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Upper ocean mixing plays a key role in the atmosphere-ocean heat transfer and sea ice extent and thickness via modulating the upper ocean temperatures in the Arctic Ocean. Observations of diffusivities in the Arctic that directly indicate the ocean mixing properties are sparse. Therefore, the spatiotemporal pattern and magnitude of diapycnal diffusivities and kinetic energy dissipation rates in the upper Arctic Ocean are important for atmosphere-ocean heat transfers and sea ice changes. These were first estimated from the Ice-Tethered Profilers dataset (2005–2019) using a strain-based fine-scale parameterization. The resultant mixing properties showed significant geographical inhomogeneity and temporal variability. Diapycnal diffusivities and dissipation rates in the Atlantic sector of the Arctic Ocean were stronger than those on the Pacific side. Mixing in the Atlantic sector increased significantly during the observation period; whereas in the Pacific sector, it weakened before 2011 and then strengthened. Potential impact factors include wind, sea ice, near inertial waves, and stratification, while their relative contributions vary between the two sectors of the Arctic Ocean. In the Atlantic sector, turbulent mixing dominated, while in the Pacific sector, turbulent mixing was inhibited by strong stratification prior to 2011, and is able to overcome the stratification gradually after 2014. The vertical turbulent heat flux constantly increased in the Atlantic sector year by year, while it decreased in the Pacific sector post 2010. The estimated heat flux variability induced by enhanced turbulent mixing is expected to continue to diminish sea ice in the near future. [ABSTRACT FROM AUTHOR]

The dynamics of the wind-generated near-inertial internal wave field in the Canada Basin of the Arctic Ocean are investigated using the drifting Ice-Tethered Profiler dataset for the years 2005 to 2014, during a decade when sea ice extent and thickness decreased dramatically. This time series, with nearly 10 years of measurements and broad spatial coverage, is used to quantify a seasonal cycle and interannual trend for internal waves in the Arctic, using estimates of the amplitude of near-inertial waves derived from isopycnal displacements. The internal wave field is found to be most energetic in summer when sea ice is at a minimum, with a second maximum in early winter during the period of maximum wind speed. Amplitude distributions for the near-inertial waves are quantifiably different during summer and winter, due primarily to seasonal changes in sea ice properties that affect how the ice responds to the wind, which can be expressed through the 'wind factor'-the ratio of sea ice drift speed to wind speed. A small positive interannual trend in near-inertial wave energy is linked to pronounced sea ice decline during the last decade. Overall variability in the internal wave field increases significantly over the second half of the record, with an increased probability of larger-than-average waves in both summer and winter. This change is linked to an overall increase in variability in the wind factor and sea ice drift speeds, and reflects a shift in year-round sea ice characteristics in the Arctic, with potential implications for dissipation and mixing associated with internal waves. [ABSTRACT FROM AUTHOR]

Ice-tethered profiler (ITP) measurements from the Arctic Ocean's Canada Basin indicate an ocean surface layer beneath sea ice with significant horizontal density structure on scales of hundreds of kilometers to the order 1 km submesoscale. The observed horizontal gradients in density are dynamically important in that they are associated with restratification of the surface ocean when dense water flows under light water. Such restratification is prevalent in wintertime and competes with convective mixing upon buoyancy forcing (e.g., ice growth and brine rejection) and shear-driven mixing when the ice moves relative to the ocean. Frontal structure and estimates of the balanced Richardson number point to the likelihood of dynamical restratification by isopycnal tilt and submesoscale baroclinic instability. Based on the evidence here, it is likely that submesoscale processes play an important role in setting surface-layer properties and lateral density variability in the Arctic Ocean. [ABSTRACT FROM AUTHOR]

A comprehensive measurement program was conducted during 16 days of a 3 week long ice pack drift, from 15 August to 1 September 2008 in the central Amundsen Basin, Arctic Ocean. The data, sampled as part of the Arctic Summer Cloud Ocean Study (ASCOS), included upper ocean stratification, mixing and heat transfer as well as transmittance solar radiation through the ice. The observations give insight into the evolution of the upper layers of the Arctic Ocean in the transition period from melting to freezing. The ocean mixed layer was found to be heated from above and, for summer conditions, the net heat flux through the ice accounted for 22% of the observed change in mixed layer heat content. Heat was mixed downward within the mixed layer and a small, downward heat flux across the base of the mixed layer accounted for the accumulated heat in the upper cold halocline during the melting season. On average, the ocean mixed layer was cooled by an ocean heat flux at the ice/ocean interface (1.2 W m-2) and heated by solar radiation through the ice (-2.6 W m-2). An abrupt change in surface conditions halfway into the drift due to freezing and snowfall showed distinct signatures in the data set and allowed for inferences and comparisons to be made for cases of contrasting forcing conditions. Transmittance of solar radiation was reduced by 59 % in the latter period. From hydrographic observations obtained earlier in the melting season, in the same region, we infer a total fresh water equivalent of 3.3 m accumulated in the upper ocean, which together with the observed saltier winter mixed layer indicates a transition towards a more seasonal ice cover in the Arctic. [ABSTRACT FROM AUTHOR]

Abstract: North Atlantic Water (NAW) plays a central role in the ocean climate of the Nordic Seas and Arctic Ocean. Whereas the pathways of the NAW in the Nordic Seas are mostly known, those into the Arctic Ocean are yet to be fully understood. To elucidate these routes the results of a high-resolution global coupled ice–ocean model are used. We demonstrate that in 1989–2004 the NAW inflow was equally divided between the Fram Strait and Barents Sea. We find that salt influx within the branches is comparable but that most of the heat entered the Arctic Ocean through Fram Strait. The model shows complex NAW circulation patterns in the Barents Sea. Two mode waters in the Barents Sea branch are identified: a halocline water produced by surface cooling at shallow convective sites in the northern Barents Sea, and bottom water formed from NAW in the southeastern Barents Sea via full-depth convection and mixing. These two modes continue into the Nansen Basin along two separate routes: one through the northern Barents Sea shelf, and the other through the southeastern Barents Sea with halocline mode water dominating the outflow. Overall, less than half of the NAW coming into the Nordic Seas reaches the Arctic Ocean relatively unmodified, and the rest of it will have been modified in the Barents and Kara Seas with a large fraction re-circulating into the North Atlantic. [Copyright &y& Elsevier]

Abstract: We use a 9-km pan-Arctic ice–ocean model to better understand the circulation and exchanges in the Bering Sea, particularly near the shelf break. This region has, historically, been undersampled for physical, chemical, and biological properties. Very little is known about how water from the deep basin reaches the large, shallow Bering Sea shelf. To address this, we examine here the relationship between the Bering Slope Current and exchange across the shelf break and resulting mass and property fluxes onto the shelf. This understanding is critical to gain insight into the effects that the Bering Sea has on the Arctic Ocean, especially in regard to recent indications of a warming climate in this region. The Bering Sea shelf break region is characterized by the northwestward-flowing Bering Slope Current. Previously, it was thought that once this current neared the Siberian coast, a portion of it made a sharp turn northward and encircled the Gulf of Anadyr in an anticyclonic fashion. Our model results indicate a significantly different circulation scheme whereby water from the deep basin is periodically moved northward onto the shelf by mesoscale processes along the shelf break. Canyons along the shelf break appear to be more prone to eddy activity and, therefore, are associated with higher rates of on-shelf transport. The horizontal resolution configured in this model now allows for the representation of eddies with diameters greater than 36km; however, we are unable to resolve the smaller eddies. [Copyright &y& Elsevier]

Evidence of microbial zonation in the open ocean is rapidly accumulating, but while the distribution of communities is often described according to depth, the other physical factors structuring microbial diversity and function remain poorly understood. Here we identify three different water masses in the North Water (eastern Canadian Arctic), defined by distinct temperature and salinity characteristics, and show that they contained distinct archaeal communities. Moreover, we found that one of the water masses contained an increased abundance of the archaeal alpha-subunit of the ammonia monooxygenase gene ( amoA) and accounted for 70% of the amoA gene detected overall. This indicates likely differences in putative biogeochemical capacities among different water masses. The ensemble of our results strongly suggest that the widely accepted view of depth stratification did not explain microbial diversity, but rather that parent water masses provide the framework for predicting communities and potential microbial function in an Arctic marine system. Our results emphasize that microbial distributions are strongly influenced by oceanic circulation, implying that shifting currents and water mass boundaries resulting from climate change may well impact patterns of microbial diversity by displacing whole biomes from their historic distributions. This relocation could have the potential to establish a substantially different geography of microbial-driven biogeochemical processes and associated oceanic production. [ABSTRACT FROM AUTHOR]

According to observations, the Arctic Ocean circulation beneath a shallow thermocline can be schematized by cyclonic rim currents along shelves and over ridges. In each deep basin, the circulation is also believed to be cyclonic. This circulation pattern has been used as an important benchmark for validating Arctic Ocean models. However, modeling this grand circulation pattern with some of the most sophisticated ocean–ice models has been often difficult. The most puzzling and thus perhaps the most interesting finding from the Arctic Ocean Model Intercomparison Project (AOMIP), an international consortium that runs 14 Arctic Ocean models by using the identical forcing fields, is that its model results can be grouped into two nearly exact opposite patterns. While some models produce cyclonic circulation patterns similar to observations, others do the opposite. This study examines what could be possibly responsible for such strange inconsistency. It is found here that the flux of potential vorticity (PV) from the subarctic oceans strongly controls the circulation directions. For a semienclosed basin like the Arctic, the PV integral over the whole basin yields a balance between the net lateral PV inflow and the PV dissipation along the boundary. When an isopycnal layer receives a net positive PV through inflow/outflow, the circulation becomes cyclonic so that friction can generate a flux of negative PV to satisfy the integral balance. For simplicity, a barotropic ocean model is used in this paper but its application to the 3D models will be discussed. In the first set of experiments, the model with a realistic Arctic bathymetry is forced by observed inflows and outflows. In this case, there is a net positive PV inflow to the basin, due to the fact that inflow layer is thinner than that of outflow. The model produces a circulation field that is remarkably similar to the one from observations. In the second experiment, the model bathymetry at Fram Strait is modified so that the same inflows and outflows of water masses lead to a net negative PV flux into the Arctic. The circulation is reversed and becomes nearly the opposite of the first experiment. In the third experiment, the net PV flux is made to be zero by modifying again the sill depth at Fram Strait. The circulation becomes two gyres, a cyclonic one in the Eurasian Basin and an anticyclonic one in the Canada Basin. To elucidate the control of the PV integral, a second set of model experiments is conducted by using an idealized Arctic bathymetry so that the PV dynamics can be better explained without the complication of rough topography. The results from five additional experiments that used the idealized topography will be discussed. While the model used in this study is one layer, the same PV-integral constraint can be applied to any isopycnal layer in a three-dimensional model. Variables that affect the PV fluxes to this density layer at any inflow/outflow channel, such as layer thickness and water volume flux, can affect the circulation pattern. The relevance to 3D models is discussed in this paper. [ABSTRACT FROM AUTHOR]