Concern about impacts of climate change in the Bering Sea prompted several research programs to elucidate mechanistic links between climate and ecosystem responses. Following a detailed literature review, Hunt et al. (2011) (Deep-Sea Res. II, 49, 2002) developed a conceptual framework, the Oscillating Control Hypothesis (OCH), linking climaterelated changes in physical oceanographic conditions to stock recruitment using walleye pollock (Theragra chalcogramma) as a model. The OCH conceptual model treats zooplankton as a single box, with reduced zooplankton production during cold conditions, producing bottom-up control of apex predators and elevated zooplankton production during warm periods leading to top-down control by apex predators. A recent warming trend followed by rapid cooling on the Bering Sea shelf permitted testing of the OCH. During warm years (2003-06), euphausiid and Calanus marshallae populations declined, post-larval pollock diets shifted from a mixture of large zooplankton and small copepods to almost exclusively small copepods, and juvenile pollock dominated the diets of large predators. With cooling from 2006-09, populations of large zooplankton increased, post-larval pollock consumed greater proportions of C. marshallae and other large zooplankton, and juvenile pollock virtually disappeared from the diets of large pollock and salmon. These shifts in energy flow were accompanied by large declines in pollock stocks attributed to poor recruitment between 2001 and 2005. Observations presented here indicate the need for revision of the OCH to account for shifts in energy flow through differing food-web pathways due to warming and cooling on the southeastern Bering Sea shelf.

A 15-year duration record of mooring observations from the eastern (>70°E) Eurasian Basin (EB) of the Arctic Ocean is used to show and quantify the recently increased oceanic heat flux from intermediate-depth (∼150-900 m) warm Atlantic Water (AW) to the surface mixed layer (SML) and sea ice. The upward release of AW heat is regulated by the stability of the overlying halocline, which we show has weakened substantially in recent years. Shoaling of the AW has also contributed, with observations in winter 2017-2018 showing AW at only 80 m depth, just below the wintertime surface mixed layer (SML), the shallowest in our mooring records. The weakening of the halocline for several months at this time implies that AW heat was linked to winter convection associated with brine rejection during sea ice formation. This resulted in a substantial increase of upward oceanic heat flux during the winter season, from an average of 3-4 W/m2 in 2007-2008 to >10 W/m2 in 2016-2018. This seasonal AW heat loss in the eastern EB is equivalent to a more than a two-fold reduction of winter ice growth. These changes imply a positive feedback as reduced sea ice cover permits increased mixing, augmenting the summer-dominated ice-albedo feedback.

Arctic Ocean primary productivity is limited by light and inorganic nutrients. With sea ice cover declining in recent decades, nitrate limitation has been speculated to become more prominent. Although much has been learned about nitrate supply from general patterns of ocean circulation and water column stability, a quantitative analysis requires dedicated turbulence measurements that have only started to accumulate in the last dozen years. Here we present new observations of the turbulent vertical nitrate flux in the Laptev Sea, Baffin Bay, and Young Sound (North-East Greenland), supplementing a compilation of 13 published estimates throughout the Arctic Ocean. Combining all flux estimates with a Pan-Arctic database of in situ measurements of nitrate concentration and density, we found the annual nitrate inventory to be largely determined by the strength of stratification and by bathymetry. Nitrate fluxes explained the observed regional patterns and magnitudes of both new primary production and particle export on annual scales. We argue that with few regional exceptions, vertical turbulent nitrate fluxes can be a reliable proxy of Arctic primary production accessible through autonomous and large-scale measurements. They may also provide a framework to assess nutrient limitation scenarios based on clear energetic and mass budget constraints resulting from turbulent mixing and freshwater flows.

We quantify Atlantic Water heat loss north of Svalbard using year-long hydrographic and current records from three moorings deployed across the Svalbard Branch of the Atlantic Water boundary current in 2012-2013. The boundary current loses annually on average 16 W m −2 during the eastward propagation along the upper continental slope. The largest vertical fluxes of >100 W m −2 occur episodically in autumn and early winter. Episodes of sea ice imported from the north in November 2012 and February 2013 coincided with large ocean-to-ice heat fluxes, which effectively melted the ice and sustained open water conditions in the middle of the Arctic winter. Between March and early July 2013, a persistent ice cover-modulated air-sea fluxes. Melting sea ice at the start of the winter initiates a cold, up to 100-m-deep halocline separating the ice cover from the warm Atlantic Water. Semidiurnal tides dominate the energy over the upper part of the slope. The vertical tidal structure depends on stratification and varies seasonally, with the potential to contribute to vertical fluxes with shear-driven mixing. Further processes impacting the heat budget include lateral heat loss due to mesoscale eddies, and modest and negligible contributions of Ekman pumping and shelf break upwelling, respectively. The continental slope north of Svalbard is a key example regarding the role of ocean heat for the sea ice cover. Our study underlines the complexity of the ocean's heat budget that is sensitive to the balance between oceanic heat advection, vertical fluxes, air-sea interaction, and the sea ice cover. Plain Language SummaryThe Atlantic Water boundary current carries heat into the Arctic Ocean as it flows through Fram Strait and along the continental slope north of Svalbard. Using observations from bottom-mounted instruments, we investigated different processes leading to heat loss from the Atlantic Water layer in the region north of Svalbard. Most of the changes recorded over the course of 1 year from September 2012 to September 2013 at 81.5 ∘ N, 31 ∘ E are driven by changes further upstream and by air-sea heat exchange. However, significant local heat loss can be caused by mixing due to wind or tides. Seasonal differences are large and predominantly caused by absence or presence of sea ice (autumn/early winter versus spring/early summer), influence of melt water and wind on the stability of the water column, and a seasonally changing light regime.

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Siberian river water is a first-order contribution to the Arctic freshwater budget, with the Ob, Yenisey, and Lena supplying nearly half of the total surface freshwater flux. However, few details are known regarding where, when, and how the freshwater transverses the vast Siberian shelf seas. This paper investigates the mechanism, variability, and pathways of the fresh Kara Sea outflow through Vilkitsky Strait toward the Laptev Sea. We utilize a high-resolution ocean model and recent shipboard observations to characterize the freshwater-laden Vilkitsky Strait Current (VSC), and shed new light on the little-studied region between the Kara and Laptev Seas, characterized by harsh ice conditions, contrasting water masses, straits, and a large submarine canyon. The VSC is 10-20 km wide, surface intensified, and varies seasonally (maximum from August to March) and interannually. Average freshwater (volume) transport is 500 6 120 km 3 a 21 (0.53 6 0.08 Sv), with a baroclinic flow contribution of 50-90%. Interannual transport variability is explained by a storagerelease mechanism, where blocking-favorable summer winds hamper the outflow and cause accumulation of freshwater in the Kara Sea. The year following a blocking event is characterized by enhanced transports driven by a baroclinic flow along the coast that is set up by increased freshwater volumes. Eventually, the VSC merges with a slope current and provides a major pathway for Eurasian river water toward the western Arctic along the Eurasian continental slope. Kara (and Laptev) Sea freshwater transport is not correlated with the Arctic Oscillation, but rather driven by regional summer pressure patterns.

Arctic Ocean properties and processes are highly relevant to the regional and global coupled climate system, yet still scarcely observed, especially in winter. Team OCEAN conducted a full year of physical oceanography observations as part of the Multidisciplinary drifting Observatory for the Study of the Arctic Climate (MOSAiC), a drift with the Arctic sea ice from October 2019 to September 2020. An international team designed and implemented the program to characterize the Arctic Ocean system in unprecedented detail, from the seafloor to the air-sea ice-ocean interface, from sub-mesoscales to pan-Arctic. The oceanographic measurements were coordinated with the other teams to explore the ocean physics and linkages to the climate and ecosystem. This paper introduces the major components of the physical oceanography program and complements the other team overviews of the MOSAiC observational program. Team OCEAN’s sampling strategy was designed around hydrographic ship-, ice- and autonomous platform-based measurements to improve the understanding of regional circulation and mixing processes. Measurements were carried out both routinely, with a regular schedule, and in response to storms or opening leads. Here we present along-drift time series of hydrographic properties, allowing insights into the seasonal and regional evolution of the water column from winter in the Laptev Sea to early summer in Fram Strait: freshening of the surface, deepening of the mixed layer, increase in temperature and salinity of the Atlantic Water. We also highlight the presence of Canada Basin deep water intrusions and a surface meltwater layer in leads. MOSAiC most likely was the most comprehensive program ever conducted over the ice-covered Arctic Ocean. While data analysis and interpretation are ongoing, the acquired datasets will support a wide range of physical oceanography and multi-disciplinary research. They will provide a significant foundation for assessing and advancing modeling capabilities in the Arctic Ocean.

[1] Hydrographic and stable isotope (d 18 O) data from four summer surveys in the Laptev Sea are used to derive fractions of sea-ice meltwater and river water. Sea-ice meltwater fractions are found to be correlated to river water fractions. While initial heat of river discharge is too small to melt the observed 0-158 km 3 of sea-ice meltwater, arctic rivers contain suspended particles and colored dissolved organic material that preferentially absorb solar radiation. Accordingly, heat content in surface waters is correlated to river water fractions. But in years when river water is largely absent within the surface layer, absolute heat content values increase to considerably higher values with extended exposure time to solar radiation and sensible heat. Nevertheless, no net sea-ice melting is observed on the shelf in years when river water is largely absent within the surface layer. The total freshwater volume of the central-eastern Laptev Sea (72-76 N, 122-140 E) varies betweeñ 1000 and 1500 km 3 (34.92 reference salinity). It is dominated by varying river water volumes (~1300-1800 km 3 ) reduced by an about constant freshwater deficit (~350-400 km 3 ) related to sea-ice formation. Net sea-ice melt (~109-158 km 3 ) is only present in years with high river water budgets. Intermediate to bottom layer (>25 salinities) contaiñ 60% and 30% of the river budget in years with low and high river budgets, respectively. The average mean residence time of shelf waters was~2-3 years during