Modification of the deep salinity-maximum in the Southern Ocean by circulation in the Antarctic Circumpolar Current and the Weddell Gyre

The evolution of the deep salinity-maximum associated with the Lower Circumpolar Deep Water (LCDW) is assessed using a set of 37 hydrographic sections collected over a 20 year period in the Southern Ocean as part of the WOCE/CLIVAR programme. A circumpolar decrease in the value of the salinity maximum is observed eastwards from the North Atlantic Deep Water (NADW) in the Atlantic sector of the Southern Ocean through the Indian and Pacific sectors to Drake Passage. Isopycnal mixing processes are limited by circumpolar fronts, and in the Atlantic sector this acts to limit the direct poleward propagation of the salinity signal. Limited entrainment occurs into the Weddell Gyre, with LCDW entering primarily through the eddydominated eastern limb. A vertical mixing coefficient, κV of (2.86 ± 1.06) x 10^-4 m^2 s^-1 and an isopycnal mixing coefficient, κI of (8.97 ± 1.67) x 10^2 m^2 s^-1 are calculated for the eastern Indian and Pacific sectors of the Antarctic Circumpolar Current (ACC). A κV of (2.39 ± 2.83) x 10-5 m^2 s^-1, an order of magnitude smaller, and a κI of (2.47 ± 0.63) x 10^2 m^2 s^-1, three times smaller, are calculated for the southern and eastern Weddell Gyre reflecting a more turbulent regime in the ACC and a less turbulent regime in the Weddell Gyre. In agreement with other studies, we conclude that the ACC acts as a barrier to direct meridional transport and mixing in the Atlantic sector evidenced by the eastward propagation of the deep salinity-maximum signal, insulating the Weddell Gyre from short-term changes in NADW characteristics

The Association of Antarctic Krill Euphausia superba with the Under-Ice Habitat

The association of Antarctic krill Euphausia superba with the under-ice habitat was investigated in the Lazarev Sea (Southern Ocean) during austral summer, autumn and winter. Data were obtained using novel Surface and Under Ice Trawls (SUIT), which sampled the 0–2 m surface layer both under sea ice and in open water. Average surface layer densities ranged between 0.8 individuals m−2 in summer and autumn, and 2.7 individuals m−2 in winter. In summer, under-ice densities of Antarctic krill were significantly higher than in open waters. In autumn, the opposite pattern was observed. Under winter sea ice, densities were often low, but repeatedly far exceeded summer and autumn maxima. Statistical models showed that during summer high densities of Antarctic krill in the 0–2 m layer were associated with high ice coverage and shallow mixed layer depths, among other factors. In autumn and winter, density was related to hydrographical parameters. Average under-ice densities from the 0–2 m layer were higher than corresponding values from the 0–200 m layer collected with Rectangular Midwater Trawls (RMT) in summer. In winter, under-ice densities far surpassed maximum 0–200 m densities on several occasions. This indicates that the importance of the ice-water interface layer may be under-estimated by the pelagic nets and sonars commonly used to estimate the population size of Antarctic krill for management purposes, due to their limited ability to sample this habitat. Our results provide evidence for an almost year-round association of Antarctic krill with the under-ice habitat, hundreds of kilometres into the ice-covered area of the Lazarev Sea. Local concentrations of postlarval Antarctic krill under winter sea ice suggest that sea ice biota are important for their winter survival. These findings emphasise the susceptibility of an ecological key species to changing sea ice habitats, suggesting potential ramifications on Antarctic ecosystems induced by climate change

Modelling the Weddell Gateway: An Inverse Approach to Determining Volume Transport a the Lazarev Sea

The Lazarev Sea is the deep water gateway to the Weddell Sea, with the topographic constraints of Maud Rise and Astrid Ridge having a noticeable impact upon the distribution of the hydrographic properties of the Warm Deep Water. The Lazarev Sea Krill Experiment (LAKRIS) cruises conducted by the RV Polarstern between 2004 and 2008 provide a suitable density of CTD sections to detect the variable hydrographic properties of the region. These patterns highlight key circulation features including a jet on the northern flank of Maud Rise, the Taylor column above the rise, and the apparent pooling of Warm Deep Water to the south-west of the rise. We are developing an inverse box-model in order to infer the circulation of a given region using CTD data from cruises with multiple parallel sections: providing a grid of data. The model is based upon the multiple linear regression of mass conservation and Dunhem–Margules equations for all of the boxes in the grid, where each box is composed of four neighbouring CTD stations. The model also includes observed current referencing (such as from ADCP data) to calibrate the geostrophic flow. The regression provides an estimate of the volume transport across each neighbouring station pair face and for a specifiable number of layers. The model is currently set-up to use the density data from the LAKRIS cruises, but could be adapted for other parameters, regions and programmes. We aim to evaluate the robustness of our approach in determining the general and localised transport of regional scale ocean areas such as the Lazarev Sea and present our inverse model results for the 4 LAKRIS cruises

NACA Research Memorandums

Report discussing a study of the effects of puncturing the heat shield of an intercontinental-ballistic-missile warhead by small projectiles. Calculations were created for both rod and sphere projectiles and experimental testing was performed on a missile model with holes drilled in the heat shield. The possibility that a projectile could have enough energy to cause mechanical damage to the interior of the warhead is also presented

Ice draft and current measurements from the north-western Barents Sea, 1993-96

From 1993 to 1996, three oceanographic moorings were deployed in the north-western Barents Sea, each with a current meter and an upward-looking sonar for measuring ice drafts. These yielded three years of current and two years of ice draft measurements. An interannual variability of almost 1 m was measured in the average ice draft. Causes for this variability are explored, particularly its possible connection to changes in atmospheric circulation patterns. We found that the flow of Northern Barents Atlantic-derived Water and the transport of ice from the Central Arctic into the Barents Sea appears to be controlled by winds between Nordaustlandet and Franz Josef Land, which in turn may be influenced by larger-scale variations such as the Arctic Oscillation/North Atlantic Oscillation

Mesopelagic carbon remineralization during the European Iron Fertilization Experiment

[1] The impact of iron fertilization on mesopelagic carbon (C) remineralization was assessed during the European Iron Fertilization Experiment (EIFEX) in the Southern Ocean by following the temporal change of excess particulate barium (also called biogenic or Ba(xs)) in the mesopelagic waters of a mesoscale eddy. Before the iron infusion the site was already sustaining a significant vertical flux of particles leading to organic remineralization in the mesopelagic. Approximately 35 d after the fertilization, mesopelagic Baxs contents provided evidence of changes in surface particulate export and of enhanced and particularly fast remineralization extending down to 1000 m. At the end of the experiment (day 36) both export and remineralization decreased. Organic carbon remineralization took place mainly in the lower part of the mesopelagic zone (500 - 1000 m) and increased more than 5-fold during EIFEX, reaching values up to 92 +/- 15 mg C m(-2) d(-1). However, such remineralization rates are not particularly high when compared to other values reported for the natural Southern Ocean during summer. Though export and remineralization reached peak values around day 34, remineralization integrated over the 150 to 1000 m layer accounted only for 13 +/- 1.4% of the export at 150 m, suggesting that a substantial amount of exported carbon reached deeper in the water column. Compared to natural blooms in high-nutrient low-chlorophyll (HNLC) waters of the Southern Ocean, the Fe-induced bloom during EIFEX resulted in a lower ratio of remineralized organic carbon over exported carbon, similar to what we recently observed for natural Fe-replete conditions prevailing above the Kerguelen Plateau. Unexpected similarities and phase lags of profiles in the mesopelagic between casts sampled inside and outside the fertilized patch during EIFEX indicated that eddy dynamics were a determinant in setting particle patterns on a broad spatial scale exceeding the extent of the fertilization, thereby homogenizing the signals of mesopelagic remineralization

Physico-biogeochemical differences along the ACC in the Atlantic Sector during one late summer - first results obtained from Eddy Pump

The multi-disciplinary project Eddy Pump was designed to study processes which exert a control on the physical and biological pumps of carbon in the Southern Ocean. The data for Eddy Pump were collected 2012 during Polarstern Cruise ANT-XXVIII/3, which started January 7 in Cape Town and ended March 11 in Punta Arenas. The study sites visited were selected in order to represent different biogeographic regimes found along the Antarctic Circumpolar Current (ACC) in the Atlantic sector: 1. the central ACC with its regular separation in different frontal jets (investigated by a meridional transect along 10°E), 2. a transient months-long lasting large-scale bloom that occurred west of 10°W adjacent to chlorophyll-poor waters to the immediate east, and 3. the region in the wake of South Georgia, which regularly features a dense bloom presumably stimulated by iron released from that island and its shelf. After presenting an overview of the obtained first results we will put up for discussion possible explanations for the observed patterns

Zetameter for microelectrophoresis studies of the oxide/water interface at temperatures up to 200 °C

The zeta potential (ZP) is an important and measurable parameter related to the electrical double layer structure at a solid-aqueous solution interface. A high temperature zetameter based on the microelectrophoresis technique was developed to determine the zeta potential and the isoelectric point (IEP) of the metal oxide/water interfaces at temperatures up to 200 °C and pressures up to 50 bar. Design of the microelectrophoresis cell, the main unit of the zetameter, utilized a flow-through concept and the cell internals were made from corrosion resistant materials in order to minimize materials degradation and solution contamination. Two sapphire windows were installed to the microelectrophoresis cell to enable observation of the particle movement under an imposed electrical field. A ZrO 2 powder was used to test the zetameter. The ZP for the ZrO 2 /water system was measured over wide ranges of temperature and p H . The IEP of the ZrO 2 /water system was found equal to 6.05 at room temperature, 5.00 at 120 °C, and 4.67 at 200 °C. Thus, at room temperature, the IEP obtained was within the confidence interval of the averaged literature data. In addition, it was found that the IEP of ZrO 2 was sensitive to temperature and the difference between the IEP and 0.5pK w remained constant (1.00±0.2) with temperatures up to 200 °C

Seasonal changes in the vertical distribution and community structure of Antarctic macrozooplankton and micronekton

The macrozooplankton and micronekton community of the Lazarev Sea (Southern Ocean) was investigated at 3 depth layers during austral summer, autumn and winter: (1) the surface layer (0–2 m); (2) the epipelagic layer (0–200 m); and (3) the deep layer (0–3000 m). Altogether, 132 species were identified. Species composition changed with depth from a euphausiid-dominated community in the surface layer, via a siphonophore-dominated community in the epipelagic layer, to a chaetognath-dominated community in the deep layer. The surface layer community predominantly changed along gradients of surface water temperature and sea ice parameters, whereas the epipelagic community mainly changed along hydrographical gradients. Although representing only 1% of the depth range of the epipelagic layer, mean per-area macrofauna densities in the surface layer ranged at 8% of corresponding epipelagic densities in summer, 6% in autumn, and 24% in winter. Seasonal shifts of these proportional densities in abundant species indicated different strategies in the use of the surface layer, including both hibernal downward and hibernal upward shift in the vertical distribution, as well as year-round surface layer use by Antarctic krill. These findings imply that the surface layer, especially when it is ice-covered, is an important functional node of the pelagic ecosystem that has been underestimated by conventional depth-integrated sampling in the past. The exposure of this key habitat to climate-driven forces most likely adds to the known susceptibility of Antarctic pelagic ecosystems to temperature rise and changing sea ice conditions