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Presentation of SIMBA buoy ice & snow thickness retrievals(first preliminary version for Arctic PASSION) in relation to the MOSAiC drift experiment in 2019/2020. Part of the session on "MOSAiC Observations in the Context of Historical Data" during the 2nd MOSAiC Science conference in Boulder, Colorado, US (February 13-17, 2023). Conference website: 2nd MOSAiC Science Conference &nbsp

Plastic pollution has become ubiquitous even in remote polar ecosystems such as the Arctic. Previous research at HAUSGARTEN observatory with Fram infrastructure has shown that microplastic concentrations are particularly high in water and sediment samples taken close to the marginal ice zone and in sea ice cores and zooplankton grazers associated with sea ice. Here, we analysed samples of the ice algae Melosira arctica that attach to ice floes and often form long filaments, curtains, nets and other growth forms, which could capture microplastics released from melting sea ice. Samples and field and procedural blanks were taken opportunistically in Fram Strait during a helicopter-based ice sampling campaign in summer 2021 and analysed by µ-Raman. Microplastics were found in all samples in significant quantities across a variety of polymer types and particles sizes. Whilst more data are needed our results indicate that ice algae could be an important sink and also accelerate the transport of microplastics to the deep seafloor, since ice algae are known to form aggregates that accelerate their sinking rates. Ice algae could thus also be vectors for microplastics into benthic or zooplankton food webs. Also see: https://micro2022.sciencesconf.org/427180/document, In MICRO 2022, Online Atlas Edition: Plastic Pollution from MACRO to nano

Sea-ice thickness is a key factor and indicator in understanding the impact of the global climate change. Deriving basin-wide sea-ice thickness estimates from satellite laser and radar altimetry relies on freeboard measurements. The freeboard-to-thickness conversion in turn requires information of snow mass and the density of the sea-ice layer that have unknown spatio-temporal variabilities and trends directly translating into the uncertainty of decadal sea-ice thickness data records. In addition, inter-mission biases arise from, e.g., different sensor types and frequencies as well as varying footprint sizes affected by surface roughness across regions and seasons. Therefore, carrying out validation and inter-calibration studies is crucial for reliable and continuous observation of the Earth’s cryosphere. To achieve this, it is beneficial to have simultaneous measurements of freeboard, snow depth, and sea-ice thickness, which provide reference data for both direct satellite observations and geophysical target parameters. Here, we present Alfred Wegener Institute’s (AWI) IceBird program, which is a series of fixed-wing aircraft campaigns to measure Arctic sea ice and to monitor its change. During two late-winter campaigns in the western Arctic Ocean in 2017 and 2019, we have carried out surveys with the unique scientific instrument configuration including an airborne laser scanner (ALS) for surface topography and freeboard measurements, a tethered electromagnetic induction sounding instrument (EM-Bird) for total (snow+ice) thickness measurements, and an ultrawideband frequency-modulated continuous-wave microwave radar to measure snow thickness. Therefore, we are able to observe all three bounding interfaces in the sea-ice–snow system in high resolution along survey tracks on regional scales. During the ship-based drift expedition Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) between October 2019 and September 2020, helicopter surveys were carried out in high spatio-temporal resolution throughout the year, including the polar night, to measure freeboard and roughness with the ALS both in local grid pattern and in larger scale. Coincident EM-Bird ice thickness data and information from snow measurements on the ground will help linking these parameters and monitor them and their effect on satellite retrievals for a full seasonal cycle. The individual parameters are important for describing and monitoring the state of the Arctic sea ice and validating retrievals from satellite data, but combined they offer further possibilities to characterise sea ice. By assuming isostatic equilibrium, we are able to estimate up-to-date bulk density values for different sea-ice types from the IceBird data and to derive a parametrisation of sea-ice bulk density based on sea-ice freeboard. These data allow us to explore spatio-temporal variations in sea-ice parameters observable from space and to evaluate the validity of the freeboard-to-thickness conversion in satellite altimetry through comparison against dedicated satellite overpasses and orbit collections.

Moored upward-looking Acoustic Doppler Current Profilers (ADCPs) can be used to observe sea ice draft. While previous studies relied on the availability of auxiliary pressure sensors to measure the instrument depth of the ADCP, we present an adaptive approach that infers instrument depth from the ADCPs default bottom track (BT) mode measurements of error velocity and range. We demonstrate that this method can be used to obtain daily mean sea ice draft time series with an estimated uncertainty of 0.1 m. The ADCP-derived ice draft time series are validated with data from adjacent Upward-Looking Sonar (ULS) moorings in the Laptev Sea. This new approach provides a low-cost opportunity to derive daily mean ice draft time series accessing existing ADCP data. Applying this method to ADCP data from the sparsely-sampled Laptev Sea allows an extension of mooring-based in situ ice draft measurements from 2 to about 13 years. The Laptev Sea is an important region for net ice production and a major contributor to the Transpolar Drift System. Recent studies show that sea ice area and volume exports from the Laptev Sea are increasing. This increased export accelerates summer sea ice retreat in the Laptev Sea and has far-reaching consequences for the entire Arctic sea ice balance. The newly acquired Laptev Sea ice draft data archive is used to analyse seasonal and interannual changes in sea ice thickness from 2003 to 2016. In addition, it provides unique data for the comparison with sea ice thickness data records derived from ENVISAT, CryoSat-2 and SMOS satellite measurements in an area where large-scale validation data sets are currently unavailable.

In February and March 2018 a unique polynya of open water had opened in the Wandel Sea north of Greenland due to unusually strong southerly winds. The polynya subsequently refroze and the area of the growing young ice strongly decreased by ice convergence due to northerly winds, constituting a natural, well-constrained, full-scale ice deformation experiment. We have carried out an airborne electromagnetic ice thickness survey of the young ice one month after the polynya began to close. It showed a thickness distribution with a modal ice thickness of 1 m and mean thickness of 2 m, representative of the contributions of thermodynamic and dynamic growth in the one month since the ice had begun to grow. We used time series of Sentinel 1 SAR images to back-track the surveyed young ice to its location of initial formation. Results showed that the area of young ice approximately halved between the time of its formation and the time of the ice thickness survey. This is in good qualitative agreement with the result of the thickness survey, showing that its mean thickness was twice its modal thickness, i.e. mean ice thickness doubled while the region's area halved. These results and the SAR observed deformation processes provide valuable information for improving representation of ice rheology and thickness redistribution in sea ice models.

This sixth World Ocean Review (WOR) focuses on the Arctic and the Antarctic – two regions which are, in a very real sense, polar opposites, with some of the world’s most extreme conditions. Besides presenting a wealth of facts and figures about the history and exploration of the polar regions, WOR 6 builds a deeper awareness of their key role for life on our planet. It highlights the changes that can be observed in their flora and fauna and analyses the already dramatic impacts of global warming on these extremely fragile regions.

In recent years, sea-ice conditions in the Arctic Ocean changed substantially toward a younger and thinner sea-ice cover. To capture the scope of these changes and identify the differences between individual regions, in situ observations from expeditions are a valuable data source. We present a continuous time series of in situ measurements from the N-ICE2015 expedition from January to June 2015 in the Arctic Basin north of Svalbard, comprising snow buoy and ice mass balance buoy data and local and regional data gained from electromagnetic induction (EM) surveys and snow probe measurements from four distinct drifts. The observed mean snow depth of 0.53 m for April to early June is 73% above the average value of 0.30 m from historical and recent observations in this region, covering the years 1955–2017. The modal total ice and snow thicknesses, of 1.6 and 1.7 m measured with ground-based EM and airborne EM measurements in April, May, and June 2015, respectively, lie below the values ranging from 1.8 to 2.7 m, reported in historical observations from the same region and time of year. The thick snow cover slows thermodynamic growth of the underlying sea ice. In combination with a thin sea-ice cover this leads to an imbalance between snow and ice thickness, which causes widespread negative freeboard with subsequent flooding and a potential for snow-ice formation. With certainty, 29% of randomly located drill holes on level ice had negative freeboard.

The Arctic Ocean is undergoing substantial warming and sea ice loss which are likely to cause changes in primary production, export fluxes and productivity of the Arctic marine ecosystems. Yet, we lack sufficient information about the ecosystem productivity, organic carbon cycling and cryo-pelago-benthic coupling processes in the Arctic Ocean. Particularly such information is scarce for deep Arctic basins and the spring to summer transition time. Our aim was to provide carbon budget estimations under different regimes and bloom development stages in the region north of Svalbard. We conducted ecological and biogeochemical early spring process studies in sea ice covered areas, from the shelf to the basins of the European Arctic margin and on the Yermak Plateau during the TRANSSIZ cruise PS 92 on the ice breaker R/V Polarstern in May/June 2015. We identified the potential characteristics of carbon production of primary producers in the sea ice and water column, and secondary production of zooplankton and benthos. We looked into the organic carbon production fate and export, including respiration and burial, as well as identified similarities and differences in ecosystem functioning along topography- , sea ice- and water mass-related gradients. This scientific initiative was undertaken by Arctic in Rapid Transition (ART) International Arctic Science Committee (IASC) network .

Ziel dieser Arbeit ist, die auf dem Forschungseisbrecher Polarstern verfügbaren Modell- und Satellitendaten hinsichtlich ihrer Anwendbarkeit als Grundlage für nautische Entscheidungsfindungsprozesse zur Planung und Durchführung von Einsätzen im Packeis zu beurteilen. Die Daten stammen von verschiedenen Providern und werden über das bordeigene ECS System (das IceGIS) automatisch übermittelt. In dieser Arbeit werden zunächst die verfügbaren Datenprodukte beschrieben und im Weiteren deren potenzielle Anwendung zur Planung und Durchführung von Einsätzen in eisbedeckten Gewässern aufgeführt. Die Analyse setzt sich kritisch mit den gegebenen Unsicherheiten und Einschränkungen der jeweiligen Datenprodukte auseinander und zeigt so die Grenzen der Anwendbarkeit auf.

Established in the Fram Strait in 1999, the LTER (Long-Term Ecological Research) observatory HAUSGARTEN enables us to study changes on the deep Arctic seafloor. Repeated deployments of a towed camera system (Ocean Floor Observation System) along the same tracks allowed us to build a time series longer than a decade (2004 - 2015). Here, we present the first time-series results from a northern and the southernmost station of the observatory (N3 and S3, ~2650m and 2350m depth respectively) obtained via the analysis of still imagery. We assess temporal variability in community structure, megafaunal densities and diversity, and use a range of biotic and abiotic factors to explain the patterns observed. There were significant temporal differences in megafaunal abundances, diversity and abiotic factors at both stations. A particularly high increase in megafaunal abundance was recorded at N3 from 12.08 (±0.39; 2004) individuals m-2 to 35.21 (±0.97; 2007) ind. m-2 alongside a ten-fold increase in (drop-)stones. At S3, megafaunal densities peaked in 2015 (22.74 ±0.61 ind. m-2) after an increasing trend since 2004 (12.44 ±0.32 ind. m-2). Holothurians showed particularly striking temporal differences: densities of the small sea cucumber Elpidia heckeri densities rose ten-fold from 0.31 ind. m-2 (±0.04; 2004) to 3.74 ind. m-2 (±0.14; 2015) at S3, coinciding with a sustained increase in phytodetritial matter (chloroplastic pigment equivalents) at the seafloor. Initially entirely absent from N3, densities of the larger holothurian Kolga hyalina peaked in 2007 (5.87 ±0.22 ind. m-2) and declined continuously since then. Overall diversity (γ) increased at both stations over the course of the study, however, with varying contributions of α and β diversities. Our results highlight the importance of time-series studies as megafaunal community composition is characterised by continuous changes. This indicates that epibenthic communities from the deep seafloor are reactive and dynamic, with no “null” community state. To continue to monitor them is therefore crucial in understanding natural and anthropogenic impacts in an area exposed to the effects of climate change.

FRAM (FRontiers in Arctic Marine Monitoring) targets a modern vision of integrated underwater infrastructure. FRAM enhances sustainable knowledge for science, society and maritime economy as it enables truly year round observations from surface to depth in the remote and harsh arctic sea. Cutting edge technologies are being (further) developed and used to record essential ocean variables to improve our understanding of the Arctic and it’s ongoing processes. Data will be made freely available to the public via the AWI data portal.

Snow on sea ice alters the properties of the underlying ice cover as well as associated physical and biological processes at the interfaces between atmosphere, sea ice and ocean. The Antarctic snow cover persists during most of the year and contributes significantly to the sea-ice mass due to the widespread surface flooding and related snow-ice formation. Snow also enhances the sea-ice surface reflectivity of incoming shortwave radiation and determines therefore the amount of light being reflected, absorbed, and transmitted to the upper ocean. Here, we present results of a case study of spectral solar radiation measurements under Antarctic pack ice with an instrumented Remotely Operated Vehicle in the Weddell Sea in 2013. In order to identify the key variables controlling the spatial distribution of the under-ice light regime, we exploit under-ice optical measurements in combination with simultaneous characterization of surface properties, such as sea-ice thickness and snow depth. Our results reveal how the distinction between flooded and non-flooded sea-ice regimes dominates the spatial scales of under-ice light variability for areas smaller than 100m-by-100m. However, the heterogeneous and widely metamorphous snow on Antarctic pack ice prevents a direct correlation between surface properties and the under-ice light field. Compared with Arctic sea ice, light levels under Antarctic pack ice are extremely low during spring (< 0.1%). This is mostly a result of the distinctly different dominant sea ice and snow properties with seasonal snow cover (including strong melt and summer melt ponds) in the Arctic and a year-round snow cover and strong surface flooding in the Southern Ocean.

Freshwater and sea ice in the Arctic Ocean (AO) both respond to climate forcing but changes in their distribution may in turn also significantly affect Northern Hemisphere climate. The exact mechanisms controlling their transport are, however, still subject of ongoing research. Radiogenic neodymium (Nd) isotopes (expressed as ��Nd) and rare earth elements (REEs) have successfully been used to trace ocean circulation in the AO and can provide valuable information on the origin, transport and fate of Arctic freshwater and drifting sea ice. Here, we present first sea-ice data and discuss the applicability of these tracers to determine the pathways of Siberian freshwater and the source areas of Arctic sea ice within the Eurasian Basin. Freshwaters originating from various Siberian rivers are well mixed and diluted prior to their advection to the central AO and the Fram Strait, thereby inhibiting the assessment of the individual contributions of the different rivers for these regions. Recent investigations of seawater Nd isotope and REE distributions on the Siberian shelves now allow for a more detailed determination of the sources of transpolar freshwater transport, and suggest that freshwater from the Lena River can at least be qualitatively traced further away from the shelves. In addition, sea ice has the potential to preserve marine and riverine ��Nd and REE signatures of the Siberian shelf source waters transported across the AO to the Fram Strait thereby avoiding the effects of mixing. Here, we test this potential for the first time by comparing ��Nd and REE signatures determined from unfiltered but essentially sediment-free sea-ice samples recovered in the Eurasian Basin of the central AO in 2012 with corresponding signatures of surface seawater in the formation regions of the ice. The sampled sea ice is dominantly characterized by ��Nd values near -8, in agreement with the incorporation of radiogenic freshwater (��Nd = -6) from the Yenisei and Ob rivers and its formation east of Vilkitsky Strait as identified by a backtracking approach based on satellite-derived ice drift and concentration data. The latter also indicates that sea ice with a less radiogenic ��Nd signature near -11 formed north of the Laptev Sea, in agreement with mixed contributions of radiogenic waters from the Kara Sea and northward flowing highly unradiogenic Lena River water (��Nd ��� -15). Compared to the REE concentrations in Siberian shelf waters, those in the sea ice are depleted, indicating rejection of REEs together with salt during sea-ice formation. Despite potential modifications during incorporation into sea ice, its REE distribution patterns indicate a predictable relationship to those of the source waters, thus in addition supporting preservation of at least some of the marine and riverine REE characteristics during sea-ice formation and transport., Polarforschung

During the Polarstern cruise PS101 to the Karasik seamount in the Central Arctic, we deployed in total 26 sea-ice buoys. Due to persistent low clouds, helicopter flying was limited to a minimum, so that most buoys had to be deployed along the ship‘s track either during longer ROV ice-stations or quick 30 minute stops accessing the ice with the mummy chair.

There is an increasing public interest in sea ice information from both Polar Regions, which requires up-to-date background information and data sets at different levels for various target groups. In order to serve this interest and need, seaiceportal.de (originally: meereisportal.de) was developed as a comprehensive German knowledge platform on sea ice and its snow cover in the Arctic and Antarctic. It was launched in April 2013. Since then, the content and selection of data sets increased and the data portal received increasing attention, also from the international science community. Meanwhile, we are providing near-real time and archive data of many key parameters of sea ice and its snow cover. The data sets result from measurements acquired by various platforms as well as numerical simulations. Satellite observations of sea ice concentration, freeboard, thickness and drift are available as gridded data sets. Sea ice and snow temperatures and thickness as well as atmospheric parameters are available from autonomous platforms (buoys). Additional ship observations, ice station measurements, and mooring time series are compiled as data collections over the last decade. In parallel, we are continuously extending our meta-data and uncertainty information for all data sets. In addition to the data portal, seaiceportal.de provides general comprehensive background information on sea ice and snow as well as expert statements on recent observations and developments. This content is mostly in German in order to complement the various existing international sites for the German speaking public. We will present the portal, its content and function, but we are also asking for direct user feedback.

There is an increasing public interest in sea ice information from both Polar Regions, which requires up-to-date background information and data sets at different levels for various target groups. In order to serve this interest and need, seaiceportal.de (originally: meereisportal.de) was developed as a comprehensive German knowledge platform on sea ice and its snow cover in the Arctic and Antarctic. It was launched in April 2013. Since then, the content and selection of data sets increased and the data portal received increasing attention, also from the international science community. Meanwhile, we are providing near-real time and archive data of many key parameters of sea ice and its snow cover. The data sets result from measurements acquired by various platforms as well as numerical simulations. Satellite observations of sea ice concentration, freeboard, thickness and drift are available as gridded data sets. Sea ice and snow temperatures and thickness as well as atmospheric parameters are available from autonomous platforms (buoys). Additional ship observations, ice station measurements, and mooring time series are compiled as data collections over the last decade. In parallel, we are continuously extending our meta-data and uncertainty information for all data sets. In addition to the data portal, seaiceportal.de provides general comprehensive background information on sea ice and snow as well as expert statements on recent observations and developments. This content is mostly in German in order to complement the various existing international sites for the German speaking public. We will present the portal, its content and function, but we are also asking for direct user feedback.

To increase the capabilities for spatially extensive under-ice investigations, the Helmholtz Infrastructure Initiative FRAM (FRontiers in Arctic marine Monitoring) enabled the Alfred-Wegener-Institute to commission a ROV system for sea ice studies. The system is composed of two redundant vehicle platforms with an extensive interdisciplinary sensor suite. It was built by Ocean Modules (Åtvidaberg, Sweden) in 2016 and immediately after acceptance tests taken on expedition to the Central Arctic. During the expedition PS101 of the german research icebreaker Polarstern, the vehicle was deployed during six sea-ice stations achieving a total of 10 different dives with over 22 hours of diving time, travelling a total horizontal distance of 11.5 km.

To support ship navigation, station planning, and scientific data evaluation, a new near real time sea-ice information system was installed onboard FS Polarstern during summer 2016.

Thinning sea ice and shrinking sea-ice coverage is currently changing production regimes in the pelagic and sympagic Arctic Ocean. It usually remains unknown, how these changes affect export fluxes and hence, benthic communities and remineralization rates in the Arctic deep sea. To tackle this question, we took sediment samples from two bathymetric transects in the Arctic Fram Strait - East Greenland continental slope and West Spitsbergen continental slope (i.e. HAUSGARTEN observatory) – characterized by annual high and low sea-ice coverage, respectively. We measured benthic oxygen consumption rates along with various biogenic parameters and characterized macro- and meiofauna communities. A comparison of the two bathymetric transects suggest that low sea-ice coverage may lead to increased food availability for deep-sea benthic communities and enhanced remineralization rates down to a depth of 2000 m. Below 2000 m depth, food availability and remineralization rates are less or even not affected by sea-ice coverage. Furthermore, our data indicate that from high to low sea-ice covered areas macrofauna abundances shifts from polychaete-dominated to nematode-dominated. Such a shift has not been found for macrofauna biomass and meiofauna abundance. This comparative study provides insights into deep-sea benthic activities and community structure in a region strongly influenced by global change. It could help to assess the fate of Arctic benthic ecosystems under future climate scenarios.

The LTER (Long-Term Ecological Research) observatory HAUSGARTEN, in the eastern Fram Strait, provides us the unique ability to study the composition of benthic megafaunal communities through the analysis of seafloor photographs. This, in combination with extensive annual sampling campaigns, which have yielded a unique data set on faunal, bacterial, biogeochemical and geological properties, as well as on hydrography and sedimentation patterns, allows us to address the question of why variations in megafaunal community structure and species distribution exist within regional (60-110 km) and local (

The sea-ice thickness distribution is one of the most important sea-ice properties, but also one of the less frequently observed ones so far. Satellite retrievals of Antarctic sea-ice thickness are currently limited to laser and radar altimetry observations of snow freeboard with large uncertainties, and electromagnetic measurements have been obtained only sporadically. For the investigation of the variability and changes in the sea-ice thickness distribution over the last decades, data are mainly available from very basic methods such as drilling or ship-based observations following the Antarctic Sea Ice Processes and Climate (ASPeCt) protocol. Thereby it is an advantage that those data also include information on the snow-depth and partly on the sea-ice freeboard distribution, which are as sparse as information on the sea-ice thickness distribution, in particular during winter conditions. The most recent data based on those methods were obtained during austral winter 2013, when various sea-ice parameters were measured in the Weddell Sea as part of the Antarctic Winter Ecosystem Climate Study (AWECS). Here, we present first results of the sea-ice thickness, freeboard and snow-depth distribution obtained by drill-hole measurements from this expedition. The new data set is compared with results from three previous winter campaigns done in 1989, 1992 and 2006 in the Weddell Sea in order to determine the long-term variability of sea-ice thickness, snow depth and freeboard. A challenge in comparing all those data is that measurement sites are based only on individual floes, which are expected to be representative for an entire region. In addition, sampling rates differ between the considered field experiments. Therefore, drill-hole thicknesses are cross-correlated with ground-based EM-measurements in order to identify for the newest data set, how representative the chosen study areas have been for the respective sea-ice floes and which consequences different measurement spacing has for the comparison of data from different years.

Oceanographic moorings and conductivity-temperature-depth (CTD) surveys from September 2009 to September 2010 are used to describe recent changes in the Laptev Sea hydrography and to highlight wind- and ice-driven surface Ekman transport as the mechanism to translate these changes from the outer- to the inner-shelf bottom waters. In February 2010, moored oceanographic instruments recorded a sudden increase in temperature (+0.8�C) and salinity (+ >3) near the bottom of the inner Laptev Sea shelf. Such warm and saline waters had not been previously observed on the inner shelf in winter. They likely originated from the basin and were first observed during a summer 2009 CTD survey in the northwestern shelf break region, subsequently spreading east and shoreward across the Laptev Sea shelf. The changes were introduced to the mooring site by the first of a series of bottom-intensified flow events with velocities reaching 20 cm s�1, topographically guided along a relic submarine river valley. Each of the flow events coincided with negative pressure anomalies at the mooring site and offshore-directed (upwelling-favorable) winds and ice drift. We suggest that the observations to first order resemble a simplified two-dimensional two-layered ocean, where offshore surface Ekman transport is compensated for by a barotropic shoreward response flow near the bottom. In this paper, we use one of the first comprehensive long-term Laptev Sea datasets to highlight ice-ocean-atmosphere interactions in early and late winter and discuss the role of freshwater, stratification, and ice mobility on under-ice circulation on the Laptev Sea shelf.

Hydrographic and stable oxygen isotope (H2 18O/H2 16O) sampling was carried out within the West New Siberian (WNS) coastal polynyas in the southern Laptev Sea in late winters 2008 and 2009. The impact of sea ice formation on the water column was quantified by a salinity/d18O mass balance. Several stations had vertically homogeneous physical properties in April/May 2008 and featured polynya-formed local bottom water with elevated signals of brine released during sea ice formation and elevated fractions of river water. The polynya-formed bottom water was fresher than surrounding bottom waters. At other stations, salinity/d18O correlation showed well-defined mixing lines for bottom and surface layers. In March–April 2009, surface waters were strongly influenced by Lena River water, and local polynya activity with elevated brine signals reached to intermediate depth but did not penetrate the bottom layer in the highly stratified water column. Inventory values of sea ice formation were comparable in both years, but freshwater distributions from the preceding summers were different. Therefore, the observed difference in the impact of polynya activity on the water column is not primarily controlled by the amount of sea ice formed during winter but by preconditioning from the preceding summer. Only in years when the river plume is mostly absent in the polynya region is stratification weak and allows winter sea ice formation to reach the bottom layer. Thus summer stratification controls the influence of local polynya water on the shelf’s bottom hydrography and, as bottom water is exported, impacts on the source water of shelf-derived halocline waters.

Salt expelled during the formation of ice in polynyas leads to a downward precipitation of brine that causes thermohaline convection and erodes the density stratification of the water column. In this thesis we investigate by means of flux models and satellite data the ability of the Western New Siberian (WNS) flaw polynya to modify the stratification of the water column and to form saline bottom water. The accuracy of existent microwave satellite-based polynya monitoring methods is assessed by a comparison of derived estimates with airborne electromagnetic ice thickness measurements and aerial photographs taken across the polynya. The cross-validation indicates that in the narrow flaw polynyas of the Laptev Sea the coarse resolution of commonly used microwave channel combinations provokes errors through mixed signals at the fast and pack ice edges. Likewise, the accuracy of flux models is tested by comparing model results to ice thickness and ice production estimates derived from high-resolution thermal infrared satellite observations. We find that if a realistic fast ice boundary and parameterization of the collection depth H is used and if the movement of the pack ice edge is prescribed correctly, the model is an appropriate tool for studying polynya dynamics and estimating associated fluxes. Hence, a flux model is used to examine the effect of ice production on the stratification of the water column. The ability of the polynya to form dense shelf bottom water is investigated by adding the brine released during an except ionally strong WNS polynya event in 2004 to the average winter density stratification of the water body. Owing to the strong density stratification and the apparent lack of extreme polynya events in the eastern Laptev Sea, we find the likelihood of convective mixing down to the bottom to be extremely low. We conclude that the recently observed breakdown of the stratification during polynya events is therefore predominantly related to wind- and tidally-driven turbulent mixing.

The detection, monitoring, and forecasting of sea-ice conditions, including their extremes, is very important for ship navigation and offshore activities, and for monitoring of sea-ice processes and trends. We summarize here recent advances in the monitoring of sea-ice conditions and their extremes from satellite data as well as the development of sea-ice seasonal forecasting capabilities. Our results are the outcome of the three-year (2015–2018) SPICES (Space-borne Observations for Detecting and Forecasting Sea-Ice Cover Extremes) project funded by the EU’s Horizon 2020 programme. New SPICES sea-ice products include pancake ice thickness and degree of ice ridging based on synthetic aperture radar imagery, Arctic sea-ice volume and export derived from multisensor satellite data, and melt pond fraction and sea-ice concentration using Soil Moisture and Ocean Salinity (SMOS) radiometer data. Forecasts of July sea-ice conditions from initial conditions in May showed substantial improvement in some Arctic regions after adding sea-ice thickness (SIT) data to the model initialization. The SIT initialization also improved seasonal forecasts for years with extremely low summer sea-ice extent. New SPICES sea-ice products have a demonstrable level of maturity, and with a reasonable amount of further work they can be integrated into various operational sea-ice services.

Year-round observations of the physical snow and ice properties and processes that govern the ice pack evolution and its interaction with the atmosphere and the ocean were conducted during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition of the research vessel Polarstern in the Arctic Ocean from October 2019 to September 2020. This work was embedded into the interdisciplinary design of the 5 MOSAiC teams, studying the atmosphere, the sea ice, the ocean, the ecosystem, and biogeochemical processes. The overall aim of the snow and sea ice observations during MOSAiC was to characterize the physical properties of the snow and ice cover comprehensively in the central Arctic over an entire annual cycle. This objective was achieved by detailed observations of physical properties and of energy and mass balance of snow and ice. By studying snow and sea ice dynamics over nested spatial scales from centimeters to tens of kilometers, the variability across scales can be considered. On-ice observations of in situ and remote sensing properties of the different surface types over all seasons will help to improve numerical process and climate models and to establish and validate novel satellite remote sensing methods; the linkages to accompanying airborne measurements, satellite observations, and results of numerical models are discussed. We found large spatial variabilities of snow metamorphism and thermal regimes impacting sea ice growth. We conclude that the highly variable snow cover needs to be considered in more detail (in observations, remote sensing, and models) to better understand snow-related feedback processes. The ice pack revealed rapid transformations and motions along the drift in all seasons. The number of coupled ice-ocean interface processes observed in detail are expected to guide upcoming research with respect to the changing Arctic sea ice.

The Arctic sea ice cover is strongly connected to the global climate system and therefore not only subject to internal variability but also in a phase of significant change related to the ongoing increase in global mean surface temperatures. The most important parameters to monitor and describe sea ice are its areal extent, thickness, and motion. While reliable, long-term satellite measurements of sea ice concentration, which is used to derive the area covered by sea ice, exist since the late 1970s, sea ice thickness and motion data sets of comparable quality and length are currently not available. The overarching goal of this dissertation is to contribute to the improvement of sea ice thickness observations and to understand and quantify the impact of ongoing sea ice thickness changes and variability in the most important regions of sea ice formation on the overall Arctic sea ice budget. To achieve that, the first study presented in this dissertation focuses on extending the knowledge about sea ice thickness variability in the Laptev Sea by developing a new method to derive sea ice thickness time series from moored sonars. It is shown that daily mean sea ice thickness time series can be inferred from basic, moored upward-looking Acoustic Doppler Current Profilers. This adaptive approach allows to revisit data sets from past mooring deployments in the Laptev Sea and exploit them to extend the available sea ice thickness records and close observational gaps in a region that, due to its limited accessibility, is vastly under-sampled. These new data sets are the basis for the validation of multiple satellite sea ice thickness products, including the longest available one introduced by the European Space Agency, which provides Arctic-wide sea ice thickness since 2002. It is shown that in the first-year ice dominated Laptev Sea the investigated satellite products provide the most frequently occurring (modal) rather than the mean sea ice thickness. This important discovery allows for a better interpretation of the available satellite records, especially for the investigation of sea ice volume transports, and underlines their deficiencies in representing dynamically deformed sea ice. Based on the knowledge gained in the Laptev Sea, the final study presented in this dissertation follows the Arctic sea ice life cycle from the regions of ice formation along the Transpolar Drift towards Fram Strait and analyses whether sea ice thickness anomalies induced in the source regions of Arctic sea ice propagate to the central Arctic Ocean and beyond. More specifically, it is investigated which particular processes are potentially responsible for the induced anomalies in the source regions and whether their signals persist until the end of the Transpolar Drift. In the absence of a single-source Arctic-wide, high temporal and spatial resolution sea ice thickness data product, this final part promotes the combination of different techniques and tools for the investigation of this complex Arctic climate parameter. At the center of the investigation is an extended long-term electromagnetic induction sounding-based sea ice thickness time series, which shows a general thinning and decreasing age of sea ice at the end of the Transpolar Drift between 2001 and 2020. Due to its length, this unique time series also permits to put ice thickness measurements conducted during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition into the historical context. Lagrangian ice tracking and modelling of thermodynamic sea ice growth along the pathways of Arctic sea ice reveal a potential preconditioning effect of observed increases in upward ocean heat fluxes in the eastern Arctic, termed Atlantification, that decelerates sea ice growth. The presented efforts are an important contribution to the better understanding of Arctic sea ice thickness variability and change and can be seen as starting points for more targeted analyses of the driving mechanisms behind them. In addition, the acquisition, validation, and extension of sea ice thickness observations provide the basis for more detailed sea ice modelling, which will improve not only the monitoring but also the prediction of Arctic sea ice thickness changes in the future.