homogeneous water column over the bank is close to the freezing point. Obtained results suggest that in the warmer climate the role of sea ice in winter transformation of thermohaline conditions over the bank is opposite to what it was in the ‘normal’ climate: imported sea ice blocks convection, thus making the water in the dense ‘dome’ warmer than it typically was throughout the 20th century.

How to cite: Ivanov, V. and Tuzov, F.: Formation of dense water dome over the Central Bank under conditions of reduced ice cover in the Barents Sea, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7786, https://doi.org/10.5194/egusphere-egu21-7786, 2021.

Almost 4000 operational Argo floats covering the world's ocean provide near-real-time data on its state. The Arctic is less covered than other waters, but observations collected by Argo floats are gaining in importance. By delivering year-round measurements from the water column down to 2000 m (or to the bottom) along float trajectories, they complement and enhance the synoptic data collected during ship campaigns or by fixed moorings. However, oceanographic measurements with autonomous platforms are significantly limited in the Arctic regions by the presence of sea ice.

Here we present results obtained by Argo floats deployed in 2012-2020 by the Institute of Oceanology Polish Academy of Sciences (IOPAN) during summer campaigns of RV Oceania. In most years, the Argo floats were launched in the eastern branch (core) and in the western branch of the West Spitsbergen Current (WSC) within the Atlantic water inflow towards the Arctic Ocean. Floats deployed in the WSC core drift predominantly northward over the shelf break and upper slope west of Svalbard. After passing Fram Strait the floats usually turn eastward and continue over the northern Svalbard shelf brake, being advected with the Svalbard Branch of the Atlantic inflow into the Arctic Ocean Boundary Current. The easternmost position reached by the IOPAN Argo float was 39.6°E. Ultimately all deployed floats submerge under the sea ice north of Svalbard or farther to the east and die under the ice. Argo floats deployed in the western WSC branch over the underwater ridges, usually recirculate to the west and continue southward with the East Greenland Current. The float WMO 3901851 that drifted to the Labrador Sea, reached the southernmost latitude of 52.5°N and have been working until now for 4.5 years, which is unusual in the Arctic conditions.    

The measurements collected in the Marginal Ice Zone are particularly interesting for studying the ocean-atmosphere-ice interactions at the boundary between open and ice-covered ocean as well as they can be used for developing the ice avoidance algorithms for the Argo floats and other under ice sensors and platforms. A number of profiles obtained by Argo floats under the sea ice provide unique measurements in the upper ocean layer that is usually inaccessible from other platforms (e.g., moorings). In 2020 several profiles were collected under the ice cover by Argo floats north of Svalbard and transmitted after the float emerged in the polynya. The eastward flow of warm (up to 4° C at 80 m depth) Atlantic water was observed along the float trajectory over the shelf break. Measurements by Argo floats, revealing the dynamics and transformation of the Atlantic water entering the Arctic Ocean, are compared with ship-borne observations collected during the IOPAN long-term observational program AREX and year-round data from IOPAN moorings deployed north of Svalbard under the A-TWAIN and INTAROS projects.

How to cite: Walczowski, W., Beszczyńska-Möller, A., and Merchel, M.: Lagrangian measurements in the West Spitsbergen Current by Argo floats, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13135, https://doi.org/10.5194/egusphere-egu21-13135, 2021.

Mesoscale activity in the Arctic Ocean remains largely unexplored, owing primarily to the challenges of i) observing eddies in this ice-covered region and ii) modelling at such small deformation radius. In this talk, we will use results from a simulation performed with a high-resolution, eddy resolving model to investigate the spatial and temporal variations of the eddy kinetic energy (EKE) in the Arctic Basin. On average and in contrast to the typical open ocean conditions, the levels of mean and eddy kinetic energy are of the same order of magnitude, and EKE is intensified along the boundary and in the subsurface. On long time scales (interannual to decadal), EKE levels do not respond as expected to changes in the large scale circulation. This can be exemplified when looking at the spin up of the gyre that occurred in response to a strong surface input of momentum in 2007-2008. On seasonal time scales, the estimation of a Lorenz energy cycle allows us to investigate the drivers behind the peculiarities of the EKE field, and to understand the relative roles played by the atmospheric forcing for them.

How to cite: Lique, C., Regan, H., Meneghello, G., and Talandier, C.: Understanding the variability of the Eddy Kinetic Energy in the Arctic Ocean., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10314, https://doi.org/10.5194/egusphere-egu21-10314, 2021.

The dynamics of the boundary layer of the ocean significantly affect the interaction between ocean and atmosphere and, as a result, global climate. The sub-ice boundary layer of the ocean and its dynamics have not been thoroughly studied because of the extremely difficult conditions for observation, in particular during winter. Current understanding of spatial-temporal variability of (sub)mesoscales of the upper Arctic Ocean is extremely limited.
At the same time, one of the most important features of the upper ocean layers are the small-scale processes that influence and possibly determine the vertical and horizontal transport of heat, salt, and biologically relevant substances. As a consequence, mathematical models, in particular climate models, experience serious difficulties in parameterization of processes not resolved by the models because of the lack sufficient knowledge to detail the spatial variability at the (sub-)mesoscale.
To a better characterization and understanding of (sub)mesoscale dynamics and its role in vertical transport of energy and mass we apply a 3D regional ocean model FESOM-C. The observed vertical hydrological structure and a corresponding reconstructed horizontal temperature and salinity fields were imposed as a part of the forcing for the numerical model. These fields and information about the vertical hydrological structure were utilized by the model as initial conditions and for constraining (nudging) during the spin-up period. After the initial spin-up period, once the model had adjusted to our initial conditions, we performed several free runs.
We expect that our 3D numerical studies of eddy properties will contribute to a better characterisation and understanding of (sub)mesoscale dynamics in the Arctic Ocean and its role in the vertical transport of energy and mass.

How to cite: Kuznetsov, I., Fang, Y.-C., Rabe, B., Androsov, A., Hoppmann, M., Mohrholz, V., Tippenhauer, S., Schulz, K., Fofonova, V., Janout, M. A., Fer, I., Baumann, T., Stanton, T. P., Liu, H., and Mallet, M.: Modeling of the upper ocean winter (sub)mesoscale variability in the central Arctic Ocean during the MOSAiC drift., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12183, https://doi.org/10.5194/egusphere-egu21-12183, 2021.

The Atlantic water boundary current north of Svalbard is a major heat and salt source to the Arctic Ocean. Yet, the mechanisms controlling the lateral transport of Atlantic water properties are not well understood. Model simulations suggest mesoscale eddies may be important for transporting heat away from the boundary current, but supporting observations are sparse.

Between September and November in 2018, a Seaglider was deployed north of Svalbard as part of the Nansen Legacy project to investigate intraseasonal variations in the boundary current and the transformation of Atlantic water. It made several transects across the boundary current and a transect across the Sofia deep. Warm core eddies originating from the boundary current were detected in the Sofia deep. Combining the Seaglider data with two year-long mooring arrays north of Svalbard, deployed in 2018 within the Nansen Legacy framework, we investigate mesoscale eddies using eddy recognition algorithms applied to glider transects and timeseries from moorings. Initial results indicate that mesoscale eddies frequently occur in the boundary current, with radius less than 10 km and velocity maxima as high as 0.35 m/s.

How to cite: Kolås, E. H., Kalhagen, K., Koenig, Z., Fer, I., and Nilsen, F.: Warm core mesoscale eddies along the boundary current and in the Sofia Deep north of Svalbard, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5627, https://doi.org/10.5194/egusphere-egu21-5627, 2021.

In the global ocean, mesoscale eddies are routinely observed from satellite observation. In the Arctic Ocean, however, their observation is impeded by the presence of sea ice, although there is a growing recognition that eddy may be important for the evolution of the sea ice cover. In this talk, we will present a new method of surface ocean eddy detection based on their signature in sea ice vorticity retrieved from Synthetic Aperture Radar (SAR) images. A combination of Feature Tracking and Pattern Matching algorithm is used to compute the sea ice drift from pairs of SAR images. We will mostly focus on the case of one eddy in October 2017 in the marginal ice zone of the Canadian Basin, which was sampled by mooring observations, allowing a detailed description of its characteristics. Although the eddy could not be identified by visual inspection of the SAR images, its signature is revealed as a dipole anomaly in sea ice vorticity, which suggests that the eddy is a dipole composed of a cyclone and an anticyclone, with a horizontal scale of 80-100 km and persisted over a week. We will also discuss the relative contributions of the wind and the surface current to the sea ice vorticity. We anticipate that the robustness of our method will allow us to detect more eddies as more SAR observations become available in the future.

How to cite: Cassianides, A., Lique, C., and Korosov, A.: Oceanic eddy signature on SAR-derived sea ice drift and vorticity, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2233, https://doi.org/10.5194/egusphere-egu21-2233, 2021.

Inflowing Atlantic Water forms a significant heat reservoir in the Arctic Ocean. In the Barents Sea, where the Atlantic Water layer resides close to the surface, strong upward heat fluxes reduce the sea ice cover. Along with a warming climate, an eastward progression of these conditions typical for the Barents Sea is anticipated. These new conditions have the potential to cause dramatic regime shifts in the Laptev Sea region, where the sea ice and the oceanic surface layer are currently sheltered from the warm Atlantic Water by a permanent halocline. Understanding and quantifying the dominant mixing processes in the Siberian Seas is hence crucial to predict how mixing and sea ice conditions, as well as particle and nutrient transport pathways will evolve in the future.

Based on recent temperature and current velocity profiles from this region, we quantify the Atlantic Water heat loss along its pathway around the Arctic basin margins. Contemporaneous turbulent microstructure measurement reveal that only 20% of this heat loss takes place in the deep basin, emphasizing the important role of stronger mixing in the continental slope region. Observed boundary mixing processes include:

• Mixing in the frictional near bottom layer, strongly enhanced at the lee side of a topographic features and where large temperature gradients associated with the upper bound of the Atlantic Water layer are present in the turbulent near bottom layer.

• Spatially confined but energetic mixing events over the whole water column. These events are ephemeral but re-occurring and can homogenize the intermediate water column down to a depth of over 300m, with substantial implications for heat transport, the vertical distribution of nutrients and cross-slope particle transport.

The presented results provide new insights into the complex mixing and transport patterns at the Arctic basin margins, and further emphasize the importance of boundary mixing across disciplines.

How to cite: Schulz, K., Janout, M., Lenn, Y.-D., Ruiz-Castillo, E., Polyakov, I., Mohrholz, V., Tippenhauer, S., Reeve, K. A., Hölemann, J., Rabe, B., and Vredenborg, M.: Boundary current heat loss and mixing processes at the Arctic Ocean continental margin, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10631, https://doi.org/10.5194/egusphere-egu21-10631, 2021.

Ocean turbulence measurements under the Arctic sea ice cover are sparse, especially in winter conditions. During the drift of the MOSAiC main camp, we collected vertical profiles of ocean microstructure in the upper 50-80 m using an ascending vertical microstructure profiler. Each profile terminated when the profiler hit the sea ice or broke through the surface in leads, which resolved the turbulent structure up to the ice or surface. These sporadic profile measurements were supplemented by an ice-moored system equipped with fast-response thermistors, collecting continuous time series at approximately 50 m below the ice. Both instruments are manufactured by Rockland Scientific, Canada. While the profiling was conducted from mid-February to mid-September 2020, the moored measurements were in the period between mid-December 2019 and late April 2020, spatially covering from 88°N30' to 84°N in the Amundsen Basin. From the vertical profiler, dissipation rate of turbulent kinetic energy, ε was estimated using the shear probes and the relatively standard methods applied to shear spectra. From the moored records, ε and dissipation rate of temperature variance, χ, were estimated using the high-resolution temperature records and maximum likelihood spectra fitting to the Batchelor spectrum using 75 s segments. This gives an exceptionally high time resolution of turbulence estimates, albeit from a fixed depth. Estimates ranged between 10^-11 to 10^-6 W/kg for ε , and 10^-12 to 10^-6 C²/s for χ. The vertical distribution of ε in the upper 50 m and the time variability and statistics of moored estimates will be discussed in relation to various environmental forcing conditions including storm events and convection.

How to cite: Fer, I., Baumann, T., Fang, Y.-C., Hoppmann, M., Koenig, Z., Kuznetsov, I., Muilwijk, M., Schaffer, J., Schulz, K., Sukhikh, N., and Tippenhauer, S.: Turbulent structure in the upper ocean during the MOSAiC drift, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4211, https://doi.org/10.5194/egusphere-egu21-4211, 2021.

Ocean mixing governs the vertical exchange of matter, heat and salt in the water column. In the Arctic Ocean, the vertical transport of heat due to turbulent mixing is ultimately coupled to the sea-ice cover, with immediate and far-reaching impacts on the climate and ecosystem. A detailed understanding and quantification of turbulent mixing is crucial to assess and predict the state of the changing Arctic Ocean. However, direct observations of turbulent mixing are complicated, expensive and sparse.

Finescale parameterization of turbulent energy dissipation allows for the quantification of mixing based on standard hydrographic observations such as velocity and density profiles. This method is based on the assumption that energy dissipation is achieved exclusively by cascading energy from large, observable scales to small scales by wave-to-wave interactions in the internal wave field, which in turn can be related to vertical diffusivity and hence turbulent fluxes. While the finescale parameterization is proved to be reliable at mid-latitudes, the Arctic Ocean internal wave field is distinct from the canonical mid-latitude spectrum and the applicability of the parameterization is not certain. Furthermore, in the historically quiescent Arctic, the application of finescale parameterization suffers from a generally low signal to noise ratio and processes violating the assumptions in the parameterization, such as double diffusion.  During the year-long MOSAiC expedition, both standard observations as well as specialized microstructure measurements were carried out continuously. We analyse dissipation rate and stratification measurements (from an MSS90L profiler) and 8-m vertical resolution current measurements (from a 75 kHz RDI acoustic Doppler current profiler) in the depth range from 70 -198 m, in the absence of thermohaline staircases or double-diffusive intrusions. Although the range of dissipation measurements is limited and spans 1e^-11 W kg^-1 to 8.8e^-7 W kg^-1, direct comparisons between in-situ observations of dissipation rate and finescale parameterization provide a detailed insight into the capabilities and limitations of this method in various meteorological, oceanographic and geographic conditions. The aim is to provide guidance in how far standard oceanographic observations may be utilized to quantify mixing in past, current and future states of the Arctic Ocean.

How to cite: Baumann, T., Fer, I., Schulz, K., Mohrholz, V., Schaffer, J., Ying-Chih, F., Hoppmann, M., Kuznetsov, I., Tippenhauer, S., Koenig, Z., and Muilwijk, M.: Quantifying mixing from standard observations: revisiting finescale parameterization in the Arctic Ocean, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14839, https://doi.org/10.5194/egusphere-egu21-14839, 2021.

Sea-ice drift becomes most energetic at last moment in summer when refreezing is about to onset. Perennial ice floes, surviving over all seasons, tend to experience a number of deformation events over yearlong drift, with uneven distribution in thickness. Deformed ice floes protrude tall keels into water of ice-ocean boundary, and then stir it up. Consequently, combination of fast ice drift and deformation-experienced perennial ice could be a primary source of momentum/thermal energy for upper waters through propagation of internal waves. In this study, during MOSAiC expedition, we attempted to perform direct observation of wave generation in ice-ocean boundary layer underneath a drifting ice floe in the central Arctic Ocean. Time series of turbulent signals, represented by Reynolds stress <u'w'> and eddy heat flux <w'T'>, were obtained by an eddy covariance system (ECS), coupling a high-frequency (34 Hz) single-point current meter and a temperature sensor. Vertical/temporal properties of near-inertial waves were obtained by a downward-looking ADCP, collocated with ECS on the same ice floe. At same time, a triangle of high-precision GPS systems tracked ice movement to represent mean drift speed, rotation and deformation about the same floe seamlessly in time. Preliminary analyses of those combined data suggested that pronounced signals of inertial motion occurred in early September of 2020 as sheer ice keels dragged underlying waters, stratified by accumulation of melt water. It then allowed occurrence of near-inertial internal waves that tend to be trapped within the interfacial boundary layer, located within top 20 m. At the conference, we will present latest and quantitative knowledges from the MOSAiC expedition.

How to cite: Kawaguchi, Y., Koenig, Z., Hoppman, M., Nomura, D., Granskog, M., Inoue, J., Katlein, C., and Nicolaus, M. and the MOSAiC OCEAN Team: Interfacial generation of internal waves and turbulent heat flux due to enhanced inertial motion for deformed sea-ice floe: Preliminary results from MOSAiC expedition, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6778, https://doi.org/10.5194/egusphere-egu21-6778, 2021.

The report discusses issues related to the influence of the increased discharge of Arctic rivers on the thermohaline structure of waters outside the Arctic shelf and, in particular, on the variability of Arctic Ocean heat content. The three-dimensional numerical model of the ocean and sea ice SibCIOM (Siberian Coupled Ice-Ocean Model), developed at the Institute of Computational Mathematics and Mathematical Geophysics SB RAS to study the climatic variability of the Arctic Ocean, and the NCEP/NCAR atmospheric reanalysis data are used.

To reveal the sensitivity of the model fields to the intensity of river runoff, numerical experiments assume the inclusion of variations in river discharge with unchanged remaining conditions, starting from 2000. The deviations of the monthly average values in a numerical experiment with increased discharge of individual Arctic rivers from the basic situation based on the monthly average climatic runoff assignment are considered.

An analysis of the numerical results obtained with increased discharge of the major Siberian rivers (Ob, Yenisei, Lena) by 1.3 times showed an increase in the Kara Sea's bottom temperature. This was followed by the warming of the subsurface layer of the waters propagating along the continental slope and increasing the heat content of the upper 200-meter layer of the Eastern Eurasian Basin. The heat preservation entering the deep-water part through the Kara Sea straits was facilitated by an increase in stratification's stability and a decrease of the mixed layer depth by 5-10 m on the continental slope of the Eurasian Basin. A similar process with a time delay (6-7 years) and on a smaller scale is developing on the Amerasian basin's continental slope and the Chukchi Sea shelf.

In the numerical experiment with an increased discharge of the Mackenzie River, deviations in the Beaufort Sea heat and freshwater content appear during the first two years. Still, their values are too small under the river's small discharge compared to the Siberian rivers' discharge.

The study is supported by the Russian Foundation for Basic Research, Grant No. 20-05-00536 A.

How to cite: Tarkhanova, M. and Golubeva, E.: Analysis of the effects of increased river runoff on the Arctic Ocean hydrology using numerical modeling, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14113, https://doi.org/10.5194/egusphere-egu21-14113, 2021.

The annual runoff of river water into the Laptev Sea is 745 km³, most of the runoff belongs to the Lena River - 525 km³.  Long-term variability in the volume of the Lena River runoff play a significant role in the variability of the scale of distribution of freshwater lenses in the Laptev Sea. The processes that take place in the area of ​​intense river runoff have an impact both in the shelf zone and in the open part of the sea due to the transfer of large-area lenses of freshened water. The influence of river runoff is considered from the Lena Delta to the continental slope of the Laptev Sea.

The data on physical and chemical properties of the Laptev Sea shelf used in this investigation was obtained during the expeditions of the Shirshov Institute of Oceanology in 2015 and 2017 and the Pacific Oceanological Institute in 2018-2020.

The distribution of hydrochemical parameters in the Lena Delta area in 2019 was typical for the river-sea mixing zone. The distribution of silicate was mixed, i.e. horizontal stratification prevailed in the near-surface layers, and vertical stratification in the bottom layers. The maximum values ​​were observed in the near-mouth area, reaching indicators over 30 µM / L, which generally coincides with the values ​​of this indicator in 2015 and more than in 2017.

When considering the distribution of specific alkalinity (total alkalinity-salinity ratio), which serves as a proxie of riverine water, it is worth noting the deepening of the boundary by 0.07 units. In 2019, this border was at depths of 20 to 40 meters, which is an atypical indicator for this water area. Apparently, this has happened owing to an increase in the supply of carbonate ions, which is noticeable from an increase in the values ​​of carbonate alkalinity in the Lena River waters (Arctic Great Rivers Observatory data).

The calculation of the parts of fresh water, based on salinity data in 2019, showed that the maximum values ​​were observed near the Lena River delta and amounted to 30-35%. Northward, the part of riverine water was up to 10% only in the surface layer. Comparing with similar calculations performed for the 2015 and 2017 sections, it should be noted that the part of fresh water has decreased. Perhaps this is due to the inflow of continental runoff in 2019 was the lowest over the considered period.

Funding: The work was carried out within the framework of the Shirshov Institute of Oceanology state assignment (theme No. 0149-2019-0008), with funding of the Russian Scientific Foundation (project No. 19-17-00196) and the grant of the President of the Russian Federation MK-860.2020.5.

How to cite: Rogozhin, V., Polukhin, A., Yakushev, E., and Semiletov, I.: Features of the Lena River runoff influence on the adjacent Laptev Sea shelf, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14630, https://doi.org/10.5194/egusphere-egu21-14630, 2021.

As a result of the analysis of the NOAA surface temperature observational data (Huang et al., 2020), the periods corresponding to "marine heatwaves" in the northeastern Pacific Ocean (2013-2019) were identified. Marine heatwaves were defined as exceeding the 90th percentile threshold. The same analysis of the temperature in the Bering Strait's immediate vicinity showed anomalously warm waters in the same years. Analysis of the pressure field, which forms the atmosphere's dynamic state and affects the water circulation system of the Bering Sea, allowed us to assume the inflow of anomalously warm Pacific waters into the Chukchi Sea. To analyze the North Pacific heatwaves' consequences for the Arctic Ocean, we carried out two numerical experiments using the regional ocean and sea ice model SibCIOM (Golubeva et al., 2018) and NCEP/NCAR atmospheric reanalysis data (Kalnay et al., 1996). The first numerical experiment was carried out to calculate hydrodynamic and ice fields from January 2000 to November 2020 (Experiment 1). On the Arctic and the Pacific Ocean boundary in the Bering Strait, we used the monthly average climatic values ​​of the transport, temperature, and salinity of waters coming from the Pacific Ocean. Experiment 2 was carried out from 2014 to November 2020. The calculated values ​​of hydrological and ice characteristics obtained in Experiment 1 were used as the initial state for this experiment. In contrast to Experiment 1,  a heat flux exceeding the average climatic values ​​was set at the Bering Strait in Experiment 2. Its assignment was provided by using temperature values ​​from observational data in the Bering Strait vicinity (Huang et al., 2020). Comparison of monthly average hydrological and ice fields obtained in two numerical experiments and analysis of numerical results showed that an increase in the temperature of the Pacific waters entering the Arctic shelf through the Bering Strait leads to an increase in the heat content of the Chukchi Sea waters, heat transfer by currents in the surface and subsurface layers, a gradual increase in the heat content of the Beaufort Sea, and the reduction of Arctic ice cover. The increase in heat content in Experiment 2 for the Beaufort Sea was obtained in both the upper 50-meter and 250-meter layers.

The research is supported by the Russian Science Foundation, grant №. 19-17-00154.

How to cite: Golubeva, E., Platov, G., and Kraineva, M.: Numerical modeling of the consequences of "marine heatwaves" in the North Pacific for the Arctic Ocean, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6921, https://doi.org/10.5194/egusphere-egu21-6921, 2021.

How to cite: Bertosio, C., Provost, C., Sennéchael, N., Artana, C., Athanase, M., Boles, E., Lellouche, J.-M., and Garric, G.: The Western Eurasian Basin Halocline in 2017: Insights From Autonomous NO Measurements and the Mercator Physical System, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7983, https://doi.org/10.5194/egusphere-egu21-7983, 2021.

The rapidly changing conditions of the Arctic sea ice system have cascading impacts on the biogeochemical cycles of the ocean. Sea ice transports sediments, nutrients, trace metals, pollutants, and gases from the extensive continental shelves into the more isolated central basins. However, it is difficult to assess the net contribution of this supply mechanism on nutrients in the surface ocean. In this study, we used Manganese (Mn), a micronutrient and tracer which can integrate source fluctuations in space and time, to understand the net impact of the long range transport of sea ice for Mn.

We developed a three-dimensional dissolved Mn model within a subdomain of the 1/12 degree Arctic and Northern Hemispheric Atlantic (ANHA12) configuration of NEMO centred on the Canadian Arctic Archipelago, and evaluated this model with in situ observations from the 2015 Canadian GEOTRACES cruises. The Mn model incorporates parameterizations for the contributions from river discharge, sediment resuspension, atmospheric deposition of aerosols directly to the ocean and via melt from sea ice, release of sediment from sea ice, and reversible scavenging, while the NEMO-TOP engine takes care of the advection and diffusion of the tracers. 

Simulations with this model from 2002 to 2019 indicate that the majority of external Mn contributed annually to the Canada Basin surface is released by sediment from sea ice, much of which originates from the Siberian shelves. Reduced sea ice longevity in the Siberian shelf regions has been postulated to result in the disruption of the long range transport of sea ice by the transpolar drift. This reduced sea ice supply has the potential to decrease the Canada Basin Mn surface maximum and downstream Mn supply, with implications for other nutrients (such as Fe) contained in ice-rafted sediments as well. These results demonstrate some of the many changes to the biogeochemical supply mechanisms expected in the near-future in the Arctic Ocean and the subpolar seas.

How to cite: Rogalla, B., Allen, S. E., Colombo, M., Myers, P. G., and Orians, K. J.: Dirty sea ice drives higher Mn concentrations in the Canada Basin, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8940, https://doi.org/10.5194/egusphere-egu21-8940, 2021.

Recently, the Arctic has undergone substantial changes in sea ice cover and the hydrologic cycle, both of which strongly impact the freshwater storage in, and export from, the Arctic Ocean. Here we analyze Arctic freshwater storage and fluxes in 7 climate models from the Coupled Model Intercomparison Project phase 6 (CMIP6) and assess their agreement over the historical period (1980-2000) and in two future emissions scenarios, SSP1-2.6 and SSP5-8.5. In the historical simulation, few models agree closely with observations over 1980-2000. In both future scenarios the models show an increase in liquid (ocean) freshwater storage in conjunction with a reduction in solid storage and fluxes through the major Arctic gateways (Bering Strait, Fram Strait, Davis Strait, and the Barents Sea Opening) that is typically larger for SSP5-8.5 than SSP1-2.6. The liquid fluxes through the gateways exhibit a more complex pattern, with models exhibiting a change in sign of the freshwater flux through the Barents Sea Opening and little change in the flux through the Bering Strait in addition to increased export from the remaining straits by the end of the 21st century. A decomposition of the liquid fluxes into their salinity and volume contributions shows that the Barents Sea flux changes are driven by salinity changes, while the Bering Strait flux changes are driven by compensating salinity and volume changes. In the straits west of Greenland (Nares, Barrow, and Davis straits), the models disagree on whether there will be a decrease, increase, or steady liquid freshwater export in the early to mid 21st century, although they mostly show increased liquid freshwater export in the late 21st century. The underlying cause of this is a difference in the magnitude and timing of a simulated decrease in the volume flux through these straits. Although the models broadly agree on the sign of late 21st century storage and flux changes, substantial differences exist between the magnitude of these changes and the models’ Arctic mean states, which shows no fundamental improvement in the models compared to CMIP5.

How to cite: Zanowski, H., Jahn, A., and Holland, M.: Arctic Freshwater in CMIP6: Declining Sea Ice, Increasing Ocean Storage and Export, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6391, https://doi.org/10.5194/egusphere-egu21-6391, 2021.

Arctic summer sea ice has decreased dramatically over the last few decades, particularly in the summer months. The observed decline is faster than most CMIP5 models, but if internal variability is considered, models and observations are not inconsistent. With only one realization of reality in observations, it is difficult to disentangle the role of internal variability from the forced response. We directly compare one metric of internal variability by resampling both observations and models. So far we have compared five CMIP5 models from the CLIVAR multi-model large ensemble archive (CanESM2, CESM1, CSIRO MK36, GFDL ESM2M, and MPI ESM1). For the pan-Arctic, these models were found to have higher internal variability than observed by approximately 10-50% across models and seasons. Spatially, we find the variability in ice edge region is consistently modelled well in March. In September, although the member mean of the models shows both smaller absolute declines and smaller variation of such declines with resampling, the models have at least one member consistent with observations. This allows us to conclude that the models’ representation of this specific metric of internal variability is consistent with observations.

How to cite: Wyburn-Powell, C., Jahn, A., and England, M.: Realism of simulated internal variability in Arctic sea ice, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6215, https://doi.org/10.5194/egusphere-egu21-6215, 2021.

How to cite: Petty, A., Keeney, N., Cabaj, A., Kushner, P., Kurtz, N., Bagnardi, M., and Kwok, R.: Winter Arctic sea ice freeboard, snow depth and thickness variability from ICESat-2 and NESOSIM, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13779, https://doi.org/10.5194/egusphere-egu21-13779, 2021.

Global warming in the recent decades has been accompanied by a rapid recline of the Arctic sea ice area most pronounced in summer (10% per decade). To understand the relative contribution of external forcing and natural variability to the modern and future sea ice area changes, it is necessary to evaluate a range of long-term variations of the Arctic sea ice area in the period before a significant increase in anthropogenic emissions of greenhouse gases into the atmosphere. Available observational data on the spatiotemporal dynamics of Arctic sea ice until 1950s are characterized by significant gaps and uncertainties. In the recent years, there have appeared several reconstructions of the early 20^th century Arctic sea ice area that filled the gaps by analogue methods or utilized combined empirical data and climate model’s output. All of them resulted in a stronger that earlier believed negative sea ice area anomaly in the 1940s concurrent with the early 20^th century warming (ETCW) peak. In this study, we reconstruct the monthly average gridded sea ice concentration (SIC) in the first half of the 20th century using the relationship between the spatiotemporal features of SIC variability, surface air temperature over the Northern Hemisphere extratropical continents, sea surface temperature in the North Atlantic and North Pacific, and sea level pressure. In agreement with a few previous results, our reconstructed data also show a significant negative anomaly of the Arctic sea ice area in the middle of the 20th century, however with some 15% to 30% stronger amplitude, about 1.5 million km² in September and 0.7 million km² in March. The reconstruction demonstrates a good agreement with regional Arctic sea ice area data when available and suggests that ETWC in the Arctic has been accompanied by a concurrent sea ice area decline of a magnitude that have been exceeded only in the beginning of the 21^st century.

How to cite: Semenov, V. and Matveeva, T.: Gridded Arctic sea ice concentration reconstruction for the first half of the 20th century based on different proxy data, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3106, https://doi.org/10.5194/egusphere-egu21-3106, 2021.

The recent retreat of Arctic sea ice area is overlaid by strong internal variability on all timescales. In winter, sea ice retreat and variability are currently dominated by the Barents Sea, primarily driven by variable ocean heat transport from the Atlantic. Climate models from the latest intercomparison project CMIP6 project that the future loss of winter Arctic sea ice spreads throughout the Arctic Ocean and, hence, that other regions of the Arctic Ocean will see increased sea-ice variability. It is, however, not known how the influence of ocean heat transport will change, and to what extent and in which regions other drivers, such as atmospheric circulation or river runoff into the Arctic Ocean, will become important. Using a combination of observations and simulations from the Community Earth System Model Large Ensemble (CESM-LE), we analyze and contrast the present and future regional drivers of the variability of the winter Arctic sea ice cover. We find that for the recent past, both observations and CESM-LE show that sea ice variability in the Atlantic and Pacific sector of the Arctic Ocean is influenced by ocean heat transport through the Barents Sea and Bering Strait, respectively. The two dominant modes of large-scale atmospheric variability – the Arctic Oscillation and the Pacific North American pattern – are only weakly related to recent regional sea ice variability. However, atmospheric circulation anomalies associated with regional sea ice variability show distinct patterns for the Atlantic and Pacific sectors consistent with heat and humidity transport from lower latitudes. In the future, under a high emission scenario, CESM-LE projects a gradual expansion of the footprint of the Pacific and Atlantic inflows, covering the whole Arctic Ocean by 2050-2079. This study highlights the combined importance of future Atlantification and Pacification of the Arctic Ocean and improves our understanding of internal climate variability which essential in order to predict future sea ice changes under anthropogenic warming.   

How to cite: Dörr, J., Årthun, M., Eldevik, T., and Madonna, E.: Atmospheric and oceanic drivers of regional Arctic winter sea-ice variability in present and future climates, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2231, https://doi.org/10.5194/egusphere-egu21-2231, 2021.

This modelling study reveals that the changes in the ocean state induced by wind perturbations can significantly influence the Arctic sea ice drift, thickness, concentration and deformation rates even after the wind perturbations have been eliminated for years. Wind perturbations can change the Arctic Ocean liquid freshwater content locally or basin-wide, thus changing the sea surface height and ocean surface geostrophic current accordingly. Such changes in the ocean can last for many years, which enforces long-lasting and strong imprint on sea ice. Both the changes in sea surface height gradient force (due to changes in sea surface height) and ocean-ice stress (due to changes in ocean geostrophic velocity) are found to be important in determining the overall impacts on sea ice. Depending on the preceding atmospheric mode driving the ocean, the ocean’s memory of wind forcing can lead to changes in Arctic sea ice characteristics with very different spatial patterns. We identified these spatial patterns associated with Arctic Oscillation, Arctic Dipole Anomaly and Beaufort High modes through dedicated numerical simulations in this study. Our results suggest the importance of initial ocean state in sea ice prediction on subseasonal to decadal time scales.

How to cite: Wang, Q., Danilov, S., Mu, L., Sidorenko, D., and Wekerle, C.: Long-lasting impacts of winds on Arctic sea ice through the ocean’s memory, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6699, https://doi.org/10.5194/egusphere-egu21-6699, 2021.

Arctic sea ice is rapidly decreasing during the recent period of global warming. One of the significant factors of the Arctic sea ice loss is oceanic heat transport from lower latitudes. For months of sea ice formation, the variations in the sea surface temperature over the Pacific Arctic region were highly correlated with the Pacific Decadal Oscillation (PDO). However, the seasonal sea surface temperatures recorded their highest values in autumn 2018 when the PDO index was neutral. It is shown that the anomalous warm seawater was a rapid ocean response to the southerly winds associated with episodic atmospheric blocking over the Bering Sea in September 2018. This warm seawater was directly observed by the R/V Mirai Arctic Expedition in November 2018 to significantly delay the southward sea ice advance. If the atmospheric blocking forms during the PDO positive phase in the future, the annual maximum Arctic sea ice extent could be dramatically reduced.

How to cite: Kodaira, T., Waseda, T., Nose, T., and Inoue, J.: Record high Pacific Arctic seawater temperatures and delayed sea ice advance in response to episodic atmospheric blocking, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9312, https://doi.org/10.5194/egusphere-egu21-9312, 2021.

Over the past few decades, Arctic sea ice volume has been decreasing faster in summer than winter; winter sea ice growth has been increasing, helping to restore the ice pack, despite the fact that Arctic warming is most intense in the winter. This raises the questions: why? And for how long can we expect winter ice growth to keep increasing? We pose these questions with a regional focus on the Kara and Laptev seas. These seas are often termed the ice factories of the Arctic because of their outsized contributions to the Arctic sea ice budget, a consequence of their divergent settings. Using the CESM climate model ensemble, we separate out the influence of different levers on ice factory productivity (the ice growth rate), and show that 20th Century and RCP8.5 changes can be skilfully reconstructed by a linear model incorporating 2 m temperature, snow thickness, September sea ice area, total (gross) divergence and ice export. Ocean temperatures, meanwhile, help to explain the timing of the onset of freezing. Increasing air temperatures naturally decrease the growth rate, while positive contributions to growth rate are made by a decreasing September sea ice area, increasing divergence and increasing export. These positive influences are all associated with a thinning, more mobile ice pack: they are negative feedbacks on sea ice loss. In CESM, once the September sea ice area in the Kara-Laptev seas approaches zero, the year-on-year productivity of the ice factories starts to decline. We place these results in the context of observations and discuss the prospects for the productivity of the Arctic Ocean’s ice factories.

How to cite: Cornish, S., Johnson, H., Richards, A., Kostov, Y., and Dörr, J.: Decline of the Arctic 'ice factories' delayed by negative feedbacks, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10387, https://doi.org/10.5194/egusphere-egu21-10387, 2021.