Icebergs boost phytoplankton growth in the Southern Ocean

Icebergs which calve from the Antarctic ice shelves and drift in the Southern Ocean deliver fresh water, dust and minerogenic particles to the surface ocean along the iceberg's path. Each of these components may have an effect on growth conditions for phytoplankton, as might the mechanical effects of the iceberg keel disturbing the water. Although anecdotal evidence and small-scale surveys suggest that drifting icebergs increase local primary production, no large-scale studies have reported on this possibility in detail. A combination of satellite and automated iceberg tracking data presented here shows that the probability of increased surface phytoplankton biomass was two-fold higher in the wake of a tracked iceberg compared to background biomass fluctuations. Only during the month of February were the effects of icebergs on surface biomass likely to be negative. These results confirm icebergs as a factor affecting phytoplankton in the Southern Ocean and highlight the need for detailed process studies so that responses to future changes in the Antarctic ice sheets may be predicted

Rapid submarine ice melting in the grounding zones of ice shelves in West Antarctica.

Enhanced submarine ice-shelf melting strongly controls ice loss in the Amundsen Sea embayment (ASE) of West Antarctica, but its magnitude is not well known in the critical grounding zones of the ASE's major glaciers. Here we directly quantify bottom ice losses along tens of kilometres with airborne radar sounding of the Dotson and Crosson ice shelves, which buttress the rapidly changing Smith, Pope and Kohler glaciers. Melting in the grounding zones is found to be much higher than steady-state levels, removing 300-490 m of solid ice between 2002 and 2009 beneath the retreating Smith Glacier. The vigorous, unbalanced melting supports the hypothesis that a significant increase in ocean heat influx into ASE sub-ice-shelf cavities took place in the mid-2000s. The synchronous but diverse evolutions of these glaciers illustrate how combinations of oceanography and topography modulate rapid submarine melting to hasten mass loss and glacier retreat from West Antarctica

Pathways of ocean heat towards Pine Island and Thwaites grounding lines

In the Amundsen Sea, modified Circumpolar Deep Water (mCDW) intrudes into ice shelf cavities, causing high ice shelf melting near the ice sheet grounding lines, accelerating ice flow, and controlling the pace of future Antarctic contributions to global sea level. The pathways of mCDW towards grounding lines are crucial as they directly control the heat reaching the ice. A realistic representation of mCDW circulation, however, remains challenging due to the sparsity of in-situ observations and the difficulty of ocean models to reproduce the available observations. In this study, we use an unprecedentedly high-resolution (200 m horizontal and 10 m vertical grid spacing) ocean model that resolves shelf-sea and sub-ice-shelf environments in qualitative agreement with existing observations during austral summer conditions. We demonstrate that the waters reaching the Pine Island and Thwaites grounding lines follow specific, topographically-constrained routes, all passing through a relatively small area located around 104°W and 74.3°S. The temporal and spatial variabilities of ice shelf melt rates are dominantly controlled by the sub-ice shelf ocean current. Our findings highlight the importance of accurate and high-resolution ocean bathymetry and subglacial topography for determining mCDW pathways and ice shelf melt rates

Carbon dynamics of the Weddell Gyre, Southern Ocean

The accumulation of carbon within the Weddell Gyre and its exchanges across the gyre boundaries are investigated with three recent full-depth oceanographic sections enclosing this climatically important region. The combination of carbonmeasurements with ocean circulation transport estimates from a box inverse analysis reveals that deepwater transports associated with Warm Deep Water (WDW) and Weddell Sea Deep Water dominate the gyre’s carbon budget, while a dual-cell vertical overturning circulation leads to both upwelling and the delivery of large quantities of carbon to the deep ocean. Historical sea surface pCO2 observations, interpolated using a neural network technique, confirm the net summertime sink of 0.044 to 0.058 ± 0.010 Pg C / yr derived from the inversion. However, a wintertime outgassing signal similar in size results in a statistically insignificant annual air-to-sea CO2 flux of 0.002± 0.007 Pg C / yr (mean 1998–2011) to 0.012 ± 0.024 Pg C/ yr (mean 2008–2010) to be diagnosed for the Weddell Gyre. A surface layer carbon balance, independently derived fromin situ biogeochemical measurements, reveals that freshwater inputs and biological drawdown decrease surface ocean inorganic carbon levels more than they are increased by WDW entrainment, resulting in an estimated annual carbon sink of 0.033 ± 0.021 Pg C / yr. Although relatively less efficient for carbon uptake than the global oceans, the summertime Weddell Gyre suppresses the winter outgassing signal, while its biological pump and deepwater formation act as key conduits for transporting natural and anthropogenic carbon to the deep ocean where they can reside for long time scales

Oceans Melting Greenland: Early Results from NASA's Ocean-Ice Mission in Greenland

Melting of the Greenland Ice Sheet represents a major uncertainty in projecting future rates of global sea level rise. Much of this uncertainty is related to a lack of knowledge about subsurface ocean hydrographic properties, particularly heat content, how these properties are modified across the continental shelf, and about the extent to which the ocean interacts with glaciers. Early results from NASA’s five-year Oceans Melting Greenland (OMG) mission, based on extensive hydrographic and bathymetric surveys, suggest that many glaciers terminate in deep water and are hence vulnerable to increased melting due to ocean-ice interaction. OMG will track ocean conditions and ice loss at glaciers around Greenland through the year 2020, providing critical information about ocean-driven Greenland ice mass loss in a warming climate

Ueber die Tiefenwasserausbreitung im Weddellmeer und in der Scotia-See: Numerische Untersuchungen der Transport- und Austauschprozesse in der Weddell-Scotia-Konfluenz-Zone

The deep Scotia Sea is filled with ventilated Weddell Sea Deep Water (WSDW).This in turn is an essential contributor to the ventilation of theWorld Ocean abyss.Depending on the formation process and/or its location along the Weddell Seaperiphery, deep and bottom water masses follow different routes to crossthe South Scotia Ridge. A primitive equation, hydrostatic, ocean generalcirculation model (BRIOS1.1) with terrain-following coordinate is usedto investigate the water mass export from the Weddell Sea.The model is circumpolar focusing on the Weddell Sea, with particularlyhigh resolution (~ 20 km) in the DOVETAIL area.The northern limb of the Weddell Gyre exhibits an eastward Weddell Sea DeepWater transport across 44°W of 24 Sv.Export rates of Weddell Sea Deep Water through gaps in theSouth Scotia Ridge are estimated to be 6.4 Sv with asemi-annual cycle of ± 0.6 Sv, which can be correlated toatmospheric cyclone activity and Weddell Gyre strength.Sensitivity studies considering extreme sea ice conditions in the WeddellSea show higher (lower) exports in years of minimum (maximum) winter seaice extent. This can be attributed to the local change of the surfacestress achieved by wind and ice.Lagrangian particle trajectories, the so-called synthetic floats,illustrate the pathways of water masses from the inner Weddell Seainto the Scotia Sea through Orkney and Philip Passage, the major gapsin the South Scotia Ridge.They support the existing flow divergence known from observationson the northwestern continental shelf with one branch entering Bransfield Straitand the other continuingeastwards subsequently filling the deep Weddell and Scotia seas.The floats also highlight the interannual variability of the flow field.Water masses flowing through the major gaps originate from thesouthwestern and southeastern Weddell Sea continental shelves.However, water masses formed east of the Weddell Sea (e.g., Prydz Bay)also seem to feed the deep Scotia Sea, since a large portion of floatsflowing northward through the gaps of the South Scotia Ridge have beenin contact with the mixed layer processes outside the inner Weddell Sea.The propagation of dense water masses spreading from the southerncontinental shelf to the South Atlantic Ocean is estimated to be about8 years not including residence times on the continental shelf

Southern Boundaries in Global Ocean Models: Can We Do Better?

Dense water masses form on the continental shelves around Antarctica, sink,and spread as a ma jor contributor to the overturning circulation into theglobal abyss. Global ocean circulation models often do not adequately re-solve high latitude processes due to, for example, a poor representation ofthe continental shelves and an insufficient spatial resolution. Without theseprocesses which are important for the lower part of the global overturningcirculation, the characteristics and flow of deep and bottom waters often re-main unrealistic in these models. We present two approaches to treat thesouthern boundary in global ocean models and in turn improve the hydrog-raphy of the Southern Ocean, the Antarctic Circumpolar Current and theglobal circulation in those models. First, a modified Large Scale GeostrophicModel (LSG) with global data assimilation and an integration time scaleof 13 years is constrained further by hydrographic sections in the Ross andWeddell seas. These sections are obtained from an accurate regional model.Secondly, in a long (3000 years) forward integration, a global 2 degree general cir-culation model takes into account high latitude processes by restoring thesouthern boundaries at 76S in the Ross and Weddell seas to hydrographyand velocity values from the same regional model. Sensitivity experimentswith both model configurations shed light on the influence of the additionaldata in the individual basins (Ross and Weddell Sea) on a regional andglobal scale. The Weddell Sea circulation significantly affects the course ofthe Antarctic Circumpolar Current with consequences for Southern Oceanwater mass characteristics and the spreading of deep and bottom waters inthe South Atlantic. Regional changes in the Pacific sector of the SouthernOcean can be attributed to the additional data in the Ross Sea. In spiteof different physics, time scales of integration, and methods of incorporatingthe regional model data between the two global models, the effects of theadditional information are consistent in both global models