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Discussion papers
https://doi.org/10.5194/acp-2019-300
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2019-300
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 15 May 2019

Submitted as: research article | 15 May 2019

Review status
This discussion paper is a preprint. A revision of the manuscript is under review for the journal Atmospheric Chemistry and Physics (ACP).

Variability in a four-network composite of atmospheric CO2 differences between three primary baseline sites

Roger J. Francey, Jorgen S. Frederiksen, L. Paul Steele, and Ray L. Langenfelds Roger J. Francey et al.
  • CSIRO Oceans and Atmosphere, Aspendale, Victoria, Australia

Abstract. Spatial differences in the monthly baseline CO2 since 1992 from Mauna Loa, (mlo, 19.5° N, 155.6° W, 3379 m), Cape Grim (cgo, 40.7° S, 144.7° E, 94 m) and South Pole (spo, 90° S, 2810 m), are examined for consistency between four monitoring networks. For each site pair, a composite based on the average of NOAA, CSIRO and two independent SIO analysis methods is presented. Averages of the monthly standard deviations are 0.25, 0.23 and 0.16 ppm for mlo-cgo, mlo-spo and cgo-spo respectively. This high degree of consistency and near-monthly temporal differentiation (compared to CO2 growth rates) provides an opportunity to use the composite differences for verification of global carbon cycle model simulations. Interhemispheric CO2 variation is predominantly imparted by the mlo data. The peaks and dips of the seasonal variation in interhemispheric difference act largely independently. The peaks mainly occur in May, near the peak of Northern Hemisphere terrestrial respiration. Boreal spring is when interhemispheric exchange via eddy processes dominates, with increasing contributions from mean transport into boreal summer. The dips occur in September, when the CO2 partial pressure difference is near zero, just after the peak in the mean interhemispheric exchange via the Hadley circulation. Surface-air terrestrial flux anomalies would need to be up to an order of magnitude larger than found in order to explain the peak and dip CO2 variations (large enough to significantly influence short-term northern hemisphere growth rate variations). Recent features in the composite records, inconsistent in timing and amplitude with air-surface fluxes, are largely consistent with interhemispheric transport variations. These include the remarkable stability in annual CO2 inter-hemispheric difference in the 5-year relatively ENSO-quiet period 2010–2014, and the 2017 recovery in the CO2 interhemispheric gradient from the unprecedented ENSO event in 2015–16.

Roger J. Francey et al.
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Status: final response (author comments only)
AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment
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Roger J. Francey et al.
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Short summary
25-year composites of interhemispheric baseline CO2 differences demonstrate close agreement between 4 monitoring networks. Variability from monthly to multiyear timeframes mostly reflects variability in upper troposphere dynamical indices chosen to represent eddy and mean transport interhemispheric exchange. Monthly interhemispheric atmospheric fluxes are much larger than air-surface terrestrial exchanges. The composite differences offer unusual constraints on transport in global carbon models.
25-year composites of interhemispheric baseline CO2 differences demonstrate close agreement...
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