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This paper presents a methodological tool for dynamic reconstruction of the state of the ocean, based, as an example, on observations from the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) experiment. The data used in this study were collected in the Amundsen Basin between October 2019 and January 2020. Analysing observational data to assess tracer field and upper-ocean dynamics is highly challenging when measurement platforms drift with the ice pack due to continuous drift speed and direction changes. We have equipped the new version of the coastal branch of the global Finite-volumE sea ice–Ocean Model (FESOM-C) with a nudging method. Model nudging was carried out assuming a quasi-steady state. Overall, the model can reproduce the lateral and vertical structure of the temperature, salinity, and density fields, which allows for projecting dynamically consistent features of these fields onto a regular grid. We identify two separate depth ranges of enhanced eddy kinetic energy located around two maxima in buoyancy frequency: the depth of the upper halocline and the depth of the warm (modified) Atlantic Water. Simulations reveal a notable decrease in surface layer salinity and density in the Amundsen Basin towards the north but no significant gradient from east to west. However, we find a mixed-layer deepening from east to west, with a 0.084 m km -1 gradient at 0.6 m km -1 standard deviation, compared to a weak deepening from south to north. The model resolves several stationary eddies in the warm Atlantic Water and provides insights into the associated dynamics. The model output can be used to further analyse the thermohaline structure and related dynamics associated with mesoscale and submesoscale processes in the central Arctic, such as estimates of heat fluxes or mass transport. The developed nudging method can be utilized to incorporate observational data from a diverse set of instruments and for further analysis of data from the MOSAiC expedition. [ABSTRACT FROM AUTHOR]

Trace gases have demonstrated their strength for oceanographic studies, with applications ranging from the tracking of glacial meltwater plumes to estimates of the abyssal overturning duration. Yet measurements of such passive tracers in the ice-covered Arctic Ocean are sparse. We here present a unique data set of trace gases collected during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition, during which R/V Polarstern drifted along with the Arctic sea ice from the Laptev Sea to Fram Strait, from October 2019 to September 2020. During the expedition, trace gases from anthropogenic origin (chlorofluorocarbon 12 (CFC-12), sulfur hexafluoride (SF 6), and tritium) along with noble gases (helium and neon) and their isotopes were collected at a weekly or higher temporal resolution throughout the entire water column (and occasionally in the snow) from the ship and from the ice. We describe the sampling procedures along with their challenges, the analysis methods, and the data sets, and we present case studies in the central Arctic Ocean and Fram Strait to illustrate possible usage for the data along with their robustness. Combined with simultaneous hydrographic measurements, these trace gas data sets can be used for process studies and water mass tracing throughout the Arctic in subsequent analyses. The two data sets can be downloaded via PANGAEA: 10.1594/PANGAEA.961729 (Huhn et al., 2023a) and 10.1594/PANGAEA.961738 (Huhn et al., 2023b). [ABSTRACT FROM AUTHOR]

This paper investigates the role of semidiurnal tides in the Hanna Shoal region on the northeastern Chukchi Sea shelf to evaluate their impact on the regional shelf dynamics. The study is based on 2‐year long velocity time series from five oceanographic moorings. These records indicate the dominance of wind‐generated near‐inertial energy during the summer season with low ice cover. However, when the ocean is fully covered by sea ice, tides dominate the variability in the semidiurnal energy band. The records reveal considerable seasonal variability as well as regional differences, where barotropic tides dominate in the well‐mixed waters west of Hanna Shoal while bottom‐trapped internal (depth‐dependent) tides are observed east of Hanna Shoal, where stratification can persist year‐round. Resulting tide‐driven shear in winter east of Hanna Shoal under stratified conditions can occasionally reach the level of a Richardson number below 1 and can be as low as ∼0.25, implying the likelihood of shear instability and potentially eroding lower water column stability. This may lead to upward fluxes of near‐bottom nutrient‐rich Winter Water and thus carries ecosystem implications. Our study indicates that the internal tides east of Hanna Shoal are modulated by the spring‐neap cycle and result from the interaction of barotropic tides with local bathymetry and stratification. Plain Language Summary: We investigated currents, temperature, and salinity records from five oceanographic moorings around Hanna Shoal in the northeast Chukchi Sea in the Arctic Ocean. The data indicate different water column structures and therefore different vertical structures of the tides between the east and west sides of Hanna Shoal. The water column on the west side is mainly homogeneous during winter, while the east side can remain stratified year‐round. The presence of stratification then leads to vertical differences in tidal currents, which are larger in the lower part of the water column. Overall, our results indicate that tides interact with stratification and bottom topography east of Hanna Shoal. The resultant vertical tidal velocity differences have a potential to promote water exchange in the water column, even underneath an ice‐covered ocean in winter and spring. This mechanism may supply important nutrients to the sunlit upper ocean in the spring and summer, and is thus potentially important for the Chukchi Sea ecosystem. Key Points: Tides dominate the semidiurnal variability of the Hanna Shoal region in winter during periods of high sea ice coverSemidiurnal internal tides exist in the eastern Hanna Shoal region, while the tides are largely barotropic in the westSemidiurnal internal tides induce current shear that, potentially, erodes lower‐layer stratification in winter [ABSTRACT FROM AUTHOR]

Measurements targeting mesoscale and smaller-scale processes in the ice-covered part of the Arctic Ocean are sparse in all seasons. As a result, there are significant knowledge gaps with respect to these processes, particularly related to the role of eddies and fronts in the coupled ocean–atmosphere–sea ice system. Here we present a unique observational dataset of upper ocean temperature and salinity collected by a set of buoys installed on ice floes as part of the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) Distributed Network. The multi-sensor systems, each equipped with five temperature and salinity recorders on a 100 m long inductive modem tether, drifted together with the main MOSAiC ice camp through the Arctic Transpolar Drift between October 2019 and August 2020. They transmitted hydrographic in situ data via the iridium satellite network at 10 min intervals. While three buoys failed early due to ice dynamics, five of them recorded data continuously for 10 months. A total of four units were successfully recovered in early August 2020, additionally yielding internally stored instrument data at 2 min intervals. The raw data were merged, processed, quality controlled, and validated using independent measurements also obtained during MOSAiC. Compilations of the raw and processed datasets are publicly available at 10.1594/PANGAEA.937271 and 10.1594/PANGAEA.940320 , respectively. As an important part of the MOSAiC physical oceanography program, this unique dataset has many synergies with the manifold co-located observational datasets and is expected to yield significant insights into ocean processes and to contribute to the validation of high-resolution numerical simulations. While this dataset has the potential to contribute to submesoscale process studies, this paper mainly highlights selected preliminary findings on mesoscale processes. [ABSTRACT FROM AUTHOR]

We examined mixing processes within the ice–ocean boundary layer (IOBL) close to the geographic North Pole, with an emphasis on wind‐driven sea ice drift. Observations were conducted from late August to late September 2020, during the final leg of the international Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition. Measurements of ice motion, and profiles of currents, hydrography, and microstructure turbulence were conducted. The multifarious direct observations of sea ice and the upper ocean were used to quantify the transport of momentum, heat, and salt in the IOBL. The ice drift was mostly characterized by the inertial oscillation at a semi‐diurnal frequency, which forced an inertial current in the mixed layer. Observation‐derived heat and salinity fluxes at the ice–ocean interface suggest early termination of basal melting and transitioning to refreezing, resulting from a rise in the freezing point temperature by the presence of freshened near‐surface water. Based on the friction velocity, the measured dissipation rate (ε) of turbulent energy can be approximated as 1.4–1.7 times of the "Law of the Wall" criterion. We also observed a spiraling Ekman flow and find its vertical extent in line with the estimate from ε‐based diffusivity. Following passage of a storm, the enhanced oscillatory motions of the ice drift caused trapping of the near‐inertial waves (NIWs) that exclusively propagated through the base of the weakly stratified mixed layer. We accounted Holmboe instabilities and NIWs for the observed distinct peak of the dissipation rate near the bottom of the mixed layer. Plain Language Summary: We examined how sea ice drift in the Arctic Ocean affects the movement of seawater directly under the ice, and how this impacts freezing and melting of the ice itself. During the Multidisciplinary drifting Observatory for the Study of Arctic Climate expedition, we used a number of different instruments to measure air, sea ice and ocean properties on an ice floe between late August and late September 2020. We recorded the drift tracks of the ice, and linked the ice motion to the currents, temperature and salinity within the upper 50 m of the ocean. Strong winds triggered wavy fluctuations and water mixing, in particular close to where the ice and ocean meet. During a storm in mid‐September, the ice stopped melting and started to refreeze even though the water and the air were still relatively warm. We explain this by the presence of surface water that was less salty, and therefore froze faster at higher temperature, and by the ice moving faster than usually observed in the region. In combination, these factors provided favorable conditions for sea ice formation. Our results suggest that the distribution of sea ice meltwater need to be accounted for in order to better predict Arctic sea ice conditions in the future. Key Points: Enhanced ice drift and near‐surface freshened water jointly promoted early termination of basal melting and preconditioning of refreezingTurbulent dissipation rate can be scaled by 1.4–1.7 times of the "Law of the Wall" criterion, with surface buoyancy flux being negligibleNear‐inertial wave was trapped by weakly stratified water in lower mixed layer, which produced turbulence by the Holmboe instability [ABSTRACT FROM AUTHOR]

The formation of platelet ice is well known to occur under Antarctic sea ice, where subice platelet layers form from supercooled ice shelf water. In the Arctic, however, platelet ice formation has not been extensively observed, and its formation and morphology currently remain enigmatic. Here, we present the first comprehensive, long‐term in situ observations of a decimeter thick subice platelet layer under free‐drifting pack ice of the Central Arctic in winter. Observations carried out with a remotely operated underwater vehicle (ROV) during the midwinter leg of the MOSAiC drift expedition provide clear evidence of the growth of platelet ice layers from supercooled water present in the ocean mixed layer. This platelet formation takes place under all ice types present during the surveys. Oceanographic data from autonomous observing platforms lead us to the conclusion that platelet ice formation is a widespread but yet overlooked feature of Arctic winter sea ice growth. Plain Language Summary: Platelet ice is a particular type of ice that consists of decimeter sized thin ice plates that grow and collect on the underside of sea ice. It is most often related to Antarctic ice shelves and forms from supercooled water with a temperature below the local freezing point. Here we present the first comprehensive observation of platelet ice formation in freely drifting pack ice in the Arctic in winter during the international drift expedition MOSAiC. We investigate its occurrence under the ice with a remotely controlled underice diving robot. Measurements of water temperature from automatic measurement devices distributed around the central MOSAiC ice floe show that supercooled water and thus platelet ice occur widely in the winter Arctic. This way of ice formation in the Arctic has been overlooked during the last century, as direct observations under winter sea ice were not available and contrary to typical Antarctic observations; manifestation of platelet ice in Arctic ice core stratigraphy has been more challenging to identify. Key Points: Here we present extensive observations of platelet ice formation under Arctic winter sea iceThe subice platelet layer appears to form locally due to seed crystals in ocean surface supercooling [ABSTRACT FROM AUTHOR]

Abstract: We measured northeastern Chukchi Sea surface currents using high‐frequency radar systems (HFR) during the ice‐free periods of August to October from 2010–2014. We analyzed these data, along with regional winds, using Self‐Organizing Maps (SOM) to develop a set of surface current‐wind patterns. Temporal changes in the SOM patterns consist predominantly of two patterns comprising northeastward and southwestward surface currents. A third pattern represents a transitional stage established during the onset of strong northeasterly winds. These patterns are analogous to the first two eigenmodes of an empirical orthogonal function analysis of the HFR data. The first principal component (PC1) is significantly correlated (∼0.8) to that of the winds and is directly related to the time series of SOM‐derived patterns. The sign of PC1 changes when the speed of local northeasterly winds exceeds ∼6 m s−1, at which point the northeastward surface currents reverse to the southwest. This finding agrees with previous models and observations that suggest this wind threshold is needed to overcome the pressure gradient between the Pacific and Arctic Oceans. The transitional stage is characterized by alongshore currents bifurcating in the vicinity of Icy Cape and wind‐driven Ekman currents north of 71.5°N. Its development is a manifestation of interactions among the poleward pressure gradient, wind stress, and geostrophic flow due to the coastal setdown. [ABSTRACT FROM AUTHOR]

This study investigates the applicability of the optimal interpolation (OI) method proposed by Kim et al. for estimating ocean surface currents from high-frequency radar (HFR) in the northeastern Chukchi Sea, where HFR siting is dictated by power availability rather than optimal locations. Although the OI technique improves data coverage when compared to the conventional unweighted least squares fit (UWLS) method, biased solutions can emerge. The quality of the HFR velocity estimates derived by OI is controlled by three factors: 1) the number of available incorporating radials (AR), 2) the ratio of the incorporating radials from multiple contributing site locations [ratio of overlapping radial velocities (ROR) or radar geometry], and 3) the positive definiteness [condition number (CN)] of the correlation matrix. Operationally, ROR does not require knowledge of the angle covariance matrix used to compute the geometric dilution of precision (GDOP) in the UWLS method and can be computed before site selection to optimize coverage or after data processing to assess data quality when applying the OI method. The Kim et al. method is extended to examine sensitivities to data gaps in the radial distribution and the effects on OI estimates. [ABSTRACT FROM AUTHOR]

The northeastern Chukchi Sea shelf is one of the most productive areas in the polar oceans, and a foraging area for large marine mammals such as gray whales and walruses. This ecosystem is fundamentally sustained by the inflow of nutrient-rich Pacific Water (PW) through Bering Strait. PW also affects Arctic climate, as it contributes substantial amounts of freshwater and heat into the basin. We present several new findings derived from multiyear mooring observations with emphasis on the flow and hydrographic characteristics around the previously undersampled Hanna Shoal (HS) region of this shelf. The goal is to better quantify how PW is modified as it flows through the HS pathway. We describe the existence of a zonal density gradient at HS, which evolves seasonally and becomes more pronounced in winter. It appears that the local stratification along the eastern flank of HS is maintained year-round, whereas the water column along the western side becomes well-mixed in winter. This causes a local baroclinic flow that counteracts the background barotropic currents and which suggests that flow convergence and shelf-basin exchange are substantial on the northern side of HS. Our results provide new insights into the complex HS region regarding PW circulation and shelf-basin interaction with the Arctic basin. [ABSTRACT FROM AUTHOR]