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Ilker Fer

Abstract

Observations were made in April 2007 of horizontal currents, hydrography, and shear microstructure in the upper 500 m from a drifting ice camp in the central Arctic Ocean. An approximately 4-day-long time series, collected about 10 days after a storm event, shows enhanced near-inertial oscillations in the first half of the measurement period with comparable upward- and downward-propagating energy. Rough estimates of wind work and near-inertial flux imply that the waves were likely generated by the previous storm. The near-inertial frequency band is associated with dominant clockwise rotation in time of the horizontal currents and enhanced dissipation rates of turbulent kinetic energy. The vertical profile of dissipation rate shows elevated values in the pycnocline between the relatively turbulent underice boundary layer and the deeper quiescent water column. Dissipation averaged in the pycnocline is near-inertially modulated, and its magnitude decays approximately at a rate implied by the reduction of energy over time. Observations suggest that near-inertial energy and internal wave–induced mixing play a significant role in vertical mixing in the Arctic Ocean.

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Anders Sirevaag and Ilker Fer

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From several drifting ice stations north of Svalbard, Norway, observations were made in early spring of the ocean turbulent characteristics in the upper 150 m using a microstructure profiler and close to the under-ice surface using eddy correlation instrumentation. The dataset is used to obtain average heat fluxes at the ice–water interface, in the mixed layer, across the main pycnocline, as well over different water masses in the region. The results are contrasted with proximity to the branches of the warm and saline Atlantic water current, the West Spitsbergen Current (WSC), which is the main oceanic heat and salinity source both to the region and to the Arctic Ocean. Hydrographic properties show that the surface water mass modification is typically due to atmospheric cooling with relatively less influence of ice melting. Surface heat fluxes of O(100) W m−2 are found within the branches of the WSC and over shelf areas with elevated levels of mixing due to strong tides. Away from the shelves and WSC, however, ocean-to-ice turbulent heat fluxes are typical of the central Arctic. Deeper in the water column, entrainment from below together with equally important horizontal advection and diffusion increase the heat content of the mixed layer and contribute to the heat flux maximum in the upper layers. The results in this study emphasize the importance of mixing along the boundaries, over shelves, and topography for the cooling of the Atlantic water layer in the Arctic in general, and for the regional heat budget, hence the ice cover and cooling of the WSC north of Svalbard, in particular.

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Ilker Fer and Mostafa Bakhoday Paskyabi

Abstract

An internally recording, autonomous instrument has been tested for measurements of ocean turbulence from a mooring line. Measurements were made at a single level in the water column, but for an extended period of time, at a predetermined duty cycle. The instrument is designed to measure, independently, in two different parts of the turbulence wavenumber spectrum: eddy correlation measurements in the inertial subrange and small-scale shear and temperature gradient measurements in the dissipation subrange using shear probes and fast-response thermistors. For the deployment reported here, the instrument is located in the wave-affected layer, and only the dissipation subrange from the shear probes can be confidently utilized for turbulence measurements. The velocity spectra in the inertial subrange are severely contaminated by platform motion and noise, and the dissipation range of the temperature gradient spectrum is not satisfactorily resolved. The shear spectra are found to be relatively free of contamination in the 1–20-Hz frequency range and are used for dissipation rate calculations. The quality of the measurements is constrained by the angle of attack and the magnitude of mean flow relative to the wave oscillatory velocities. Dissipation rates are consistent with a scaling expected from breaking long waves, when background shear is weak, and are elevated when the gradient Richardson number is small, consistent with additional turbulence production by shear. While limited to a single depth, the instrument makes it possible to collect time series for 3 weeks continuously or for 3 months at a 25% duty cycle.

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Elin Darelius, Ilker Fer, and Detlef Quadfasel

Abstract

The Faroe Bank Channel is the deepest connection through the Greenland–Scotland Ridge, where dense water formed north of the ridge flows southward over the sill crest, contributing to the formation of North Atlantic Deep Water. The overflow region is characterized by high mesoscale variability and energetic oscillations, accompanied by a high degree of sea surface level variability. Here, 2-month-long time series of velocity and temperature from 12 moorings deployed in May 2008 are analyzed to describe the oscillations and explore their generation and propagation. The observed 2.5–5-day oscillations in velocity and temperature are highly coherent both horizontally and vertically, and they are associated with 100–200-m-thick boluses of cold plume water flowing along the slope. A positive correlation between temperature and relative vorticity and the distribution of clockwise/counterclockwise rotation across the slope suggest a train of alternating warm cyclonic and cold anticyclonic eddies, where the maximum plume thickness is located downslope of the eddy center. The along-slope phase velocity is found to be 25–60 cm s−1, corresponding to a wavelength of 75–180 km, while the vertical phase propagation is downward. The oscillations are present already in the sill region. The observations do not match predictions for eddies generated either by vortex stretching or baroclinic instability but agree broadly with properties of topographic Rossby waves.

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Achim Randelhoff, Ilker Fer, and Arild Sundfjord

Abstract

Every summer, intense sea ice melt around the margins of the Arctic pack ice leads to a stratified surface layer, potentially without a traditional surface mixed layer. The associated strengthening of near-surface stratification has important consequences for the redistribution of near-inertial energy, ice–ocean heat fluxes, and vertical replenishment of nutrients required for biological growth. The authors describe the vertical structure of meltwater layers and quantify their seasonal evolution and their effect on turbulent mixing in the oceanic boundary layer by analyzing more than 450 vertical profiles of velocity microstructure in the seasonal ice zone north of Svalbard. The vertical structure of the density profiles can be summarized by an equivalent mixed layer depth h BD, which scales with the depth of the seasonal stratification. As the season progresses and melt rates increase, h BD shoals following a robust pattern, implying stronger vertical stratification, weaker vertical eddy diffusivity, and reduced vertical extent of the mixing layer, which is bounded by h BD. Through most of the seasonal pycnocline, the vertical eddy diffusivity scales inversely with buoyancy frequency (K ρN −1). The presence of mobile sea ice alters the magnitude and vertical structure of turbulent mixing primarily through stronger and shallower stratification, and thus vertical eddy diffusivity is greatly reduced under sea ice. This study uses these results to develop a quantitative model of surface layer turbulent mixing during Arctic summer and discuss the impacts of a changing sea ice cover.

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Ilker Fer, Keith Makinson, and Keith W. Nicholls

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Observations were made of ocean microstructure and horizontal currents adjacent to Brunt Ice Shelf in the southeastern Weddell Sea. Periods of in situ supercooled water extending as deep as 65 m were associated with ice nucleation and frazil formation at depth. Ascending ice crystals due to convection lead to increased dissipation rates. The main outflow of potentially supercooled water from deep beneath ice shelf is suggested to be in the deep channel northeast of the measurement site. Because this water is advected southward along the front, it becomes in situ supercooled, leading to suspended ice formation, thermohaline convection, and enhanced dissipation.

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Ilker Fer, Ragnheid Skogseth, and Florian Geyer

Abstract

Observations were made of oceanic currents, hydrography, and microstructure in the southern Yermak Plateau in summer 2007. The location is in the marginal ice zone at the Arctic Front northwest of Svalbard, where the West Spitsbergen Current (WSC) carries the warm Atlantic Water into the Arctic Ocean. Time series of approximately 1-day duration from five stations (upper 520 m) and of an 8-day duration from a mooring are analyzed to describe the characteristics of internal waves and turbulent mixing. The spectral composition of the internal-wave field over the southern Yermak Plateau is 0.1–0.3 times the midlatitude levels and compares with the most energetic levels in the central Arctic. Dissipation rate and eddy diffusivity below the pycnocline increase from the noise level on the cold side of the front by one order of magnitude on the warm side, where 100-m-thick layers with average diffusivities of 5 × 10−5 m2 s−1 lead to heat loss from the Atlantic Water of 2–4 W m−2. Dissipation in the upper 150 m is well above the noise level at all stations, but strong stratification at the cold side of the front prohibits mixing across the pycnocline. Close to the shelf, at the core of the Svalbard branch of the WSC, diffusivity increases by another factor of 3–6. Here, near-bottom mixing removes 15 W m−2 of heat from the Atlantic layer. Internal-wave activity and mixing show variability related to topography and hydrography; thus, the path of the WSC will affect the cooling and freshening of the Atlantic inflow. When generalized to the Arctic Ocean, diapycnal mixing away from abyssal plains can be significant for the heat budget. Around the Yermak Plateau, it is of sufficient magnitude to influence heat anomaly pulses entering the Arctic Ocean; however, diapycnal mixing alone is unlikely to be significant for regional cooling of the WSC.

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Chuncheng Guo, Mehmet Ilicak, Ilker Fer, Elin Darelius, and Mats Bentsen

Abstract

The generation mechanism of mesoscale eddies in the Faroe Bank Channel (FBC) overflow region and their spatiotemporal characteristics are examined using the high-resolution regional Massachusetts Institute of Technology general circulation model (MITgcm). From the modeled overflow, it is found that the volume transport downstream of the FBC sill exhibits strong variability with a distinct period of ~4 days. Energetic, alternating cyclonic and anticyclonic eddies appear at ~40 km downstream of the sill. They grow side by side in the nascent stage, but later the cyclones migrate along the 800-m isobath to the south of Iceland, whereas the anticyclones descend downslope across the isobath and gradually dissipate. Analysis of the eddy characteristics shows that the cyclones are associated with a larger plume thickness and width, larger volume transport, colder and denser water, and a plume core located farther downslope, whereas the opposite is true for the anticyclones. The oscillatory structure developed at the lower boundary of the mean plume and the following generation of alternating cyclones and anticyclones are typical features of baroclinic instability. A linear instability analysis of a two-layer analytical baroclinic model yields a most unstable mode that agrees favorably with the simulations. The calculation of the divergent eddy heat flux shows a substantial rightward (upslope)-directed component downstream of the FBC sill. This region is also associated with a strong baroclinic conversion rate. The above arguments constitute evidence for the generation of unstable plume and mesoscale eddies in the FBC region by baroclinic instability.

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Nicholas Beaird, Ilker Fer, Peter Rhines, and Charles Eriksen

Abstract

Turbulent mixing is an important process controlling the descent rate, water mass modification, and volume transport augmentation due to entrainment in the dense overflows across the Greenland–Scotland Ridge. These overflows, along with entrained Atlantic waters, form a major portion of the North Atlantic Deep Water, which pervades the abyssal ocean. Three years of Seaglider observations of the overflows across the eastern Greenland–Scotland Ridge are leveraged to map the distribution of dissipation of turbulent kinetic energy on the Iceland–Faroe Ridge. A method has been applied using the finescale vertical velocity and density measurements from the glider to infer dissipation. The method, termed the large-eddy method (LEM), is compared with a microstructure survey of the Faroe Bank Channel (FBC). The LEM reproduces the patterns of dissipation observed in the microstructure survey, which vary over several orders of magnitude. Agreement between the inferred LEM and more direct microstructure measurements is within a factor of 2. Application to the 9432 dives that encountered overflow waters on the Iceland–Faroe Ridge reveals three regions of enhanced dissipation: one downstream of the primary FBC sill, another downstream of the secondary FBC sill, and a final region in a narrow jet of overflow along the Iceland shelf break.

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Ilker Fer, Algot K. Peterson, and Jenny E. Ullgren

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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 ε is measured using airfoil shear probes. A comparison 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 × 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/u t < 20, where U is the flow past the sensor, u t = (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.

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