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Southern ocean inversions: interannual variability and sink trend

Rachel Law and Richard Matear

April 26 2007

CSIRO Marine and Atmospheric Research

Background

• Le Quéré et al., Weakening of the Southern Ocean sink of CO2, revised, awaiting final editorial decision.

•Lenton and Matear, Interannual variability: SAM and CO2 uptake, GBC, in press.

• Lovenduski et al., Enhanced CO2 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 1970-1999 with NCEP nudging

Ocean CO2 from Matear model with NCEP forcing

Biosphere CO2 using model of Friedlingstein et al., GBC, 1995

Fossil CO2 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 CO2 only

Ocean flux (gC m-2 y-1)

Matear, 1990-1999 average

Takahashi – 1999 version

Region map

Red: Basic network, Black: extra sites

Sample result

Network choice

SPO: F = 0.76 C -0.40, R2=0.47

+PSA: F = 0.89 C -0.41, R2=0.59

+MQA: F = 0.94 C -0.44, R2=0.62

+CRZ: F = 0.95 C -0.44, R2=0.62

+MAA: F = 0.96 C -0.43, R2=0.62

+HBA, JBN, SYO: F = 1.01 C -0.44, R2=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 segments Blue: trend over 10 years

Red: trend over 20 years

Flux trends from inversions

10 year trend 20 year trend

Red: fluxes from ocean model, blue: inversions

NB smaller y-range

Inversion of observed CO2

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 CO2 flux

Need to check sensitivity to data uncertainty choice

Trends

10 year trend 20 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 CO2 not very similar to ocean model fluxes. Need to test sensitivity to data uncertainty (and probably other things too).