Sandra Tippenhauer

Climatic changes in the physical environment modulate biogeochemical cycles, biodiversity, and trophic interactions in the Central Arctic Ocean (CAO). Physical processes and sea-ice conditions are highly seasonal in the CAO and dependent on interactions that occur throughout the evolution of the upper ocean–sea ice–lower atmosphere system. Understanding these seasonal interactions is critical to comprehending and predicting the long-term trends as the CAO moves towards ice-free summers and to informing future policy decisions at the core of ongoing discussions concerning the CAO Fisheries agreement, for example, at the Arctic Council and International Council for Exploration of the Sea working group on the CAO. Here, we review current knowledge of the physical environment, biogeochemical cycles, and biodiversity in the waters of the CAO, identify emerging research questions, and introduce the science plan for the first Tara Polaris drift onboard the Tara Polar Station to advance knowledge and address these questions. Despite increased observational programmes in the CAO over the past years, for example, the Nansen and Amundsen Basin Observational System (NABOS) and Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC), extensive knowledge gaps remain in relation to ocean stratification, sea ice and lightscape, nitrogen fixation and nutrient fluxes, carbon export and transfer, sympagic-pelagic coupling, aerosol production, contaminant transport and transformation, chronobiology, and fish distribution. Further knowledge on overall CAO biodiversity, ecosystem functionality and interannual variability is also critically needed. We describe a way forward to address these knowledge gaps using ice-tethered and profiling instruments coupled with multi-omics, culturing, and imagery approaches deployed from Tara Polar Station during the first of ten Tara Polaris drifts designed to facilitate detection of interannual variability and change over time.

Wintertime observations in the central Arctic Ocean are sparse. We report a unique early winter data set containing atmospheric, hydrographic, and ocean velocity observations obtained during the first leg of the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition, with sea ice concentration at nearly 100%. One storm and four other periods of strong wind were encountered during the drift. This work focuses on the impact of the storm on oceanic variability in the upper 200 m. Variable oceanic surface mixed layers and pervasive horizontal density gradients were observed in the upper 100 m. Several anomalously large horizontal density gradients which lasted 1–2 days can be related to intrahalocline eddies. The nearly mixed upper 20-m water column and the deeper heavily stratified layer belonged to two separate dynamical regimes and appeared unaffected by the storm. Signatures of forced currents due to high ice drift speeds and velocity maxima resulting from intrahalocline eddies were seen above and below the base of the mixed layer, respectively. Near-inertial internal waves (NIWs) were detected among ice floes underneath the base of the mixed layer and were prevalent in the upper 100 m. Some NIWs were observed at depths deeper than 100 m with downward energy propagation. These results imply NIWs were generated at the surface. The single storm potentially injected half the overall near-inertial energy into the mixed layer, underlining the role of storm systems in the Arctic winter. Our results suggest that internal ice stress dampens momentum transfer from wind to the upper ocean, rendering wind forcing insufficient to erode preexisting stratification under full sea ice cover. This dampening contributes to the persistent thermohaline gradients observed. Instead, sea ice moving coherently in response to the winds acts as the main surface forcing driving upper-ocean processes such as NIWs.

The Southern Ocean plays a central role in the global climate system by regulating large-scale circulation, facilitating interbasin exchange, absorbing large amounts of anthropogenic heat and carbon and influencing Antarctic Ice Sheet stability. Yet, observational data, including rare earth element measurements such as neodymium (Nd) isotopes and samarium (Sm), have so far remained sparse in the Indian sector and along the East Antarctic continental margin, thereby limiting our understanding of circulation, water mass transformation, and sediment-ocean interactions in a changing climate.Water masses in the Southern Ocean are traditionally characterized using hydrographic parameters (e.g., potential temperature, salinity, neutral density). During physico-chemical weathering along the East Antarctic margin, old continental crust supplies a distinctly unradiogenic neodymium isotope signature (low εNd) to regional shelf waters that interact with more radiogenic Antarctic Circumpolar Current waters (higher εNd) further north. This isotopic difference makes neodymium isotopes an especially powerful tracer of regional circulation and mixing along the East Antarctic continental margin. We present the first high-resolution dataset of dissolved εNd, together with Nd and Sm concentrations, from the Indian sector of the Southern Ocean. Gridded water column samples in conjunction with bottom water samples extracted from multicorer sediment supernatants, collected during expedition EASI-2 onboard RV Polarstern (Dec 2023–Feb 2024), provide a meridional transect from the Denman Glacier front (~66°S) to ~45°S along 100°E. Combined with hydrographic observations, this dataset provides a detailed framework for examining water mass structure, mixing, and regional boundary fluxes along the transect.Away from direct Antarctic continental influences, the εNd distributions show largely conservative behavior in intermediate to deep waters and allow clear identification of major Southern Ocean water masses. A striking feature is the persistence of a remnant North Atlantic Deep Water εNd signal within lower Circumpolar Deep Water, highlighting long-range interbasin connectivity. Near the Denman Glacier, warm and radiogenic modified Circumpolar Deep Water (mCDW) intrudes onto the continental shelf, evident in both physical properties and εNd signatures below ~400 meters water depth. As seen in earlier studies, a pronounced mCDW tongue was observed to reach close to the Denman Glacier front, with associated high basal melt rates evident from potential temperature and salinity in sampled local East Antarctic shelf waters.Nd and Sm concentrations increase linearly with depth north of the Polar Front, but exhibit substantial enrichment south of the front, reflecting deep-water upwelling, biogenic scavenging, and a latitudinally gradual boundary exchange. Pronounced variations in εNd and rare earth element concentrations in bottom waters point to substantial benthic additions in the southern reaches of the transect driven by weathering inputs from ambient terrigenous sediments, whereas particle-related scavenging appears to dominate offshore.This study closes a critical observational gap in the Indian sector of the Southern Ocean and provides new constraints on the present-day circulation, water mass structure, and the influence of Antarctic crustal and benthic Nd additions, while demonstrating the value of εNd as a tracer in modern and paleoceanographic contexts.

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.

Chromophoric dissolved organic matter (CDOM) is a ubiquitous component in marine environments, and substantial changes in its sources and distribution, related to the carbon cycle in the Arctic Ocean, are expected due to Arctic warming. In this study, we present unique CDOM data in the Eurasian Arctic Ocean derived from the year‐round MOSAiC expedition. We used CDOM absorbance spectra and fluorescence excitation‐emission matrices in combination with parallel factor analysis to characterize differences in DOM sources and composition. Our results suggested that terrestrial DOM was less sensitive to seasonal changes but controlled by regionality in hydrography. Elevated dissolved organic carbon (DOC) levels in polar surface water were primarily derived from terrigenous sources as identified by CDOM absorption and fluorescence characteristics. In the Amundsen Basin and western Fram Strait surface waters, to which terrestrial DOM is primarily transported by the Transpolar Drift, we found, on average, a 188% larger meteoric water fraction and a 40% higher DOC concentration compared to the Atlantic water that dominated western Nansen Basin and Yermak Plateau. In the Amundsen Basin, the DOC concentration in summer of surface water was only 13% higher compared to winter season. Additionally, autochthonous DOM and chlorophyll‐a concentrations were relatively low in surface water and exhibited significant differences compared to those observed in summer, while there were significant differences between autochthonous DOM and chlorophyll‐a. We also observed that sea ice melt contributed to autochthonous DOM in summer, while storms in winter affected the vertical distribution of terrestrial and autochthonous DOM in the subsurface.

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

The Arctic Ocean is a region important for global and regional climate. Although generally quiescent compared to mid-latitudes, the upper Arctic ocean hosts mesoscale and smaller scale processes. These processes can have a profound impact on vertical ocean fluxes, stratification, and feedback with the sea ice and atmosphere. Sparse and non-synoptic in-situ observations of the polar oceans are limited by the distribution of manual observing platforms and autonomous instrumentation. Analyzing observational data to assess tracer field gradients and upper ocean dynamics becomes highly challenging when measurement platforms drift with the ice pack due to continuous changes in drift speed direction. This work presents a dynamical reconstruction of the ocean state, based on observations of the Multidisciplinary Observatory for the Study of Arctic Climate (MOSAiC) experiment. Overall, the model can reproduce the lateral and vertical structure of the temperature, salinity, and density fields, which allows for projecting dynamically consistent features of these fields onto a regular grid. We identify two separate depth ranges of enhanced eddy kinetic energy, which are located around two maxima in buoyancy frequency: the depth of the upper halocline and the depth of the warm (modified) Atlantic Water. Simulations reveal a notable decrease in surface layer salinity and density towards the north, accompanied by high variability in the mixed layer depth in the south-north direction. And no significant horizontal gradients in salinity and density fields but an increase in mixed layer depth from west to east 0.084 m/km gradient with 0.6 m/km standard deviation, indicating opposite characteristics compared to the south-north direction. The model resolves several stationary eddies in the warm Atlantic Water and provides insights into the associated dynamics. The obtained three-dimensional fields of temperature and salinity can be used for further analysis of the thermohaline structure and related dynamics associated with submesoscale processes in the Central Arctic. Dynamic characteristics and eddy fields can be used for further analysis and comparison with state-of-the-art climate and Earth System Models. The developed nudging method can be used to utilize future observational data obtained from a diverse set of instruments.

The Arctic Ocean is a region important for global and regional climate. Although generally quiescent compared to mid-latitudes, the upper Arctic ocean hosts mesoscale and smaller scale processes. These processes can have a profound impact on vertical ocean fluxes, stratification, and feedback with the sea ice and atmosphere. Sparse and non-synoptic in-situ observations of the polar oceans are limited by the distribution of manual observing platforms and autonomous instrumentation. Analyzing observational data to assess tracer field gradients and upper ocean dynamics becomes highly challenging when measurement platforms drift with the ice pack due to continuous changes in drift speed direction. This work presents a dynamical reconstruction of the ocean state, based on observations of the Multidisciplinary Observatory for the Study of Arctic Climate (MOSAiC) experiment. Overall, the model can reproduce the lateral and vertical structure of the temperature, salinity, and density fields, which allows for projecting dynamically consistent features of these fields onto a regular grid. We identify two separate depth ranges of enhanced eddy kinetic energy, which are located around two maxima in buoyancy frequency: the depth of the upper halocline and the depth of the warm (modified) Atlantic Water. Simulations reveal a notable decrease in surface layer salinity and density towards the north, accompanied by high variability in the mixed layer depth in the south-north direction. And no significant horizontal gradients in salinity and density fields but an increase in mixed layer depth from west to east 0.084 m/km gradient with 0.6 m/km standard deviation, indicating opposite characteristics compared to the south-north direction. The model resolves several stationary eddies in the warm Atlantic Water and provides insights into the associated dynamics. The obtained three-dimensional fields of temperature and salinity can be used for further analysis of the thermohaline structure and related dynamics associated with submesoscale processes in the Central Arctic. Dynamic characteristics and eddy fields can be used for further analysis and comparison with state-of-the-art climate and Earth System Models. The developed nudging method can be used to utilize future observational data obtained from a diverse set of instruments.

Microalgae are the main source of the omega‐3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), essential for the healthy development of most marine and terrestrial fauna including humans. Inverse correlations of algal EPA and DHA proportions (% of total fatty acids) with temperature have led to suggestions of a warming‐induced decline in the global production of these biomolecules and an enhanced importance of high latitude organisms for their provision. The cold Arctic Ocean is a potential hotspot of EPA and DHA production, but consequences of global warming are unknown. Here, we combine a full‐seasonal EPA and DHA dataset from the Central Arctic Ocean (CAO), with results from 13 previous field studies and 32 cultured algal strains to examine five potential climate change effects; ice algae loss, community shifts, increase in light, nutrients, and temperature. The algal EPA and DHA proportions were lower in the ice‐covered CAO than in warmer peripheral shelf seas, which indicates that the paradigm of an inverse correlation of EPA and DHA proportions with temperature may not hold in the Arctic. We found no systematic differences in the summed EPA and DHA proportions of sea ice versus pelagic algae, and in diatoms versus non‐diatoms. Overall, the algal EPA and DHA proportions varied up to four‐fold seasonally and 10‐fold regionally, pointing to strong light and nutrient limitations in the CAO. Where these limitations ease in a warming Arctic, EPA and DHA proportions are likely to increase alongside increasing primary production, with nutritional benefits for a non‐ice‐associated food web.

Trace gases have demonstrated their strength for oceanographic studies, with applications ranging from the tracking of glacial meltwater plumes to estimates of the abyssal overturning duration. Yet measurements of such passive tracers in the ice-covered Arctic Ocean are sparse. We here present a unique data set of trace gases collected during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition, during which R/V Polarstern drifted along with the Arctic sea ice from the Laptev Sea to Fram Strait, from October 2019 to September 2020. During the expedition, trace gases from anthropogenic origin (chlorofluorocarbon 12 (CFC-12), sulfur hexafluoride (SF₆), and tritium) along with noble gases (helium and neon) and their isotopes were collected at a weekly or higher temporal resolution throughout the entire water column (and occasionally in the snow) from the ship and from the ice. We describe the sampling procedures along with their challenges, the analysis methods, and the data sets, and we present case studies in the central Arctic Ocean and Fram Strait to illustrate possible usage for the data along with their robustness. Combined with simultaneous hydrographic measurements, these trace gas data sets can be used for process studies and water mass tracing throughout the Arctic in subsequent analyses. The two data sets can be downloaded via PANGAEA: https://doi.org/10.1594/PANGAEA.961729 (Huhn et al., 2023a) and https://doi.org/10.1594/PANGAEA.961738 (Huhn et al., 2023b).