The penetrative mixing in the Laptev Sea coastal polynya pycnocline layer

The large recurrent areas of open water and/or thin ice (polynyas) producing cold brine-enriched waters off the fast-ice edge are evident in the Laptev Sea in winter time. A number of abrupt positively correlated transitions in temperature and salinity were recorded in the bottom and intermediate layers at a mooring station in the West New Siberian (WNS) polynya in February-March 2008. Being in the range of -0.5 degrees C and -1.6 psu these changes are induced by horizontal motions across the polynya and correspond to temperature and salinity horizontal gradients in the range of 0.3-1.0 degrees C/10 km and 1.4-3.5 psu/10 km, respectively. The events of distinct freshening and temperature decrease coincide with a northward current off the fast-ice edge, while southward currents brought saltier and warmer waters at intermediate depths. We suggest that the observed transitions are connected to altering pycnocline depths across the polynya. The source of relatively fresher waters at the intermediate depths in polynya is supposed to originate from penetrative mixing of surface low salinity waters to intermediate water depth. Several forcing processes that could be responsible for a penetrative mixing through the density interface in polynya are discussed. These are penetrative convection and shear-driven mixing that originates from two-layer water dynamics and/or baroclinic tidal motions. The heavily ridged seaward fast-ice edge could produce an additional source of turbulent mixing even through a shear-free density interface due to the increased roughness at the ice-water interfac

On the Variability of Stratification in the Freshwater-Influenced Laptev Sea Region

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

Inorganic carbon and nutrient fluxes on the Arctic Shelf

Historic data from the Russian-American Hydrochemical Atlas of Arctic Ocean together with data from the TRANSDRIFT II 1994 and TUNDRA 1994 cruises have been used to assess the spatial and inter-annual variability of carbon and nutrient fluxes, as well as air–sea CO2 exchange in the Laptev and western East Siberian Seas during the summer season. Budget computations using summer data of dissolved inorganic phosphate (DIP), dissolved inorganic nitrogen (DIN) and dissolved inorganic carbon (DIC) gives that the Laptev Sea shelf is a net sink of DIP and DIN of 2.5×106, 23.2×106 mol d−1, respectively, while it is a net source of DIC (excluding air–sea exchange) of 1249×106 mol d−1. In the East Siberian Seas the budget computations give 0.5×106, −11.4×106 and −173×106 mol d−1 (minus being a sink) for DIP, DIN, and DIC, respectively. In summers, the Laptev Sea Shelf is net autotrophic while the East-Siberian Sea Shelf is net heterotrophic, and both systems are weak net denitrifying. The Laptev Sea Shelf takes up 2.1 mmol CO2 m−2 d−1 from atmosphere, whereas the western part of the East-Siberian Sea Shelf loose 0.3 mmol CO2 m−2 d−1 to the atmosphere. The variability of DIP, DIN and DIC fluxes during summer in the different regions of the Laptev and East Siberian Seas depends on bottom topography, river runoff, exchange with surrounding seas and wind field

Identifying Drivers of Seasonality in Lena River Biogeochemistry and Dissolved Organic Matter Fluxes

Warming air temperatures, shifting hydrological regimes and accelerating permafrost thaw in the catchments of the Arctic rivers is affecting their biogeochemistry. Arctic river monitoring is necessary to observe changes in the mobilization of dissolved organic matter (DOM) from permafrost. The Lena River is the second largest Arctic river and 71% of its catchment is continuous permafrost. Biogeochemical parameters, including temperature, electrical conductivity (EC), stable water isotopes, dissolved organic carbon (DOC) and absorption by colored dissolved organic matter (aCDOM) have been measured as part of a new high-frequency sampling program in the central Lena River Delta. The results show strong seasonal variations of all biogeochemical parameters that generally follow seasonal patterns of the hydrograph. Optical indices of DOM indicate a trend of decreasing aromaticity and molecular weight from spring to winter. High-frequency sampling improved our estimated annual fluvial flux of annual dissolved organic carbon flux (6.79 Tg C). EC and stable isotope data were used to distinguish three different source water types which explain most of the seasonal variation in the biogeochemistry of the Lena River. These water types match signatures of (1) melt water, (2) rain water, and (3) subsurface water. Melt water and rain water accounted for 84% of the discharge flux and 86% of the DOC flux. The optical properties of melt water DOM were characteristic of fresh organic matter. In contrast, the optical properties of DOM in subsurface water revealed lower aromaticity and lower molecular weights, which indicate a shift toward an older organic matter source mobilized from deeper soil horizons or permafrost deposits. The first year of this new sampling program sets a new baseline for flux calculations of dissolved matter and has enabled the identification and characterization of water types that drive the seasonality of the Lena River water properties

Ocean Colour remote sensing in the Southern Laptev Sea: evaluation and applications

Enhanced permafrost warming and increased arctic river discharges have heightened concern about the input of terrigeneous matter into Arctic coastal waters. We used optical operational satellite data from the Ocean Colour sensor MERIS onboard the ENVISAT satellite mission for synoptic monitoring of the pathways of terrigeneous matter in the southern Laptev Sea. MERIS satellite data from 2006 on to 2011 were processed using the Case2Regional Processor, C2R, installed in the open-source software ESA BEAM-VISAT. Since optical remote sensing using Ocean Colour satellite data has seen little application in Siberian Arctic coastal and shelf waters, we assess the applicability of the calculated MERIS parameters with surface water sampling data from the Russian-German ship expeditions LENA2010 and TRANSDRIFT-XVII taking place in August and September 2010 in the southern Laptev Sea. The surface waters of the southern Laptev Sea are characterized by low transparencies, due to turbid river water input, terrestrial input by coastal erosion, resuspension events and, therefore, high background concentrations of Suspended Particulate Matter, SPM, and coloured Dissolved Organic Matter, cDOM. The mapped calculated optical water parameters, such as the first attenuation depth, Z90, the attenuation coefficient, k, and Suspended Particulate Matter, SPM, visualize resuspension events that occur in shallow coastal and shelf waters indicating vertical mixing events. The mapped optical water parameters also visualize that the hydrography of the Laptev Sea is dominated by frontal meanders with amplitudes up to 30 km and eddies and filaments with diameters up to 100 km that prevail throughout the ice-free season. The meander crests, filaments and eddy-like structures that become visible through the mapped MERIS C2R parameters indicate enhanced vertical and horizontal transport energy for the transport of terrigenous and living biological matter in the surface waters during the ice-free season

Observations of supercooling and frazil ice formation in the Laptev Sea coastal polynya

This paper examines a hydrographic response to the wind‐driven coastal polynya activity over the southeastern Laptev Sea shelf for April–May 2008, using a combination of Environmental Satellite (Envisat) advanced synthetic aperture radar (ASAR) and TerraSAR‐X satellite imagery, aerial photography, meteorological data, and SBE‐37 salinity‐temperature‐depth and acoustic Doppler current profiler land‐fast ice edgemoored instruments. When ASAR observed the strongest end‐of‐April polynya event with frazil ice formation, the moored instruments showed maximal acoustical scattering within the surface mixed layer, and the seawater temperatures were either at or 0.02°C below freezing. We also find evidence of the persistent orizontal temperature and salinity gradients across the fast ice edge to have the signature of geostrophic flow adjustment as predicted by polynya models

Nutrient and Silicon Isotope Dynamics in the Laptev Sea and Implications for Nutrient Availability in the Transpolar Drift

Realistic prediction of the near-future response of Arctic Ocean primary productivity to ongoing warming and sea ice loss requires a mechanistic understanding of the processes controlling nutrient bioavailability. To evaluate continental nutrient inputs, biological utilization and the influence of mixing and winter processes in the Laptev Sea, the major source region of the Transpolar Drift, we compare observed with preformed concentrations of dissolved inorganic nitrogen (DIN), phosphorus (DIP), silicic acid (DSi) and silicon isotope compositions of DSi (δ30SiDSi) obtained for two summers (2013, 2014) and one winter (2012). In summer, preformed nutrient concentrations persisted in the surface layer of the southeastern Laptev Sea, while diatom-dominated utilization caused intense northward drawdown and a pronounced shift in δ30SiDSi from +0.91 to +3.82 ‰. The modeled Si isotope fractionation suggests that DSi in the northern Laptev Sea originated from the Lena River during the spring freshet, while in the southeastern Laptev Sea it was continuously supplied by it during the summer. Primary productivity fueled by river-borne nutrients was enhanced by admixture of DIN- and DIP-rich Atlantic-sourced waters to the surface, either by convective mixing during the previous winter or by occasional storm-induced stratification breakdowns in late summer. Substantial enrichments of DSi (+240 %) and DIP (+90 %) beneath the Lena River plume were caused by sea ice-driven redistribution and remineralization. Predicted weaker stratification on the outer Laptev shelf will enhance DSi utilization and removal through greater vertical DIN supply, which will limit DSi export and reduce diatom-dominated primary productivity in the Transpolar Drift. Key Points - Surface DIN, DIP, DSi and Si isotope dynamics are controlled by marine and riverine inputs and uptake by phytoplankton - Strong DIP and DSi enrichments beneath the Lena River plume are due to sea ice-driven nutrient redistribution and remineralization - Enhanced DSi utilization in the Laptev Sea will lead to a reduced diatom-dominated primary productivity in the Transpolar Drif