Atlantic Water Boundary Current Along the Southern Yermak Plateau, Arctic Ocean

The major ocean current that carries heat into the Arctic Ocean splits into three main branches of Atlantic Water (AW) and recirculations when it encounters the Yermak Plateau (YP) located north of Svalbard. While the branches that cross the plateau and recirculations have been extensively studied, there has been limited observation of the transport and variability of the Yermak branch. In this study, we present year-round observations from an array of three moorings that were deployed across the boundary current on the southern slope of the YP. The temporal-averaged sections show a surface-intensified AW core, which is strongest in winter but also persistent throughout the record within the upper 500 m. The volume transport of AW is highest in fall (1.4 ± 0.2 Sv; 1 Sv = 106 m3 s−1) and decreases to 0.8 ± 0.1 Sv in summer. Beneath a surface-intensified core, the velocity profile has a minimum at middepth, gradually increasing toward the bottom. This cold, bottom-intensified current is detectable in all seasons and reaches a maximum transport of 1.5 Sv in spring. The transport of AW is regulated by wind stress curl and coastal upwelling along the northwestern shelf of Svalbard. A positive wind stress curl increases the volume transport in the Yermak branch, thereby reducing the Svalbard branch transport. Eddy kinetic energy is surface-intensified and decreases to negligible values below 500 m. In the upper 500 m, the average baroclinic conversion in winter and summer is about 1 × 10−5 W m−3, which is 4–10 times the barotropic conversion rates.publishedVersio

Dissipation measurements using temperature microstructure from an underwater glider

Microstructure measurements of temperature and current shear are made using an autonomous underwater glider. The glider is equipped with fast-response thermistors and airfoil shear probes, providing measurements of dissipation rate of temperature variance, χχ, and of turbulent kinetic energy, εε, respectively. Furthermore, by fitting the temperature gradient variance spectra to a theoretical model, an independent measurement of εε is obtained. Both Batchelor (εBεB) and Kraichnan (εKεK) theoretical forms are used. Shear probe measurements are reported elsewhere; here, the thermistor-derived εBεB and εKεK are compared to the shear probe results, demonstrating the possibility of dissipation measurements using gliders equipped with thermistors only. A total of 152 dive and climb profiles are used, collected during a one-week mission in the Faroe Bank Channel, sampling the turbulent dense overflow plume and the ambient water above. Measurement of εε with thermistors using a glider requires careful consideration of data quality. Data are screened for glider flight properties, measurement noise, and the quality of fits to the theoretical models. Resulting dissipation rates from the two independent methods compare well for dissipation rates below 2×10−7 W kg−1. For more energetic turbulence, thermistors underestimate dissipation rates significantly, caused primarily by increased uncertainty in the time response correction. Batchelor and Kraichnan spectral models give very similar results. Concurrent measurements of εε and χχ are used to compute the dissipation flux coefficient ΓΓ (or so-called apparent mixing efficiency). A wide range of values is found, with a mode value of Γ≈0.14Γ≈0.14, in agreement with previous studies. Gliders prove to be suitable platforms for ocean microstructure measurements, complementary to existing methods

Microstructure measurements using a glider in the Faroe Bank Channel Overflow

The application of turbulence instrumentation on underwater gliders is addressed, and two methods for glider-inferred dissipation rates of turbulent kinetic energy are evaluated against a ship-based vertical microstructure profiler. The well-established ship-based measurements are used as a reference for the analysis. A Slocum glider was deployed for one week in the Faroe Bank Channel, equipped with a MicroRider with turbulence sensors for velocity shear and temperature microstructure. Dissipation rates of turbulent kinetic energy are calculated from velocity shear by integrating the wavenumber spectrum after fitting it to the Nasmyth universal spectrum. Survey-averaged profiles from the glider's shear-derived dissipation rates have similar shape as that measured by the vertical microstructure profiler, but overestimate dissipation rates by up to a factor of 3 in the vicinity the turbulent interface, attributed to the glider's slanted path and inability to penetrate sufficiently undisturbed through the swift plume interface. Microstucture temperature profiles are used to calculate dissipation rates, which is done by fitting temperature gradient spectra to the universal Batchelor form using the maximum likelihood estimate. This method allows for automatic rejection criteria, which are applied to remove bad fits. Results compare reasonably well with the vertical microstructure profiler measurements, but are underestimated close to the bottom, which is a caveat of the Batchelor fit, consistent with a previous study. Overall, measurement of dissipation rates from gliders is a powerful addition to traditional shipborne turbulence profilers, as they make it possible to survey large areas by deploying several gliders. Measurements are reasonably accurate, and require much less dedicated ship time

Mixing processes in the changing Arctic Ocean

The Arctic has undergone tremendous changes the last decades, including a strong decline in sea ice extent and thickness. The rapid pace of Arctic changes relative to the global changes are known as Arctic amplification, and has been referred to as the ‘canary in the coalmine’ of the present climate changes. Factors contributing to the accelerated changes are the ice-albedo effect, and the vast heat reservoir of Atlantic water flowing in the ocean below. This study has aimed to describe and quantify the influence of oceanic heat on the heat budget at the ocean’s upper boundary. There is a delicate heat balance at the interface between the atmosphere, the sea ice and the ocean. A small change in heat flux can have large effect on the ice cover. While the Arctic Ocean is generally not a very energetic one, the recent changes has raised concern about whether internal wave energy and the importance of vertical mixing processes are increasing. Reductions in sea ice extent may allow for more momentum transfer from the atmosphere to the ocean, either mixing the surface layer directly, or initiating inertial oscillations in the boundary layer. Near-inertial internal waves may propagate into the interior and cause mixing away from the surface boundary layer. An increase in vertical mixing in the Arctic Ocean may bring up more heat from the underlying warm Atlantic Water, posing a further threat to the diminishing Arctic sea ice. The study is based on observations from two different campaigns, both located in the region north of Svalbard. First, under-ice boundary layer and upper ocean measurements made during the winter-to-spring drift campaign N-ICE2015. Second, a yearlong deployment of three moorings on the slope of the Yermak Plateau is used to study the near-inertial wave field by the plateau. From an under-ice turbulence mast, a unique data set of winter-time measurements over the deep basin is obtained. Direct measurements of heat fluxes are weakly positive, even in winter, which are roughly doubled during storm events. Individual events can cause an order of magnitude increase in fluxes. A one-dimensional vertical diffusion model based on the observations from the drift satisfactorily reproduced observed changes in upper ocean winter hydrography. The model further suggests that observed salinity increase in the mixed layer was dominated by entrainment of saline water from below, rather than brine rejection from ice growth. In spring, coincident with drift over the shallower topography, where the warm Atlantic Water is found at shallow depths, heat fluxes below the sea ice are much higher. Varying by one to two orders of magnitude, heat fluxes are highly dependent on the depth of the warm water layers, the wind forcing and its effect on the ice cover. Highest heat fluxes exceeded 100Wm−2 over several hours, during a wind event in the marginal ice zone. From a subset of the under-ice turbulence measurements, during sea ice melt in June, heat and salt fluxes are found to be inversely correlated. This is contrary to expectations of positive heat- and salt fluxes during sea ice melt. This is hypothesized to origin in salt released from the melting sea ice. Objective criteria are used to identify 131 salty plumes descending past the measurement volume, accounting for 9% of the salt fluxes in only 0.5% of the time. The accumulated salt flux indicate a near full desalination of the sea ice. The reduction in bulk salinity of two nearby ice cores, taken three days apart, agree with accumulated salt flux within a factor of two. Plumes have previously only been observed from land-fast ice in a Svalbard fjord. The study confirms its existence on drifting Arctic sea ice, with implications for the understanding of salt and freshwater distribution in the under-ice boundary layer, and brine drainage in sea ice. From the southwestern Yermak Plateau, the near-inertial field was analyzed in yearlong records from three moorings. The near-inertial signal is clockwise dominant, indicative of downward energy propagation. The clockwise polarization is stronger closer to the surface, further suggesting surface generation by winds. Examples of wind-generated near-inertial wave propagation are presented, and wave group properties are calculated. At mid-depth and in the deep, episodic events of elevated near-inertial horizontal kinetic energy can be caused by surface generation at a remote location, or by tidal currents interacting with the rough topography. Theoretical characteristic beam paths initiated at the shelf break are consistent with the mid-depth elevation in near-inertial horizontal kinetic energy. The sum of these observations further highlight the importance and complexity of ocean mixing processes, both at the ice-ocean interface and at depth. The Yermak Plateau is a region of significant internal wave generation and energetic turbulence, and will be an important and interesting region for further studies. The diapycnal mixing taking place here is key in determining the vertical exchange of heat between inflowing Atlantic water and the surface, and the fate of this heat in the Arctic basins

Observations of brine plumes below Arctic sea ice

In sea ice, interconnected pockets and channels of brine are surrounded by fresh ice. Over time, brine is lost by gravity drainage and flushing. The timing of salt release and its interaction with the underlying water can impact subsequent sea ice melt. Turbulence measurements 1 m below melting sea ice north of Svalbard reveal anti-correlated heat and salt fluxes. From the observations, 131 salty plumes descending from the warm sea ice are identified, confirming previous observations from a Svalbard fjord. The plumes are likely triggered by oceanic heat through bottom melt. Calculated over a composite plume, oceanic heat- and salt fluxes during the plumes account for 6% and 9% of the total fluxes, respectively, while only lasting in total 0.5% of the time. The observed salt flux accumulates to 7.6 kg m−2, indicating nearly full desalination of the ice. Bulk salinity reduction between two nearby ice cores agree with accumulated salt fluxes to within factor of two. The increasing fraction of younger, more saline ice in the Arctic suggests an increase in desalination processes with the transition to the ’new Arctic’

Tidal forcing, energetics, and mixing near the Yermak Plateau

The Yermak Plateau (YP), located northwest of Svalbard in Fram Strait, is the final passage for the inflow of warm Atlantic Water into the Arctic Ocean. The region is characterized by the largest barotropic tidal velocities in the Arctic Ocean. Internal response to the tidal flow over this topographic feature locally contributes to mixing that removes heat from the Atlantic Water. Here, we investigate the tidal forcing, barotropic-to-baroclinic energy conversion rates, and dissipation rates in the region using observations of oceanic currents, hydrography, and microstructure collected on the southern flanks of the plateau in summer 2007, together with results from a global high-resolution ocean circulation and tide model simulation. The energetics (depth-integrated conversion rates, baroclinic energy fluxes and dissipation rates) show large spatial variability over the plateau and are dominated by the luni-solar diurnal (K1) and the principal lunar semidiurnal (M2) constituents. The volume-integrated conversion rate over the region enclosing the topographic feature is approximately 1 GW and accounts for about 50% of the M2 and approximately all of the K1 conversion in a larger domain covering the entire Fram Strait extended to the North Pole. Despite the substantial energy conversion, internal tides are trapped along the topography, implying large local dissipation rates. An approximate local conversion–dissipation balance is found over shallows and also in the deep part of the sloping flanks. The baroclinic energy radiated away from the upper slope is dissipated over the deeper isobaths. From the microstructure observations, we inferred lower and upper bounds on the total dissipation rate of about 0.5 and 1.1 GW, respectively, where about 0.4–0.6 GW can be attributed to the contribution of hot spots of energetic turbulence. The domain-integrated dissipation from the model is close to the upper bound of the observed dissipation, and implies that almost the entire dissipation in the region can be attributed to the dissipation of baroclinic tidal energy

Microstructure Measurements from an Underwater Glider in the Turbulent Faroe Bank Channel Overflow

Measurements of ocean microstructure are made in the turbulent Faroe Bank Channel overflow using a turbulence instrument attached to an underwater glider. Dissipation rate of turbulent kinetic energy « is measured using airfoil shear probes.Acomparison is made between 152 profiles from dive and climb cycles of the glider during a 1-week mission in June 2012 and 90 profiles collected from the ship using a vertical microstructure profiler (VMP). Approximately one-half of the profiles are collocated. For 96% of the dataset, measurements are of high quality with no systematic differences between dives and climbs. The noise level is less than 5 x 10-¹¹ W kg-¹, comparable to the best microstructure profilers. The shear probe data are contaminated and unreliable at the turning depth of the glider and for U/ut < 20, where U is the flow past the sensor, ut =(ε/N)¹/² is an estimate of the turbulent velocity scale, and N is the buoyancy frequency. Averaged profiles of ε from the VMP and the glider agree to better than a factor of 2 in the turbulent bottom layer of the overflow plume, and beneath the stratified and sheared plume–ambient interface. The glider average values are approximately a factor of 3 and 9 times larger than the VMP values in the layers defined by the isotherms 3°–6° and 6°–9°C, respectively, corresponding to the upper part of the interface and above. The discrepancy is attributed to a different sampling scheme and the intermittency of turbulence. The glider offers a noise-free platform suitable for ocean microstructure measurements

One-dimensional evolution of the upper water column in the Atlantic sector of the Arctic Ocean in winter

A one-dimensional model is employed to reproduce the observed time evolution of hydrographic properties in the upper water column during winter, between 26 January and 11 March 2015, in a region north of Svalbard in the Nansen Basin of the Arctic Ocean. From an observed initial state, vertical diffusion equations for temperature and salinity give the hydrographic conditions at a later stage. Observations of microstructure are used to synthesize profiles of vertical diffusivity, K, representative of varying wind forcing conditions. The ice-ocean heat and salt fluxes at the ice-ocean interface are implemented as external source terms, estimated from the salt and enthalpy budgets, using friction velocity from the Rossby similarity drag relation, and the ice core temperature profiles. We are able to reproduce the temporal evolution of hydrography satisfactorily for two pairs of measured profiles, suggesting that the vertical processes dominated the observed changes. Sensitivity tests reveal a significant dependence on K. Variation in other variables, such as the temperature gradient of the sea ice, the fraction of heat going to ice melt, and the turbulent exchange coefficient for heat, are relatively less important. The increase in salinity as a result of freezing and brine release is approximately 10%, significantly less than that due to entrainment (90%) from beneath the mixed layer. Entrainment was elevated during episodic storm events, leading to melting. The results highlight the contribution of storms to mixing in the upper Arctic Ocean and its impact on ice melt and mixed-layer salt and nutrient budgets

Mixing rates and vertical heat fluxes north of Svalbard from Arctic winter to spring

Mixing and heat flux rates collected in the Eurasian Basin north of Svalbard during the N-ICE2015 drift expedition are presented. The observations cover the deep Nansen Basin, the Svalbard continental slope, and the shallow Yermak Plateau from winter to summer. Mean quiescent winter heat flux values in the Nansen Basin are 2 W m−2 at the ice-ocean interface, 3 W m−2 in the pycnocline, and 1 W m−2 below the pycnocline. Large heat fluxes exceeding 300 W m−2 are observed in the late spring close to the surface over the Yermak Plateau. The data consisting of 588 microstructure profiles and 50 days of high-resolution under-ice turbulence measurements are used to quantify the impact of several forcing factors on turbulent dissipation and heat flux rates. Wind forcing increases turbulent dissipation seven times in the upper 50 m, and doubles heat fluxes at the ice-ocean interface. The presence of warm Atlantic Water close to the surface increases the temperature gradient in the water column, leading to enhanced heat flux rates within the pycnocline. Steep topography consistently enhances dissipation rates by a factor of four and episodically increases heat flux at depth. It is, however, the combination of storms and shallow Atlantic Water that leads to the highest heat flux rates observed: ice-ocean interface heat fluxes average 100 W m−2 during peak events and are associated with rapid basal sea ice melt, reaching 25 cm/d

Turbulent heat and momentum fluxes in the upper ocean under Arctic sea ice

We report observations of heat and momentum fluxes measured in the ice-ocean boundary layer from four drift stations between January and June 2015, covering from the typical Arctic basin conditions in the Nansen Basin to energetic spots of interaction with the warm Atlantic Water branches near the Yermak Plateau and over the North Spitsbergen slope. A wide range of oceanic turbulent heat flux values are observed, reflecting the variations in space and time over the five month duration of the experiment. Oceanic heat flux is weakly positive in winter over the Nansen Basin during quiescent conditions, increasing by an order of magnitude during storm events. An event of local upwelling and mixing in the winter-time Nansen basin highlights the importance of individual events. Spring-time drift is confined to the Yermak Plateau and its slopes, where vertical mixing is enhanced. Wind events cause an approximate doubling of oceanic heat fluxes compared to calm periods. In June, melting conditions near the ice edge lead to heat fluxes of O(100 W m−2). The combination of wind forcing with shallow Atlantic Water layer and proximity to open waters leads to maximum heat fluxes reaching 367 W m−2, concurrent with rapid melting. Observed ocean-to-ice heat fluxes agree well with those estimated from a bulk parameterization except when accumulated freshwater from sea ice melt in spring probably causes the bulk formula to overestimate the oceanic heat flux