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

Sustained observations in the Weddell Sea spanning more than 20 years show gradual increase of the deep water heat content

Beginning in 1989, Eberhard Fahrbach established and maintained until his premature death an observational programme in the Weddell Sea, which outstandingly contributed to alleviate the grave problem of undersampling of the Southern Ocean. Continuation of his legacy by the Alfred-Wegener-Institut has yielded a time series that now extends into 2013, hence covers almost 24 years. Here we analyse this data set for long-term changes of the heat content in the deep Weddell Sea. We exclusively evaluate the calibrated temperature records obtained with ship-lowered CTD (conductivity-temperature-depth sonde) casts at repeated hydrographic stations and along repeated sections. Using this approach we avoid introducing potential temperature offsets that can result from combination of different measurement technologies and potential biases resultant from differences in geographic positions. Our results show that the deep water masses below 700 m gradually warmed over the past two decades by 0.001 – 0.004 K a-1. Superimposed inter-annual to multi-annual variations appear as largely uncorrelated horizontally across the Weddell Gyre. The long-term (21 – 24 years) trends of increasing temperatures in different depth layers below 700 m at all stations and sections can be approximated by linear regression that explains between 27 and 91 % of the variance, where the coefficients of correlation tend to increase with depth. No significant trends are found in the top 700 m. The heating rate of the water masses below 700 m is estimated to 0.79 ± 0.14 W m-2, which is more than twice as high as determined for the global deep ocean in general. Our results hence corroborate the view that Southern Ocean processes make an above-average contribution to the deep ocean warming, and so add to bring global estimates of the deep ocean heating rate and of the net energy flux into the Earth’s climate system at the top of the atmosphere of 0.5 - 1 W m-2 closer in line with each other. Thus they help to resolve the problem of the ‘missing heat’ or ‘missing energy’, respectively, terms coined to grasp the observation that surface temperatures of planet Earth have stalled rising since about 15 years while radiation-affecting atmospheric CO2 concentrations continued to increase. Our results support the finding that excess energy which results from changes in the Earth’ radiation balance is transferred into heating of the deep ocean, where it does not contribute to an increase of surface temperatures but inevitably enhances thermosteric sea level rise

Mesoscale features create hotspots of carbon uptake in the Antarctic Circumpolar Current

The influence of eddy structures on the seasonal depletion of dissolved inorganic carbon (DIC) and carbon dioxide (CO2) disequilibrium was investigated during a trans-Atlantic crossing of the Antarctic Circumpolar Current (ACC) in austral summer 2012. The Georgia Basin, downstream of the island of South Georgia (54-55°S, 36-38°W) is a highly dynamic region due to the mesoscale activity associated with the flow of the Subantarctic Front (SAF) and Polar Front (PF). Satellite sea-surface height and chlorophyll-a anomalies revealed a cyclonic cold core that dominated the northern Georgia Basin that was formed from a large meander of the PF. Warmer waters influenced by the SAF formed a smaller anticyclonic structure to the east of the basin. Both the cold core and warm core eddy structures were hotspots of carbon uptake relative to the rest of the ACC section during austral summer. This was most amplified in the cold core where greatest CO2 undersaturation (-78 µatm) and substantial surface ocean DIC deficit (5.1 mol m-2) occurred. In the presence of high wind speeds, the cold core eddy acted as a strong sink for atmospheric CO2 of 25.5 mmol m-2 day-1. Waters of the warm core displayed characteristics of the Polar Frontal Zone (PFZ), with warmer upper ocean waters and enhanced CO2 undersaturation (-59 µatm) and depletion of DIC (4.9mol m-2). A proposed mechanism for the enhanced carbon uptake across both eddy structures is based on the Ekman eddy pumping theory: (i) the cold core is seeded with productive (high chlorophyll-a) waters from the Antarctic Zone and sustained biological productivity through upwelled nutrient supply that counteracts DIC inputs from deep waters; (ii) horizontal entrainment of low-DIC surface waters (biological uptake) from the PFZ downwell within the warm core and cause relative DIC-depletion in the upper water column. The observations suggest that the formation and northward propagation of cold core eddies in the region of the PF could project low-DIC waters towards the site of Antarctic Intermediate Water formation and enhance CO2 drawdown into the deep ocean

Controls of primary production in two phytoplankton blooms in the Antarctic Circumpolar Current

The Antarctic Circumpolar Current has a high potential for primary production and carbon sequestration through the biological pump. In the current study, two large-scale blooms observed in 2012 during a cruise with R.V. Polarstern were investigated with respect to phytoplankton standing stocks, primary productivity and nutrient budgets. While net primary productivity was similar in both blooms, chlorophyll a –specific photosynthesis was more efficient in the bloom closer to the island of South Georgia (39 °W, 50 °S) compared to the open ocean bloom further east (12 °W, 51 °S). We did not find evidence for light being the driver of bloom dynamics as chlorophyll standing stocks up to 165 mg m-2 developed despite mixed layers as deep as 90 m. Since the two bloom regions differ in their distance to shelf areas, potential sources of iron vary. Nutrient (nitrate, phosphate, silicate) deficits were similar in both areas despite different bloom ages, but their ratios indicated more pronounced iron limitation at 12 °W compared to 39 °W. While primarily the supply of iron and not the availability of light seemed to control onset and duration of the blooms, higher grazing pressure could have exerted a stronger control toward the declining phase of the blooms

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