scholarly journals Opening the window to the Southern Ocean: The role of jet dynamics

2018 ◽  
Vol 4 (10) ◽  
pp. eaao4719 ◽  
Author(s):  
Andreas Klocker
Keyword(s):  
Southern Ocean ◽  
Mixed Layer ◽  
Global Climate ◽  
Control Valve ◽  
Intermediate Water ◽  
Surface Mixed Layer ◽  
Last Deglaciation ◽  
Antarctic Intermediate Water ◽  
Topographic Features ◽  
Ocean Ventilation

The surface waters of the Southern Ocean act as a control valve through which climatically important tracers such as heat, freshwater, and CO2 are transferred between the atmosphere and the ocean. The process that transports these tracers through the surface mixed layer into the ocean interior is known as ocean ventilation. Changes in ocean ventilation are thought to be important for both rapid transitions of the ocean’s global overturning circulation during the last deglaciation and the uptake and storage of excess heat and CO2 as a consequence of anthropogenic climate change. I show how the interaction between Southern Ocean jets, topographic features, and ocean stratification can lead to rapid changes in Southern Ocean ventilation as a function of wind stress. For increasing winds, this interaction leads from a state in which tracers are confined to the surface mixed layer to a state in which tracers fill the ocean interior. For sufficiently high winds, the jet dynamics abruptly change, allowing the tracer to ventilate a water mass known as Antarctic Intermediate Water in the mid-depth Southern Ocean. Abrupt changes in Antarctic Intermediate Water ventilation have played a major role in rapid climate transitions in Earth’s past, and combined with the results presented here, this would suggest that jet dynamics could play a prominent role in contributing to, or even triggering, rapid transitions of the global climate system.

scholarly journals Silicon isotopes in Antarctic sponges: an interlaboratory comparison

2010 ◽  
Vol 23 (1) ◽  
pp. 34-42 ◽  
Author(s):  
Katharine R. Hendry ◽  
Melanie J. Leng ◽  
Laura F. Robinson ◽  
Hilary J. Sloane ◽  
Jerzy Blusztjan ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Matrix Effects ◽  
Global Climate ◽  
Intermediate Water ◽  
Silicon Isotopes ◽  
Important Player ◽  
Si Isotopes ◽  
Si Isotope ◽  
Additional Reference ◽  
Antarctic Sponges

AbstractCycling of deepwater silicon (Si) within the Southern Ocean, and its transport into other ocean basins, may be an important player in the uptake of atmospheric carbon, and global climate. Recent work has shown that the Si isotope (denoted by δ29Si or δ30Si) composition of deep sea sponges reflects the availability of dissolved Si during growth, and is a potential proxy for past deep and intermediate water silicic acid concentrations. As with any geochemical tool, it is essential to ensure analytical precision and accuracy, and consistency between methodologies and laboratories. Analytical bias may exist between laboratories, and sponge material may have matrix effects leading to offsets between samples and standards. Here, we report an interlaboratory evaluation of Si isotopes in Antarctic and sub-Antarctic sponges. We review independent methods for measuring Si isotopes in sponge spicules. Our results show that separate subsamples of non-homogenized sponges measured by three methods yield isotopic values within analytical error for over 80% of specimens. The relationship between δ29Si and δ30Si in sponges is consistent with kinetic fractionation during biomineralization. Sponge Si isotope analyses show potential as palaeoceaongraphic archives, and we suggest Southern Ocean sponge material would form a useful additional reference standard for future spicule analyses.

scholarly journals Mechanisms of dissolved and labile particulate iron supply to shelf waters and phytoplankton blooms off South Georgia, Southern Ocean

2018 ◽  
Vol 15 (16) ◽  
pp. 4973-4993 ◽  
Author(s):  
Christian Schlosser ◽  
Katrin Schmidt ◽  
Alfred Aquilina ◽  
William B. Homoky ◽  
Maxi Castrillejo ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Water Column ◽  
Mixed Layer ◽  
Surface Mixed Layer ◽  
Pore Waters ◽  
Phytoplankton Blooms ◽  
Faecal Pellets ◽  
South Georgia ◽  
Shelf Sediments ◽  
Bottom Waters

Abstract. The island of South Georgia is situated in the iron (Fe)-depleted Antarctic Circumpolar Current of the Southern Ocean. Iron emanating from its shelf system fuels large phytoplankton blooms downstream of the island, but the actual supply mechanisms are unclear. To address this, we present an inventory of Fe, manganese (Mn), and aluminium (Al) in shelf sediments, pore waters, and the water column in the vicinity of South Georgia, alongside data on zooplankton-mediated Fe cycling processes, and provide estimates of the relative dissolved Fe (DFe) fluxes from these sources. Seafloor sediments, modified by authigenic Fe precipitation, were the main particulate Fe source to shelf bottom waters as indicated by the similar Fe ∕ Mn and Fe ∕ Al ratios for shelf sediments and suspended particles in the water column. Less than 1 % of the total particulate Fe pool was leachable surface-adsorbed (labile) Fe and therefore potentially available to organisms. Pore waters formed the primary DFe source to shelf bottom waters, supplying 0.1–44 µmol DFe m−2 d−1. However, we estimate that only 0.41±0.26 µmol DFe m−2 d−1 was transferred to the surface mixed layer by vertical diffusive and advective mixing. Other trace metal sources to surface waters included glacial flour released by melting glaciers and via zooplankton egestion and excretion processes. On average 6.5±8.2 µmol m−2 d−1 of labile particulate Fe was supplied to the surface mixed layer via faecal pellets formed by Antarctic krill (Euphausia superba), with a further 1.1±2.2 µmol DFe m−2 d−1 released directly by the krill. The faecal pellets released by krill included seafloor-derived lithogenic and authigenic material and settled algal debris, in addition to freshly ingested suspended phytoplankton cells. The Fe requirement of the phytoplankton blooms ∼ 1250 km downstream of South Georgia was estimated as 0.33±0.11 µmol m−2 d−1, with the DFe supply by horizontal/vertical mixing, deep winter mixing, and aeolian dust estimated as ∼0.12 µmol m−2 d−1. We hypothesize that a substantial contribution of DFe was provided through recycling of biogenically stored Fe following luxury Fe uptake by phytoplankton on the Fe-rich shelf. This process would allow Fe to be retained in the surface mixed layer of waters downstream of South Georgia through continuous recycling and biological uptake, supplying the large downstream phytoplankton blooms.

scholarly journals Silicon pool dynamics and biogenic silica export in the Southern Ocean inferred from Si-isotopes

Ocean Science ◽  
2011 ◽  
Vol 7 (5) ◽  
pp. 533-547 ◽  
Author(s):  
F. Fripiat ◽  
A.-J. Cavagna ◽  
F. Dehairs ◽  
S. Speich ◽  
L. André ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Silicic Acid ◽  
Mixed Layer ◽  
Deep Water ◽  
Biogenic Silica ◽  
Polar Front ◽  
Intermediate Water ◽  
Subtropical Zone ◽  
Isotopic Signatures ◽  
The Antarctic

Abstract. Silicon isotopic signatures (δ30Si) of water column silicic acid (Si(OH)4) were measured in the Southern Ocean, along a meridional transect from South Africa (Subtropical Zone) down to 57° S (northern Weddell Gyre). This provides the first reported data of a summer transect across the whole Antarctic Circumpolar Current (ACC). δ30Si variations are large in the upper 1000 m, reflecting the effect of the silica pump superimposed upon meridional water transfer across the ACC: the transport of Antarctic surface waters northward by a net Ekman drift and their convergence and mixing with warmer upper-ocean Si-depleted waters to the north. Using Si isotopic signatures, we determine different mixing interfaces: the Antarctic Surface Water (AASW), the Antarctic Intermediate Water (AAIW), and thermoclines in the low latitude areas. The residual silicic acid concentrations of end-members control the δ30Si alteration of the mixing products and with the exception of AASW, all mixing interfaces have a highly Si-depleted mixed layer end-member. These processes deplete the silicic acid AASW concentration northward, across the different interfaces, without significantly changing the AASW δ30Si composition. By comparing our new results with a previous study in the Australian sector we show that during the circumpolar transport of the ACC eastward, the δ30Si composition of the silicic acid pools is getting slightly, but significantly lighter from the Atlantic to the Australian sectors. This results either from the dissolution of biogenic silica in the deeper layers and/or from an isopycnal mixing with the deep water masses in the different oceanic basins: North Atlantic Deep Water in the Atlantic, and Indian Ocean deep water in the Indo-Australian sector. This isotopic trend is further transmitted to the subsurface waters, representing mixing interfaces between the surface and deeper layers. Through the use of δ30Si constraints, net biogenic silica production (representative of annual export), at the Greenwich Meridian is estimated to be 5.2 ± 1.3 and 1.1 ± 0.3 mol Si m−2 for the Antarctic Zone and Polar Front Zone, respectively. This is in good agreement with previous estimations. Furthermore, summertime Si-supply into the mixed layer of both zones, via vertical mixing, is estimated to be 1.6 ± 0.4 and 0.1 ± 0.5 mol Si m−2, respectively.

scholarly journals Mid-depth South Atlantic ocean circulation and chemical stratification during MIS-10 to 12: implications for atmospheric CO<sub>2</sub>

2008 ◽  
Vol 4 (3) ◽  
pp. 667-695 ◽  
Author(s):  
A. J. Dickson ◽  
M. J. Leng ◽  
M. A. Maslin
Keyword(s):  
Southern Ocean ◽  
Ocean Circulation ◽  
North Atlantic ◽  
Sediment Cores ◽  
Convective Cell ◽  
Intermediate Water ◽  
Ocean Ventilation ◽  
Cape Basin ◽  
Glacial Periods ◽  
Atlantic Margin

Abstract. A detailed record of benthic foraminifera carbon isotopes from the South East Atlantic margin shows little glacial-interglacial variability between MIS-12 to MIS-10, suggesting that Glacial North Atlantic Intermediate Water (GNAIW) consistently penetrated to at least 30° S. Millennial-scale increases in either the mass or flux of GNAIW over the core site occur alongside reductions in Lower North Atlantic Deep Water recorded in North Atlantic sediment cores and show that the lower and intermediate limb of the Atlantic deepwater convective cell oscillated in anti-phase during previous glacial periods. In addition, a 500 yr resolution record of the Cape Basin intermediate-deep δ13C gradient shows that a reduction in deep Southern Ocean ventilation at the end of MIS-11 was consistent with a modelled CO2 drawdown of ~21–30 ppm. Further increases in the Southern Ocean chemical divide during the transition into MIS-10 were completed before minimum CO2 levels were reached, suggesting that other mechanisms such as alkalinity changes were responsible for the remaining ~45 ppm drawdown.

scholarly journals Silicon pool dynamics and biogenic silica export in the Southern Ocean, inferred from Si-isotopes

2011 ◽  
Vol 8 (2) ◽  
pp. 639-674 ◽  
Author(s):  
F. Fripiat ◽  
A.-J. Cavagna ◽  
F. Dehairs ◽  
S. Speich ◽  
L. André ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Silicic Acid ◽  
Mixed Layer ◽  
Deep Water ◽  
Biogenic Silica ◽  
Water Masses ◽  
Intermediate Water ◽  
Subtropical Zone ◽  
Isotopic Signatures ◽  
The Antarctic

Abstract. Water column silicon isotopic signatures (δ30Si) of silicic acid (Si(OH)4) in the Southern Ocean were measured along a meridional transect from South Africa (Subtropical Zone) down to 57° S (northern Weddell Gyre). These data are the first reported for a summer transect across the whole Antarctic Circumpolar Current (ACC). δ30Si variations are large in the upper 1000 m, reflecting the effect of the silica pump superimposed upon meridional transfer across the ACC: the transport of Antarctic surface waters northward by a net Ekman drift and their convergence and mixing with warmer upper-ocean Si-depleted waters to the north. Using Si isotopic signatures, we determined different mixing interfaces between ACC water masses: the Antarctic Surface Water (AASW), the Antarctic Intermediate Water (AAIW), and the thermoclines in the low latitude areas. The residual silicic acid concentrations of end-members control the δ30Si alteration of the mixing products. With the exception of AASW, all mixing interfaces have a highly Si-depleted mixed layer end-member. These processes deplete the silicic acid AASW concentration across the different interfaces northward without significantly changing the AASW δ30Si. By comparing our new results with a previous study in the Australian sector we show that during the circumpolar transport of the ACC eastward, there is a slight but significant Si-isotopic lightening of the silicic acid pools from the Atlantic to the Australian sectors. This results either from the dissolution of biogenic silica in the deeper layers and/or from an isopycnal mixing with the deep water masses in the different oceanic basins: North Atlantic Deep Water in the Atlantic, and Indian Ocean deep water in the Indo-Australian sector. This eastward lightening is further transmitted to the subsurface waters, representing mixing interfaces between the surface and deeper layers. Using the Si-isotopic constraint, we estimate for the Greenwich Meridian a net biogenic silica production which should be representative of the annual export, at 4.5 ± 1.1 and 1.5 ± 0.4 mol Si m−2 for the Antarctic Zone and Polar Front Zone, respectively, in agreement with previous estimations. The summertime Si-supply into the mixed layer via vertical mixing was also assessed at 1.5 ± 0.4 and 0.1 ± 0.5 mol Si m−2, respectively.

scholarly journals Rapid shifts in circulation and biogeochemistry of the Southern Ocean during deglacial carbon cycle events

2020 ◽  
Vol 6 (42) ◽  
pp. eabb3807
Author(s):  
Tao Li ◽  
Laura F. Robinson ◽  
Tianyu Chen ◽  
Xingchen T. Wang ◽  
Andrea Burke ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Ocean Circulation ◽  
Deep Sea ◽  
Deep Ocean ◽  
Biological Pump ◽  
Last Deglaciation ◽  
Proxy Data ◽  
Ocean Ventilation ◽  
Carbon Release ◽  
The Last Deglaciation

The Southern Ocean plays a crucial role in regulating atmospheric CO2 on centennial to millennial time scales. However, observations of sufficient resolution to explore this have been lacking. Here, we report high-resolution, multiproxy records based on precisely dated deep-sea corals from the Southern Ocean. Paired deep (∆14C and δ11B) and surface (δ15N) proxy data point to enhanced upwelling coupled with reduced efficiency of the biological pump at 14.6 and 11.7 thousand years (ka) ago, which would have facilitated rapid carbon release to the atmosphere. Transient periods of unusually well-ventilated waters in the deep Southern Ocean occurred at 16.3 and 12.8 ka ago. Contemporaneous atmospheric carbon records indicate that these Southern Ocean ventilation events are also important in releasing respired carbon from the deep ocean to the atmosphere. Our results thus highlight two distinct modes of Southern Ocean circulation and biogeochemistry associated with centennial-scale atmospheric CO2 jumps during the last deglaciation.

scholarly journals Southern Ocean heat storage, reemergence, and winter sea ice decline induced by summertime winds

2020 ◽  
pp. 1-47
Author(s):  
Edward W. Doddridge ◽  
John Marshall ◽  
Hajoon Song ◽  
Jean-Michel Campin ◽  
Maxwell Kelley
Keyword(s):  
Southern Ocean ◽  
Sea Ice ◽  
Mixed Layer ◽  
Vertical Mixing ◽  
Heat Content ◽  
Ice Cover ◽  
Sea Surface ◽  
Surface Mixed Layer ◽  
Surface Cooling ◽  
Westerly Winds

AbstractThe observational record shows a substantial 40-year upward trend in summertime westerly winds over the Southern Ocean, as characterised by the Southern Annular Mode (SAM) index. Enhanced summertime westerly winds have been linked to cold summertime sea surface temperature (SST) anomalies. Previous studies have suggested that Ekman transport or upwelling is responsible for this seasonal cooling. Here, another process is presented in which enhanced vertical mixing, driven by summertime wind anomalies, moves heat downwards, cooling the sea surface and simultaneously warming the subsurface waters. The anomalously cold SSTs draw heat from the atmosphere into the ocean, leading to increased depth-integrated ocean heat content. The subsurface heat is returned to the surface mixed layer during the autumn and winter as the mixed layer deepens, leading to anomalously warm SSTs and potentially reducing sea ice cover. Observational analyses and numerical experiments support our proposed mechanism, showing that enhanced vertical mixing produces subsurface warming and cools the surface mixed layer. Nevertheless, the dominant driver of surface cooling remains uncertain; the relative importance of advective and mixing contributions to the surface cooling is model dependent. Modeling results suggest that sea ice volume is more sensitive to summertime winds than sea ice extent, implying that enhanced summertime westerly winds may lead to thinner sea ice in the following winter, if not lesser ice extent. Thus, strong summertime winds could precondition the sea ice cover for a rapid retreat in the following melt season.

Diapycnal and Isopycnal Transports in the Southern Ocean Estimated by a Box Inverse Model

2013 ◽  
Vol 43 (11) ◽  
pp. 2270-2287 ◽  
Author(s):  
K. Katsumata ◽  
B. M. Sloyan ◽  
S. Masuda
Keyword(s):  
Southern Ocean ◽  
Mixed Layer ◽  
Deep Water ◽  
General Circulation ◽  
General Circulation Models ◽  
Inverse Model ◽  
Ekman Transport ◽  
Surface Mixed Layer ◽  
Circumpolar Deep Water ◽  
Eddy Transport

Abstract Quantitative descriptions of Circumpolar Deep Water upwelling and evolution into a lighter mode and heavier bottom waters in the Southern Ocean are still not well constrained. Here, data from two occupations of eight hydrographic sections are combined and used in a box inverse model to estimate isopycnal and diapycnal transports in the Southern Ocean. A mixed layer box allows diapycnal transports in the surface mixed layer to be estimated separately. Current velocity at 1000 dbar was constrained by the mean velocity field estimated from subsurface float drift data. The estimated isopycnal transports are largely consistent with past estimates and with outputs of three ocean general circulation models. The estimated subduction and upwelling at the base of the Southern Ocean mixed layer show that Upper Circumpolar Deep Water upwells [16 ± 15 and 17 ± 21 Sv (where 1 Sv ≡ 106 m3 s−1) by different inversion methods] and evolves into heavier Lower Circumpolar Deep Water (5 ± 13 and 6 ± 18 Sv) and Bottom Water (8 ± 9 and 8 ± 13 Sv) or lighter Mode and Intermediate Waters (9 ± 18 and 13 ± 24 Sv). Meridional transport in the surface mixed layer is due to northward Ekman transport and mostly southward eddy transport. In seasonal ice-covered areas near Antarctica, a significant (14 ± 14 Sv) southward transport was found. The southward eddy transport is largest north of the Antarctic Circumpolar Current and decreases poleward because of the poleward decrease in the eddy diffusivity. The interior diapycnal transports, which can be either upward (gaining buoyancy) or downward (gaining density), are comparable in magnitude to the horizontal diapycnal transports within the surface mixed layer.

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