Ocean Science
www.ocean-sci.net
1812-0784
1812-0792
2
1
2006
10.5194/os-2-43-2006
http://www.ocean-sci.net/2/43/2006/
http://www.ocean-sci.net/2/43/2006/os-2-43-2006.html
http://www.ocean-sci.net/2/43/2006/os-2-43-2006.pdf
43
60
2006-07-17
Interannual-to-decadal variability of North Atlantic air-sea CO<sub>2</sub> fluxes
S. Raynaud
stephane.raynaud@cea.fr
J. C. Orr
O. Aumont
K. B. Rodgers
P. Yiou
LSCE/IPSL, Laboratoire des Sciences du Climat et de l'Environnement, CEA-CNRS-UVSQ, Gif-sur-Yvette, France
LOCEAN/IPSL, Laboratoire d'Oc\'eanographie et du Climat: Expérimentation et Approches Numériques, CNRS-IRD-UPMC, Paris, France
now at: Atmospheric and Oceanic Sciences (AOS) Program, Princeton University, Princeton, USA
The magnitude of the interannual variability of North Atlantic air-sea
CO<sub>2</sub> fluxes remains uncertain. Interannual extremes simulated by
atmospheric inverse approaches are typically about ±0.3 Pg C yr<sup>−1</sup>,
whereas those from ocean models are less than ±0.1 Pg C yr<sup>−1</sup>. Thus
variability in the North Atlantic is either about 60% or less than
20% of the global variability of about ±0.5 Pg C yr<sup>−1</sup> (as estimated
by both approaches). Here we explore spatiotemporal variability within
the North Atlantic basin of one ocean model in order to more fully
describe potential counteracting trends in different regions that may
explain why basin-wide variability is small relative to global-scale
variability. Typical atmospheric inverse approaches separate the North
Atlantic into at most a few regions and thus cannot properly simulate
such counteracting effects. For this study, two simulations were made
with a biogeochemical model coupled to a global ocean general
circulation model (OGCM), which itself was forced by 55-year NCEP
reanalysis fields. In the first simulation, atmospheric CO<sub>2</sub> was
maintained at the preindustrial level (278 ppmv); in the second
simulation, atmospheric CO<sub>2</sub> followed the observed increase.
Simulated air-sea CO<sub>2</sub> fluxes and associated variables were then
analysed with a statistical tool known as multichannel singular
spectrum analysis (MSSA). We found that the subtropical gyre is not
the largest contributor to the overall, basin-wide variability, in
contrast to previous suggestions. The subpolar gyre and the
inter-gyre region (the transition area between subpolar and
subtropical gyres) also contribute with multipolar anomalies at
multiple frequencies: these tend to cancel one another in terms of the
basin-wide air-sea CO<sub>2</sub> flux. We found a strong correlation between
the air-sea CO<sub>2</sub> fluxes and the North Atlantic Oscillation (NAO), but
only if one takes into account time lags as does MSSA (maximum
<i>r</i>=0.64 for lags between 1 and 3 years). The effect of increasing
atmospheric CO<sub>2</sub> (the anthropogenic perturbation) on total variability
was negligible at interannual time scales, whereas at the decadal
(13-year) time scale, it increased variability by 30%.
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