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Variability of surface water masses of the Laptev and the East Siberian seas in August–September 2018 is studied using in situ and satellite data. In situ data were collected during the ARKTIKA-2018 expedition and then complemented with satellite-derived sea surface temperature (SST), salinity (SSS), sea surface height, wind speed, and sea ice concentration. The estimation of SSS fields is challenging in high-latitude regions, and the precision of soil moisture and ocean salinity (SMOS) SSS retrieval is improved by applying a threshold on SSS weekly error. For the first time in this region, the validity of DMI (Danish Meteorological Institute) SST and SMOS SSS products is thoroughly studied using ARKTIKA-2018 expedition continuous thermosalinograph measurements and conductivity–temperature–depth (CTD) casts. They are found to be adequate to describe large surface gradients in this region. Surface gradients and mixing of the river and the sea water in the ice-free and ice-covered areas are described with a special attention to the marginal ice zone at a synoptic scale. We suggest that the freshwater is pushed northward, close to the marginal ice zone (MIZ) and under the sea ice, which is confirmed by the oxygen isotope analysis. The SST-SSS diagram based on satellite estimates shows the possibility of investigating the surface water mass transformation at a synoptic scale and reveals the presence of river water on the shelf of the East Siberian Sea. The Ekman transport is calculated to better understand the pathway of surface water displacement on the shelf and beyond.

Highlights • First comprehensive seawater Nd isotope and REE data set for the Barents Sea • Water masses traced with Nd isotopes, salinity and stable oxygen isotopes • No release of particulate REEs to the dissolved load except for cerium • Transformation of Atlantic Water accompanied by pronounced REE removal from the dissolved phase Abstract Nearly half the inflow of warm and saline Atlantic Water (AW) to the Arctic Ocean is substantially cooled and freshened in the Barents Sea, which is therefore considered a key region for water mass transformation in the Arctic Mediterranean. Numerous studies have focused on this transformation and the increasing influence of AW on Arctic climate and biodiversity, yet geochemical investigations of these processes have been scarce. Using the first comprehensive data set of the distributions of dissolved radiogenic neodymium (Nd) isotopes (expressed as ɛNd), rare earth elements (REE) and stable oxygen isotope (δ18O) compositions from this region we are able to constrain the transport and transformation of AW in the Barents Sea and to investigate which processes change the chemical composition of the water masses beyond what is expected from circulation and mixing. Inflowing AW and Norwegian Coastal Water (NCW) both exhibit distinctly unradiogenic ɛNd signatures of −12.4 and −14.5, respectively, whereas cold and dense Polar Water (PW) has considerably more radiogenic ɛNd signatures reaching up to −8.1. Locally formed Barents Sea Atlantic Water (BSAW) and Barents Sea Arctic Atlantic Water (BSAAW) are encountered in the northeastern Barents Sea and have intermediate ɛNd values resulting from admixture of PW containing small amounts of riverine freshwater from the Ob (

The last 10 years of Arctic Ocean observations showed the dramatic changes and new records of the sea ice minimum. The largest variations were observed in the Eastern Arctic: the Kara, the Laptev, the East-Siberian seas. This region is a key area of the important freshwater input from the great Siberian rivers (Ob’, Yenisei, Lena). This remote area remains one of the less studied in the Arctic Ocean, although several regular expeditions (such as NABOS or Transdrift) together with special expeditions following the Northern Route, such as Tara-2013 expedition, or recent Transarktika-2019 expedition help to monitor the changes of surface waters in recent years.The use of new satellite-derived datasets, (e.g., SST blended product from Danish Meteorological Institute or REMSS, SSS SMOS from LOCEAN University of Sorbonne) fill the gaps and help to better understand the complex dynamics of surface waters in the Eastern Arctic ocean.In this work, we discuss the surface waters variations using in situ and satellite data at different scales. Synoptic scales are studied with continuous and point in situ measurements (thermosalinographs and CTD data). The recent scientific results of Transarktika-2019 expedition are presented. In the summer season of 2019 (July-October) Transarktika expedition did oceanographic measurements following the Northern Route twice, from Vladivistok to Murmansk and back to Vladivostok. The seasonal variations are analyzed over the period of 10 years, comparing with climatological data. The difference between the climatological values of SST or SSS can reach 5 or more units in some areas of the Eastern Arctic. The results of interannual variations analysis using satellite data, suggest the salinification (“Atlantification”) of the southern areas and freshening of the northern parts of the Eastern Arctic.The development of SSS SMOS Arctic product was supported by the French CNES-TOSCA SMOS-OCEAN project. Anastasiia Tarasenko, Nikita Kusse-Tiuz, Mikhail Makhotin and Vladimir Ivanov acknowledge financial support from the Ministry of Science and Higher Education of the Russian Federation, project RFMEFI61619X0108

During the last few decades we observe fast climatic changes. These changes are well expressed in the Polar Region that is more sensitive to all environmental shifts. The temperature and salinity anomalies were observed not only on the surface but also in the Pacific origin halocline layer. Due to hydrostatic imbalance and atmospheric circulation, water from the North Pacific flows through the Bering Strait, transits the upper levels of the Arctic Ocean, and penetrates to the North Atlantic. Pacific Summer Water (PSW) (flows through the Bering Strait in summer) is a main freshwater source in the Canadian Basin and influences thermohaline structure of the whole Arctic Ocean. Based on the field oceanographic data obtained during 1991-2014 we found the high interannual variability of PSW extent and maximum temperatures of PSW core. Since 1991 the area of PSW distribution decreased and the boundary of PSW extension shifted from the Makarov Basin towards the Canada Basin. At the end of the 2000s the PSW boundary extension returned to approximately early 1990s conditions. Rapid changes in the Arctic wind forcing regime occurred in 2007 led to anomaly extension of the PSW boundary towards the Lomonosov Ridge in 2008. For study the distribution of PSW under the conditions of a lack of field data, we used the modeling data of the GLORYS12V1 product of Copernicus Marine Environment Monitoring Service (http://marine.copernicus.eu/). The model component is the NEMO platform driven at the surface by ECMWF ERA-Interim reanalysis. Based on the calculated data we revealed the maximum temperatures and extension of PSW core in the Arctic Ocean from 1993 to 2018. As a result of comparing of field and calculated data, we found a good correspondence the values of maximum temperatures and the position of the PSW core in the centre of the Canada Basin. For example, in 2011 observed (1.0 °С) and calculated (0.9 °С) maximum temperature of PSW were very close. Based on model data we calculated the heat content of PSW which reached maximum values in recent years. The research was supported by the Ministry of Science and Higher Education of the Russian Federation (project RFMEFI61619X0108).

Variability of surface water masses of the Laptev and the East-Siberian seas in August–September 2018 is studied using in situ and satellite data. In situ data was collected during ARKTIKA-2018 expedition and then completed with satellite estimates of sea surface temperature (SST) and salinity (SSS), sea surface height, satellite-derived wind speeds and sea ice concentrations. Derivation of SSS is still challenging in high latitude regions, and the quality of Soil Moisture and Ocean Salinity (SMOS) SSS retrieval was improved by applying a threshold on SSS weekly error. The validity of SST and SSS products is demonstrated using ARKTIKA-2018 continuous thermosalinograph measurements and CTD casts. The surface gradients and mixing of river and sea waters in the free of ice and ice covered areas is described with a special attention to the marginal ice zone. The Ekman transport was calculated to better understand the pathway of surface water displacement. T-S diagram using surface satellite estimates shows a possibility to investigate the surface water masses transformation in detail.

Dedicated to the memory of our colleague Klaus Hochheim, who tragically lost his life in the Arctic expedition in September 2013. A distinct, subsurface density front along the eastern St. Anna Trough in the northern Kara Sea is inferred from hydrographic observations in 1996 and 2008–2010. Direct velocity measurements show a persistent northward subsurface current (~ 18 cm s−1) along the St. Anna Trough eastern flank. This sheared flow, carrying the outflow from the Barents and Kara seas to the Arctic Ocean, is also evident from shipboard observations as well as from geostrophic velocities and numerical model simulations. Although we cannot substantiate our conclusions by direct observation-based estimates of mixing rates in the area, we hypothesize that the enhanced vertical mixing along the St. Anna Trough eastern flank favors the upward heat loss from the intermediate warm Atlantic water layer. Modeling results support this hypothesis. The upward heat flux inferred from hydrographic data and model simulations is of O(30–100) W m−2. The region of lowered sea ice thickness and concentration seen both in sea ice remote sensing observations and model simulations marks the Atlantic water pathway in the St. Anna Trough and adjacent Nansen Basin continental margin. In fact, the sea ice shows a delayed freeze-up onset during fall and a reduction in the sea ice thickness during winter. This is consistent with our results on the enhanced Atlantic water heat loss along the Atlantic water pathway in the St. Anna Trough.

This paper discusses the results of unique direct observations over the current velocities in the eastern part of the St. Anna deep-water trough, made in the period from August 2009 until September 2010, and analyzes the physical ways how the temporal variability of currents is formed. The stable northward barotropic transport with an average annual velocity of about 20 cm/s was discovered. It was found that changes in velocity with a characteristic timescale of several weeks occurred synchronously in the entire water column and were determined by the deformation of the sea level field due to long-period disturbances of the large-scale field of ground wind above the northern parts of Barents and Kara seas. For the winds of the southwest and west directions, the sea level’s gradient is formed across the St. Anna Trough and the northward meridional water transport is intensified owing to the geostrophic adjustment. These are verified by the results of numerical simulation.

This paper examines the role of atmospheric forcing in modifying the pathways of riverine water on the Laptev Sea shelf, using summer-to-winter hydrographic surveys from 2007 to 2009. Over the two consecutive winter seasons of 2007–2008 and 2008–2009 in the area of the winter coastal polynya, our data clearly link winter surface salinity fields to the previous summer conditions, with substantially different winter salinity patterns preconditioned by summer atmospheric forcing. In the summer of 2007, dominant along-shore westerly winds in the cyclonic regime force the Lena River runoff to flow eastward. In contrast, in the summer of 2008, dominant along-shore easterly winds over the East Siberian Sea and on-shore northerly winds over the Laptev Sea in the anticyclonic regime lock the riverine water in the vicinity of the Lena Delta. Over the coastal polynya area in the southeastern Laptev Sea these patterns precondition a surface salinity difference of 8–16 psu between the winters of 2008 and 2009. Overall, this indicates a residence time of at least half a year for riverine water on the Laptev Sea shelf. Future climate change associated with an enhanced summer cyclonicity over the eastern Arctic may turn more riverine water eastward along the eastern Siberian coast, resulting in weaker vertical density stratification over the Laptev Sea shelf, with possible impact on the efficiency of vertical mixing and polynya dense water production.

This work is devoted to study of interannual variability of characteristics of Pacific summer waters (PSWs) supplied into the AO in summer. The distribution area, volume, and heat content of PSWs have been calculated for the first time for the entire Arctic Basin in the periods of 1950–1989 and 2008–2009 demonstrating the presence of substantial interannual variability. From 1953 until 1983 a negative trend and since 1984 a positive trend have been observed; the latter lasted until 2009, when the heat content, volume, and distribution area of PSWs reached their maximal values for the entire period considered. It has been shown that PSW quantity in the Arctic Basin, identified by temperature, rather depends on the intensity of heat flux through the Bering Strait, and the calculated value for PSW life in the Arctic Basin is seven years.

A distinct, subsurface density front along the eastern St. Anna Trough in the northern Kara Sea is inferred from hydrographic observations in 1996 and 2008–2010. Direct velocity measurements show a persistent northward subsurface current (~ 20 cm s−1) along the St. Anna Trough eastern flank. This sheared flow, carrying the outflow from the Barents and Kara Seas to the Arctic Ocean, is also evident from shipboard observations as well as from geostrophic velocities and numerical model simulations. Although no clear evidence for the occurrence of shear instabilities could be obtained, we speculate that the enhanced vertical mixing along the St. Anna Trough eastern flank promoted by a vertical velocity shear favors the upward heat loss from the intermediate warm Atlantic water layer. The associated upward heat flux is inferred to 50–100 W m−2 using hydrographic data and model simulations. The zone of lowered sea ice thickness and concentration essentially marks the Atlantic water pathway in the St. Anna Trough and adjacent Nansen Basin continental margin from both sea-ice remote sensing observations and model simulations. In fact, the seaice shows a consistently delayed freeze-up onset during fall and a reduction in the seaice thickness during winter. This is consistent with our results on the enhanced Atlantic water heat loss along the Atlantic water pathway in the St. Anna Trough.1 1Dedicated to the memory of our colleague Klaus Hochheim who tragically lost his life in the Arctic expedition in September 2013

The seasonal hydrographic cycle explains about 25–75% of the entire salinity variability spectrum of the Siberian shelf hydrography. Quasi-decadal variations in the seasonal salinity difference over the Laptev and East Siberian sea shelves derived from continuous summer-to-winter historical records from the 1960s–1990s are attributed to atmospheric vorticity quasi-decadal variations. Summer cyclonic vorticity results in riverine water accumulation on the shelf, increasing the salinity summer-to-winter difference. Summer anticyclonic wind pattern enhances fresh water movement from the shelf towards the Arctic Ocean that tends to weaken the seasonal salinity amplitude.