Southern ocean inversions: interannual variability and sink trend Rachel Law and Richard Matear April CSIRO Marine and Atmospheric Research

Background Le Quéré et al., Weakening of the Southern Ocean sink of CO 2, revised, awaiting final editorial decision. Lenton and Matear, Interannual variability: SAM and CO 2 uptake, GBC, in press. Lovenduski et al., Enhanced CO 2 outgassing in the Southern Ocean from a positive phase of the Southern Annular mode, submitted. Butler et al., Observed relationship between the Southern Annular mode and atmospheric carbon dioxide, submitted to GBC.

Testing inversion capability Create pseudo-obs  Run CCAM for with NCEP nudging  Ocean CO 2 from Matear model with NCEP forcing  Biosphere CO 2 using model of Friedlingstein et al., GBC, 1995  Fossil CO 2 from C4MIP Invert using CRC-MATCH  116 regions  GCM winds (no IAV)  Inversion set-up similar to Baker et al., 2006 including fossil90, fossil95, CASA and Takahashi99 presubs  Use monthly mean CO 2 only

Ocean flux (gC m -2 y -1 ) Matear, average Takahashi – 1999 version

Region map Red: Basic network, Black: extra sites

Sample result

Network choice SPO: F = 0.76 C -0.40, R 2 =0.47 +PSA: F = 0.89 C -0.41, R 2 =0.59 +MQA: F = 0.94 C -0.44, R 2 =0.62 +CRZ: F = 0.95 C -0.44, R 2 =0.62 +MAA: F = 0.96 C -0.43, R 2 =0.62 +HBA, JBN, SYO: F = 1.01 C -0.44, R 2 =0.68

Regional fluxes – North/South split Blue: total Green: South Red: North +PSA +CRZ

Regional fluxes: basin split Blue: Indian, Green: Pacific Red: Atlantic +PSA +MQA

Flux trend from ocean model Linear regression, 10 year segmentsBlue: trend over 10 years Red: trend over 20 years

Flux trends from inversions 10 year trend20 year trend Red: fluxes from ocean model, blue: inversions NB smaller y-range

Inversion of observed CO 2 Blue: BRW, SMO, SPO (NOAA) CHR, MLO, SPO, SMO (SIO) Red: + PSA (NOAA) Cyan: +HBA (NOAA) Magenta: +SYO (NOAA) Purple: +MQA (CSIRO) Green: +CRZ (NOAA), MAA(CSIRO) Black : ocean model flux Annual mean southern ocean CO 2 flux Need to check sensitivity to data uncertainty choice

Trends 10 year trend20 year trend NB y-range different IAV too large to reliably say anything about trend ??

Conclusions Inversions of synthetic data suggest that IAV of total southern ocean flux can be retrieved with some skill even with very small networks Regional information is less certain Trends appear difficult to interpret given magnitude of IAV Fluxes from inversion of observed CO 2 not very similar to ocean model fluxes. Need to test sensitivity to data uncertainty (and probably other things too).

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