Uptake, Storage, and Transport:

1
Estimates of air-sea anthropogenic carbon
dioxide flux from ocean interior carbon
measurements and OGCMs Sara Mikaloff
Fletcher1, Nicolas Gruber1, Andrew Jacobson2,
Manuel Gloor2, Jorge Sarmiento2, and the Ocean
Inversion Project Modellers 1.
Institute of Geophysics and Planetary Physics,
UCLA, California 2. Program
in Atmospheric and Oceanic Sciences, Princeton
University
Inverse Flux Estimates In a first attempt to
quantify the uncertainty associated with the
choice of transport model, we present inverse
estimates using basis functions from seven
OGCMs, shown in Table 1. The models used in this
experiment include five different versions of
Princetons Modular Ocean Model (MOM) that
represent different configurations of the
vertical mixing, along-isopycnal mixing, and
other parameters. These five versions of MOM
have been shown to span the range of tracer
transport from a much larger suite of models
(Matsumoto et al., 2004). In addition, basis
functions from the Massachusetts Institute of
Technology (MIT) and Estimating the Circulation
and the Climate of the Oceans (ECCO) OGCMs were
used. Table 1. Comparison of
the global total anthropogenic flux estimates
from forward model simulations and inverse
estimates. Figure 4. Inverse
flux estimates averaged across all OGCMs. The
error bars shown here indicate the range of model
estimates. The 22 model regions shown in Figure
2 have been aggregated to 11 regions after the
inversion for simplicity in this figure.
Figure 5. Difference
between forward flux estimates and inverse flux
estimates using the MOM suite of models. Forward
model simulations followed the protocols of the
Ocean Carbon Model Intercomparison Project. The
22 model regions shown in Figure 2 have been
aggregated to 11 regions after the inversion for
simplicity in this figure.
Introduction The exchange of
anthropogenic carbon dioxide across the air-sea
interface cannot be measured directly however,
the concentration of anthropogenic carbon in the
ocean can be determined from DIC and nutrient
data using the ?C method (Gruber et al., 1996).
Using a recently developed inversion technique,
global and regional air-sea fluxes of
anthropogenic carbon dioxide have been estimated
using the spatial distribution of anthropogenic
carbon in the ocean, and pathways and rates of
ocean transport and mixing given by an Ocean
General Circulation Model (OGCM).
Previous sensitivity studies have shown that
model circulation is one of the most important
sources of error in the ocean inversion (Gloor et
al., 2001). We present estimates of
anthropogenic carbon exchange using a suite of
six different OGCM's in order quantify the
robustness of our results and explore the role of
different representations of ocean circulation.
Uptake, Storage, and Transport
Figure 6. Zonal
integral of uptake, storage, and transport of
anthropogenic carbon for all seven OGCMs.
Storage and transport are calculated from the
inverse flux estimates and basis functions. The
strong agreement between the storage estimates
using different models is expected because the
inverse estimates were constrained by
anthropogenic carbon concentrations in the ocean.
Uptake
- - - - ECCO - - - - MIT MOM - RDS MOM -
LL MOM - HH MOM - LHS MOM - PSS
Storage
Model OCMIP-2 forward model Inverse Model
MIT NA 2.21
ECCO NA 2.11
MOM-LL 1.84 1.95
MOM-HH 2.35 2.31
MOM-LHS 1.98 2.06
MOM-PSS 2.29 2.27
MOM-RDS (Standard) 2.16 2.19
Transport (PositiveNorth)
Observations
Figure 1. Column inventory of the anthropogenic
carbon estimates used to infer surface fluxes of
anthropogenic carbon (µmol/kg). The
anthropogenic carbon signal is extracted from
ocean interior observations from the WOCE/JGOFS
global carbon dioxide survey.

• Conclusions
• The inverse models estimate a global total
  anthropogenic carbon uptake of 1.95 to 2.31 Pg
  C/yr. Forward model simulations using the same
  suite of models indicate a larger range of
  anthropogenic carbon uptake, 1.84 to 2.35 Pg
  C/yr.
• Most of the broad features of uptake and
  transport are robust over all models.
• The greatest uptake occurs at high latitudes and
  in the tropics, with the maximum in the Southern
  Ocean between 44S and 58S.
• Little uptake occurs at mid-latitudes however,
  most anthropogenic carbon is stored at mid
  latitudes. From the uptake and storage, we
  calculate equator-ward transport of anthropogenic
  carbon from high latitude regions and pole-ward
  transport from the tropics.
• The anthropogenic carbon inventories and inverse
  estimates suggest a small amount of northward
  transport across the equator
• However, model transport is an important source
  of uncertainty in the inverse estimates, with a
  between-model range of up to 135 of the uptake
  in the Southern Ocean.

• Inverse Model
• We use a Greens function inverse technique
  which is analogous to atmospheric inversions used
  to infer surface fluxes from observations of the
  spatiotemporal distribution of trace gases in the
  atmosphere. First, the surface of the ocean is
  divided into discrete regions (Figure 2). Then,
  an OGCM is used to create a Greens function for
  each surface region (Figure 3), which describes
  how fluxes at the surface influence
  concentrations in the ocean interior. The
  estimates of anthropogenic carbon are treated as
  a linear combination of the Greens functions
  multiplied by the surface fluxes.
• ?Cjant Anthropogenic carbon calculated from
  observations at site j
• xi Magnitude of the flux from region i
• Hi,j The modelled response of a unit flux from
  region i at station j, or basis functions

References Gloor, M., N. Gruber, J.L.
Sarmiento, C.S. Sabine, R.A. Feely, and C.
Rödenbeck, A first estimate of present and
pre-industrial air-sea CO2 flux patterns based
on ocean interior carbon measurements and
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doi10.1029/2002GL015594, 2003. Gruber, N.,
J.L. Sarmiento and T.F. Stocker, An improved
method for detecting anthropogenic CO2 in the
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809-837, 1996. Matsumoto, K., J. L. Sarmiento,
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Follows, Y. Gao, A. Gnanadesikan, N. Gruber, A.
Ishida, F. Joos, K. Lindsay, E. Maier-Reimer, J.
C. Marshall, R. J. Matear, P. Monfray, R.
Najjar, G.-K. Plattner, R. Schlitzer, R. Slater,
P. S. Swathi, I. J. Totterdell, M.-F. Weirig, Y.
Yamanaka, A. Yool, J. C. Orr, Evaluation of
ocean carbon cycle models with data- based
metrics, Geophysical Research Letters, 31,
L07303, doi10.1029/2003GL018970, 2004.
Figure 2. Inverse model region definitions.
Circles denote the locations of observational
data used to constrain the inversion.
Contact Information Sara E. Mikaloff
Fletcher E-mail fletcher_at_igpp.ucla.edu Tel
[phone] Web http//quercus.igpp.ucla.edu/Oc
eanInversion
Figure 3 Column integral of sample basis
function describing anthropogenic carbon flux
into a region in the South Atlantic, outlined in
black (mol/cm2).