The HAUSGARTEN observatory consists of 21 stations at which, since 1999, abiotic and biotic processes in Fram Strait are being monitored. As the Transpolar Drift continuously transports sea ice from the central Arctic Ocean toward Fram Strait, some stations are covered seasonally or even year-round by closed ice pack. Previous studies have shown that when sea ice is present, it can exert direct or indirect effects on the underlying water column, extending all the way down to the seafloor. In this manuscript, we provide an overview and a more thorough assessment of the sea-ice conditions at the N3-5 and EGI-IV stations, focusing on both seasonal variability and long-term changes in satellite data. The western stations (EGI-IV) show significantly higher ice coverage, whereas the northeastern stations (N3-5) persistently exhibit lower ice concentrations. Somewhat unexpectedly, and in contrast to the rest of the Arctic, sea-ice concentration at these stations shows no significant trends-neither on an annual nor on a monthly timescale. Although no direct changes in ice coverage are apparent, noticeable changes in ice properties do occur. The age of the ice at both locations has declined sharply. Its origin has shifted northward, thereby reducing the proportion of ice formed in shallow-water regions and interrupting the transport of ice-rafted matter from the Siberian shelves towards Fram Strait.

The Arctic is undergoing rapid warming, resulting in retreating sea ice and glaciers1, yet how cryospheric changes propagate into the deep ocean remains poorly understood2. Here we identify a climate-driven mechanism linking accelerating glacier disintegration to an increase in deep-sea hard-bottom habitats far beyond calving fronts. Seafloor observations in Fram Strait show a localized increase in the density and patchiness of dropstones delivered by debris-laden icebergs. At the same time, four decades of shipboard records show that the occurrence of icebergs increased abruptly in the early 2000s. Backtracking links these icebergs to the main outlet glaciers in northeast Greenland and the Russian High Arctic. In northeast Greenland, the timing of glacier destabilization coincides with this rise, whereas sparse satellite coverage in the Russian sector limits temporal attribution despite indications of enhanced glacier activity. A model sensitivity study shows that, apart from intensified calving, a more dynamic sea ice cover enhances downstream transport of glacial ice. Along these pathways, increased iceberg activity could reshape deep-sea habitats through enhanced melt and associated lithogenic input, and elevate navigational hazards as maritime traffic expands in the Arctic. Although modest compared with the iceberg discharges of Pleistocene Heinrich events, this mechanism provides a modern analogue of long-range cryospheric influence on the seafloor in a warming climate.

Abstract. The NASA airborne Arctic Radiation-Cloud-aerosol-Surface-Interaction Experiment (ARCSIX) collected a unique data set providing a near-simultaneous characterization of radiative fluxes, surface, cloud, and aerosol particle properties to address science questions on the surface radiation budget, the processes governing the cloud lifecycle, atmospheric composition, and the interactions between the surface and atmosphere. The overarching goal of ARCSIX was to quantify the contributions of surface, clouds, aerosol particles, and precipitation to summer sea ice melt. ARCSIX consisted of two deployments in 2024 (Spring: 2024-05-28 through 2024-06-13 and Summer: 2024-07-25 through 2024-08-15) to capture pre- and post-melt conditions. ARCSIX provided coordinated remote sensing and in situ sampling using three aircraft in a high-flyer/low-flyer configuration. The NASA G-III served as the high-flying remote sensing platform with two lower flying in situ and near-target remote sensor observing platforms, NASA P-3B and SPEC Inc. Learjet. ARCSIX data are well-suited to improve satellite remote sensing capabilities in the Arctic. ARCSIX included an array of sea ice mass balance buoys deployed in the Lincoln Sea that were regularly overflown during the campaign. ARCSIX research flights spanned the Baffin Bay, Lincoln Sea, west and north of the Canadian Archipelago, and the Greenland north and northeast coasts. During the spring deployment, 19 research flights took place covering 114 flight hours: 10 flights and 68 hours by the P-3B and nine flights and 46 hours by the G-III. During summer, 24 research flights covered 136 flight hours: nine flights and 75 hours by the P-3B, five flights and 26 hours by the G-III, and 10 flights and 35 hours by the Learjet. A total of 13 coordinated flights with 2+ aircraft were carried out. This paper describes the ARCSIX flight strategy, instrumentation, and data set access, and usage details. ARCSIX data are publicly available at https://doi.org/10.5067/SUBORBITAL/ARCSIX/DATA001.

Abstract. The East Greenland Current (EGC) is the main conveyor of cold, fresh Polar Water and sea ice from the Arctic Ocean through Fram Strait to the subpolar North Atlantic. Seven years of continuous mooring observations on the East Greenland continental slope at 79° N (2018–2025) reveal that whilst the EGC typically maintains a southward flow year-round, it is punctuated by periodic northward flow events on the upper continental slope. The majority are short-lived local events (lasting fewer than 5 days), consistent with mesoscale eddy activity. However, two prolonged deep-reaching events — in April–May 2019 (29 days) and May–June 2025 (at least 35 days) — were anomalous in their duration, intensity (i.e. exceeding the climatological mean by 2–3 standard deviations), and vertical coherence (down to at least 500 m depth). During both events, hydrographic observations show anomalous warm Atlantic-origin waters at depths typically occupied by cooler Arctic Atlantic Water, weakened vertical stratification, and temperature-salinity properties consistent with recent West Spitsbergen Current recirculation across Fram Strait. An extended mooring record at 78°50' N (2003–2019) identified a further 7 deep-reaching northward flow events, demonstrating that episodic northward flow is a recurring intrinsic feature of the EGC system rather than a recent phenomenon. A persistent sea-level pressure dipole with anomalous northward winds along the East Greenland margin is identified all strong deep northward flow events. It is proposed that this forcing drives a lateral reorganisation of the boundary current where anomalous eastward Ekman transport displaces the EGC core offshore whilst geostrophically adjusted flow on the inner slope produces a net northward current inshore. The anomalously warm AW at depth thus arises through northward advection of recirculating AW and upwelling over the slope. These anomalous flow events establish an episodic northward pathway for warm Atlantic-origin water with a potential advective reach of perhaps hundreds of kilometres beyond the mooring location — a previously unidentified pathway of Atlantic heat transport into the Arctic interior. The co-occurrence of the 2025 event with a record low sea-ice area in July in the northeast Greenland region suggests that these oceanic events influence regional sea ice variability through dynamic atmospheric export and ocean-ice interactions, though a direct causal link remains to be established. Whether these events represent rare features of a naturally variable boundary current system or early signals of ongoing change in the Arctic-Atlantic exchange remains an open question.

Highly pathogenic avian influenza of subtype H5N1 (HPAIV H5N1) reached Antarctica in late 2023/2024 breeding season, but the host ecology and movement processes shaping its incursion and spread remain poorly resolved. We investigated these mechanisms by integrating viral genomics, multi-season serology, and seabird movement data at King George Island (Fildes Peninsula, South Shetland Islands), a likely gateway between South America and the Antarctic Peninsula. Across three breeding seasons (2022/23–2024/25), we collected swabs and blood serum from key scavenging and predatory seabirds (Brown skua, South Polar skua, Southern Giant petrel) and paired these data with light-level geolocator tracks (10–16 full annual tracks per species) and breeding-season GPS tracks (10–20 individuals per species). Phylogenetic analyses including three newly generated HPAIV H5N1 genomes from infected Brown skuas placed these viruses within the South Georgia–Antarctic lineage, consistent with introduction via South Georgia followed by onward spread within the region. Longitudinal serology revealed a rapid increase in cumulative exposure in scavenging species, with antibodies against influenza A virus’ nucleoprotein and hemagglutinin subtype H5 rising to high levels by 2024/25 in Brown skuas (52.2%) and Southern Giant petrels (62.5%), but remaining markedly lower in South Polar skuas (12.5%), consistent with their more pelagic and less scavenging foraging behaviour. Tracking showed that migratory connectivity was greatest during the non-breeding period but contracted rapidly during colony return and early breeding. In contrast, network simulations parameterized by high-percentile breeding-season foraging distances indicated strong peninsula-wide connectivity and non-zero potential links toward South Georgia and other sub-Antarctic stepping-stones. Together, these results provide a mechanistic, host-based framework suggesting that HPAIV H5N1 likely reached Antarctica through the South Georgia pathway, but that its regional spread was shaped primarily by foraging ecology and breeding-season movement networks rather than by long-distance migration.

Coupled Numerical Weather Prediction (NWP) models have only recently been implemented for short-term environmental prediction and both challenges and benefits are evident in polar regions. Their simulation of surface exchange over sea ice depends on the model's sea-ice characteristics, however these are hard to constrain due to a lack of in situ and accurate remotely sensed observations. We focus on the Fram Strait region during peak melt conditions and during the passage of an Arctic cyclone: very challenging conditions for coupled NWP. We use in situ aircraft observations from the Arctic Summertime Cyclones field campaign in July-August 2022, plus satellite products, to evaluate a set of 5-day forecasts from the Met Office Unified Model. Our model set ups are based on operational GC4 (Global Coupled 4) and developmental GC5 (Global Coupled 5) configurations, which use the CICE5.1 and SI3 sea-ice models respectively. We find a combination of deficiencies in the simulated sea-ice field, due to initialization and modeling problems. An initially low concentration of sea ice results in excessive absorption of shortwave radiation by the ocean, leading to excessive basal melting of the sea ice, and further sea-ice loss; leading to relatively poorly simulated sea-ice fields in general. In contrast, the passage of an Arctic cyclone and its impact on sea-ice velocities are captured well. Although we demonstrate several deficiencies in the short-term forecasts of two state-of-the-art coupled NWP models, we also find promising aspects of model performance and some clear benefits from a fully coupled atmosphere-ice-ocean system.

During Arctic summer, meltwater inputs and a fragmented ice cover impede quantifying the role of boundary stress for turbulent mixing in the ice-ocean boundary layer. Here, we show that less than two-thirds of the turbulent kinetic energy (TKE) generated from mean flow shear under drifting sea ice is dissipated, and the remainder can be attributed to balancing stabilizing buoyancy fluxes. We deployed a high-resolution acoustic Doppler current profiler under an ice floe to estimate Reynolds stress, shear production, and dissipation rate of TKE. At 0.75 m below the interface, dissipation rates from 1.5 x 10(-9) to 4.2 x 10(-7) m(2) s(-3) and shear production from 6.9 x 10(-10) to 7.7 x 10(-7) m(2) s(-3) were measured (5%-95% percentiles), with shear production exceeding dissipation on average. The turbulent stress was largest during an event with similar to 9.2-h-period oscillations in the upper ocean, consistent with tidally forced lee waves generated near steep topography. An overall estimate of the quadratic skin drag coefficient representative of the ice floe is CD0=7.0x10-4 . We further identified three qualitative regimes of atmosphere-ice-ocean coupling in our observations: a high-frequency range [>4 cycles per day (cpd)] in which the ice acts like a rigid lid atop the ocean, an intermediate range, and a low-frequency range (<0.8 cpd), where wind-driven ice drift determines the under-ice current. As the latter only contained half of the variance of the ice-relative flow, we emphasize that resolving subdaily time scales is crucial in observing and modeling atmosphere-ice-ocean coupling.

Melt ponds play a crucial role in modulating the energy balance of Arctic sea ice by reducing surface albedo. While Arctic sea ice is becoming younger and smoother, this raises questions about how these changes affect melt pond characteristics, as the effect of ice deformation features, such as pressure ridges, on pond development on large spatial scales remains insufficiently understood. Here, we analyze 70 km2 of high resolution airborne optical imagery and coincident laser altimeter data from three research flights north of Greenland to investigate the relationship between ridge fraction and melt pond properties. Our results reveal that melt pond fractions on heavily deformed multi-year ice can reach values comparable to those on smooth ice, with similar distributions observed for both. Furthermore, we find that ridge fraction influences both the size distribution and geometry of melt ponds on various typical ice types. Whether ridges constrain pond geometry in a way that increases or decreases pond shape complexity depends on pond size: small ponds are on average more complex in the presence of ridges, whereas large ponds are restricted in their complexity. This shift in behavior occurs around the characteristic size scale of 102 m2 that coincides with the transition in pond fractal dimension. These results demonstrate the role of ice morphology in shaping melt pond characteristics and provide valuable insights for improving melt pond parameterizations in sea ice models.

The Transpolar Drift (TPD) plays a crucial role in regulating Arctic climate and ecosystems by transporting fresh water and key substances, such as terrestrial nutrients and pollutants, from the Siberian Shelf across the Arctic Ocean to the North Atlantic. However, year-round observations of the TPD remain scarce, creating significant knowledge gaps regarding the influence of sea ice drift and ocean surface circulation on the transport pathways of Siberian fresh water and associated matter. Using geochemical provenance tracer data collected over a complete seasonal cycle, our study reveals substantial spatiotemporal variability in the dispersal pathways of Siberian matter along the TPD. This variability reflects dynamic shifts in contributions of individual Siberian rivers as they integrate into a large-scale current system, followed by their rapid and extensive redistribution through a combination of seasonal ice-ocean exchanges and divergent ice drift. These findings emphasize the complexity of Arctic ice-ocean transport pathways and highlight the challenges of forecasting their dynamics in light of anticipated changes in sea ice extent, river discharge, and surface circulation patterns.

Snow cover plays a pivotal role in the Arctic's climate, hydrology, and ecology, making the understanding of its deposition and accumulation dynamics crucial. Snow depth and its duration can directly influence soil temperature: the insulating properties of snow increase with greater snow depth, which prevents soil temperatures from declining in winter. Trail Valley Creek, NWT, Canada, is located at the northern boundary of the tundra-taiga transition zone, approximately 45 km north of Inuvik, and is underlain by continuous permafrost. The region’s rapid warming points to a trend of vegetation changes such as shrub expansion northwards into the tundra.Topography and vegetation cover are the main drivers of spatial variation of snow depth across different landscapes, while wind significantly influences snow redistribution. This reallocation causes snow to accumulate preferably in terrain features such as valleys and leeward sides of ridges, and taller vegetation, as their height and intricate structure can favour snow trapping. Understanding the relationships among snow distribution, topography features, and vegetation types is vital, though it is often limited by the scarcity of high-resolution data with broad spatial cover.To investigate the spatial snow distribution in Trail Valley Creek, we analyzed how snow depth varies according to different topography classes and slope aspects, as well as the region’s different vegetation classes and heights. For this purpose, we explored records from Aerial Laser Scanning (ALS) collected during both winter and summer of 2023, covering an area of over 170 km2. We generated a high-resolution Digital Elevation Model (DEM) from the winter snow-covered surface (2023-04-02), a Digital Terrain Model (DTM) from the summer snow-free terrain (2023-07-10), and by combining both, created a 1-m resolution snow depth map of the area. Additionally, we used 3129 Magnaprobe ground-based snow depth measurements for validation (2023-03-26 to 2023-03-29). For the topography analysis, we classified the slope aspects, and subdivided the terrain into 10 geomorphological classes using the geomorphons approach. This method calculates terrain forms, such as plateaus, slopes, ridges and valleys, and their associated geometry using a machine vision approach. To analyze the role of vegetation cover, we used a 13-class map that categorizes land-cover features and vegetation types, such as graminoids, shrubs and trees, and vegetation height rasters, derived from the ALS summer data.Snow is the main driver of the hydrological system in Trail Valley Creek, and the outcomes of this study will provide insights in the important interplay between vegetation, snow depth and terrain characteristics in a permafrost landscape.

Pressure ridges, formed by sea ice deformation, affect momentum transfer in the Arctic Ocean and support a larger biomass than the surrounding-level ice. Although trends in Arctic sea ice thickness and concentration are well documented, changes in ridge morphology remain unclear. This study provides airborne-based evidence of a shift towards a smoother ice surface, with fewer pressure ridges and reduced surface drag, attributed to the loss of old ice. Furthermore, an increase in seasonal ice cover enhances overall deformation in the Arctic and acts as a negative feedback mechanism on pan-Arctic ridge morphology: the greater the proportion of seasonal ice, the higher the pan-Arctic mean ridge rate, dampening an overall decline in ridges with age. While thinner and less frequent ridges benefit industries such as shipping, these changes are likely to have profound impacts on the energy and mass balance and the ecosystem of the Arctic Ocean. Pressure ridges, a characteristic feature of Arctic sea ice, play an important role in the ecosystem but pose challenges to shipping. Here the authors use aircraft measurements to document a decline in both the frequency and height of these pressure ridges in recent decades.

Globally, the most intense uptake of anthropogenic carbon dioxide (CO2) occurs in the Atlantic north of 50°N, and it has been predicted that atmospheric CO2 sequestration in the Arctic Ocean will increase as a result of ice-melt and increased primary production. However, little is known about the impact of pan-Arctic sea-ice decline on carbon export processes. We investigated the potential ballasting effect of sea-ice derived material on settling aggregates and carbon export in the Fram Strait by combining 13 years of vertical flux measurements with benthic eDNA analysis, laboratory experiments, and tracked sea-ice distributions. We show that melting sea-ice in the Fram Strait releases cryogenic gypsum and terrigenous material, which ballasts sinking organic aggregates. As a result, settling velocities of aggregates increased ≤10-fold, resulting in ≤30% higher carbon export in the vicinity of the melting ice-edge. Cryogenic gypsum is formed in first-year sea-ice, which is predicted to increase as the Arctic is warming. Simultaneously, less sea-ice forms over the Arctic shelves, which is where terrigenous material is incorporated into sea-ice. Supporting this, we found that terrigenous fluxes from melting sea-ice in the Fram Strait decreased by >80% during our time-series. Our study suggests that terrigenous flux will eventually cease when enhanced sea-ice melt disrupts trans-Arctic sea-ice transport and thus, limit terrigenous-ballasted carbon flux. However, the predicted increase in Arctic primary production and gypsum formation may enhance gypsum-ballasted carbon flux and compensate for lowered terrigenous fluxes. It is thus unclear if sea-ice loss will reduce carbon export in the Arctic Ocean.

Microplastic (MP) pollution has reached the remotest areas of the globe, including the polar regions. In the Arctic Ocean, MPs have been detected in ice, snow, water, sediment, and biota, but their temporal dynamics remain poorly understood. To better understand the transport pathways and drivers of MP pollution in this fragile environment, this study aims to assess MPs (≥ 11 μm) in sediment trap samples collected at the HAUSGARTEN observatory (Fram Strait) from September 2019 to July 2021. MP fluxes determined by μ-Fourier transform infrared (FTIR) imaging ranged from 0 to 2.9 MP m-2 d-1, peaking in April 2020 and April 2021, with all detected MPs being <300 μm in size. There was no strong correlation between MPs and any of the recorded biogeochemical and physical variables, as each MP flux event was associated with different variables such as biogenic matter, sea ice concentration, or origin. By providing time series data over 21 months, this study provides a baseline for future MP flux assessments in Fram Strait, Arctic.

Central Arctic properties and processes are important to the regional and global coupled climate system. The Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) Distributed Network (DN) of autonomous ice-tethered systems aimed to bridge gaps in our understanding of temporal and spatial scales, in particular with respect to the resolution of Earth system models. By characterizing variability around local measurements made at a Central Observatory, the DN covers both the coupled system interactions involving the ocean-ice-atmosphere interfaces as well as three-dimensional processes in the ocean, sea ice, and atmosphere. The more than 200 autonomous instruments (“buoys”) were of varying complexity and set up at different sites mostly within 50 km of the Central Observatory. During an exemplary midwinter month, the DN observations captured the spatial variability of atmospheric processes on sub-monthly time scales, but less so for monthly means. They show significant variability in snow depth and ice thickness, and provide a temporally and spatially resolved characterization of ice motion and deformation, showing coherency at the DN scale but less at smaller spatial scales. Ocean data show the background gradient across the DN as well as spatially dependent time variability due to local mixed layer sub-mesoscale and mesoscale processes, influenced by a variable ice cover. The second case (May–June 2020) illustrates the utility of the DN during the absence of manually obtained data by providing continuity of physical and biological observations during this key transitional period. We show examples of synergies between the extensive MOSAiC remote sensing observations and numerical modeling, such as estimating the skill of ice drift forecasts and evaluating coupled system modeling. The MOSAiC DN has been proven to enable analysis of local to mesoscale processes in the coupled atmosphere-ice-ocean system and has the potential to improve model parameterizations of important, unresolved processes in the future.

Arctic marine ecosystems are becoming more boreal due to climate change. Predictions of ecosystem change focus mainly on Arctic inflow or interior shelves, with few comprehensive studies on Arctic outflow regions. During September–October 2017, soft-bottom communities were sampled and benthic ecosystem processes were quantified at 12 stations on the Northeast Greenland shelf (outflow shelf) and compared to the last regional ecosystem study, conducted in 1992 and 1993. The benthic habitat was characterized in terms of sediment granulometry, pigment concentrations, and porewater chemistry (dissolved inorganic carbon, nutrients). Total abundance and biomass of macrobenthos and meiobenthos, bacterial abundance, porewater dissolved inorganic carbon and ammonium concentrations were higher on the outer shelf compared to locations adjacent to the Nioghalvfjerdsfjorden glacier at 79°N and the inner shelf stations (e.g., macrofauna: 1,964–2,952 vs. 18–1,381 individuals m−2). These results suggest higher benthic production in the outer parts of the NEG shelf. This difference was also pronounced in macrobenthic and meiobenthic community structure, which was driven mainly by food availability (pigments with 1.3–4.3 vs. 0.3–0.9 µg g−1 sediment, higher total organic carbon content and bacterial abundance). Compared to the early 1990s, warmer bottom water temperatures, increased number of sea-ice-free days and lower sediment pigment concentrations in 2017 were accompanied by decreased polychaete and increased nematode abundance and diversity, and a different community structure of nematode genera. The present study confirms previous reports of strong pelagic-benthic coupling on the NEG shelf, but highlights a possible weakening since the early 1990s, with a potential shift in importance from macrofauna to meiofauna in the benthic community. Increasing inflow of Atlantic water and decreasing volume transport and thickness of sea ice through the Fram Strait, probably affecting the Northeast Water Polynya, may be responsible, suggesting ecosystem-wide consequences of continued changes in sea-ice patterns on Arctic shelves.

Plastic pollution has become ubiquitous with very high quantities detected even in ecosystems as remote as Arctic sea ice and deep-sea sediments. Ice algae growing underneath sea ice are released upon melting and can form fast-sinking aggregates. In this pilot study, we sampled and analyzed the ice algaeMelosira arcticaand ambient sea water from three locations in the Fram Strait to assess their microplastic content and potential as a temporary sink and pathway to the deep seafloor. Analysis by μ-Raman and fluorescence microscopy detected microplastics (≥2.2 μm) in all samples at concentrations ranging from 1.3 to 5.7 × 104 microplastics (MP) m-3 in ice algae and from 1.4 to 4.5 × 103 MP m-3 in sea water, indicating magnitude higher concentrations in algae. On average, 94% of the total microplastic particles were identified as 10 μm or smaller in size and comprised 16 polymer types without a clear dominance. The high concentrations of microplastics found in our pilot study suggest thatM. arctica could trap microplastics from melting ice and ambient sea water. The algae appear to be a temporary sink and could act as a key vector to food webs near the sea surface and on the deep seafloor, to which its fast-sinking aggregates could facilitate an important mechanism of transport.

Marine sinking particles sequester atmospheric carbon dioxide to the deep ocean via the biological carbon pump. Understanding how environmental shifts drive changes in the microbial composition of particles, and how these affect the export of organic matter from the surface to the deep ocean, is critical, especially in the rapidly changing Arctic Ocean. Here, we applied next generation sequencing of the 18S and 16S rRNA genes to sediment trap samples from around 200 m water depth in the eastern Fram Strait, covering a time frame of more than one decade (2000-2012). The aim was to characterize their microbial composition during annual highest particulate organic carbon flux events. The bimodal annual spring and summer export fluxes were representative of the strong seasonality in the region. Furthermore, the study period was characterized by considerable interannual variation, marked especially by a warm water anomaly between 2005 and 2007. During this period changes in the hydrography and sea ice cover also led to measurable changes in the microbial composition of particles. The warm water period was marked by a decrease in diatoms affiliated with Chaetoceros, an increase of small phytoplankton and an increase in sequence abundance of the bacterial taxa Oceanospirillales, Alteromonadales and Rhodobacterales on the particles. The resulting changes in microbial composition and the associated microbial network structure suggest the emergence of a more developed retention system in the surface ocean. Our results provide the first long-term assessment of the microbial composition of sinking particles in the Arctic Ocean, and stress the importance of sea ice and hydrography for particle composition and subsequent flux of organic matter to deeper waters.

Using operational sea-ice maps, we provide first insight into the seasonal evolution of fast ice in the East Siberian Sea for the period between 1999 and 2021. The fast ice season tends to start later by 4.7 d per decade and to end earlier by 9.7 d per decade. As a result, there is a trend towards a shorter length of fast ice season by 2 weeks per decade. The analysis of air temperatures indicates that onset and end of the fast ice season are largely driven by thermodynamic processes. Two spatial modes (large, L-mode and small, S-mode) of East Siberian fast ice cover which have significant areal differences were distinguished. The occurrence of L- and S-modes was linked to the polarity of the Arctic Oscillation (AO) index. Negative AO phase leads to increased sea-ice convergence in the region, which in turn favours sea-ice grounding and promotes the development of large fast ice extent (L-mode). Lower deformation rates in the region during positive AO phase does not allow the formation of grounded features which results in small fast ice extent (S-mode). An analysis of sea-ice divergence confirms that L-mode seasons are characterised by higher on-shore convergence compared with S-mode seasons.