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. Sallée and Serguei Sokolov CSIRO Marine and Atmospheric Research Antarctic Climate and Ecosystems CRC Wealth from Oceans National Research Flagship Hobart, Tasmania , Australia www.csiro.au

Southern Ocean connects the upper and lower limbs of global overturning circulation Rintoul, 2001

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.

How is water transferred from the surface layer to the ocean interior (ie subducted)? Sallée et al, JPO, 2010

Subduction hot-spots

Inferred subduction hot-spots consistent with interior PV distribution γ n = 26.9 γ n = 27.1 Sallee, Speer, Rintoul + Wijffels, JPO, 2010

… and interior distribution of anthropogenic carbon dioxide Sallée, Matear, Lenton and Rintoul, submitted

Can the Gent-McWilliams (GM) parameterisation represent eddy contribution to subduction? Eddy subduction Eddy + Ekman Sallée and Rintoul, Ocean Modelling, submitted

Eddy subduction: dependence on diffusivity

Large, regionally-varying κ required

The deep limb of the SO overturning Orsi et al., 1999

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.

Freshening of RSBW at 150E

AABW freshening

Summary and conclusions Eddies largely compensate Ekman contribution to subduction. Regional pattern set by lateral induction. Subduction pattern consistent with interior distribution of PV and anthropogenic CO 2 . 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 of Mertz glacier contributed.

Mertz Polynya region, summer 2008

Mertz Polynya region, 2011 (red), 2008 (blue)

0.041 Less dense water formed after calving

Freshening of Ross Sea shelf waters Trend of 0.03 / decade, 1958 – 2008 (r = -0.90) Whitworth, 2002, GRL Jacobs et al., 2002 Jacobs 2004, 2006 Jacobs +Giulivi, JCLIM, 2010

Water mass changes: freshening of Antarctic Bottom Water neutral density salinity

Water mass changes: freshening of Antarctic Bottom Water neutral density salinity

Martin Riddle and Aaron Spurr CAML photo

Regulation of chemical exposure by the overturning circulation

Elevated methyl mercury in the Southern Ocean methyl mercury dissolved oxygen reactive mercury a b Cossa et al., 2010, submitted

Ocean, ice and atmosphere combine to produce elevated methyl mercury at high latitude 3. Mercury in atmosphere reacts with bromine released during sea ice 2. Upwelling of low oxygen formation and is deposited deep water increases on ice-ocean surface. methylmercury concentrations in surface water. 4. Mercury is scavenged 1. Bacteria consume by sinking organic particles oxygen and methylate in productive sea ice zone. mercury in deep water Bacteria consume organic matter and methylate mercury.

Ocean currents protecting ecosystems from exposure to contaminants (eg PFCs). Bengtson-Nash et al., 2010

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

A Southern Ocean Observing System: www.scar.org/soos

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

Mixed layer depth anomalies consistent with air- sea heat flux anomalies associated with SAM Mixed layer depth anomalies Expected mixed layer depth regressed on SAM anomalies from 1-D heat budget.