The imprint of surface fluxes and transport on variations in total column carbon dioxide
New observations of the vertically integrated CO_2 mixing ratio, ⟨CO_2⟩, from ground-based remote sensing show that variations in CO_2⟩ 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 ⟨CO_2⟩ and CO_2 concentrations in the free troposphere show that large-scale spatial gradients induce synoptic-scale temporal variations in ⟨CO_2⟩ in the Northern Hemisphere midlatitudes through horizontal advection. Rather than obscure the signature of surface fluxes on atmospheric CO_2, these synoptic-scale variations provide useful information that can be used to reveal the meridional flux distribution. We estimate the meridional gradient in ⟨CO_2⟩ from covariations in ⟨CO_2⟩ 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 ⟨CO_2⟩ 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.
© 2012 Author(s). This work is distributed under the Creative Commons Attribution 3.0 License. Published by Copernicus Publications on behalf of the European Geosciences Union. Received: 30 June 2011; Published in Biogeosciences Discuss.: 27 July 2011; Revised: 2 January 2012; Accepted: 9 February 2012; Published: 1 March 2012. Support for this work from NASA Carbon Cycle Program grant NNX08AI86G is gratefully acknowledged. GKA acknowledges fellowships from NSF and AAUW. The simulations used in this study were performed on the Caltech Division of Geological and Planetary Sciences Dell Cluster. Lauder TCCON measurements are funded by New Zealand Foundation of Research Science and Technology contracts C01X0204, C01X0703, and C01X0406. HIPPO is supported by the National Science Foundation and the National Ocean and Atmosphere Administration. CarbonTracker 2009 results were provided by NOAA ESRL, Boulder, Colorado, USA from the website at http://carbontracker.noaa.gov. LEF flux tower observations were made possible with assistance from A. Andrews (NOAA), J. Thom (UW), D. Baumann and M. Kubiske (USFS), and R. Strand and J. Ayers of the Wisconsin Educational Communications Board, and supported by Department of Energy (DOE) Office of Biological and Environmental Research (BER) National Institute for Climatic Change Research (NICCR) Midwestern Region Subagreement 050516Z19 and the National Science Foundation (NSF) Biology Directorate Grant DEB-0845166. RJA was sponsored by US Department of Energy, Office of Science, Biological and Environmental Research (BER) programs and performed at Oak Ridge National Laboratory (ORNL) under US Department of Energy contract DE-AC05-00OR22725. We acknowledge financial support by the Senate of Bremen and the EU projects IMECC and GEOmon as well as maintainance and logistical work provided by AeroMeteo Service (Bialystok).
Published - bg-9-875-2012.pdf