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), Prabir Patra (JAMSTEC), Jonathan Bent (Scripps), Colm Sweeney (NOAA/CU)

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

Southern Ocean air-sea CO2 and O2 fluxes

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

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]

January 2009

Drake PassageOctober 1998

SeaWiFS Summer Chlorophyll a

Southern Ocean atmospheric O2 observations

SPO

CGO

PSAAMS SYO

MCQ

BHD

Scripps flask

Princeton flask

NIWA in situ

Model and data products evaluated

Dissolved-gas climatologiesSeasonal ocean O2 and N2: Garcia and Keeling, 2001Ocean CO2: Takahashi et al., 2002 and 2009

Ocean inversionsMean ocean O2: Gruber et al., 2001Mean ocean N2: Gloor et al., 2001

Fossil-fuel fluxesCO2 and O2: CDIAC and EDGAR

Ocean biogeochemistry modelsORCA-PISCES-T: Le Quere et al. 2007CCSM-3: Doney et al. 2009

Atmospheric inversionsCO2 ACTM 64 reg: Patra et al., 2011

Atmospheric transport modelsACTM (~ 2.8°): Patra et al., 2009TM3 (4° x 5 °) : Heimann and Körner, 2009

• 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 CO2 outgassing in early winter

• Limitations in biological submodels and/or lack of eddy-mixing may be important

What’s going on with the climatologies and models?

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

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 pCO2 climatologies 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