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www.csiro.au
Towards a better understanding of the Southern Ocean overturning - - PowerPoint PPT Presentation
Towards a better understanding of the Southern Ocean overturning - - PowerPoint PPT Presentation
Towards a better understanding of the Southern Ocean overturning circulation Steve Rintoul, J. B. Salle and Serguei Sokolov CSIRO Marine and Atmospheric Research Antarctic Climate and Ecosystems CRC Wealth from Oceans National Research
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Role of the Southern Ocean in the Earth system
The Southern Ocean: acts as a valve controlling exchange between the surface and the deep ocean. plays roughly equal role with North Atlantic in ventilating the deep ocean. stores more heat and anthropogenic carbon than any other latitude band. is the primary return path for nutrients and carbon exported to the deep sea. influences the rate of mass loss by the Antarctic ice sheet and therefore sea-level rise.
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How is water transferred from the surface layer to the ocean interior (ie subducted)?
Sallée et al, JPO, 2010
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Subduction hot-spots
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Inferred subduction hot-spots consistent with interior PV distribution γn = 26.9 γn = 27.1
Sallee, Speer, Rintoul + Wijffels, JPO, 2010
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… and interior distribution of anthropogenic carbon dioxide
Sallée, Matear, Lenton and Rintoul, submitted
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Can the Gent-McWilliams (GM) parameterisation represent eddy contribution to subduction?
Sallée and Rintoul, Ocean Modelling, submitted
Eddy subduction Eddy + Ekman
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Eddy subduction: dependence on diffusivity
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Large, regionally-varying κ required
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The deep limb of the SO overturning
Orsi et al., 1999
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Kerguelen Deep Western Boundary Current Experiment
12 Sv ± 1.2 Sv of AABW (<0°C) to north. Net AABW transport across array is 6.8 Sv. 2-yr mean speeds > 20 cm s-1 at 4000 m. Fukamachi et al., Nature Geoscience, 2010.
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Freshening of RSBW at 150E
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AABW freshening
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Summary and conclusions
Eddies largely compensate Ekman contribution to
- subduction. Regional pattern set by lateral induction.
Subduction pattern consistent with interior distribution
- f PV and anthropogenic CO2.
Must get eddies, mixed layer depth and surface currents right to get heat and carbon storage right. Need to use a large, spatially-varying diffusivity in GM to parameterise eddy contribution to subduction. Antarctic Bottom Water continues to freshen. Calving
- f Mertz glacier contributed.
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Mertz Polynya region, summer 2008
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Mertz Polynya region, 2011 (red), 2008 (blue)
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Less dense water formed after calving
0.041
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Freshening of Ross Sea shelf waters
Jacobs +Giulivi, JCLIM, 2010
Trend of 0.03 / decade, 1958 – 2008 (r = -0.90)
Whitworth, 2002, GRL Jacobs et al., 2002 Jacobs 2004, 2006
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Water mass changes: freshening of Antarctic Bottom Water
neutral density salinity
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Water mass changes: freshening of Antarctic Bottom Water
neutral density salinity
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CAML photo
Martin Riddle and Aaron Spurr
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Regulation of chemical exposure by the
- verturning circulation
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dissolved oxygen
Cossa et al., 2010, submitted
a b
Elevated methyl mercury in the Southern Ocean
methyl mercury reactive mercury
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Ocean, ice and atmosphere combine to produce elevated methyl mercury at high latitude
- 1. Bacteria consume
- xygen and methylate
mercury in deep water
- 2. Upwelling of low oxygen
deep water increases methylmercury concentrations in surface water.
- 3. Mercury in atmosphere
reacts with bromine released during sea ice formation and is deposited
- n ice-ocean surface.
- 4. Mercury is scavenged
by sinking organic particles in productive sea ice zone. Bacteria consume organic matter and methylate mercury.
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Ocean currents protecting ecosystems from exposure to contaminants (eg PFCs).
Bengtson-Nash et al., 2010
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Open questions
Future of Southern Ocean overturning Likelihood and magnitude of climate feedbacks
Overturning Carbon cycle Sea ice
Ocean impact on Antarctic ice sheet mass balance Ecosystem response to global change
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A Southern Ocean Observing System: www.scar.org/soos
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Conclusions
The Southern Ocean has a profound influence on the earth system. Changes in the Southern Ocean will affect climate, sea level, biogeochemical cycles and biological productivity. Recent progress:
mechanism of subduction & heat/carbon uptake role of eddies physical controls on biological productivity pathways of overturning circulation nature and causes of Southern Ocean change
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