ALBERT

Description: 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 water column stratification on the Laptev Sea shelf. 2. Summer winds increasingly control the extent of the region of freshwater influence under decreasing sea ice. 3. Further reductions in sea ice increases surface warming, heat storage, and the interannual variability in mixed layer depth.

Description: The gridded sea ice thickness (SIT) climate data record (CDR) produced by the European Space Agency (ESA) Sea Ice Climate Change Initiative Phase 2 (CCI-2) is the longest available, Arctic-wide SIT record covering the period from 2002 to 2017. SIT data are based on radar altimetry measurements of sea ice freeboard from the Environmental Satellite (ENVISAT) and CryoSat-2 (CS2). The CCI-2 SIT has previously been validated with in situ observations from drilling, airborne remote sensing, electromagnetic (EM) measurements and upward-looking sonars (ULSs) from multiple ice-covered regions of the Arctic. Here we present the Laptev Sea CCI-2 SIT record from 2002 to 2017 and use newly acquired ULS and upward-looking acoustic Doppler current profiler (ADCP) sea ice draft (VAL) data for validation of the gridded CCI-2 and additional satellite SIT products. The ULS and ADCP time series provide the first long-term satellite SIT validation data set from this important source region of sea ice in the Transpolar Drift. The comparison of VAL sea ice draft data with gridded monthly mean and orbit trajectory CCI-2 data, as well as merged CryoSat-2–SMOS (CS2SMOS) sea ice draft, shows that the agreement between the satellite and VAL draft data strongly depends on the thickness of the sampled ice. Rather than providing mean sea ice draft, the considered satellite products provide modal sea ice draft in the Laptev Sea. Ice drafts thinner than 0.7 m are overestimated, while drafts thicker than approximately 1.3 m are increasingly underestimated by all satellite products investigated for this study. The tendency of the satellite SIT products to better agree with modal sea ice draft and underestimate thicker ice needs to be considered for all past and future investigations into SIT changes in this important region. The performance of the CCI-2 SIT CDR is considered stable over time; however, observed trends in gridded CCI-2 SIT are strongly influenced by the uncertainties of ENVISAT and CS2 and the comparably short investigation period.

Description: In September 2019, the research icebreaker Polarstern started the largest multidisciplinary Arctic expedition to date, the MOSAiC (Multidisciplinary drifting Observatory for the Study of Arctic Climate) drift experiment. Being moored to an ice floe for a whole year, thus including the winter season, the declared goal of the expedition is to better understand and quantify relevant processes within the atmosphere–ice–ocean system that impact the sea ice mass and energy budget, ultimately leading to much improved climate models. Satellite observations, atmospheric reanalysis data, and readings from a nearby meteorological station indicate that the interplay of high ice export in late winter and exceptionally high air temperatures resulted in the longest ice-free summer period since reliable instrumental records began. We show, using a Lagrangian tracking tool and a thermodynamic sea ice model, that the MOSAiC floe carrying the Central Observatory (CO) formed in a polynya event north of the New Siberian Islands at the beginning of December 2018. The results further indicate that sea ice in the vicinity of the CO (〈40 km distance) was younger and 36 % thinner than the surrounding ice with potential consequences for ice dynamics and momentum and heat transfer between ocean and atmosphere. Sea ice surveys carried out on various reference floes in autumn 2019 verify this gradient in ice thickness, and sediments discovered in ice cores (so-called dirty sea ice) around the CO confirm contact with shallow waters in an early phase of growth, consistent with the tracking analysis. Since less and less ice from the Siberian shelves survives its first summer (Krumpen et al., 2019), the MOSAiC experiment provides the unique opportunity to study the role of sea ice as a transport medium for gases, macronutrients, iron, organic matter, sediments and pollutants from shelf areas to the central Arctic Ocean and beyond. Compared to data for the past 26 years, the sea ice encountered at the end of September 2019 can already be classified as exceptionally thin, and further predicted changes towards a seasonally ice-free ocean will likely cut off the long-range transport of ice-rafted materials by the Transpolar Drift in the future. A reduced long-range transport of sea ice would have strong implications for the redistribution of biogeochemical matter in the central Arctic Ocean, with consequences for the balance of climate-relevant trace gases, primary production and biodiversity in the Arctic Ocean.

Description: The data set includes the light absorption coefficient of colored dissolved organic matter (CDOM) from 200 nm to 700 nm wavelength together with physical oceanographic observations (temperature and salinity) from the Laptev Sea. Sampling was carried out September 2010. During the ship-based expedition, we operated a CTD profiler (conductivity temperature-depth; Seabird 19+) to sample the basic hydrographic parameters. The CTD is operated on a carousel water sampler (SBE 32SC) allowing automated water sampling at pre-selected depths. The data set is part of a multi-year study (different seasons over a period of 9 years: 2010-2019) of the dissolved organic matter (CDOM), which includes observations in marine waters from the coastal zone to the shelf edge of the Laptev Sea and East Siberian Sea, the river Lena and ice and water samples from the land-fast in the south-eastern Laptev Sea.

Description: The data set includes the light absorption coefficient of colored dissolved organic matter (CDOM) from 200 nm to 700 nm wavelength and dissolved organic carbon (DOC) together with physical oceanographic observations (temperature and salinity) from the Laptev Sea. Sampling was carried out August and September 2018. During the ship-based expedition, we operated a CTD profiler (conductivity temperature-depth; Seabird 911+) to sample the basic hydrographic parameters. The CTD is operated on a carousel water sampler (SBE 32), which can take water samples at the selected depths. The dataset is part of a multi-year study (different seasons over a period of 9 years: 2010-2019) of the dissolved organic matter (CDOM), which includes observations in marine waters from the coastal zone to the shelf edge of the Laptev Sea and East Siberian Sea, the river Lena and ice and water samples from the land-fast in the south-eastern Laptev Sea.

Description: The data set includes the light absorption coefficient of colored dissolved organic matter (CDOM) from 200 nm to 700 nm wavelength and dissolved organic carbon (DOC) together with physical oceanographic observations (temperature and salinity) from the Laptev Sea and East Siberian Sea. Sampling was carried out September and October 2019. During the ship-based expedition, we operated a CTD profiler (conductivity temperature-depth; Seabird 911+) to sample the basic hydrographic parameters. The CTD is operated on a carousel water sampler (SBE 32), which can take water samples at the selected depths. The dataset is part of a multi-year study (different seasons over a period of 9 years: 2010-2019) of the dissolved organic matter (CDOM), which includes observations in marine waters from the coastal zone to the shelf edge of the Laptev Sea and East Siberian Sea, the river Lena and ice and water samples from the land-fast in the south-eastern Laptev Sea.

Description: The data set includes the light absorption coefficient of colored dissolved organic matter (CDOM) from 200 nm to 700 nm wavelength and dissolved organic carbon (DOC) together with physical oceanographic observations (temperature and salinity) from the Laptev Sea. Sampling was carried out August and September 2011. During the ship-based expedition, we operated a CTD profiler (conductivity temperature-depth; Seabird 19+) to sample the basic hydrographic parameters. The CTD is operated on a carousel water sampler (SBE 32SC) allowing automated water sampling at pre-selected depths. The dataset is part of a multi-year study (different seasons over a period of 9 years: 2010-2019) of the dissolved organic matter (CDOM), which includes observations in marine waters from the coastal zone to the shelf edge of the Laptev Sea and East Siberian Sea, the river Lena and ice and water samples from the land-fast in the south-eastern Laptev Sea.

Description: The data set includes the light absorption coefficient of colored dissolved organic matter (CDOM) from 200 nm to 700 nm wavelength and dissolved organic carbon (DOC) from the land-fast ice in the Laptev Sea. Sampling was carried out March and April 2012. Samples were collected with the help of helicopters in March and April 2012 from camps on the land-fast ice of the south eastern Laptev Sea. During the winter expedition a standalone CTD (Seabird 19+) was lowered through an ice hole along with Nansen water samplers. Six Ice cores (9 cm diameter) from different ice types were collected in 2012 at three sampling sites. Subsamples from the ice cores were taken at 10 cm intervals to represent the different periods in which the ice formed, with the youngest ice occurring in the lower section and the oldest ice from the first phases of ice formation during early winter in the uppermost sections of the ice core. In addition to the CDOM, DOC and salinity measurements in the ice cores, we measured CDOM, DOC, temperature and salinity in the water column under the ice.