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Long-term observations of atmospheric O 2 :CO 2 ratios over the Southern Ocean

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Long-term observations of atmospheric O 2 :CO 2 ratios over the Southern Ocean. Britton Stephens (NCAR), Ralph Keeling (Scripps), Gordon Brailsford (NIWA), Andrew Manning (UEA), Sara Mikaloff -Fletcher (NIWA), Prabir Patra (JAMSTEC), Jonathan Bent (Scripps), Colm Sweeney (NOAA/CU).

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Long-term observations of atmospheric O2:CO2 ratios

over the Southern Ocean

Britton Stephens (NCAR), Ralph Keeling (Scripps), Gordon Brailsford (NIWA), Andrew Manning (UEA), Sara Mikaloff-Fletcher (NIWA), PrabirPatra (JAMSTEC), Jonathan Bent (Scripps), Colm Sweeney (NOAA/CU)


Southern Ocean air-sea CO2 and O2 fluxes

  • Measurements of atmospheric O2 are valuable because:
  • Solubility and biological processes have strong and reinforcing effects on atmospheric O2
  • Solubility, biological, and anthropogenic effects have distinct O2:CO2 signatures
  • The atmosphere integrates over large ocean regions
  • Future changes will involve subtle combinations of all processes
  • However, applications to date have been limited

75 S 65 S 55 S

dO2/N2 (per meg) =

[(O2/N2)sample/(O2/N2)reference - 1] x 106


Basic problem:

    • Quantitatively relating atmospheric variations in O2 and CO2 to underlying ocean processes continues to be limited by errors in atmospheric transport models
  • Southern Ocean strategy:
    • Find ways to use atmospheric data to constrain fluxes and test ocean models that are independent of atmospheric transport models

Seasonal cycle amplitude from 9 TransCom models forced with identical seasonal ocean O2 fluxes [T. Blaine, Dissertion, 2005]


Idea 1: Look at zonally and vertically integrated O2 concentrations

Idea 2: Look at O2:CO2 ratios in concentration measurements

Drake Passage

October 1998

January 2009

001 Poster B1538: Jonathan Bent et al.

Assessing biogeochemical models with high-resolution airborne observations of the O2/N2 ratio over the Southern Ocean

[Stephens et al., Tellus 2003]


Southern Ocean atmospheric O2 observations

Scripps flask

Princeton flask

NIWA in situ








SeaWiFS Summer Chlorophyll a


Model and data products evaluated

Dissolved-gas climatologies

Seasonal ocean O2 and N2: Garcia and Keeling, 2001

Ocean CO2: Takahashi et al., 2002 and 2009

Ocean inversions

Mean ocean O2: Gruber et al., 2001

Mean ocean N2: Gloor et al., 2001

Fossil-fuel fluxes

CO2and O2: CDIAC and EDGAR

Ocean biogeochemistry models

ORCA-PISCES-T: Le Quere et al. 2007

CCSM-3: Doney et al. 2009

Atmospheric inversions

CO2 ACTM 64 reg: Patra et al., 2011

Atmospheric transport models

ACTM (~ 2.8°): Patra et al., 2009

TM3 (4° x 5 °) : Heimann and Körner, 2009


What’s going on with the climatologies and models?

  • In general, O2 and CO2 PSA-SPO gradients are uncorrelated, and CO2 gradients are often too small.
  • The Garcia and Keeling O2 fluxes have about a 1-month phase lead (by design)
  • Takahashi 2009 CO2 fluxes are an improvement but may still overestimate uptake in late summer (data coverage? coarse grid? gas-exchange formulation?)
  • The O2:CO2 correlations are worse in the models
    • ORCA-PISCES-T appears to have CO2 flux in wrong direction (uptake) in early winter and no CO2 uptake in summer
    • CCSM-3 appears to overestimate CO2outgassing in early winter
    • Limitations in biological submodels and/or lack of eddy-mixing may be important

Baring Head, New Zealand: 10 years of O2:CO2 ratios in synoptic variations during steady-CO2 Southerly wind episodes



June 2012 initiation of shipboard O2 and CO2 measurements will greatly increase Southern Ocean data coverage

R/V Lawrence M. Gould

Sweeney et al., 004 Poster B2052



  • Atmospheric O2 and CO2 measurements have potential for testing surface-data based and ocean biogeochemistry model fluxes in the Southern Ocean
  • Monthly surface gradient (PSA-SPO) O2:CO2 ratios suggest that both pCO2climatologies and model seasonal CO2 fluxes still require improvement in the Southern Ocean
  • High hopes for O2:CO2 ratios measured in synoptic variations in Drake Passage
    • isolated from terrestrial influences
    • strong signals
    • model predictions of synoptic ratios well-behaved