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We assessed the spatial and temporal variability of the Arctic Boundary Current (ABC) using seven oceanographic moorings, deployed across the continental slope north of Severnaya Zemlya in 2015–2018. Transports and individual water masses were quantified based on temperature and salinity recorders and current profilers. Our results were compared with observations from the northeast Svalbard and the central Laptev Sea continental slopes to evaluate the hydrographic transformation along the ABC pathway. The highest velocities (>0.30 m s−1) of the ABC occurred at the upper continental slope and decreased offshore to below 0.03 m s−1 in the deep basin. The ABC showed seasonal variability with velocities two times higher in winter than in summer. Compared to upstream conditions in Svalbard, water mass distribution changed significantly within 20 km of the shelf edge due to mixing with- and intrusion of shelf waters. The ABC transported 4.15 ± 0.3 Sv in the depth range 50–1,000 m, where 0.88 ± 0.1, 1.5 ± 0.2, 0.61 ± 0.1 and 1.0 ± 0.15 Sv corresponded to Atlantic Water (AW), Dense Atlantic Water (DAW), Barents Sea Branch Water (BSBW) and Transformed Atlantic Water (TAW). 62–70% of transport was constrained to within 30–40 km of the shelf edge, and beyond 84 km, transport increases were estimated to be 0.54 Sv. Seasonality of TAW derived from local shelf-processes and advection of seasonal-variable Fram Strait waters, while BSBW transport variability was dominated by temperature changes with maximum transport coinciding with minimum temperatures. Further Barents Sea warming will likely reduce TAW and BSBW transport leading to warmer conditions along the ABC pathway.

Remobilization of soil carbon as a result of permafrost degradation in the drainage basin of the major Siberian rivers combined with higher precipitation in a warming climate potentially increase the flux of terrestrial derived dissolved organic matter (tDOM) into the Arctic Ocean. The Laptev (LS) and East Siberian Seas (ESS) receive enormous amounts of tDOM-rich river water, which undergoes at least one freeze-melt cycle in the Siberian Arctic shelf seas. To better understand how freezing and melting affect the tDOM dynamics in the LS and ESS, we sampled sea ice, river and seawater for their dissolved organic carbon (DOC) concentration and the colored fraction of dissolved organic matter. The sampling took place in different seasons over a period of 9 years (2010–2019). Our results suggest that the main factor regulating the tDOM distribution in the LS and ESS is the mixing of marine waters with freshwater sources carrying different tDOM concentrations. Of particular importance in this context are the 211 km3 of meltwater from land-fast ice from the LS, containing ~ 0.3 Tg DOC, which in spring mixes with 245 km3 of river water from the peak spring discharge of the Lena River, carrying ~ 2.4 Tg DOC into the LS. During the ice-free season, tDOM transport on the shelves takes place in the surface mixed layer, with the direction of transport depending on the prevailing wind direction. In winter, about 1.2 Tg of brine-related DOC, which was expelled from the growing land-fast ice in the LS, is transported in the near-surface water layer into the Transpolar Drift Stream that flows from the Siberian Shelf toward Greenland. The actual water depth in which the tDOM-rich brines are transported, depends mainly on the density stratification of the LS and ESS in the preceding summer and the amount of ice produced in winter. We suspect that climate change in the Arctic will fundamentally alter the dynamics of tDOM transport in the Arctic marginal seas, which will also have conse

In this paper, we investigate the seasonal and spatial variability of stratification on the Siberian shelves with a case study from the Laptev Sea based on shipboard hydrographic measurements, year-round oceanographic mooring records from 2013 to 2014 and chemical tracer-based water mass analyses. In summer 2013, weak onshore-directed winds caused spreading of riverine waters throughout much of the eastern and central shelf. In contrast, strong southerly winds in summer 2014 diverted much of the freshwater to the northeast, which resulted in 50% less river water and significantly weaker stratification on the central shelf compared with the previous year. Our year-long records additionally emphasize the regional differences in water column structure and stratification, where the northwest location was well-mixed for 6 months and the central and northeast locations remained stratified into spring due to the lower initial surface salinities of the river-influenced water. A 26 year record of ocean reanalysis highlights the region’s interannual variability of stratification and its dependence on winds and sea ice. Prior the mid-2000s, river runoff to the perennially ice-covered central Laptev Sea shelf experienced little surface forcing and river water was maintained on the shelf. The transition toward less summer sea ice after the mid-2000s increased the ROFI’s (region of freshwater influence) exposure to summer winds. This greatly enhanced the variability in mixed layer depth, resulting in several years with well-mixed water columns as opposed to the often year-round shallow mixed layers before. The extent of the Lena River plume is critical for the region since it modulates nutrient fluxes and primary production, and further controls intermediate heat storage induced by lateral density gradients, which has implications for autumnal freeze-up and the eastern Arctic sea ice volume. MAIN POINTS 1. CTD surveys and moorings highlight the regional and temporal variations in

In this paper, we investigate the seasonal and spatial variability of stratification on the Siberian shelves with a case study from the Laptev Sea based on shipboard hydrographic measurements, year-round oceanographic mooring records from 2013 to 2014 and chemical tracer-based water mass analyses. In summer 2013, weak onshore-directed winds caused spreading of riverine waters throughout much of the eastern and central shelf. In contrast, strong southerly winds in summer 2014 diverted much of the freshwater to the northeast, which resulted in 50% less river water and significantly weaker stratification on the central shelf compared with the previous year. Our year-long records additionally emphasize the regional differences in water column structure and stratification, where the northwest location was well-mixed for 6 months and the central and northeast locations remained stratified into spring due to the lower initial surface salinities of the river-influenced water. A 26 year record of ocean reanalysis highlights the region’s interannual variability of stratification and its dependence on winds and sea ice. Prior the mid-2000s, river runoff to the perennially ice-covered central Laptev Sea shelf experienced little surface forcing and river water was maintained on the shelf. The transition toward less summer sea ice after the mid-2000s increased the ROFI’s (region of freshwater influence) exposure to summer winds. This greatly enhanced the variability in mixed layer depth, resulting in several years with well-mixed water columns as opposed to the often year-round shallow mixed layers before. The extent of the Lena River plume is critical for the region since it modulates nutrient fluxes and primary production, and further controls intermediate heat storage induced by lateral density gradients, which has implications for autumnal freeze-up and the eastern Arctic sea ice volume.

The presence of optically active water constituents is known to attenuate the light penetration in the ocean and impact the ocean heat content. Here, we investigate the influence of colored dissolved organic matter (CDOM) and total suspended matter (TSM) on the radiative heating of the Laptev Sea shelf waters. The Laptev Sea region is heavily influenced by the Lena River, one of the largest river systems in the Arctic region. We simulate the radiative heating by using a coupled atmosphere-ocean radiative transfer model (RTM) and in situ measurements from the TRANSDRIFT XVII expedition carried out in September 2010. The results indicate that CDOM and TSM have significant influence on the energy budget of the Laptev Sea shelf waters, absorbing most of the solar energy in the first 2 m of the water column. In the station with the highest CDOM absorption (aCDOM(443) = 1.77 m−1) ~43% more energy is absorbed in the surface layer compared to the station with the lowest aCDOM(443) (~0.2 m−1), which translates to an increased radiative heating of ~0.6°C/day. The increased absorbed energy by the water constituents also implies increased sea ice melt rate and changes in the surface heat fluxes to the atmosphere. By using satellite remote sensing and RTM we quantify the spatial distribution of the radiative heating in the Laptev Sea for a typical summer day. The combined use of satellite remote sensing, RT modeling and in situ observations can be used to improve parameterization schemes in atmosphere-ocean circulation models to assess the role of the ocean in the effect of Arctic amplification.

According to historical oceanographic data, anomalies in the dynamic topography during the winter period were calculated and two climatic stages of dynamic condition of the Arctic Ocean were defined: 1949–1993 and 2007–2013. The associativity of opposition of anomalies in the dynamic topography of the Eurasian and Amerasian basins with fluctuations in the thermal condition of the Northern Atlantic and Pacific oceans, the indexes of atmospheric circulation, river runoff, and change in the area of ice in August of the Laptev, East Siberian, and Chukchi seas is established.

Sea ice is an important vehicle for sediment transport in the Arctic Ocean. On the Laptev Sea shelf (Siberian Arctic) large volumes of sediment-laden sea ice are formed during freeze-up in autumn, then exported and transported across the Arctic Ocean into Fram Strait where it partly melts. The incorporated sediments are released, settle on the sea floor, and serve as a proxy for ice-transport in the Arctic Ocean on geological time scales. However, the formation process of sediment-laden ice in the source area has been scarcely observed. Sediment-laden ice was sampled during a helicopter-based expedition to the Laptev Sea in March/April 2012. Sedimentological, biogeochemical and biological studies on the ice core as well as in the water column give insights into the formation process and, in combination with oceanographic process studies, on matter fluxes beneath the sea ice. Based on satellite images and ice drift back-trajectories the sediments were likely incorporated into the sea ice during a mid-winter coastal polynya near one of the main outlets of the Lena River, which is supported by the presence of abundant freshwater diatoms typical for the Lena River phytoplankton, and subsequently transported about 80 km northwards onto the shelf. Assuming ice growth of 12 –19 cm during this period and mean suspended matter content in the newly formed ice of 91.9 mg l− 1 suggests that a minimum sediment load of 8.4×10 4 t might have been incorporated into sea ice. Extrapolating these sediment loads for the entire Lena Delta region suggests that at least 65% of the estimated sediment loads which are incorporated during freeze-up, and up to 10% of the annually exported sediment load may be incorporated during an event such as described in this paper.

Highlights • Observations show that formation of sediment-laden sea ice occurs in coastal polynyas in winter. • Sea ice rafted sediments are a significant component of the Laptev Sea’s sediment budget. • No observational evidence for sediment entrainment into sea ice in mid-shelf polynyas at water depth greater than 20 m. Abstract Sea ice is an important vehicle for sediment transport in the Arctic Ocean. On the Laptev Sea shelf (Siberian Arctic) large volumes of sediment-laden sea ice are formed during freeze-up in autumn, then exported and transported across the Arctic Ocean into Fram Strait where it partly melts. The incorporated sediments are released, settle on the sea floor, and serve as a proxy for ice-transport in the Arctic Ocean on geological time scales. However, the formation process of sediment-laden ice in the source area has been scarcely observed. Sediment-laden ice was sampled during a helicopter-based expedition to the Laptev Sea in March/April 2012. Sedimentological, biogeochemical and biological studies on the ice core as well as in the water column give insights into the formation process and, in combination with oceanographic process studies, on matter fluxes beneath the sea ice. Based on satellite images and ice drift back-trajectories the sediments were likely incorporated into the sea ice during a mid-winter coastal polynya near one of the main outlets of the Lena River, which is supported by the presence of abundant freshwater diatoms typical for the Lena River phytoplankton, and subsequently transported about 80 km northwards onto the shelf. Assuming ice growth of 12 to 19 cm during this period and mean suspended matter content in the newly formed ice of 91.9 mg l-1 suggests that a minimum sediment load of 8.4x104 t might have been incorporated into sea ice. Extrapolating these sediment loads for the entire Lena Delta region suggests that at least 65% of the estimated sediment loads which are incorporated during freeze-up, and up to 10% of