Abstract
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 Scripps Institution of Oceanography (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) provide 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 (NH) terrestrial photosynthesis/respiration cycle. February–April is when interhemispheric exchange via eddy processes dominates, with increasing contributions from mean transport via the Hadley circulation into boreal summer (May–July). The dips occur in September, when the CO2 partial pressure difference is near zero. The cross-equatorial flux variation is large and sufficient to significantly influence short-term Northern Hemisphere growth rate variations. However, surface–air terrestrial flux anomalies would need to be up to an order of magnitude larger than found to explain the peak and dip CO2 difference variations. Features throughout the composite CO2 difference records are inconsistent in timing and amplitude with air–surface fluxes but are largely consistent with interhemispheric transport variations. These include greater variability prior to 2010 compared to the remarkable stability in annual CO2 interhemispheric difference in the 5-year relatively El Niño-quiet period 2010–2014 (despite a strong La Niña in 2011), and the 2017 recovery in the CO2 interhemispheric gradient from the unprecedented El Niño event in 2015–2016.
Highlights
Atmospheric CO2 measurements are normally introduced into global carbon budgets as a “global growth rate . . . based on the average of multiple stations selected from the marine boundary layer sites with well-mixed background air . . . , after fitting each station with a smoothed curve as a function of time, and averaging by latitude band . . . ” (Le Quéré et al, 2018)
The NOAA atmospheric sampling is generally more frequent than is the case for the Scripps Institution of Oceanography (SIO) or CSIRO programs; the size and sampling frequency in the NOAA network amplifies calibration challenges due to shorter lifetimes of reference and calibration standards. Both NOAA and SIO use non-dispersive infrared analysers (NDIR) for CO2 measurement (CSIRO flask sampling at cgo, spo, and mlo in the early 1980s used NDIR for analysis of chemically dried air, pressurized into 5 L glass flasks)
While the models have demonstrated an impressive ability to predict mid-to-highlatitude CO2 variations influenced by weather, it is less clear that short-term variations in IH exchange have been adequately captured
Summary
Atmospheric CO2 measurements are normally introduced into global carbon budgets as a “global growth rate . . . based on the average of multiple stations selected from the marine boundary layer sites with well-mixed background air . . . , after fitting each station with a smoothed curve as a function of time, and averaging by latitude band . . . ” (Le Quéré et al, 2018). A clearer indication of the global impact of regional emissions comes from sites demonstrating maximum spatial representation In this case, global significance of biogeochemical CO2 exchanges between the surface will be informed by their impact on validated baseline data with the least continental influence. The scope of this paper includes (a) reduction of measurement uncertainties in IH CO2 difference using a three decade composite of published CO2 measurement results (distinguished by maximum spatial representation and by well-documented sampling and measurement quality), and (b) demonstration of the potential uses of the composite CO2 record by comparing anomalies in the magnitude and phasing of composite IH CO2 variations with those in air–surface exchange model outputs, as well as in dynamics indices representing atmospheric IH exchange
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