Near-inertial mixing in the central Arctic Ocean

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.publishedVersio

Mean structure and seasonality of the Norwegian Atlantic Front Current along the Mohn Ridge from repeated glider transects

The poleward flow of Atlantic Water in the Nordic Seas forms the upper limb of the meridional overturning circulation driving an important heat transport. The Norwegian Atlantic Front Current along the Mohn Ridge between the Greenland and Norwegian Seas is characterized for the first time, using repeated sections over 14 months from autonomous underwater gliders and two research cruises. The Norwegian Atlantic Front Current follows the 2,550‐m isobath with a width of about 60 km and absolute geostrophic velocities peaking at about 0.45 m s−1. The mean transport of Atlantic Water is 4.6 ± 0.2 Sv (equivalent to temperature transport of 100 ± 6 TW). Seasonal variability was observed with an amplitude of 0.9 Sv and maximum values in the fall. The deep currents at 1,000 m explained most of this seasonal variation and were anticorrelated with time‐integrated wind stress curl over the Lofoten Basin. Part of this flow might recirculate within the Lofoten Basin, while the rest continues toward the Arctic.publishedVersio

Mixing in the stratified interface of the Faroe Bank Channel overflow: the role of transverse circulation and internal waves

The overflow of cold water across the Faroe Bank Channel sill is a significant volume flux of dense water to the North Atlantic Ocean. Using observations of hydrography, current and microstructure from a 1 week cruise and 2 month long time series from moored instruments, we address the role of transverse circulation and internal waves in mixing in the stratified 100 m thick plume-ambient interface. The streamwise momentum budget is dominated by a balance between the pressure gradient and bottom friction; the entrainment stress is negligible. The transverse momentum budget is in geostrophic balance, and the transverse velocity variability is governed by the internal streamwise pressure gradient. The transverse geostrophic flow in the interfacial layer is opposed by the bottom Ekman transport. The shear associated with the interfacial jet lowers the Richardson number and enhances dissipation rates. Convective overturning events observed on the upslope side suggest a link between the transverse circulation and the vertical mixing on the upper slope. Several independent threads of evidence support the transverse circulation as an important mixing mechanism for the overflow plume. In the ambient, dissipation rates inferred from fine-scale shear and density profile measurements are in good agreement with direct measurements, supporting internal wave breaking as a dominant mechanism for dissipation of turbulent energy. In the interfacial layer, spectral distribution of internal wavefield is energetic. In addition to shear-induced mixing and entrainment in the interfacial layer, internal wave breaking is likely to be important for the dissipation of turbulent energy and should not be ignored.publishedVersio

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

Mixing of the Storfjorden overflow (Svalbard Archipelago) inferred from denstity overturns

Observations were made of the dense overflow from Storfjorden from a survey conducted at closely spaced stations in August 2002. The field data set consists of conventional conductivity-temperature-depth profiles and short-term moored current meters and thermistor strings. Finestructure estimates were made by calculating Thorpe scales over identified overturns using 0.1-dbar vertically averaged density profiles. Dissipation rate of turbulent kinetic energy per unit mass, e, is estimated assuming proportionality between Thorpe and Ozmidov length scales. Vertical eddy diffusivity Kz is estimated using Osborn’s model assuming a constant mixing efficiency. Survey-averaged profiles suggest enhanced mixing near the bottom with values of Kz and e, when averaged within the overflow, equal to 10 x 10-4 m2 s-1 and 3 x 10-8 W kg-1, respectively. Kz is found to decrease with increasing buoyancy frequency as N-1.2 (±0.3), albeit values of N covered only 0.5–8 cph (1 cph = 2p/3600 s-1). Values of heat flux obtained using Kz suggest that the plume gains a considerable amount of heat, 45 ± 25 W m2, when averaged over the thickness of the plume, from overlying waters of Atlantic origin. This value is lower than but, considering the errors in estimates of Kz, comparable with 100 W m2, the rate of change of heat in the overflow derived from sections across the sill and 80 km downstream.publishedVersio

Stratified flow over complex topography: A model study of the bottom drag and associated mixing

The flow of stratified fluid over complex topography may lead to a significant drag on the fluid, exerted by the bottom obstacles. Using a 2-m resolution, three-dimensional, non-hydrostatic numerical ocean model, the drag and associated mixing on a stratified flow over real, 1-m resolution topography (interpolated to model resolution) is studied. With a typical mountain height of 12 m in 174 m water and buoyancy frequencies ranging from 0:6 102 s1 to 1:2 102 s1, resolving the topographic features leads to extensive drag exerted on the flow manifested through three different processes: (i) gravity wave drag, (ii) aerodynamic or blocked flow drag, and (iii) hydraulic drag. A parameterization of the internal wave drag based on linear, two-dimensional, hydrostatic wave solutions provides satisfactory results in terms of the turbulent kinetic energy levels. The depth of the layer where the vertical momentum flux is deposited, however, is underestimated, leading to an overestimated gravity wave drag in the layer. & 20acceptedVersio

Faroe Bank Channel Overflow: Mesoscale Variability

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 os- cillations, 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 temper- ature 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 al- ternating 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.publishedVersio

Observational validation of the diffusive convection flux laws in the Amundsen Basin, Arctic Ocean

The low levels of mechanically driven mixing in many regions of the Arctic Ocean make double diffusive convection virtually the only mechanism for moving heat up from the core of Atlantic Water towards the surface. In an attempt to quantify double diffusive heat fluxes in the Arctic Ocean, a temperature microstructure experiment was performed as a part of the North Pole Environmental Observatory (NPEO) 2013 field season from the drifting ice station Barneo located in the Amundsen Basin near the Lomonosov Ridge (89.58N, 758W). A diffusive convective thermohaline staircase was present between 150 and 250 m in nearly all of the profiles. Typical vertical heat fluxes across the high-gradient interfaces were consistently small, O(1021 )Wm22 . Our experiment was designed to resolve the staircase and differed from earlier Arctic studies that utilized inadequate instrumentation or sampling. Our measured fluxes from temperature microstructure agree well with the laboratory derived flux laws compared to previous studies, which could find agreement only to within a factor of two to four. Correlations between measured and parameterized heat fluxes are slightly higher when using the more recent Flanagan et al. [2013] laboratory derivation than the more commonly used derivation presented in Kelley [1990]. Nusselt versus Rayleigh number scaling reveals the convective exponent, g, to be closer to 0.29 as predicted by recent numerical simulations of single-component convection rather than the canonical 1/3 assumed for double diffusion. However, the exponent appears to be sensitive to how convective layer height is defined.publishedVersio

Images of internal tides near the Norwegian continental slope

Author Posting. © American Geophysical Union, 2009. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 36 (2009): L00D10, doi:10.1029/2009GL038909.Internal tides, or internal gravity waves propagating at tidal frequencies, play an important role in ocean mixing but are challenging to detect and map over large spatial sections in the ocean's interior. We present seismic images of oceanic finestructure in the Norwegian Sea that demonstrate that semidiurnal (M2) internal tidal beams can be seismically imaged. We observe bands of seismic reflections that cross isotherms and closely mimic the expected internal tide ray characteristic over hundreds of meters vertically and tens of km laterally, in an area where critical seafloor slopes are common. Coincident temperature and density profiles show that the reflections come from reversible finestructure caused by internal wave strains. Where the beams intersect the seafloor, indications of enhanced mixing are present, including finestructure disruption and enhanced internal wave energy. These results suggest that seismic oceanography can be an effective tool in studies of ocean mixing by internal tides.This work was supported by Office of Naval Research grant N00014-04-1-0585 and by NSF's Ocean Drilling Program (grant OCE-0221366) and Physical Oceanography Program (grants OCE-0337289, OCE-0452744, and OCE-0648620)