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Biogeosciences An interactive open-access journal of the European Geosciences Union
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Volume 9, issue 3 | Copyright

Special issue: Integrated Land Ecosystem-Atmosphere Processes Study (iLEAPS)...

Biogeosciences, 9, 875-891, 2012
https://doi.org/10.5194/bg-9-875-2012
© Author(s) 2012. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 01 Mar 2012

Research article | 01 Mar 2012

The imprint of surface fluxes and transport on variations in total column carbon dioxide

G. Keppel-Aleks1, P. O. Wennberg1, R. A. Washenfelder2, D. Wunch1, T. Schneider1, G. C. Toon4, R. J. Andres3, J.-F. Blavier4, B. Connor5, K. J. Davis6, A. R. Desai7, J. Messerschmidt8, J. Notholt8, C. M. Roehl1, V. Sherlock9, B. B. Stephens10, S. A. Vay11, and S. C. Wofsy12 G. Keppel-Aleks et al.
  • 1California Institute of Technology, Pasadena, CA, USA
  • 2National Oceanic and Atmospheric Administration, Boulder, CO, USA
  • 3Oak Ridge National Laboratory, Oak Ridge, TN, USA
  • 4NASA Jet Propulsion Laboratory, Pasadena, CA, USA
  • 5BC Consulting, New Zealand
  • 6The Pennsylvania State University, University Park, PA, USA
  • 7University of Wisconsin, Madison, WI, USA
  • 8University of Bremen, Bremen, Germany
  • 9National Institute of Water and Atmospheric Research, Wellington, New Zealand
  • 10National Center for Atmospheric Research, Boulder, CO, USA
  • 11NASA Langley Research Center, Langley, VA, USA
  • 12Harvard University, Cambridge, MA, USA

Abstract. New observations of the vertically integrated CO2 mixing ratio, ⟨CO2⟩, from ground-based remote sensing show that variations in CO2⟩ are primarily determined by large-scale flux patterns. They therefore provide fundamentally different information than observations made within the boundary layer, which reflect the combined influence of large-scale and local fluxes. Observations of both ⟨CO2⟩ and CO2 concentrations in the free troposphere show that large-scale spatial gradients induce synoptic-scale temporal variations in ⟨CO2⟩ in the Northern Hemisphere midlatitudes through horizontal advection. Rather than obscure the signature of surface fluxes on atmospheric CO2, these synoptic-scale variations provide useful information that can be used to reveal the meridional flux distribution. We estimate the meridional gradient in ⟨CO2⟩ from covariations in ⟨CO2⟩ and potential temperature, θ, a dynamical tracer, on synoptic timescales to evaluate surface flux estimates commonly used in carbon cycle models. We find that simulations using Carnegie Ames Stanford Approach (CASA) biospheric fluxes underestimate both the ⟨CO2⟩ seasonal cycle amplitude throughout the Northern Hemisphere midlatitudes and the meridional gradient during the growing season. Simulations using CASA net ecosystem exchange (NEE) with increased and phase-shifted boreal fluxes better fit the observations. Our simulations suggest that climatological mean CASA fluxes underestimate boreal growing season NEE (between 45–65° N) by ~40%. We describe the implications for this large seasonal exchange on inference of the net Northern Hemisphere terrestrial carbon sink.

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