Sea ice altimetry currently remains the primary method for estimating sea ice thickness from space, however, time series of such satellite-derived estimates are of limited use without having been quality-controlled against reference measurements. Such reference measurements (a term encapsulating in situ observations and remotely sensed measurements from ground, air, and below the ice) for validation of altimetry measurements over sea ice in the polar regions are sparse and rarely presented in a manner where the time-space averaging matches that of the satellite-derived products. Here, an approach to a published comprehensive collection of sea ice reference measurements repurposed for satellite altimetry observations over sea ice is presented, which includes estimates of freeboard, thickness, draft and snow depth from sea ice-covered regions in the Northern Hemisphere (NH) and the Southern Hemisphere (SH), all of which are relevant for comparison with altimetry estimates. The measurements have been collected using airborne sensors, autonomous drifting buoys, moored and submarine-mounted upward-looking sonars, and visual observations. The data package has been prepared to match the spatial (25 km for NH and 50 km for SH) and temporal (monthly) resolutions of conventional satellite altimetry-derived sea ice thickness data products for a direct evaluation of these, and the code is publicly available and distributed for users to modify depending on their aim. This data package, also known as the Climate Change Initiative (CCI) sea ice thickness (SIT) Round Robin Data Package (RRDP), was produced within the ESA CCI Sea Ice project. The current version of the CCI SIT RRDP covers the polar satellite altimetry era (1993–2024) and has ongoing efforts aimed at continuously updating the datasets. The CCI SIT RRDP has been collocated with satellite-derived sea ice thickness products from CryoSat-2, Envisat, and ERS-1/2 produced within the ESA CCI and the Fundamental Data Records for Altimetry (FDR4ALT) projects to demonstrate the overlap and inter-comparison between the reference measurements and satellite-derived products. Here, the CCI SIT RRDP is introduced along with examples of its use as a validation source for satellite altimetry products, where the averaging, collocation and uncertainty methodology is presented, and advantages and limitations are discussed. The CCI SIT RRDP dataset is available at https://doi.org/10.11583/DTU.24787341 (Olsen and Skourup, 2026a).

The projected transition of the central Arctic Ocean (CAO) into a warmer, seasonally ice-free ocean requires more knowledge of this environment to predict changes in the structure and dynamics of its ecosystems. We aimed to compare the state and underlying processes of Nansen Basin and Amundsen Basin ecosystems observed in August–September 2021 and assess impacts of Atlantic Water inflow and fresher Transpolar Drift waters, respectively, on these ecosystems. The basins differed in features of sea ice, hydrography, and chemical and biological compositions. The near-slope open water in western Nansen Basin showed a clear fingerprint of warm, saline Atlantic Water, with larger vertical turbulent fluxes facilitating nutrient transport across the pycnocline and supporting larger standing stocks of bacteria, protists, and zooplankton. Pelagic primary production and microbial and faunal stocks decreased northward and into Amundsen Basin, likely due to lower nutrient concentrations, stronger stratification, and reduced light through the more continuous and thicker ice and snow cover in Amundsen Basin, possibly also impacted by seasonally declining light levels. Transpolar Drift signals included lower salinity, stronger stratification, and higher silicate concentrations in Amundsen Basin surface waters. Similarities to earlier observations included the increase in small-sized algae from Nansen Basin into Amundsen Basin and overall low faunal abundances in the CAO, suggesting that overarching patterns remained unchanged over past decades. Examples of species range extensions and notable taxon absences relative to earlier studies, however, could be due to borealization and changes in sea-ice conditions, respectively. Higher density ecosystem sampling and consistent time series are recommended to confirm such conclusions. The distinct basin differences call for a regional approach to future management of the CAO. We especially caution against using the area of strong Atlantic Water inflow in southern Nansen Basin as representative of the entire basin, let alone Amundsen Basin or the CAO.

During winter, sea ice is moving in cohesive clusters of ice plates. These clusters – hereafter named ‘Coherent Dynamic Elements’ (CDE) are composed of several areas of deformed and level ice, that slide coherently along active sea ice fractures. The largest sea ice fractures detectable from medium resolution Synthetic Aperture Radar (SAR) satellites (about 50 m spatial resolution) are the Linear Kinematic Features (LKFs). Sea ice deformation information can be estimated from the strain rates in the LKFs and as well from the geometrical characteristics of the CDEs. However, there is a sudden seasonal transition, at the point where the sea ice warms and loses its internal strength. After this transition the delineation of LKFs and CDEs from SAR becomes challenging. In this contribution we will analyze sea ice deformation during the drift of the MOSAiC expedition from October 2019 to July 2020. During this time, the expedition drifted the entire length of the Transpolar drift from the northern Laptev Sea into the Fram Strait and the sea ice surrounding it underwent numerous deformation events. The MOSAiC sea ice deformation data and the onset of the melt period is compared to the data over the Fram Strait, where the sea ice deformation can was estimated from SAR and upward looking sonar devices on fixed moorings for the period of 2010-2023. We will present the data on the changes in the onset of the melt period and show that MOSAiC year was a typical year representative for the sea ice deformation of the recent decade.

In this study, we cover observations of the rapid consolidation and enhanced melt of Arctic sea-ice ridges. During the freezing period, the consolidated part of sea ice ridges is usually up to 1.6–1.8 times thicker than surrounding level ice. Meanwhile, during the melt season, ridges are often observed to be fully consolidated, but this process is not fully understood. We present the evolution of the morphology and temperature of a first-year ice ridge studied during MOSAiC from its formation to advanced melt. From October to May, the draft of first-year ice at the MOSAiC coring site increased from 0.3 m to 1.5 m, while from January to July, the consolidated layer thickness in the ridge reached 3.9 m. We observed several types of ridge consolidation. From the beginning of January until mid-April, the ridge consolidated slowly through heat loss to the atmosphere, with a total consolidated layer growth of 0.7 m. From mid-April to mid-June, there was a rapid increase in ridge consolidation rates, despite conductive heat fluxes not increasing. In this period, the mean thickness of the consolidated layer increased by 2.2 m. We also estimated a substantial snow mass fraction (6%–11%) of ridges using analysis of oxygen isotope composition. Our observations suggest that this sudden change was related to the transport of snow-slush inside the ridge keel via adjacent open leads that decreased ridge macroporosity, which could result in more rapid consolidation.During the summer season, sea ice melts from the surface and bottom. The melt rates substantially vary for sea ice ridges and undeformed first- and second-year ice. Ridges generally melt faster than undeformed ice, while the melt of ridge keels is often accompanied by further summer growth of their consolidated layer, which increases their survivability. We examined the spatial variability of ice melt for different types of ice from in situ drilling, coring, and multibeam sonar scans of the remotely operated underwater vehicle. Six sonar scans performed from 24 June to 21 July were analyzed and validated using seven ice drilling transects. The area investigated by the sonar (0.4 km by 0.2 km) consisted of several ice ridges, surrounded by first- and second-year ice. We show a substantial difference in melt rates for sea ice with a different draft. We also show how ridge keels decay depending on the keel draft, width, steepness, and location relative to the surrounding ridge keel edges. We also use temperature buoy data to distinguish snow, ice surface, and bottom melt rates for both ridges and level ice. These results are important for quantifying ocean heat fluxes for different types of ice during the advanced melt and for estimating the ridge contribution to the total ice mass and summer meltwater balances of the Arctic Ocean.

The Holocene Thermal Maximum (HTM) is a significant warm period of the Holocene epoch, occurring between 11 and 6 ka BP. Here we present a multidecadal-to-centennial scale resolution diatom-based quantitative reconstruction of August Sea surface temperature (aSST) from Kongsfjorden, Svalbard for the regional HTM period between 10.5 and 7.5 ka BP. We find highly variable and moderately warm surface conditions with periods of colder aSST and/or seasonal sea ice presence. Based on the reconstruction, we infer that the variability of local surface conditions during HTM was mainly influenced by insolation and glacier/sea ice melt along with changes in the proximity of oceanic frontal zones, while influence of Atlantic Water at the surface was limited. The reconstructed aSST of 3.7 degrees C on average for the reconstructed part of the HTM period and 4.2 degrees C for its warmest interval between 10.5 and 10.1 ka BP are comparable within the method uncertainty to summer sea surface temperatures observed in the area today. We further present the evidence that the diatom assemblages are preserved in the sedimentary sequence for the first time in the record during the HTM and then re-emerge only in the core top sediments, suggesting some similarity in the environmental conditions in Kongsfjorden between the modern period and the HTM.

Abstract. This study presents seven years (2012–2019) of Arctic sea-ice draft observations from upward-looking sonars combined with coincident observations of ice drift velocity from four moorings located across the Arctic outflow in the Fram Strait at 78.83° N. The data set covers in total about 150 000 km of drifting Arctic sea ice, at a 1 m spatial resolution, providing one of the most extensive spatially referenced sea-ice draft records in the Arctic available today. Level ice makes up about 40–50 % of the ice cover, with modal ice thickness varying between 1 m and 2.5 m, and thicker level ice westward towards the east Greenland shelf. Using local level-ice thickness and a variable-threshold ridge detection algorithm, we identify and quantify the sizes of sea-ice ridges, including shallow ridges with keel drafts less than 5 m deep, often overlooked by traditional methods using a fixed threshold. The study highlights ridges as a significant component of the sea-ice cover, with keels covering some 20–30 % of the ice bottom and contributing 28–55 % of the total sea-ice volume. The typical spatial density varies from 6 to 9 individual ridges per kilometer of sea-ice, with approximately 3000 to 5500 ridges per month at each site. A westward increase in ridge frequency and coverage was associated with the differences in the origin of sea ice arriving at the mooring locations. Further, we show that shallow ridges comprise up to 80 % of all ridges and 35–45 % of the ridged ice volume, and thus play an important role in the sea-ice volume budget. Thus, shallow ridges deserve greater attention, especially given the ongoing changes in the Arctic sea-ice cover.

The Svalbard archipelago is particularly sensitive to climate change due to the relatively low altitude of its main ice fields and its geographical location in the higher North Atlantic, where the effect of Arctic amplification is more significant. The largest temperature increases have been observed during winter, but increasing summer temperatures, above the melting point, have led to increased glacier melt. Here, we evaluate the impact of this increased melt on the preservation of the oxygen isotope (δ18O) signal in firn records. δ18O is commonly used as a proxy for past atmospheric temperature reconstructions, and, when preserved, it is a crucial parameter to date and align ice cores. By comparing four different firn cores collected in 2012, 2015, 2017 and 2019 at the top of the Holtedahlfonna ice field (1100 m a.s.l.), we show a progressive deterioration of the isotope signal, and we link its degradation to the increased occurrence and intensity of melt events. Our findings indicate that, starting from 2015, there has been an escalation in melting and percolation resulting from changes in the overall atmospheric conditions. This has led to the deterioration of the climate signal preserved within the firn or ice. Our observations correspond with the model's calculations, demonstrating an increase in water percolation since 2014, potentially reaching deeper layers of the firn. Although the δ18O signal still reflects the interannual temperature trend, more frequent melting events may in the future affect the interpretation of the isotopic signal, compromising the use of Svalbard ice cores. Our findings highlight the impact and the speed at which Arctic amplification is affecting Svalbard's cryosphere.

Abstract. Sea ice is important for both regional and global climate, but comprehensive sea ice records are lacking pre-1978, when global-scale spaceborne observations began. Attempts to reconstruct sea ice conditions in different regions of Antarctica with the help of methane sulphonic acid (MSA) records from ice cores have had varying success, highlighting the often-regional relationship between ice core MSA and sea ice. This study uses MSA records from three firn cores and one ice core drilled on Fimbul Ice Shelf in Dronning Maud Land, East Antarctica, to investigate the relationship to satellite-derived sea ice extent (SIE) in the Southern Ocean. Chlorophyll-a concentrations, serving as a measure of phytoplankton biomass, are correlated to the MSA records to further test the MSA – SIE relationship. The relationship to both SIE and chlorophyll-a differs largely between the different firn and ice core MSA records. We find significant correlations for the MSA records from the two higher accumulation core sites to SIE and chlorophyll-a in the Weddell Sea, Western Pacific Ocean, and Ross Sea Sectors. Furthermore, the use of stacked MSA records introduced significant correlations between MSA from the lower accumulation core sites and SIE. The absence of coherent correlation patterns between the MSA records across the four investigated cores and SIE or chlorophyll-a in the Southern Ocean suggests that the Fimbul Ice Shelf MSA records are not consistent proxies for regional SIE.

Maritime historical documentary sources of weather and state of sea surface including sea ice can aid in filling a known climate knowledge gap for the Southern Ocean and Antarctica for the first half of the 20th century. This study presents a data set of marine climate, sea ice and icebergs recovered from a collection of logbooks from mainly Norwegian whaling factory ships that operated in the Southern Ocean during 1929–1940. The data set comprises some 8000 weather and 4000 sea ice/open sea records from austral summers of the study period. This paper further discusses the structure and content of most common Norwegian maritime documentary sources of the period along with the practices of logging information relevant for the study, such as time keeping, positioning and making weather observations. An emphasis was made on recovery of notes on sea ice and icebergs and their interpretation in terms of WMO categories of sea ice concentration. Data, including ship‐related metadata from all individual documents are homogenized and structured to a common machine‐readable format that simplifies its ingestion into relevant climate data depositories.

Abstract. Sea ice altimetry currently remains the primary method for estimating sea ice thickness from space, however time-series of sea ice thickness estimates are of limited use without having been quality-controlled against reference measurements. Such reference observations for sea ice thickness validation in the polar regions are sparse and rarely presented in a format matching the satellite-derived products. Here, the first published comprehensive collection of sea ice reference observations including freeboard, thickness, draft and snow depth from sea ice-covered regions in the Northern Hemisphere (NH) and the Southern Hemisphere (SH) is presented. The observations have been collected using airborne sensors, autonomous drifting buoys, moored and submarine-mounted upward-looking sonars, and visual observations. The data package has been prepared to match the spatial (25 km for NH and 50 km for SH) and temporal (monthly) resolutions of conventional satellite altimetry-derived sea ice thickness data products for a direct evaluation of these. This data package, also known as the Climate Change Initiative (CCI) sea ice thickness (SIT) Round Robin Data Package (RRDP) was produced within the ESA CCI sea ice project. The current version of the CCI SIT RRDP covers the polar satellite altimetry era (1993–2021) and is part of ongoing efforts to keep the dataset updated. The CCI SIT RRDP has been collocated to satellite-derived sea ice thickness products from CryoSat-2, Envisat and ERS-1/2 produced within ESA CCI and the Fundamental Data Records for Altimetry (FDR4ALT) project to demonstrate the overlap and inter-comparison between the reference observations and satellite-derived products. Here, the CCI SIT RRDP is introduced along with examples of its use as a validation source for satellite altimetry products, where the averaging, collocation and uncertainty methodology is presented and their advantages and limitations are discussed.

Abstract. Satellite retrievals of Arctic sea ice thickness typically assume fixed values of sea ice bulk density (IBD), overlooking its seasonal evolution and spatial heterogeneity, which are influenced by factors such as the age, deformation, brine, and air inclusions of the sea ice. This study investigates the seasonal variability of IBD during the Arctic freezing season from October to April, across the Distributed Network (DN) scale of the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition. To estimate IBD, we combined sea ice and snow observations from ice mass balance buoys, snow pits, repeated transects, and ice cores, together with high-resolution along-track freeboard data obtained from airborne laser scanning (ALS) and the Ice, Cloud, and land Elevation Satellite-2 (ICESat-2). Assuming hydrostatic equilibrium, IBDs were determined for the level ice components of the MOSAiC ice floes, which consisted predominantly of second-year ice (SYI). Our results revealed significant seasonal variability of the IBD with two main phases during the MOSAiC freezing season at scales of DN (~ 50 km), L-sites (~ 25 km), and Main Coring Site (MCS, ~ 50 m). Throughout the freezing season, the mean IBD estimated at the DN scale (910 ± 7 kg m−3) was close to that of the SYI cores at the MCS (912 ± 2 kg m−3), highlighting the SYI-dominated regional ice properties. We also identified that sea ice freeboard, along with the ratios of ice freeboard to total freeboard or ice freeboard to thickness, are critical indicators to determine IBD at the scale of tens of kilometers. We have therefore developed parameterizations for IBD that are expected to be applicable throughout the freezing season for the SYI region, which is also the ice type that currently dominates the central Arctic Ocean. The proposed parameterizations have the potential to optimize basin-scale IBD estimation and improve satellite-derived sea ice thickness.

RationaleThe efficiency of selected ion monitoring (SIM) and selected reaction monitoring (SRM) analyses for the quantification of three mono‐, di‐ and tri‐unsaturated highly branched isoprenoid (HBI) alkenes (IP25, IPSO25 and HBI III, respectively), often used as proxies for the occurrence of Arctic and Antarctic sea ice or the adjacent open waters, was compared.MethodsGas chromatography (GC)–mass spectrometry (MS)/SIM and GC/MS/MS/SRM analyses were carried out on dilute solutions made from purified standards of these three HBIs, and then on hydrocarbon fractions of several sediment and sea ice sample extracts. More efficient and specific SRM transitions were selected after collision‐induced dissociation of each precursor ion at different collision energies.ResultsSRM analysis avoided any overestimation of IP25 resulting from the contribution of the coeluting 13C mass isotopomer of IPSO25 (M+˙ + 2) to the SIM target ion. In contrast, SRM analysis is less reliable for IPSO25 quantification in cases where several regio‐isomers are present, likely due to intense double bond migrations following electron impact. In the case of HBI III, SRM analysis constitutes a potentially suitable alternative to SIM analysis, especially in terms of improving limit of detection.ConclusionsDespite the intense migrations of HBI double bonds under electron ionization, the selected SRM transitions should be more suitable than SIM target ions for IP25 and HBI III quantification in complex hydrocarbon fractions of natural samples. However, the advantage is less evident for IPSO25 due to the presence of numerous regio‐isomers.

Sea ice ridges are one of the most under-sampled and poorly understood components of the Arctic sea ice system. Yet, ridges play a crucial role in the sea ice mass balance and have been identified as ecological hotspots for ice-associated flora and fauna in the Arctic. To better understand the mass balance of sea ice ridges, we drilled and sampled two different first-year ice (FYI) ridges in June–July 2020 during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC). Ice cores were cut into 5 cm sections, melted, then analyzed for salinity and oxygen (δ18O) isotope composition. Combined with isotope data of snow samples, we used a mixing model to quantify the contribution of snow to the consolidated sea ice ridge mass. Our results demonstrate that snow meltwater is important for summer consolidation and overall ice mass balance of FYI ridges during the melt season, representing 6%–11% of total ridged ice mass or an ice thickness equivalent of 0.37–0.53 m. These findings demonstrate that snowmelt contributes to consolidation of FYI ridges and is a mechanism resulting in a relative increase of sea ice volume in summer. This mechanism can also affect the mechanical strength and survivability of ridges, but also contribute to reduction of the habitable space and light levels within FYI ridges. We proposed a combination of two pathways for the transport of snow meltwater and incorporation into ridge keels: percolation downward through the ridge and/or lateral transport from the under-ice meltwater layer. Whether only one pathway or a combination of both pathways is most likely remains unclear based on our observations, warranting further research on ridge morphology.

Fram Strait is the major gateway connecting the Arctic Ocean and North Atlantic Ocean, where nearly 90% of the sea ice export from the Arctic Ocean takes place. The transported ice represents a broad range of thicknesses and types and exhibits an integrated history of thermodynamic growth/decay and deformation on its way across the Arctic. The present study utilizes high resolution sea ice draft data from ice profiling sonars (IPS) from the four moorings of the Fram Strait Outflow Observatory at 78.85 N and 3W to 6.5 W over the period 2006-2019. The analysis focuses on the identification of deformed ice/sea ice ridges and analysis of the seasonal and interannual variability in the number, geometry and shape of ridges. The study demonstrates a pronounced seasonal cycle in the number, probability density function of keel drafts and shape of ridges traversing FS with a maximum ridge count in March-April and minimum in August-September. An overall decline found in the annual ridge number is accompanied by a general shallowing of ridge keels. The observed changes are most pronounced in the easternmost mooring at 3W, and linked to continuing sea ice retreat in the FS over the studied period. The results are further compared with previous studies on ridge statistics from the area and placed in the context of the observed changes in Arctic sea ice over the last two decades.

Some of the most visible signs of climate change can be found in the northern high latitude region. Due to dynamic glacial-marine contrast, sedimentary geochemical proxies from western Svalbard fjords may provide a detailed account of paleoenvironmental changes in the region. We constructed a multivariate sedimentary record from a high-Arctic fjord (Kongsfjorden) using the sediment core NP14-FM. The new multi-proxy record includes sedimentary organic matter (OM) concentrations and their carbon and nitrogen isotopic composition, covering a period between AD 1106 and AD 1967. Using an unsupervised learning technique, the multivariate data was partitioned into two groups, helping us identify occurrences of warm and cold Arctic spells at multi-decadal resolution during the last millennium. Individually, the δ15N variability forms a highly coherent pattern with surface temperature records while δ13C and elemental OM concentrations reveal changing mixing proportions between marine and terrestrial sources. Furthermore, the region's interchanging warm and cold episodes, as detected by the multivariate sedimentary record, were linked to stronger and weaker summer monsoon over India, respectively. Modulation of meridional thermal gradients over the Indian monsoon domain amid contrasting climatic conditions in the Arctic region likely facilitated the observed Arctic-Indian monsoon teleconnection during the last millennium.