The Akademii Nauk ice core and solar activity

Ice cores are well established archives for paleo-environmental studies, but this requires a reliable ice core chronology. The concentration of cosmogenic radionuclides in ice cores reflects the solar activity in the past and can be used as dating tool for ice cores. Accelerator mass spectrometry (AMS) allows the determination of nuclides in high resolution. Here we present results of a 10Be study in an ice core from Akademii Nauk (Severnaya Zemlya, Russian Arctic). AMS analyses of more than 500 samples were carried out using the 6 MV accelerator facility of the Ion Beam Centre of the Helmholtz-Zentrum Dresden-Rossendorf. For the time period 400 to 2000 CE the temporal variations of 10Be reflect the centennial variations of solar activity known from similar studies of Greenlandic ice cores and from 14C production reconstructions. The 10Be peak of 775 CE, today understood as result of the strongest known solar particle storm, was found by high resolution core analysis. This peak is used as tie point (additionally to volcanic reference horizons) for the development of the depth-age relationship of the Akademii Nauk ice core. Indications of the so called “Carrington Event” of 1859 CE, 20 to 30 times weaker than 775 CE, could also be detected in the core

Linking sea ice deformation to ice thickness redistribution using high-resolution satellite and airborne observations

An unusual, large, latent-heat polynya opened and then closed by freezing and convergence north of Greenland's coast in late winter 2018. The closing presented a natural but well-constrained full-scale ice deformation experiment. We observed the closing of and deformation within the polynya with satellite synthetic-aperture radar (SAR) imagery and measured the accumulated effects of dynamic and thermodynamic ice growth with an airborne electromagnetic (AEM) ice thickness survey 1 month after the closing began. During that time, strong ice convergence decreased the area of the refrozen polynya by a factor of 2.5. The AEM survey showed mean and modal thicknesses of the 1-month-old ice of 1.96 ± 1.5 m and 1.1 m, respectively. We show that this is in close agreement with modeled thermodynamic growth and with the dynamic thickening expected from the polynya area decrease during that time. We found significant differences in the shapes of ice thickness distributions (ITDs) in different regions of the refrozen polynya. These closely corresponded to different deformation histories of the surveyed ice that we reconstructed from Lagrangian ice drift trajectories in reverse chronological order. We constructed the ice drift trajectories from regularly gridded, high-resolution drift fields calculated from SAR imagery and extracted deformation derived from the drift fields along the trajectories. Results show a linear proportionality between convergence and thickness change that agrees well with the ice thickness redistribution theory. We found a proportionality between the e folding of the ITDs' tails and the total deformation experienced by the ice. Lastly, we developed a simple, volume-conserving model to derive dynamic ice thickness change from the combination of Lagrangian trajectories and high-resolution SAR drift and deformation fields. The model has a spatial resolution of 1.4 km and reconstructs thickness profiles in reasonable agreement with the AEM observations. The modeled ITD resembles the main characteristics of the observed ITD, including mode, e folding, and full width at half maximum. Thus, we demonstrate that high-resolution SAR deformation observations are capable of producing realistic ice thickness distributions

Ocean Variability at Greenland's Largest Glacier Tongue Linked to Continental Shelf Circulation

Increased ocean‐to‐ice heat fluxes play a key role in the accelerated mass loss of Greenland’s marine‐terminating glaciers. Ocean current variability leads to variations in this heat flux. A year‐long time series of ocean currents at all gateways to the ocean cavity under Greenland’s largest remaining floating ice tongue at the Nioghalvfjerdsfjorden Glacier (79NG) was analyzed. The variability of the exchange flow at intra‐annual to near‐daily timescales was characterized. The currents exhibit considerable variability with standard deviations exceeding the time mean flow strength by a factor of 2. The inflow of warm Atlantic Intermediate Water into the cavity and the outflow via the northernmost calving front were directly coupled on intra‐annual timescales (periods, T > 30 days) with enhanced fluctuations in the winter months. A strong correlation between the variability of the deep inflow and currents in the subsurface boundary current on the continental shelf suggests a link between cavity and continental shelf circulation. Variability on higher frequencies (T < 30 days) in the outflow was only partly induced by the inflow variability. Two export branches of the cavity circulation were identified, which were potentially constrained by subglacial meltwater channels. The relative importance of the two export branches varies on monthly time scales. This research has provided evidence that the large intra‐annual ocean current variability at the 79NG is strongly influenced by the continental shelf circulation. Temporally varying preferred export routes increase the complexity of the cavity circulation

Structure and variability of the circulation at tidal to intra-seasonal time scales near the 79 North Glacier

Greenland's largest floating ice tongue at the Nioghalvfjerdsfjorden Glacier (79 North Glacier) is thinning, most likely triggered by enhanced submarine melting. The strength of the cavity circulation beneath the floating ice tongue is essentially responsible for the ocean heat flux to the glacier ice and variability in the circulation may have consequences on the basal melt rate. Analyzing four moored records (summers 2016-2017) from the 79 North Glacier calving front, this study characterizes the variability of the cavity circulation and the relative importance of its local and regional drivers. The focus lies on the variability in the currents in Dijmphna Sund, the most relevant export pathway for glacially modifed waters out of the glacial cavity. Variability is split into three groups. (1) Half of the variance of the time series is concentrated at sub-daily to daily time scales (T = 0.25 - 2 days) and is associated with barotropic tides. (2) Periods of 2-30 days comprise one third of the variance. At those time scales, currents in Dijmphna Sund are weakly linked to sea ice conditions close to the coast and enhanced wind speeds. (3) In periods larger than 30 days, 1/6 of the variance is found. Low-pass filtered Empirical Orthogonal Functions reveal a strong link between the export of glacially modified waters through Dijmphna Sund and the inflow into the glacial cavity. This intra-annual variability of the cavity circulation was also observed by a mooring on the continental shelf 170km south of the 79 North Glacier. The time series from the calving front and the one from the continental shelf are significantly correlated (R = 0.65) and shifted by a time difference of 27 - 29 hours. The time delay fits well to travel times of baroclinic waves. This suggests that large-scale wave activity may be the main driver of the intra-annual variability of the cavity circulation

Let's talk fieldwork: early-career scientists sharing practical knowledge about polar fieldwork

</jats:p

Arctic sea ice thickness variability and change

Arctic sea ice thickness variability and change and their dependence on the atmospheric and oceanic forcing are at the core of research in Subtopic 2.1, Theme: Ongoing and Future Arctic and Antarctic Climate Change. Our research is particularly focused on a better process understanding and representation in models, and observations during MOSAiC play a strong role. The poster gives examples of such process studies focused on Arctic sea ice thickness variability and change. We outline observations of the long-term and regional variability and change of sea ice thickness using satellite remote sensing, airborne surveying, and ice mass balance buoys. Thermodynamic growth and its interaction with the atmosphere over leads and level ice serves as an example for our joint research interests. The poster also gives examples of causes of sea ice thinning, like increased ocean heat flux to the ice due to Atlantification, and consequences, e.g., for reduced sea ice volume transport through Fram Strait

Asymmetries in cloud microphysical properties ascribed to sea ice leads via water vapour transport in the central Arctic

To investigate the influence of sea ice openings like leads on wintertime Arctic clouds, the air mass transport is exploited as a heat and humidity feeding mechanism which can modify Arctic cloud properties. Cloud microphysical properties in the central Arctic are analysed as a function of sea ice conditions during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition in 2019–2020. The Cloudnet classification algorithm is used to characterize the clouds based on remote sensing observations and the atmospheric thermodynamic state from the observatory on board the research vessel (RV) Polarstern. To link the sea ice conditions around the observational site with the cloud observations, the water vapour transport (WVT) being conveyed towards RV Polarstern has been utilized as a mechanism to associate upwind sea ice conditions with the measured cloud properties. This novel methodology is used to classify the observed clouds as coupled or decoupled to the WVT based on the location of the maximum vertical gradient of WVT height relative to the cloud-driven mixing layer. Only a conical sub-sector of sea ice concentration (SIC) and the lead fraction (LF) centred on the RV Polarstern location and extending up to 50 km in radius and with an azimuth angle governed by the time-dependent wind direction measured at the maximum WVT is related to the observed clouds. We found significant asymmetries for cases when the clouds are coupled or decoupled to the WVT and selected by LF regimes. Liquid water path of low-level clouds is found to increase as a function of LF, while the ice water path does so only for deep precipitating systems. Clouds coupled to WVT are found to generally have a lower cloud base and larger thickness than decoupled clouds. Thermodynamically, for coupled cases the cloud-top temperature is warmer and accompanied by a temperature inversion at the cloud top, whereas the decoupled cases are found to be closely compliant with the moist adiabatic temperature lapse rate. The ice water fraction within the cloud layer has been found to present a noticeable asymmetry when comparing coupled versus decoupled cases. This novel approach of coupling sea ice to cloud properties via the WVT mechanism unfolds a new tool to study Arctic surface–atmosphere processes. With this formulation, long-term observations can be analysed to enforce the statistical significance of the asymmetries. Furthermore, our results serve as an opportunity to better understand the dynamic linkage between clouds and sea ice and to evaluate its representation in numerical climate models for the Arctic system.</p

Methane pumping by rapidly refreezing lead ice in the ice-covered Arctic Ocean

If and how the sea ice cycle drives the methane cycle in the high Arctic is an open question and crucial to improving source/sink balances. This study presents new insights into the effects of strong and fast freezing on the physical–chemical properties of ice and offers implications for methane fluxes into and out of newly formed lead ice. During the 2019–2020 transpolar drift of the Multidisciplinary Drifting Observatory for the Study of Arctic Climate (MOSAiC), we took weekly samples of growing lead ice and underlying seawater at the same site between January and March 2020. We analyzed concentrations and stable carbon isotopic signatures (δ13C–CH4) of methane and calculated methane solubility capacities (MSC) and saturation levels in both environments. During the first month, intense cooling resulted in the growth of two-thirds of the final ice thickness. In the second month, ice growth speed decreased by 50%. Both growth phases, disentangled, exposed different freeze impacts on methane pathways. The fast freeze caused strong brine entrapment, keeping the newly formed lead ice permeable for 2 weeks. These physical conditions activated a methane pump. An increased MSC induced methane uptake at the air–ice interface, and the still-open brine channels provided top-down transport to the ocean interface with brine drainage. When the subsurface layer became impermeable, the top-down pumping stopped, but the ongoing uptake induced a methane excess on top. During the second growth phase, methane exchange exclusively continued at the ice–ocean interface. The shift in the relative abundance of the 12C and 13C isotopes between lead ice and seawater toward a 13C-enrichment in seawater reveals brine drainage as the main pathway releasing methane from aging lead ice. We conclude that in winter, refrozen leads temporarily function as active sinks for atmospheric methane and postulate that the relevance of this process may even increase when the Arctic fully transitions into a seasonally ice-covered ocean when leads may be more abundant. To highlight the relevance of methane in-gassing at the air–ice interface as a potential but still unconsidered pathway, we include estimates of the occurrence and frequency of young lead ice from satellite observations of leads during MOSAiC

Kilometer-scale digital elevation models of the sea ice surface with airborne laser scanning during MOSAiC

An integrated sensor platform including an inertial navigation system (INS) and a commercial airborne laser scanner (ALS) among other sensor was mounted in the cargo compartment in one of the Polarstern helicopters during MOSAiC. ALS data was acquired from more than 60 flights between October 2019 and September 2020 with a range of survey types intended to map changes of the sea ice surface during the full annual cycle at high spatial resolution and coverage. Here, we provide an overview of the collected data, the challenge of achieving centimeter elevation accuracy with a helicopter platform at high polar latitudes as well as the content and specifications of ALS data products. The high spatial resolution and repeated coverage of the larger area around Polarstern allow studying various surface features (e.g. pressure ridges, floes, melt ponds, snow drifts, etc.), their seasonal evolution, and their impact on atmosphere and ocean. Finally, we outline methods for planned applications, such as identifying individual floes and surface types using both measured freeboard and surface reflectance. Collocated helicopter-based optical and infrared imagery allow analyzing sea ice properties in further applications and to upscale comparable in-situ observations