Landfast ice controls on turbulence in Antarctic coastal seas

Knowledge of the ocean surface layer beneath Antarctic landfast ice is sparse. In this article surface layer turbulent and fine structure are quantified with and without landfast ice in the same West Antarctic Peninsula location. Landfast ice reduced turbulence levels locally to an order of magnitude less than ice-free values, and near-inertial energy and sub-inertial tidal energy levels to less than half their ice-free values. Vertical turbulent heat and nutrient fluxes were, respectively, 6 and 10 times greater than previously estimated. Under-ice tidal energy dissipation over the entire Antarctic continental shelf due to seasonal landfast ice cover is estimated at 788 MW. The total rate of wind-generated turbulence in the surface ocean is greatly reduced by the presence of seasonal landfast ice to an average of 14% of the ice-free value, but with large sectoral variations. Counter-intuitively, however, tides and wind contribute approximately equally to the turbulent kinetic energy budget of the upper ocean between the Antarctic coastline and the maximal landfast ice extent, with large sectoral variations, attributed to geographic variations in the strength of the barotropic tide

Modification of deep waters in Marguerite Bay, western Antarctic Peninsula, caused by topographic overflows

Circumpolar Deep Water (CDW) intrudes from the mid-layers of the Antarctic Circumpolar Current onto the shelf of the western Antarctic Peninsula, providing a source of heat and nutrients to the regional ocean. It is well known that CDW is modified as it flows across the shelf, but the mechanisms responsible for this are not fully known. Here, data from underwater gliders with high spatial resolution are used to demonstrate the importance of detailed bathymetry in inducing multiple local mixing events. Clear evidence for overflows is observed in the glider data as water flows along a deep channel with multiple transverse ridges. The ridges block the densest waters, with overflowing water descending several hundred metres to fill subsequent basins. This vertical flow leads to entrainment of overlying colder and fresher water in localised mixing events. Initially this process leads to an increase in bottom temperatures due to the temperature maximum waters descending to greater depths. After several ridges, however, the mixing is sufficient to remove the temperature maximum completely and the entrainment of colder thermocline waters to depth reduces the bottom temperature, to approximately the same as in the source region of Marguerite Trough. Similarly, it is shown that deep waters of Palmer Deep are warmer than at the same depth at the shelf break. The exact details of the transformations observed are heavily dependent on the local bathymetry and water column structure, but glacially-carved troughs and shallow sills are a common feature of the bathymetry of polar shelves, and these types of processes may be a factor in determining the hydrographic conditions close to the coast across a wider area

Three-dimensional structure of a cold-core Arctic eddy interacting with the Chukchi Slope Current

A rapid, high‐resolution shipboard survey, using a combination of lowered and expendable hydrographic measurements and vessel‐mounted acoustic Doppler current profiler data, provided a unique three‐dimensional view of an Arctic anti‐cyclonic cold‐core eddy. The eddy was situated 50 km seaward of the Chukchi Sea shelfbreak over the 1000 m isobath, embedded in the offshore side of the Chukchi Slope Current. The eddy core, centered near 150 m depth, consisted of newly ventilated Pacific winter water which was high in nitrate and dissolved oxygen. Its fluorescence signal was due to phaeopigments rather than chlorophyll, indicating that photosynthesis was no longer active, consistent with an eddy age on the order of months. Subtracting out the slope current signal demonstrated that the eddy velocity field was symmetrical with a peak azimuthal speed of order 10 cm s‐1. Its Rossby number was ~0.4, consistent with the fact that the measured cyclogeostrophic velocity was dominated by the geostrophic component. Different scenarios are discussed regarding how the eddy became embedded in the slope current and what the associated ramifications are with respect to eddy spin‐down and ventilation of the Canada Basin halocline

Observed eddy-internal wave interactions in the Southern Ocean

The physical mechanisms that remove energy from the Southern Ocean’s vigorous mesoscale eddy field are not well understood. One proposed mechanism is direct energy transfer to the internal wave field in the ocean interior, via eddy-induced straining and shearing of preexisting internal waves. The magnitude, vertical structure, and temporal variability of the rate of energy transfer between eddies and internal waves is quantified from a 14-month deployment of a mooring cluster in the Scotia Sea. Velocity and buoyancy observations are decomposed into wave and eddy components, and the energy transfer is estimated using the Reynolds-averaged energy equation. We find that eddies gain energy from the internal wave field at a rate of −2.2 ± 0.6 mW m−2, integrated from the bottom to 566 m below the surface. This result can be decomposed into a positive (eddy to wave) component, equal to 0.2 ± 0.1 mW m−2, driven by horizontal straining of internal waves, and a negative (wave to eddy) component, equal to −2.5 ± 0.6 mW m−2, driven by vertical shearing of the wave spectrum. Temporal variability of the transfer rate is much greater than the mean value. Close to topography, large energy transfers are associated with low-frequency buoyancy fluxes, the underpinning physics of which do not conform to linear wave dynamics and are thereby in need of further research. Our work suggests that eddy–internal wave interactions may play a significant role in the energy balance of the Southern Ocean mesoscale eddy and internal wave fields

Seismic reflection imaging of mixing processes in Fram Strait

The West Spitsbergen Current, which flows northward along the western Svalbard continental slope, transports warm and saline Atlantic water (AW) into the Arctic Ocean. A combined analysis of high-resolution seismic images and hydrographic sections across this current has uncovered the oceanographic processes involved in horizontal and vertical mixing of AW. At the shelf break, where a strong horizontal temperature gradient exists east of the warmest AW, isopycnal interleaving of warm AW and surrounding colder waters is observed. Strong seismic reflections characterize these interleaving features, with a negative polarity reflection arising from an interface of warm water overlying colder water. A seismic-derived sound speed image reveals the extent and lateral continuity of such interleaving layers. There is evidence of obliquely aligned internal waves emanating from the slope at 450–500 m. They follow the predicted trajectory of internal S2 tidal waves and can promote vertical mixing between Atlantic and Arctic-origin waters

Controls on turbulent mixing on the West Antarctic Peninsula shelf

The ocean-to-atmosphere heat budget of the West Antarctic Peninsula is controlled in part by the upward flux of heat from the warm Circumpolar Deep Water (CDW) layer that resides below ~200 m to the Antarctic Surface Water (AASW), a water mass which varies strongly on a seasonal basis. Upwelling and mixing of CDW influence the formation of sea ice in the region and affect biological productivity and functioning of the ecosystem through their delivery of nutrients. In this study, 2.5-year time series of both Acoustic Doppler Current Profiler (ADCP) and conductivity-temperature-depth (CTD) data are used to quantify both the diapycnal diffusivity κ and the vertical heat flux Q at the interface between CDW and AASW. Over the period of the study, a mean upward heat flux of ~1 W m−2 is estimated, with the largest heat fluxes occurring shortly after the loss of winter fast ice when the water column is first exposed to wind stress without being strongly stratified by salinity. Differences in mixing mechanisms between winter and summer seasons are investigated. Whilst tidally-driven mixing at the study site occurs year-round, but is likely to be relatively weak, a strong increase in counterclockwise-polarized near-inertial energy (and shear) is observed during the fast-ice-free season, suggesting that the direct impact of storms on the ocean surface is responsible for much of the observed mixing at the site. Given the rapid reduction in sea-ice duration in this region in the last 30 years, a shift towards an increasingly wind-dominated mixing regime may be taking place

Tracing Glacial Meltwater From the Greenland Ice Sheet to the Ocean Using Gliders

The Greenland Ice Sheet (GrIS) is experiencing significant mass loss and freshwater discharge at glacier fronts. The freshwater input from Greenland will impact the physical properties of adjacent coastal seas, including important regions of deep water formation and contribute to global sea level rise. However, the biogeochemical impact of increasing freshwater discharge from the GrIS is less well constrained. Here, we demonstrate the use of bio-optical sensors on ocean gliders to track biogeochemical properties of meltwaters off southwest Greenland. Our results reveal that fresh, coastal waters, with an oxygen isotopic composition characteristic of glacial meltwater, are distinguished by a high optical backscatter and high levels of fluorescing dissolved organic matter (FDOM), representative of the overall colored dissolved organic matter pool. Reconstructions of geostrophic velocities are used to show that these particle and FDOM-enriched coastal waters cross the strong boundary currents into the Labrador Sea. Meltwater input into the Labrador Sea is likely driven by mesoscale processes, such as eddy formation and local bathymetric steering, in addition to wind-driven Ekman transport. Ocean gliders housing bio-optical sensors can provide the high-resolution observations of both dissolved and particulate glacially derived material that are needed to understand meltwater dispersal mechanisms and their sensitivity to future climatic change

A novel organic-rich meteoritic clast from the outer solar system

The Zag meteorite which is a thermally-metamorphosed H ordinary chondrite contains a primitive xenolitic clast that was accreted to the parent asteroid after metamorphism. The cm-sized clast contains abundant large organic grains or aggregates up to 20μm in phyllosilicate-rich matrix. Here we report organic and isotope analyses of a large (~10μm) OM aggregate in the Zag clast. The X-ray micro-spectroscopic technique revealed that the OM aggregate has sp2 dominated hydrocarbon networks with a lower abundance of heteroatoms than in IOM from primitive (CI,CM,CR) carbonaceous chondrites, and thus it is distinguished from most of the OM in carbonaceous meteorites. The OM aggregate has high D/H and 15N/14N ratios (δD=2,370±74‰ and δ15N=696±100‰), suggesting that it originated in a very cold environment such as the interstellar medium or outer region of the solar nebula, while the OM is embedded in carbonate-bearing matrix resulting from aqueous activities. Thus, the high D/H ratio must have been preserved during the extensive late-stage aqueous processing. It indicates that both the OM precursors and the water had high D/H ratios. Combined with 16O-poor nature of the clast, the OM aggregate and the clast are unique among known chondrite groups. We further propose that the clast possibly originated from D/P type asteroids or trans-Neptunian Objects

Silica in Protoplanetary Disks

Mid-infrared spectra of a few T Tauri stars (TTS) taken with the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope show prominent narrow emission features indicating silica (crystalline silicon dioxide). Silica is not a major constituent of the interstellar medium; therefore, any silica present in the circumstellar protoplanetary disks of TTS must be largely the result of processing of primitive dust material in the disks surrouding these stars. We model the silica emission features in our spectra using the opacities of various polymorphs of silica and their amorphous versions computed from earth-based laboratory measurements. This modeling indicates that the two polymorphs of silica, tridymite and cristobalite, which form at successively higher temperatures and low pressures, are the dominant forms of silica in the TTS of our sample. These high temperature, low pressure polymorphs of silica present in protoplanetary disks are consistent with a grain composed mostly of tridymite named Ada found in the cometary dust samples collected from the STARDUST mission to Comet 81P/Wild 2. The silica in these protoplanetary disks may arise from incongruent melting of enstatite or from incongruent melting of amorphous pyroxene, the latter being analogous to the former. The high temperatures of 1200K-1300K and rapid cooling required to crystallize tridymite or cristobalite set constraints on the mechanisms that could have formed the silica in these protoplanetary disks, suggestive of processing of these grains during the transient heating events hypothesized to create chondrules.Comment: 47 pages, 9 figures, to appear in the 1 January, 2009 issue of the Astrophysical Journa

Local- and large-scale drivers of variability in the coastal freshwater budget of the Western Antarctic Peninsula

The west Antarctic Peninsula (WAP) is a region of marked climatic variability, exhibiting strong changes in sea ice extent, retreat of most of its glaciers, and shifts in the amount and form of precipitation. These changes can have significant impacts on the oceanic freshwater budget and marine biogeochemical processes; it is thus important to ascertain the relative balance of the drivers, and the spatial scales over which they operate. We present a novel 7‐year summer‐season (October to March; 2011 to 2018) series of oxygen isotopes in seawater (δ18O), augmented with some winter sampling, collected adjacent to Anvers Island at the WAP. These data are used to attribute oceanic freshwater changes to sea ice and meteoric sources, and to deduce information on the spatial scales over which the changes are driven. Sea ice melt shows significant seasonality (∼9% range) and marked interannual changes, with pronounced maxima in seasons 2013/14 and 2016/17. Both of these extrema are driven by anomalous winds, but reflect strongly contrasting dynamic and thermodynamic sea ice responses. Meteoric water also shows seasonality (∼7% range), with interannual variability reflecting changes in the input of accumulated precipitation and glacial melt to the ocean. Unlike sea ice melt, meteoric water extremes are especially pronounced in thin (<10 m) surface layers close to the proximate glacier, associated with enhanced ocean stratification. Isotopic tracers help to deconvolve the complex spatio‐temporal scales inherent in the coastal freshwater budget, and hence improve knowledge of the separate and cumulative physical and ecological impacts