On the Robustness of Air-Sea Flux Estimates of Anthropogenic Carbon from Ocean Inversions

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On the Robustness of Air-Sea Flux Estimates of
Anthropogenic Carbon from Ocean Inversions

• Sara Mikaloff Fletcher, Nicolas Gruber, Andrew
  Jacobson, Scott Doney, Stephanie Dutkiewicz, Mick
  Follows, Keith Lindsay, Dimitris Menemenlis, Anne
  Mouchet

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Anthropogenic Carbon (µmol/Kg)

• Cant is the component of DIC due to elevated
  atmospheric CO2
• Estimated from observations of DIC and other
  ocean tracers using the ?C method (Gruber et
  al., 1996).

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The Ocean Inversion

• The inverse model finds the combination of carbon
  fluxes from a discrete number of ocean regions
  that optimally fit the observations
• Cjant Anthropogenic carbon calculated from
  observations at site i
• xi Magnitude of the flux from region i
• Hi,j The modelled response of a unit flux from
  region i at station j, called the basis function
• eError associated with the method

The model regions and observations used in the
ocean inversion
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Key Questions

• Are the inverse estimates robust with respect to
  ocean transport?
• Are the inverse estimates robust with respect to
  uncertainty in Cant?
• What have we learned?

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Anthropogenic Carbon FluxScaled to 1995
North
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Uncertainty in Anthropogenic Carbon Estimates

• Matsumoto and Gruber (submitted, 2004)
• Stoichiometric ratios used to remove the signal
  due to biological activity (Anderson and
  Sarmiento, 1994)
• Low Rco
• High Rco

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Anthropogenic Carbon Bias (PRINCE-RDS model)
North
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Global Total Anthropogenic Carbon Uptake Scaled
to 1995 (Pg C/yr)
Model OCMIP-2 forward model Inverse Model
ECCO NA 2.10
MIT NA 2.20
MOM-LL 1.84 1.93
MOM-HH 2.35 2.33
MOM-LHS 1.98 2.06
MOM-RDS 2.29 2.20
MOM-RDS 2.16 2.28
UL 2.94 2.77
Mean (Range) 2.26 (1.84 2.94) 2.23 (1.93 2.77)
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Inverse Model Forward Model
North
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Conclusions

• The broad features of the ocean inversion are
  robust
• The greatest source of uncertainty is model
  transport
• The largest spread between models occurs in the
  Southern Ocean
• The anthropogenic carbon inventories call for
• Global anthropogenic carbon uptake of 1.95 to
  2.77 Pg C/yr
• Equator-ward transport from high-latitudes, with
  storage at mid-latitudes
• Some cross-equatorial transport
• Compared to the forward simulations, the inverse
  estimates find
• Greater uptake in the Southern Ocean between 44S
  and 58S and the Indian Ocean
• Less uptake throughout most of the pacific ocean

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Sources of Uncertainty in the Inverse Estimates

• Modeled transport
• Anthropogenic Carbon Estimates
• Implicit assumptions
• Ocean transport is in steady state
• Anthropogenic carbon uptake is proportional to
  the atmospheric carbon perturbation
• Inverse methodology
• Aggregation error