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

1. 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)

2. 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

3. 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]

4. 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]

5. Southern Ocean atmospheric O2 observations Scripps flask Princeton flask NIWA in situ PSA SPO SYO AMS CGO MCQ BHD SeaWiFS Summer Chlorophyll a

6. 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

11. 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

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

13. 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

14. Summary • 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