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Given the prospect of a merely seasonally ice-covered Arctic Ocean in the future and a consequential new quality of atmosphere-ocean coupling, understanding and quantifying oceanic processes that contribute to sea ice melt is of particular relevance.A region of intense melting is the marginal ice zone (MIZ) north of Fram Strait, where inflowing warm Atlantic Water meets sea ice advected southward by the Transpolar Drift. We present observations of the ice-ocean boundary layer (IOBL) from a cruise of German research vessel Polarstern to that region in summer 2022, where we gathered continuous-in-time hydrographic observations from autonomous drifting stations on three separate ice floes, supplemented by intense observation periods of vertical microstructure profiles and ice cores from crewed stations during three revisits per floe throughout the drifting period.The three occupied floes were oriented on a line approximately perpendicular to the ice edge, initially about 25 km apart from each other, with the southernmost floe located 75 km away from the edge. The drifting instrument platforms cover a common time period of approximately two weeks, under relatively quiescent atmospheric conditions. First results show that, while the floes exhibited similar drift trajectories dominated by superimposed diurnal and semidiurnal oscillations, the evolution of key IOBL variables, such as stratification, melt rates, friction velocity, and turbulent fluxes, varied considerably – both in time and among the occupied floes.We plan to assess how this observed variability relates to other measured properties of sea ice (e.g., ice roughness, ice thickness distribution, floe size distribution) and of the upper ocean (e.g., ice-ocean velocity shear, turbulence, surface waves, internal waves and tides) and their interaction, in order to put our preliminary findings into the broader context: ocean controls on sea ice melt in the marginal ice zone north of Fram Strait.

The marginal ice zone in Fram Strait is a highly variable environment, in which dense Atlantic Water and lighter Polar Water meet and create numerous mesoscale and submesoscale fronts. This makes it a model region for researching ocean frontal dynamics in the Arctic, as the interaction between Atlantic Water and the marginal ice zone is becoming increasingly important in an "atlantifying" Arctic Ocean. Here we present the first results of a front study conducted near the ice edge in central Fram Strait, where Atlantic Water subducted below Polar Water. We posit that the frontal dynamics associated with the sea ice edge also apply beyond, both to the open and the ice-covered ocean in the vicinity. They, in turn, can affect the structure of the marginal ice zone. The study comprises a total of 54 high resolution transects, most of which were oriented across the front. They were taken over the course of a week during July 2020 and include current velocity measurements from a vessel-mounted ADCP. Most of the transects also include either temperature and salinity measurements from an underway CTD, or temperature and salinity measurements and various biogeochemical properties from a TRIAXUS towed vehicle. Additionally, 22 CTD stations were conducted, and 31 surface drifters were deployed. This wealth of measurements gives us the opportunity to follow the temporal and spatial development of the density fronts present at the time. We discuss the dynamics of the frontal development, including the associated geostrophic motion, and the induced secondary ageostrophic circulation with subsequent subduction of Atlantic Water and biological material in a highly stratified region. Beneath the stratified upper ocean, subduction is clearly visible in the biogeochemical properties, and water samples indicate a substantial vertical transport of smaller particles. Surface drifters accumulated in locations of subduction, where sea ice, if present, would likely also accumulate. Our study thus demonstrates the importance of frontal dynamics for the vertical transport of water properties and biological material, and the highly variable development of the marginal ice zone in Fram Strait.

The Fram Strait, the deepest gateway to the Arctic Ocean, is strongly influenced by eddy dynamics. Here we analyse the output from two eddy-resolving models (ROMS and FESOM) with around 1 km mesh resolution in the Fram Strait, with focus on their representation of eddy properties and dynamics. A comparison with mooring observations shows that both models reasonably simulate hydrography and eddy kinetic energy. Despite differences in model formulation, they show relatively similar eddy properties. The eddies have a mean radius of 4.9 km and 5.6 km in ROMS and FESOM, respectively, with slightly more cyclones than anticyclones (ROMS: 54 %, FESOM: 55 %). The lifetime of detected eddies is relatively short in both simulations (ROMS: 10 days, FESOM: 11 days), and the mean travel distance is 35 km in both models. More anticyclones are trapped in deep depressions or move toward deep locations. The two models show comparable patterns of baroclinic and barotropic instability. However, ROMS has relatively stronger eddy intensity and baroclinic instability, possibly due to its smaller grid size and higher effective resolution. Overall, the relatively good agreement between the two models strengthens our confidence in their ability to realistically represent the Fram Strait ocean dynamics, and also highlights the need for very high mesh resolution.

Measuring temperature and salinity profiles in the world's oceans is crucial to understanding ocean dynamics and its influence on the heat budget, the water cycle, the marine environment and on our climate. Since 1983 the German research vessel and icebreaker Polarstern has been the platform of numerous CTD (conductivity, temperature, depth instrument) deployments in the Arctic and the Antarctic. We report on a unique data collection spanning 33 years of polar CTD data. In total 131 data sets (1 data set per cruise leg) containing data from 10 063 CTD casts are now freely available at doi:10.1594/PANGAEA.860066. During this long period five CTD types with different characteristics and accuracies have been used. Therefore the instruments and processing procedures (sensor calibration, data validation, etc.) are described in detail. This compilation is special not only with regard to the quantity but also the quality of the data – the latter indicated for each data set using defined quality codes. The complete data collection includes a number of repeated sections for which the quality code can be used to investigate and evaluate long-term changes. Beginning with 2010, the salinity measurements presented here are of the highest quality possible in this field owing to the introduction of the OPTIMARE Precision Salinometer.

Nutrient-rich water upwells offshore of northwest Africa and is subsequently advected westwards. There it forms eddies and filaments with a rich spatial structure of physical and biological/biogeochemical properties. Here we present a high-resolution (2.5 km) section through upwelling filaments and an eddy obtained in May 2018 with a TRIAXUS towed vehicle equipped with various oceanographic sensors. Physical processes at the mesoscale and submesoscale such as symmetric instability, trapping of fluid in eddies, and subduction of low potential vorticity (which we use as a water mass tracer) water can explain the observed distribution of biological production and export. We found a nitrate excess (higher concentrations of nitrate than expected from oxygen values if only influenced by production and remineralization processes) core of an anticyclonic mode water eddy. We also found a high nitrate concentration region of ≈5 km width in the mixed layer where symmetric instability appears to have injected nutrients from below into the euphotic zone. Similarly, further south high chlorophyll-a concentrations suggest that nutrients had been injected there a few days earlier. Considering that such interactions of physics and biology are ubiquitous in the upwelling regions of the world, we assume that they strongly influence the productivity of such systems and their role in CO2 uptake. The intricate interplay of different parameters at the kilometer scale needs to be taken into account when interpreting single-profile and/or bottle data in dynamically active regions of the ocean.

Warm Atlantic Water (AW) flows around the Nordic Seas in a cyclonic boundary current loop. Some AW enters the Arctic Ocean where it is transformed to Arctic Atlantic Water (AAW) before exiting through Fram Strait. There the AAW is joined by recirculating AW. Here we present the first summer synoptic study targeted at resolving this confluence in Fram Strait which forms the East Greenland Current (EGC). Absolute geostrophic velocities and hydrography from observations in 2016, including four sections crossing the east Greenland shelfbreak, are compared to output from an eddy-resolving configuration of the sea–ice ocean model FESOM. Far offshore (120 km at 80.8° N) AW warmer than 2 °C is found in northern Fram Strait. The Arctic Ocean outflow there is broad and barotropic, but gets narrower and more baroclinic toward the south as recirculating AW increases the cross-shelfbreak density gradient. This barotropic to baroclinic transition appears to form the well-known EGC boundary current flowing along the shelfbreak further south where it has been previously described. In this realization, between 80.2° N and 76.5° N, the southward transport along the east Greenland shelfbreak increases from roughly 1 Sv to about 4 Sv and the warm water composition, defined as the fraction of AW of the sum of AW and AAW (AW/(AW + AAW)), changes from 19 ± 8 % to 80 ± 3 %. Consequently, in southern Fram Strait, AW can propagate into Norske Trough on the east Greenland shelf and reach the large marine terminating glaciers there. High instantaneous variability observed in both the synoptic data and the model output is attributed to eddies, the representation of which is crucial as they mediate the westward transport of AW in the recirculation and thus structure the confluence forming the EGC.

Measuring temperature and salinity profiles in the world's oceans is crucial to understand ocean dynamics and its influence on the heat budget, the water cycle, the marine environment and on our climate. Since 1983 the German research vessel and icebreaker POLARSTERN has been the platform of numerous CTD deployments in the Arctic and the Antarctic. We report on a unique data collection spanning 33 years of polar CTD (conductivity, temperature, depth) data. In total 131 datasets (one dataset per cruise leg) containing data from 10063 CTD casts are now freely available at doi:10.1594/PANGAEA.860066. During this long period five CTD types with different characteristics and accuracies have been used. Therefore the instruments and processing procedures (sensor calibration, data validation etc.) are described in detail. This compilation is special not only with regard to the quantity, but also the quality of the data – the latter one being indicated for each dataset using defined quality codes. The complete data collection includes a number of repeated sections for which the quality code can be used to investigate and evaluate long-term changes. Beginning with 2010, the salinity measurements presented here are of the highest quality possible in this field owing to the introduction of the Optimare Precision Salinometer.